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Outdoor Free Flight Forum => Free Flight Scale => Topic started by: Prosper on June 01, 2014, 05:44:37 AM



Title: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 01, 2014, 05:44:37 AM
Hullo everyone,

For some weeks now I've been working on a Hawker Tempest II model, 1/24 scale for rubber power. It's proving (mostly) less troublesome than the Fw 190D I made over the winter - but there's a lot more of it! Also I've decided to use pendulum-controlled ailerons, which adds another layer of complexity. I didn't start a 'build' thread because I was too cowardly, but now I've got to a stage where it looks like it might actually progress to completion (gasp), so I thought I'd start a 'retrospective' topic.

The original drawings are Arthur Bentley's - extremely detailed and with an air of authority about them. I haven't come across any inconsistencies so far, despite having been researching this design for over a year now in odd hours here and there. Sometimes just a cursory study of photos of an original will demolish any sense of trust you might have had in someone's published drawings, but not in this case.

I've gone for what ought to be a sturdy design, but it's all TLAR, so I may have misjudged the strength needed, or failed to spot a critical weakpoint. Assuming I manage to finish it and am strong enough to throw it (I'm working on my dumb-bells), it will be by a large margin the biggest model I've flown. Bf109 wing area 280 cm2, flying weight 23-ish grams. Fw190D 317.7 cm2, 30-ish grams. Tempest 486cm2, estimated 40-something grams.

I'm going to break up the 'back-story' into separate posts. This a.m. I've been doing some dry-runs before assembling the stick-and-former fuselage framework, and it occurred to me that since the framework was hanging together without glue, I could mock the rest of the bits up around it. It just held together long enough for a couple of photos, then fell apart!

I've made plenty of foul-ups already, some just down to clumsiness, impatience and bodgy workmanship, some because I'm trying new things, which don't always work out. Some because making model aeroplanes is difficult :P.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 01, 2014, 05:49:11 AM

The Tempest was physically comparable to the Republic P47 - about the same size and power. The performance characteristics were very different though. The Tempest II never flew in WWII combat because despite having first flown in 1943, and being generally superior to the Sabre-engined Tempest V, there weren't enough Centaurus engines to get production going. By 1945 they were being churned out and the Tempest II was slated to go to South East Asia to help roll up the Japanese, but things ended sooner than expected, mostly courtesy of two B-29s.

Here's a picture I took of LA607 with my Kodak Instamatic in 1972, at Staverton. The Ford Capri wasn't my dad's - he was more of a Cortina Estate type of dad! LA607 was the second prototype Tempest II, and ended up with Kermit Weeks in FLA, last I heard of it.

The Tempest is a very handsome object to my eye but the gurning chinny-chin-chin of the Sabre-engined models rather detracts from the look, for me. I was going to opt for a Mk V, because the chin is a great place to mount a pendulum, but I thought, naa-aa, build what you want, and cram the pendulum in anyhow ;D. What's more, the radial engine nose is slightly longer, and I thought it would be more straightforward to build (I was wrong there). If this flies I'll probably make a Tempest V, maybe invasion stripes and whatnot, in the future.

I've chosen to represent MW800, because according to an artist's impression "off the internet" it had a red spinner. It was an early example, built in 1945. Here's a pic of some of its close relatives. Y'know, it occurs to me that whoever took this picture in 1945 was a bit more skilled than I was with my Instamatic; whadya think? :).

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on June 01, 2014, 06:56:43 AM
 ;D Nice work, Prosper! I've built one of these, not as tidy as yours, but I do prefer your full metal look.
   I believe my father used to work on the engines on these in occupation forces in Germany. sadly he is dead and I never asked him enough questions.
   As always I look forward to watching your progress.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Don McLellan on June 01, 2014, 12:55:45 PM
Another exceptionally beautiful model Stephen!  Very happy you're sharing your progress.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 01, 2014, 01:34:03 PM
Thanks Don, thanks sparkle :D. Yes, always sad to think of the experience, knowledge and insight lost when people die. Mind you, decades ago I knew a bloke who flew Tempest Vs in the war, I used to ask him questions - but he didn't want to talk about it!

I began this build with the wings - thought they'd be the most difficult bit (correct, so far). I took a lot of step-by-step photos, because I think I might have settled on a sequence of wing-building that would suit many or most wings made of aliphatic or cyano-coated balsa. Don't worry, I won't bore anyone with the whole lot!

I used aliphatic with silver (i.e. aluminium powder) pigment added instead of white. I've got quite a bit of the white sheet left over, so the rest of the airframe's white. I also tried scribing panel lines on the flat sheet, prior to assembly. This brings its own problems of precise registration of the wing skins during construction, but if I can learn to get that right it's by far the easiest way to do things.

The wing section is Hawker's own, 14% thick at the centre-section tapering to 10% at the wingtip rib. It looks symmetrical but isn't quite. It's a section with a small radius leading edge, and this meant that I could use flat sheet and not skins that had been wetted and bound to a mould to pre-form them, because the skins don't undergo much curvature (the grain of the skins runs fore-and-aft; they don't want to bend much).

Rather than the prefab spar sheet I used for earlier efforts, I thought I'd better make each spar individually, a lighter approach for deep spars. I worked out a method which is quick and easy, based on the fact that the spacing and verticality of the uprights isn't at all critical - in other words, just fix the uprights any old how, no marking, measuring or left-a-bit, right-a-bit :). The uprights are fixed each end with a small drop of thin CA run in by capillary action, and trimmed later. The tail uses spars made from prefab sheet, just the same as I used for the Fw190 and Bf109.

The two ribs and the spars are then fixed to the bottom skin and the top skin is glued on with aliphatic, which gives the longer working time needed for the job (there's plenty of left-a-bit-right-a-bit going on). I sat the bottom part in a crude cradle in order to get the washout right - trouble is the cradle was too crude - the port wing ended up with a couple or three degrees of washout, not wanted - I had then to give the right wing the same washout. I can't see from photos or the drawings that the original had any washout, and I'm not a fan of the stuff.

Before fixing the top skins on the main wing panels, I installed the pushrods and bellcranks for the ailerons, and also the landing / ID lights. The lateral pushrods (1/16" sq) now stuck out until the wings were joined - with all the work left to do it was amazing that I got through without snapping them off with my normal Mr Magoo type clumsiness. They also meant that I couldn't give the wingrib a final facing with a sanding block to render it completely flat, so the mainwing/centresection joints suffered a little.

More shortly!

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: tom arnold on June 01, 2014, 03:03:34 PM
Beautiful job, Prosper. We had a fellow here fly a Tempest II at Geneseo and it was a good flyer and eventually went missing in action after a mission at Geneseo one summer. His had a smidgin more dihedral than scale and the pilot admitted it was a bit on the edge on windy days. What sort of pendulum system are you planning on? A vertical or angled-horizontal one? And a construction question too: why was the grain of the wing skins chosen to go chord-wise vs. span-wise? Many thanks and looking forward to seeing this build.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 02, 2014, 04:32:01 AM
Thanks for the encouragement Tom, and the questions. Seems like your friend's Tempest II needed a D/T, huh? That seems to be the way of it in the FAC/Geneseo style of scale-model building, as far as I can make out ;D. My first target will be 5 seconds ;D.

The pendulum will be near-vertical and as far forward as I can get it. The slight tilt is not due to any fancy theory of operation, just down to getting the thing to fit in a way that doesn't (I hope) get fouled by the rubber motor :). As you can imagine the pendulum will be very short, pivot-to-bob distance, and this leads to a very twitchy action. If you cause the pendulum to move by yawing the fuselage quickly from side to side, the pendulum swings in a very excited fashion and takes a few cycles to damp down. A long pendulum would react less and damp better, I imagine. I made a simple, non-scale S&T test model last year and that worked. Whether this bigger effort will work is questionable though. The ailerons are much heavier than the ones in the test model. They'll have much geater aerodynamic forces acting on them too. Against this, the weight of the Tempest's pendulum will be greater than the one in the test model. Another consideration is that once you've glued the wing skins up, it's sayonara to the bellcrank/pushrod mechanism! If any friction develops due to bad construction, that's it. To get at it would need major surgery. . . The Tempest has some dihedral. If the ailerons don't work perhaps I can hope that it could fly large circles in very calm air and over long grass!

The grain runs chordwise as a convenience more than anything. I find the prospect of making ribs for tapered wings very off-putting. I know they can be made by sanding rectangular blanks squeezed between a root and a tip template, but this never seemed accurate enough to me, and it doesn't work for elliptical wings. I know you can put spacers between the blanks to increase accuracy, but by then you may as well make a whole solid wing from balsa or foam, and slice it chordwise to get your exact rib profiles. Using chordwise grain and several spars needs just a root and a tip rib to define an exact aerofoil section. The method seems much quicker and easier to me and hasn't (yet) shown any major shortcomings in flying/crashing. . . When I make a DHC2 Beaver or suchlike wing with constant chord and thickness, then I'll try ribs and spanwise grain, and see how the two methods stack up.

Regards,
Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 02, 2014, 04:35:20 AM
I mentioned landing/ID lights - the Tempest had a red and a green lamp in one bowl under the stbd wing and a retractable landing light under the port. I reshaped the end of a Sharpie pen slightly and plunge-moulded two reflector bowls from thin transparent plastic sheet stretched between two battens. This was quick because no female mould or cutout was needed. I painted the outside of the bowls with silver paint and trimmed them to shape. I cut lenses from the same thin sheet and glued them into place in cutouts made in the wing skin. This was hard because I just couldn't see the tiny transparent discs most of the time! Then I glued the bowls on the inside. You can see the Sharpie pen influence in the last pic :).

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on June 02, 2014, 07:57:47 AM
 ;D Thats mad detail!


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: billdennis747 on June 02, 2014, 09:21:31 AM


The pendulum will be near-vertical and as far forward as I can get it.


I always enjoy hearing about pendulum experiments but I don't think vertical, side to side pendulums work. Fernando Ramos showed this. As a model tightens into a turn, the pendulum will swing out and it's time to collect the bits in a bag. I suspect the tempest will fly anyway. I'm interested in why you didn't go for a shallow-angle pendulum as it would fit better, could be longer and would have better mechanical advantage?


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 02, 2014, 03:25:44 PM
Well the answers are difficult because it's based only on my no doubt half-witted ponderings. I can say certainly that an inclined, long pendulum is as much of a problem in terms of rubber motor fouling as a shorter, vertical one. You still need the pivot to be well above the bottom of the fuselage. As for centrifugal v. gravity the more I think abut it the more speculative it gets, but it helps me to recall that I made a vertical-pendulum test model last autumn that made long flights with no dihedral at all, a low-winger.

A canted pendulum simply cannot refrain from doing the wrong thing in any significant climb, or dive, depending on which end is hinged. A vertical pendulum though just becomes less lively as climb or dive angle increases.

Without a linked rudder, a pendulum controlling ailerons is just a wing-leveller (or to be precise a bank-angle maintainer). In a steady, balanced turn the vertical pendulum is central, i.e. on the aircraft's vertical axis, so it's doing nothing. The essence is in how fast the ailerons can react to any gust-or-turbulence induced excursion in roll. If they can react to restore the trimmed bank angle before the roll causes a new rate of turn, i.e. before it's muddled by a change in centrifugal force, then you're winning, and a vertical pendulum will react faster than an inclined one. I'm not saying it'll be successful in restoring the bank angle, just that it will try, up to the point of full aileron deflection. If the turn is a flat (skidding) turn, centrifugal force will swing the pendulum in the direction which causes into-turn bank, i.e. it tries to restore a balanced turn. If the turn is a slipping turn, again centrifugal force will move the pendulum in the sense that picks the lower wing up, tending to restore balance.

An inclined pendulum will respond to slips and skids likewise, only I imagine more slowly. It will also be centred in a balanced turn, if I understand aright. But in the case of picking up a dropped wing before the bank induces a heading change, I believe it's in trouble. No heading change no centrifugal force change, so the inclined pendulum will just swing full-deflection, but ponderously because of its length and inclination. In other words it will react slowly, but overreact completely.

As a model tightens into a turn, the pendulum will swing out and it's time to collect the bits in a bag.

I presume that by swing out you mean the pendulum will be affected more by centrifugal force than by gravity, thus actually causing an into-turn aileron deflection? I'm still not sure that the pendulum will go past the vertical, in other words it will swing to where there's no aileron input and the model's in the same situation as a locked-control model. certainly that's a dire-sounding situation for a heavy low-winger with little dihedral and a huge vertical tail area!

I suspect the tempest will fly anyway.

I suspect so too, but it needs to fly for a decent spell. Instability is a duration-killer. In layout and dihedral it's remarkably like a giant Chilton DW1 - but with consequently greater inertia forces, also greater wing-loading, and the lack of dihedral-enhancing trouser fairings. The dihedral on most WWII low-wing fighters seems adequate to me, but the Tempest is pushing it really hard IMO.

These are just my musings, I'm hoping to hear from anyone who can correct or add to the above.
Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: tom arnold on June 02, 2014, 10:27:22 PM
I think Bill was quoting from an article that Fernando Ramos wrote in the old Model Builder mag eons ago. He took a vertical pendulum "bread board" arrangement aloft in his full size airplane and had his passenger hold it in his lap and they observed it as he went into a diving turn much like a model will do. The pendulum bob swung to the outside of the turn due to centrifugal force immediately which (would have) increased a model's bank angle. In using a second arrangement with a horizontal pendulum canted down about 10-15 degrees and hinged at the front, it was not affected by the turn no matter how tight and always moved "down wing", so to speak. The pendulum was buried in the wing and was as long as the wing chord. His passenger, modeller Jack McCracken, then built a jumbo gas scale model of the Bellanca Columbia and it flew flawlessly and I witnessed many flights of it. However, it really did not need it but you could see the wings getting picked up at regular intervals as the system did its thing.

Having said all that, I do think a vertical pendulum would work in the beginning of a wing low situation and if it worked early enough, the turn would never develop. If the turn develops, though, well......


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 03, 2014, 03:29:55 AM
Ah, that's a very illuminating account Tom. With a front-hinged inclined pendulum, he's trading gravity against centrifugality in that one condition, a developing spiral dive. That's probably fair enough given the terminality of that condition in models with inadequate spiral stability, but I can't see how, with that setup and in a straight ahead dive of over 10-15 degrees, such as in stall recovery or glide entry, the pendulum won't go to full-stop one side or the other. The only answer would be to stay away from full stalls, and while advisable, that's not easy for models with small tailplanes, or flying in a breeze, or while trimming, or flirting with CL max to go for max endurance and scale speed. . .
 
Conversely, in a climb the aileron effectiveness of this same pendulum setup will reduce dramatically, so I don't know how it would work for a model that's tip-stalling in a climb (however if you're lucky it might sort things out in the subsequent spiral dive!).

If the turn develops, though, well......
I do regret not having opted for a rudder linkage in this model - I looked hard at it and it's not a very easy job. A rudder changes the whole story. I'd presumed on a floating rudder because of the huge fin area and small dihedral. We'll see how it works out. . .:)

Stephen.



Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 03, 2014, 03:31:39 AM
;D Thats mad detail!

Well in fact these lights turned out easy to make, so didn't make me 'mad' ;D.

I went for navlites too. The only way to do it was to use solid plastic blocks (from a toothbrush handle). Moulding something to that extreme curvature would be very time-consuming, tricky and also fragile, I decided. Solid blocks are H-E-V-V-Y, just what you don't need at the wingtips, but they're strong, and placed just where a lot of whacks occur. I drilled out the inside face to represent the coloured bulb, and dropped paint into the inside of the drilling. The result looks great from some angles, but because the highly-curved plastic acts as a powerful distorting lens, from some angles the effect is of some lurid trinket from a Christmas cracker. After lots of consideration I decided that the pros outweigh the cons.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 03, 2014, 03:35:24 AM
Making the leading edge intakes was a faff. The drawings were a bit ambiguous and although I had fantastic photos from 'walkarounds' available on the net, it was hard to get the intakes right. Put another way, I didn't get them right :). They're completely integrated with the wing and I found myself making them up as I went along, cutting out little bits of sheet and adding them to the wing skins with little balsa tabs and so on - result is a dog's dinner. I could re-do them but perhaps I'll wait to see if they get wiped off in a crash and then the decision will be made for me! These intakes convinced me that in fact the Tempest V would be an easier prospect - nice, clean leading edges and a dirty great chin intake that you could enjoy whittling from a big block without worrying about the weight.

Finally, I glued the mainwing panels to the centre section panels and braced the wings together. The mainwing/centresection joints aren't completely flush, as can be seen near the L.E. in picture 2. The difficulty in sanding aliphatic and the fact that the skins are only 0.3mm thick may lead me to accept this blemish.

After my experience with building an Fw190, where I decided to let the wing/fuselage join pretty much look after itself, and paid the price in terms of time wasted bodging the joint, I thought this one through, and the way the two mate is pretty accurate and sound. I just hope it's strong enough :-O. The weight of the whole wing assembly shown here is 11.2g.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on June 03, 2014, 04:29:12 AM
 ;D  really neat work. i seem to remember the air intakes were a bit of a mystery when I did mine. Watching this is making me think I should repair mine.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 03, 2014, 05:42:24 AM
. . .making me think I should repair mine.

You should, I've been reading the comments on Don McL's Stinson A thread, I think he's right, it's getting it down from the shelf that's the hardest part, then figgering out how to go about it, but once you start hacking, you're away! All balsa can be scarfed, spliced, butted, doubled, infilled or what have you. I admit that tissue repairs are a problem if you're hoping for a "nobody will know" look, but my covering skills mean that even a model that's just freshly covered looks like it's crashed in a thorn hedge ;D.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Yak 52 on June 03, 2014, 05:51:53 AM
Great work as always Stephen - fascinating to watch.

I'm still a bit baffled as to pendulums  :) since in a coordinated turn there is no centrifugal force discernible by the 'pilot' (pendulum) just increased G. I suppose you are using the slip condition before the spiral dive to give the aileron response? ie a left movement of the pendulum gives right roll?


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 03, 2014, 05:53:34 AM
Next installment of the back-story, AKA "here's one I made earlier". There seems to be an awful lot of it, I'll be glad when I've caught up to the present. . .

The Tempest tailplane is disastrously thick - 14% thickness-to-chord - this is a real bummer in terms of weight and aerodynamics, but looking for something positive; it won't ever warp! The fin is thick too. The fin hasn't come out well - despite its thickness it has a pretty sharp leading edge and this caused fat-finger problems for me. Also the fin has to blend with the fuselage; I got this about right but I can see that when it's painted it's going to highlight a lot of unevenness.

Spars are cut from a large premade sheet of 0.3mm balsa with 0.3mm strips running across.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 03, 2014, 06:15:09 AM
Hi Jon, yes I'm baffled too. This is what I wrote yesterday.
Without a linked rudder, a pendulum controlling ailerons is just a wing-leveller (or to be precise a bank-angle maintainer). In a steady, balanced turn the vertical pendulum is central, i.e. on the aircraft's vertical axis, so it's doing nothing. The essence is in how fast the ailerons can react to any gust-or-turbulence induced excursion in roll. If they can react to restore the trimmed bank angle before the roll causes a new rate of turn, i.e. before it's muddled by a change in centrifugal force, then you're winning, and a vertical pendulum will react faster than an inclined one. I'm not saying it'll be successful in restoring the bank angle, just that it will try, up to the point of full aileron deflection. If the turn is a flat (skidding) turn, centrifugal force will swing the pendulum in the direction which causes into-turn bank, i.e. it tries to restore a balanced turn. If the turn is a slipping turn, again centrifugal force will move the pendulum in the sense that picks the lower wing up, tending to restore balance.


So yes, if the left bank increases the pendulum moves left-er relative to the model's vertical axis which will cause right-bank aileron. I think that if a wing tips it must take a measurable time for a turn to be established. If the ailerons can pick the wing up before this happens then good. The ailerons should be deflecting as the wing is tipping. I'll be aiming to lever them to attain a good deflection even with shallow bank angles, if the layout allows this (it's hard keeping this mechanism out of sight), but the main thing is how quickly they'll act, and that requires a system with negligible friction, which is difficult too. I suppose the way to look at it is that this isn't a system that would allow a not-very stable model to fly in a gale, but which might be made to work so as to increase its margin of safety in lighter airs. Yes, in theory a model like this could complete long circling flights in calm air, but experience with low-wingers with much more dihedral than the Tempest makes me think it's unlikely, and anyway when do we get calm air? :).

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 04, 2014, 04:59:55 AM
Lastly the fuselage - the double-curved rear fuselage was made by wetting sheet and binding it to a polystyrene form. You end up with left and right shells which are joined along the centreline, like a plastic construction kit. This gives you a monocoque sleeve which is braced with a few balsa hoops, and which should slide onto the stick-and-former fuselage frame with complete accuracy (!). Should, but something has gone a bit wrong in this case, the accuracy is a bit, er, less than complete. Anyway, that sentence signals that at last I've got this story up to the present moment :) :).

The pictures of the engine exhaust collector ring and spinner show that my turning of large items has improved - they're not round, in any sense an engineer would use the word, but they're a good deal rounder than what I've generally turned out hitherto!

The metal band around the rear cowling former is silver mylar. The Tempest II in the RAF museum has this as bright, bare metal, it looks like it might be stainless steel. I imagine this is the firewall, and that this is a genuine representation of the original's colour scheme, so I thought it would be good to try to emulate it. However since then I've learned that the museum's Tempest is a 'late model' whereas the one I've decided to represent is an early one. I can't find a single photo of an early Mk II that has a bare-metal band. . .hmmm, whether I leave this feature, as a bit of bling, or whether I do the authentic thing and spray over it with grey and green, I don't think I'll know till I've got the airbrush in my hand :).

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Don McLellan on June 04, 2014, 04:55:28 PM
Great stuff Stephen.  Always fascinating watching how you do things.  Very interested in the pendulum as well.

Don


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: DavidJP on June 06, 2014, 11:35:24 AM
I have ben admiring your work generally Steve and noted the intakes.  The ones on my West Wings Sea Fury looked quite nice in the raw state; but then I started to cover the airframe in the wrapping tissue supplied. I should have realised from the outset this was a silly mistake and used Esaki and airbrushed it to suite. But no - obstinate or silly or both.  And the tissue got all inside the intake and it now looks quite a mess because it seems impossible to remove.  So I am going to see how it flys and then decide whether or not to have a general refit. but of course it is intended for "Kit Scale" so intense detail, as you are so admirably portraying, will not be required.

Well done though - masterclass stuff and all that.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 07, 2014, 06:48:15 AM
 Thanks Dave!

. . .and which should slide onto the stick-and-former fuselage frame with complete accuracy (!). Should, but something has gone a bit wrong in this case, the accuracy is a bit, er, less than complete.


Well, I had to scrap the rear fuselage shell. Once it's glued along the seams, then each station has a fixed circumference, and if that doesn't match the circumference of the former it has to be fixed to, then you're scuppered. Try to fix the skin to the former at one point and it bulges out somewhere else. Cut out relief wedges from the skin and when they're closed up they cause kinks and ridges elsewhere. I stripped the whole thing off the frame and moulded two new half-shells, being extra attentive while binding the wet skin to the polystyrene mould. Luckily I had one last sheet of aliphatic coated balsa - I didn't fancy making more at present. It was the lowest-grade sheet in terms of imperfections but it'll have to do. I was extra-extra attentive when cutting the seams of each half-shell, and when gluing the seams up.

This time it worked and I'm in business again. There's a difficulty to overcome now which is that the tail section of course has to match the rear fuselage - the join has to be as near invisible as poss. The tail section was made as a matched pair with the now discarded rear fuse. shell. Getting the new one to match exactly will be no stroll. I made a big mess of this with my Fw 190 model and have kept telling myself how I'd get it just right this time. . .hmmmm. . .

I've also fixed the belly panel, which has distorted the thin, poorly-supported wing skin it joins to. Never mind, live and learn, live and learn. . .

Also made a spinner cap. I made a balsa plug to blend with the spinner and used this to plunge-mould a cap from 40 thou plastic card. Since the plug matched the spinner exactly, this meant that the cap was ~ 40 thou too big all round, but when I reduced the length and gave it a sanding, it fitted well. The Tempest's spinner is absolutely enormulous. Just the plastic cap on this model weighs 0.5g. Probably need weight there, but still makes me cringe a bit. The weight is starting to ratchet up fast.

Pic 3 shows using a brass rod as a guide for cutting the seam of a fuselage shell, still attached to the polystyrene mould. The rod conforms to the curve of the mould. The cut is visible just above the top of the rod.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on June 07, 2014, 06:54:05 PM
 ;D Amazing work!   8)


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 10, 2014, 05:44:23 AM
Thanks again, sparkle, have you decided to repair yours yet?

I've been doing a few nerve-wracking jobs on this one. I had to cut the former out of the tail section, split the tail section open along a seam, cut a wedge out, and glue it shut again, to reduce the circumference slightly where it mates with the rear fus. The difference in circumference was only a couple or three mm but that's enough to make a step between the two that sticks out like the proverbial sore thumb. The two sections are better matched now, and although I don't think I can make the join disappear, it shouldn't be too glaring (famous last words?).

It all sounds sensible and straightforward now I'm writing this a couple of days later, but it made me hop up and down at the time. I seem to have got away with it. The knife in the pic isn't so I can commit hara-kiri if it goes wrong, but to hack a crude expanded poystyrene form to slip into the tail section and provide a firm backing for the cutting and gluing.

Making the wing fillets has been setting my teeth on edge too. I'm only half way through that job :-0. There are tight radii and quite a bit of double curvature , so the force needed to get the fillet sections in place was plenty enough to crack stringers or distort formers if misdirected. Also, more than the usual ration of hands is required to keep it all steady and get it glued up. It's the same job as on the Fw190, but the Germans were kinder to modellers by contriving to make neat, simple fillets with very little double curvature. Anyway, What I've done so far has turned out OK. So I'm breathing easy for now.
 
Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: rgroener on June 10, 2014, 06:17:09 AM
Prosper, your technique is very special and so is the look of the model. This seems the be the way for a perfect finish of a all metal plane.
I am intersted to see the finished model. Will follow closely.

Roman


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Pat D on June 10, 2014, 06:45:52 AM
Watching with huge interest.

Great craftsmanship as ever..

Pat


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 10, 2014, 08:47:30 AM
Thanks for the interest fellas, I appreciate it.

I think this aliphatic or cyano-coated balsa is a good way to represent metal or ply-covered aeroplanes too, which is why I've been chugging away at it so long. When I started modelling I thought I'd be building Golden Age classics, British lightplanes from the 1920s/30s and so on - instead I seem to be stuck with nasty warplanes! Trying to get an (nearly) opaque, stressed-skin look that wasn't unflyably heavy just kept nagging at me!

However it's early days yet. I've learned to produce the raw sheet material quite painlessly now, though it does take some time - 3-4 coats of glue are needed and each coat has to dry. Also the balsa has to be reduced from 1/32" to 0.3mm, this needs a vacuum cleaner or dust-extractor and takes time and practise to get a good result. I'm gonna ask my supplier if they can provide thinner wood.

The stuff I make now typically weighs 70gsm - that's the weight of printer paper, say, i.e. it's very heavy, but is a much superior product to paper for building with. I guess an equivalent S&T model might weigh only half as much, but wouldn't have the same look - the old trade-off, scale v.duration!

The aliphatic-coated balsa has very little resistance against splitting along the grain, once it is punctured. If something stabs through the sheet, crop stubble for instance, the split can progress until it meets an internal member. This doesn't happen with CA-coated wood.

This method doesn't produce a 'plastic-kit' style flawless surface - or at least I've never really achieved one yet :).

Aliphatic-coated balsa can't really be sanded worth a damn. Thus if you join two components and the join is imperfect you can't sand the joint fair - or not very easily anyway. You may see evidence of this as the Tempest nears completion :-0.

This style of construction entails working with the outside skin of the model right from the outset of the build - this exposes the model for the fullest time to wear-and-tear during construction, in stark opposition to S&T modelling where the airframe can be made fair and then right at the end, a virgin sheet of tissue covering acts as a 'blank canvas' for the artist to get to work on. I reckon it's easier for an expert to get a flawless finish that way.

These are a few of the cons, but there are many pros too :). Pardon a long post :).


Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Don McLellan on June 10, 2014, 10:23:01 AM
Hi Stephen,

For me your builds read like a novel where you only get a chapter every so often.  It always makes my day when I get a new 'chapter'.

Great build.

Don


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 11, 2014, 01:52:36 PM
Cheers as ever, Don, here's a very short chapter:

Been struggling a bit with the fillets, kinda got writer's builder's block, so I taped together another mockup to give me a boost - this time the only tape needed is a piece holding the spinner cap to the spinner, so it must be coming together!

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on June 11, 2014, 06:11:56 PM
 :) That really captures the atmosphere! Gotta love the first time one sticks a new model together. Sadly  :(  my Tempest is still glaring at me! but it's on the "to do" list!


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on June 12, 2014, 05:55:38 AM
 ;D hi Steven, sorry to borrow your thread! Well it seems my to do list suddenly got shorter! The poor old girl is getting some battle damage fixed and I'm moving the motor peg forward to do away with the house brick in the nose!
     I have to say that the fully sheeted look is better. something to aspire too!
   


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 14, 2014, 11:20:53 AM
Nice work sparkle, well worth repairing - doesn't look like too much work!  That looks like a scale tailplane, or nearly so. I'm a big fan of forward motor pegs, but TBH that's maybe because I have to be - my all-sheet models end up so heavy you have to do everything remotely possible to minimise noseweight. . . With both the Bf 109 and more recent Fw 190 I've made,  I've been using motors about 4x hook-to-peg distance. As OZPAF John said elsewhere, this is "pushing it a bit" :D. Sometimes you get smooth motor runs, other times you get harsh vibration and uneven RPMs, and even a seized prop when the rubber jams in a great lump in the nose.

I've been nibbling away at this one, but I'm at the stage where you have a complete-looking airframe but realise that there's still any number of tasks left to tackle. I've decided to get the thing in some kind of flyable condition ASAP because time is pressing, and my neighbour's grass is getting rapidly back to the 'too-long-to-tramp-on' stage. When it's cropped that'll leave  a harsh grass stubble with hard bare patches - not what I want for test-flying this thing.

I said up the thread that I was confused about the pendulum issue, and the rudder makes this confusion even more complete. I think the fin area is much too large for the dihedral, and originally thought that I'd link the rudder to the pendulum. This looked too fiddly to achieve in such a small airframe though, so I decided to go for a floating rudder.

Trouble is, I didn't really know what a floating rudder is. If it's just a hinged, free-flopping rudder, then as far as I can see (not very far), then regardless of air forces acting on it, its weight will bias it one way or the other as soon as the model banks. This bias will be in the direction that exacerbates a spiral dive. To prevent this I thought I ought to mass-balance the rudder, but with the weight of this model marching upwards, that didn't seem a very good plan.

I've ended up with a bit of lead on a long stick ahead of the rudder. When the model banks this should cause the rudder to move in the direction causing a slipping turn (overbalanced rudder). The overbalance is so slight that it may be irrelevant v. air forces. This is ignoring centrifugal effects. The rudder can be made a fixed rudder if that seems necessary or preferable.

The whole assembly weighs 0.4g as seen. I've decided not to dope the rudder but to rely on the acrylic paint to fill the pores and give the tissue some proofing. Acrylic is not nearly as good as dope at this IMO, but at least this is right at the tail where there should be less wear and tear than further forward.

The Tempest has very large trim tabs on both elevators and rudder. I'm wondering if the movable rudder tab will act as a servo-tab - it would be very handy if it did.

Pic 2 shows the assembly. A few of the ribs have distorted as the tissue shrank. I think applying the acrylic paint will relax these - if it doesn't I'll re-cover the rudder at some point with a slacker covering. Pic 3 shows the hinges fixed to the fin post - scale positions of course :D.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Monz on June 14, 2014, 01:03:23 PM
This whole build is great, love that rudder setup. Going to copy it for a model of mine if you don't mind ;)


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: tom arnold on June 14, 2014, 01:39:10 PM
I have put a free-flopping rudder on many of my planes and have had good luck with them. Normally I make a sheet balsa glider of whatever my subject is and cut the rudder off to make sure it still flys. The purpose is to allow the plane to climb more steeply before a roll off occurs. My hinges have been very limp fishing line or pins in a aluminum tube----whatever, they have allow the rudder to fall to either side on their own weight very easily. I have never found a built up rudder to be so heavy that it falls into the prop blast and causes a spiral, though.

The servo tab concept does work and I cannot figure out why, to tell the truth. If you make a little adjustable servo tab, bend it the OPPOSITE way you want the rudder to work (don't ask me how I finally figured this out). If you hold the model in a moving stream of air, you can see the rudder riding a little bit to one side. Here's the part that gets fuzzy, though. If the servo tab causes the rudder to yaw the model, then logic says you don't have a free floating rudder any more but fixed one of sorts, no? If that is the case, then the purpose of a free floater is negated, no? I don't know the answer.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 14, 2014, 02:26:45 PM
Hi Monique, 'course I don't mind, but be careful - if you mean copying the mass-balance bit, I've no real idea yet whether it works or is worthwhile in any meaningful way. If you mean the hinge bit, and you can see where one rudder-pin drops into a hole in a finpost bracket, and another rudder-pin clips into another finpost bracket, then be careful that the clip is made so that you can press the pin into place without over-stressing the surrounding structure, and yet be sure that no flight loads are going to cause it to come loose.

Tom, thanks for the reassurance. I kinda hope I can dispense with the balance weight - who needs lead in the tail of their model?

As for the servo tabbed-rudder, I see what you mean, if it exerts a force on the aeroplane it's attached to (causing it to yaw) then the rudder seems to be fixed in a way. It is: it's fixed by aerodynamic forces, and not by hydraulics or a pilot's big fat boot. I guess I'd call it a 'biased floating rudder'. The aerodynamic forces which bias it one way or the other act on the fin too because the air sees the fin/rudder as a single aerofoil, and with the rudder biased one way the camber of the whole foil has changed. That creates asymmetric lift, and that makes the whole plane yaw, and the genuinely free-floating (but biased) rudder says "cor, look what I did". :). That's how I see it - maybe wrong. The deflection of the servo tab 'lifts' the rudder one way, creating camber of the fin/rudder which creates a much greater lift force, enough to yaw the whole shebang.

Regards,
Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Monz on June 14, 2014, 03:46:36 PM
Hi Monique, 'course I don't mind, but be careful - if you mean copying the mass-balance bit, I've no real idea yet whether it works or is worthwhile in any meaningful way. If you mean the hinge bit, and you can see where one rudder-pin drops into a hole in a finpost bracket, and another rudder-pin clips into another finpost bracket, then be careful that the clip is made so that you can press the pin into place without over-stressing the surrounding structure, and yet be sure that no flight loads are going to cause it to come loose.



Ja, the bold bit  ;D


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on June 14, 2014, 07:32:49 PM
 :D Hi Stephen, yes lots of discussion about rudder size etc.
 I went back to my build in 2010 and I there were similar discussions! In the end I think I reduced the fin, ( I can't find any reference to it though ) i know I would have given it a tiny bit more dihedral, (also the wings detach, wire in alum tubes, so they might bend a bit in flight giving me a bit more dihedral).
 Left/ right stability for me wasn't a problem from memory. Still some sort of moving rudder might be good if your fin is scale size.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: jym6aw6 on June 15, 2014, 12:03:40 AM
Absolutely fascinating model, as is usual with all of the Prosper builds I've seen  :) .

Question re "floating" rudders: some years back in some forum it was advised that floating rudders needed to flop only a few degrees max from either side of center. Apparently some sort of stops were built on the fin to limit rudder travel. Pretty sure it was less than 5 degrees.

Almost sure that Al Backstrom made the suggestion and I've searched for his post but to no avail, maybe it was on SFA or other.

Has anyone done this? It seemed counter-intuitive to me, but what do I know?  ::)  ;D . 


Jim (6aw6)


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on June 15, 2014, 02:31:34 AM
How did I miss this? Another fascinating build and more pendulum and now servo tab operated floating rudders explained in detail. What more could you want?

Fascinating.
John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: DHnut on June 15, 2014, 02:58:03 AM
Hi Stephen,
                 As usual a really interesting build. Sitting in my workshop is a 27" Tempest V built to the KK plan.It is done in the colours and markings of 486 Sqn who had the code SA so mine is SAM! One of those that Doug McHard found, and Lindsey Smith built some while ago. His has flown but I am not sure how well and what the trim was. I have managed to get mine gliding, but for some reason I put it away after initial power flights that I seem to remember were starting to power stall. I think the choice of a four bladed plastic prop may not have been wise either and the engineering of the nose block may also be not ideal to cope with an arrival. The weight was in the 90 grm region and the nose is sheeted back to the wing, together with an undercarriage that is on the flimsey side. Now the floppy rudder debate has started me thinking that it might be worthwhile looking at it again.
     Ricky


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 16, 2014, 06:35:21 AM
. . .for some reason I put it away after initial power flights. . .
Ha! So that's at least two Tempests that this thread has shooed out of the cupboard! Sparkle's one looks good to go, to me - if I had a model with that much 'battle damage' I'd still be flying it as it is ;D.

Jim, my rudder does indeed have about 5 deg travel either side - this is dictated by the freedom of movement of the balance bar that extends into the rear fus. more than by any calculation or experience, but I could see how a rudder that was allowed to deflect too far might stall the whole fin.

Well folks, this model has flown. Inadvisedly, since the wings were held on with masking tape, various large items like the canopy were missing, the pendulum system had no stops so it could potentially fly round 360 deg and mangle the pushrods, and a few other things invalidating its C. of A. However, I have to focus on non-modelling matters for a spell, and I thought I'd give this one the dignity of at least tasting the air!

I fixed up the prop assembly. Perhaps foolishly I fixed the thrust button in place with lots of right thrust. Looked at in side elevation the button is about half-way along the spinner, so tilting it 5 deg moves the base of the spinner a good distance off-centre. This looks silly from some angles, and necessitated carving chunks off each blade root to clear the cowling. The prop disc diameter is scale! If this thing ever achieves sustained flight, I think a scale prop disc is good in terms of overall flying realism.

I made a T-hook. Separating the strands of a braided motor to get them over each arm was not easy, but I suspect it's a knack that can be developed.

The motor peg is held internally. I did this with my Fw 190 model, and although there's less access in the case of the Tempest, getting the motor anchored is still easier than prodding around with stuffing sticks while squinting down the fuselage :). All that's needed externally is two very small holes to slide the stooge-wire through.

To stop the motor, peg and all, smashing into the rear fuselage in the case of a burst motor, two small pins need to be slid through the balsa motor-peg retainers. This is easy with the Fw 190 where removing the canopy+fairing effectively takes the top off the rear fus. It could be fiddly with the Tempest. These flights used a few hand-winds only so these were omitted.

Basic airframe weight is 26g, and about 3.5g of that is pendulum gubbins. The prop/hub assembly weighs 6.5g and I stuck in 7.5g of 1/4" rubber. This gave a flying weight of 40g and a CG at 27% m.a.c. with no noseweight needed. I didn't make the nose any heavier than I thought necessary - I think the good balance is down to the heavy prop/hub and good luck in keeping the tail light. I now hope for a final weight when paint, canopy and all details are added of less than 45g.

I taped up the ailerons and made a couple of short glides, which showed that a fair bit of down-elevator was needed. Then several flights of between 60-100 turns with taped ailerons. It was windy, but the model seemed very wayward all the same. It was impossible to guess what was going on really, but alarmingly, if it stalled it would yaw dramatically, almost seeming to yaw round its own vertical axis :-0. Then I untaped the ailerons. Several more flights suggested to me that the model flies better with active ailerons than without - but, it still wandered round in a mystifying fashion. One long hop was nice and stable - but with massively crossed-controls so it did an exaggerately crabbed flight that looked like it was a slow pass at an airshow :-0. The good news is that it can fly for at least 5 seconds ;D.

Anyway, perhaps I can squeeze a few more flights if the breeze calms down this evening. Things might click into place, but I rather suspect that even if this thing's trimmable at all, it's going to be a pig. When I trimmed the little Piper Vagabond I rebuilt recently I was secretly a tiny bit disappointed at how absurdly easy it was to trim, and kind of hankered for a bit more of a challenge - they say be careful what you wish for!

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on June 16, 2014, 06:51:35 AM
Love the red spinner! 8)  i also like the internal motor peg. neat!
Well my Tempest was back in skies today  ;D and flew nicely after moving the motor peg forward and loosing some lead in the nose!  Needs alittle more glide, which i fixed, but then ran out of light!  ::)
  Stephen,i'm sure yours just needs to be spoken to nicely!  :-* :-*
 well Ricky it looks like its up to you now!  ;)
 ( see how many smiley faces one can put in a single post!)  ;)


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 17, 2014, 04:10:44 AM
( see how many smiley faces one can put in a single post!)  ;)

Sparkle, you are, and always will be, the uncontested HPA smiley-face champ as far as I'm concerned :) :) :) :). Glad the model's back flying!

The breeze did drop a good bit last evening and I had the chance for a few chucks.

In my haste to get it flying at all yesterday morning I had skipped essential pre-takeoff checks! I didn't spot that it was 'taking off' with the prop pitch fully coarse - P.O. Prune would be proud of me :). It occurred to me that a model with too-coarse pitch will wander round in a muddled and arbitrary-seeming way (I've seen this before) - perhaps that was the problem?

I twisted the blades to a lower pitch at the tips, and also reset the aileron bias so that the ailerons were completely level when the model was level. Then (in much calmer air) the model started to fly. There's a new pre-takeoff 'vital action': Controls - FULL AND FREE IN THE CORRECT SENSE :D.

Since it was now behaving more like a model should, I got a video of the last flight of four. With such short hops a good bit of the flight is spent getting the camera on target, but FWIW;

http://youtu.be/pqpbMYrezKs

That's 10 sec from 200 turns. I don't like models turning right - kinda makes me a bit seasick, so I'll be reducing right thrust probably, but I need to learn more about the effects of rudder and ailerons first. By this last flight it had lost its rudder servo tab in the grass somewhere. Despite Youtube pixelation you might see tiny twitches, which in the original video can be linked to aileron deflections. Whether this is valuable response to disturbances or just artefact caused by motor vibration, or something else, I don't know, but the motor was running very smoothly. In theory the less visible the response the better - although then you're left wondering if there was any upset to respond to in the first place! Last night I watched film of an RAF Tornado negotiating the Welsh valleys at 200ft with its terrain-following radar. The tailerons were shivering this way and that at the heck of a rate, and the 'plane was steady as a rock.

Now I have to put the Tempest to one side for a week or three. Maybe no bad thing - I've been rushing it too fast. There's a whole list of things I've forgotten to do - I need to step back, survey the whole thing and do a fair bit of work before trying to paint it.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: billdennis747 on June 17, 2014, 05:29:42 AM
Stephen - can you not just tape up the control surfaces temporarily and see what happens? I did this on my SPAD, and it stopped flying.
It's definitely flying, isn't it.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: DHnut on June 17, 2014, 06:31:22 AM
Stephen,
            Will the right thrust be needed when the torque increases as the turns on the motor are increased? The response to aileron may be rapid because the inertia's are small in relation to the control forces being applied, just a thought.
Sparke, I take your point but there is a trip to Sydney in July for the NSW And Trans Tasman event and as usual too much to do so the Tempest will have to wait. Also the weather has turned wet windy and cool.
  Ricky


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 17, 2014, 07:46:22 AM
Bill, I will do this when I have the chance to fly it again, pref. when it's flying over long grass. It's quite possible that it could have managed such a low-torque flight with locked controls, given the light breeze. I suspect it would have slipped into a spiral though. It's only when I increase turns significantly that things will become clearer.

Hi Ricky, yes I expect it will still need some right thrust, but I think I bunged in too much. I would guess that under full power with the right thrust it's got, it would want to fly more or less straight - probably good in terms of stability and duration, but bad in terms of hitting things! I did rig the ailerons to have plenty of deflection per bank angle - the only thing is that I don't know how much this is reduced by aerodynamic forces on them. Since the model looks now as if it will turn out decently light (or do I mean not catastrophically overweight), then perhaps I can add some more weight to the pendulum if necessary, which ought to increase 'control authority' further.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on June 17, 2014, 08:13:33 AM
 ;D Video flight looks promising! 8)


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: tom arnold on June 17, 2014, 10:16:11 AM
Excellent trim flight and it looks like the pendulum is working fine with the little wiggles to keep things upright. I would say everything is right on the way it is----more turns may show problems but I say things look great the way they are. The wiggles come with the pendulum and show it is working, to my way of thinking.

Don't be gone too long as this is a great thread!


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Don McLellan on June 17, 2014, 11:56:50 AM
Agree, an excellent flight. 

Don


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Marco on June 17, 2014, 03:41:29 PM
Stephen,your model is great ! Please do not rush things to see it flying (as I would do....), flights allowed only over very long grass ! it would be a pity if such a beauty would be damaged !


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on June 17, 2014, 08:31:02 PM
Thats a good looking trim flight Stephen - it looks like it will be very good on higher turns. I would suggest that you try more turns before altering the sidethrust as the turn under power, with the higher torque, will open up and you may find that you will get more height withe right turns.
It looks very safe in right turns and the glide/low power descent is good.
The slight roll wriggles definitely look like pendulum correction to me and response /inertia ratio doesn't look too bad. More power and higher speeds will prove it, no doubt.
Great effort and here's  couple of :D for the collection :D :D :D
Good luck  with it.
John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 27, 2014, 02:07:57 PM
Thanks for the generous comments folks.

I would suggest that you try more turns before altering the sidethrust as the turn under power, with the higher torque, will open up and you may find that you will get more height withe right turns.
John, I shall follow your advice re. right thrust - for one I expect you're right (I mean correct!), and secondly it saves me the chore of un-soldering and resoldering the freewheel thingy to the propshaft, which has to be done if altering the thrustline.

I have had the chance to do a couple of things in the last few days. I messed around with acrylic inks trying to mix the hues I need. I think I've got the greys sorted out. The RAF dark green had the quality of looking almost black in some lights and viewing angles, but with bright, olive green highlights in sunshine. I have a candidate, but brushing the stuff onto paper doesn't necessarily give the same result as spraying it onto an imporous, curved surface.

I also started ticking off a checklist of piddly details. The exhaust stubs and the cooling-gill arrangement behind them are now done, and also the fairings which blend the wing leading edges into the fuselage. Both these tasks were toilsome, bespoke work, there seemed no way better than to make something that might fit, try to fit it, see where it rubbed, remove it, sand it, try again. . .

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on June 29, 2014, 03:30:05 PM
Weeks ago, knowing that the Tempest had a blown canopy, I decided that this was the time to get to grips with a simple vac-moulder - but now with this project way over time I wondered if a plunge-moulding could be persuaded to adopt a bubble shape. The crude plunge-moulding frame I made for my Fw 190D has a sort of scissors-action so as you push the plug in, the plastic sheet folds up behind it to some extent. H/T Ljuba for this idea.

I shaped a canopy plug from pine. The smell of the wood was pleasant as I whittled away at it. You have to push pretty hard on the heated plastic and balsa would probably deform.

The answer was yes, you can get a sort of bubble from plunge moulding. After several attempts I got what I thought was a good effort. The Tempest had impressively minimalist canopy framing - I made the frame at the front of the canopy hood from styrene sheet (plasticard) to give the necessary rigidity, but glued it in wonky. It looks 'orrible but it'll have to do for now. I think I'll have to tackle a vac-box as a separate project in its own right, when I'm not building anything.

The second photo is meant to show the 'blown' plunge-moulded hood, but the camera focussed on the spinner - mind you it gives a good view of how far the spinner is offset by having added right-thrust.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on June 29, 2014, 04:19:50 PM
 :) looking good!  ;D


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Pat D on June 30, 2014, 04:02:04 AM
Coming along really nice Stephen..


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on June 30, 2014, 06:16:49 AM
Not bad at all Stephen- ie pretty good :D
John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: RolandD6 on June 30, 2014, 09:21:48 PM
Very impressive work Stephen.
Please keep on posting.

Paul


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 01, 2014, 04:41:21 AM
Thanks John, Paul, George, Ringo, Pat, sparkle :).

Well, here's where I do my "Let us spray" gag. Anyone missing it this time will be sure to catch it sometime in the future as I repeat it again. . .and again. . .and again. . .

I benefitted from the large difference between airbrushing in summer compared to winter. Yesterday was about 18deg indoors and with some sunshine which added buckets of radiant heat. Airbrushing in winter, say 10-12deg indoors and a good deal of radiant heat loss to the cold outdoors, is a trickier business, at least I've found it so.

The warmth allowed me to progress much faster than I'd expected. I painted the white SEAC stripes and the Sky band around the rear fus., and masked these off. I also masked the national marking areas off. For masking I used thin paper sprayed with Sprayfix. There's always the tussle here between secure fixing to keep the mask in place, v. too-secure fixing that drags paint off when the mask is removed. I've been impressed by Sprayfix hitherto, but here I got it wrong. It wasn't just that the Sprayfix took paint with it when a mask was peeled off (it did, slightly, but my fault for proceeding before the paint was much more than touch-dry) but, it left some of itself behind, in the form of tiny rubbery dots - yuk. The whole point of Sprayfix is that it's supposed to stay put on the surface you've applied it to.

Notwithstanding the masking niggles, I got the whole underside blasted with my take on "medium sea grey", which touch-dried so quickly that I felt able to start larding on the "ocean grey" topsides. I'd already decided to use the airbrush as a spraygun, and go for an even, opaque finish regardless of weight.

I got the Ocean Grey on, and because of the masking issues, decided to hand-paint a lining of Dark Green where it meets the Ocean Grey.  The lining would be wide enough to allow an airbrushed infill of the main area without risking overspray onto the Ocean Grey. I would still use masks along the border between Dark Green and the light grey underside. Right? Well, in fact it did work, apart from a couple of faint oversprays due to inattention, not visible from any distance.

So to my amazement I got the great bulk of the paintwork done in one day. Not beautiful, but serviceable. I offered up the wing to the fus. in order to see if touching-up would be needed where colour areas meet, and to anticipate a few "Eeeeeeow" engine noises. Something was wrong - what?

I had transposed the green and the grey on the port wing. They was da wrong way round.

After jumping up and down making keening noises, then chewing a chairleg and making growling noises for a while, I reflected that here was my chance to see whether paint can be removed wholesale from aliphatic-coated balsa sheet without damaging the sheet. I used tepid water and kitchen paper. This worked OK, probably because the paint wasn't fully cured. With truly cured acrylic a stronger solvent would be needed, and this would presumably attack the aliphatic.

Tomorrow I'll respray the port wing.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: billdennis747 on July 01, 2014, 04:51:06 AM
It wasn't just that the Sprayfix took paint with it when a mask was peeled off (it did, slightly, but my fault for proceeding before the paint was much more than touch-dry) but, it left some of itself behind, in the form of tiny rubbery dots - yuk. The whole point of Sprayfix is that it's supposed to stay put on the surface you've applied it to.

Lighter fuel on a tissue fetches it off. Charlie Newman told me that.
Bill


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: ILM Tarheel on July 01, 2014, 07:12:21 AM
Windex, or any window cleaner with ammonia in it, is excellent for cleaning off water based acrylic paint. The ammonia is what does the trick. I've even restored paint brushes stiff with dried acrylic paint by soaking them in Windex. Also works with Pledge Future floor finish used as a clear coat. As a test, take a "clean" used brush and re-clean it using Windex and see how much more color washes out. I have used acrylic paint from many years building 1/72 scale plastic models (IPMS), the other part of my model airplane addiction. I've never tried it on a tissue or balsa surface, so I would suggest testing on a non-essential item first.

Jimmy J



Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Don McLellan on July 01, 2014, 12:45:11 PM
Transposing camo colours is what I do best.  However in my case I just had to throw the printed tissue away.

Looking forward to the finished Scheme A(?) paint work.

Don


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 02, 2014, 12:42:56 PM
"Type A" is right Don, except when you transpose colours on one wing, when it becomes "Type A/B Air Ministry Top Secret Scheme"; this scheme is known of only by the most expert and informed modellers ;D.

Bill, I took the butane I use in my gas soldering iron to be the same as lighter fluid, and tried a bit on acrylic paint. Paint unmoved. I tried a bit where Sprayfix tack was stuck to acrylic - the tack came off! Great tip, thanks.

Thanks for the ammonia tip Jimmy J. When I track down our ammonia window spray thingy I'll try that. If it doesn't attack the aliphatic coat on the balsa then that's another trick in the bag.

Another thing I tried as a result of my port wing fiasco was to use masks when spraying the green paint, as opposed to hand-lining the green as described above. In terms of time taken and fiddliness generally I think masks win, at least on fairly flat surfaces like wings. I'm not sure how getting them into place on complex and more curved surfaces would go though.

Despite Bill's tip reducing my anxiety about Sprayfix tack ending up on the wrong surface, I decided to paint the national insignia by hand. After a quick reckoning of all the paper discs and hoops of different diameters I'd need as masks to spray the roundels, and all the airbrush rinsings and paint-changings, I suddenly grew very fatigued :). To misuse a famous phrase, I was "airbrushed out". I must admit too, that as a wannabe oil-painter, it doesn't take much persuasion for me to grab a brush and go freehand. I faintly marked an inner circle, and the outer circles were already defined by masks used when spraying the camo colours. All the other borders are painted freehand with reference to those two circles. I used a 000 brush. Within a few feet the markings are very obviously hand-painted - but then at that range this is very obviously a model! The most difficult bit of the hand painting was the transparency of the red, yellow and blue paints. They still need at least one more coat.

I'm always conscious at this stage that now we have a free flight model, and intricate finishing seems a doobious investment on something that might crash and burn at its first opportunity. If I learn to make dependable flyers then I guess I'll take more care with the detailed finishing.

Gluing the wing in is a big step. The link between the pendulum and the aileron pushrods is now inaccessible. You need access to the link to alter the aileron bias. I'm hoping the neutral aileron setting will prove okay, but if I need to bias them I plan to cut an access panel out of the belly to reach the link. The wing is fixed with PVA, so I hope I can melt it off with warm water if need be.

There's still an open-ended list of detailing and marking to do, but if weather and crop state allow, it's ready to fly again :).

Couldn't resist a black-and-white (seems to be called grey scale nowadays) pic for that period feel.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Yak 52 on July 02, 2014, 12:52:26 PM
Superb!
It's hard to believe it's as small as it is considering all the details AND the pendulum  8)
Seriously cool and congrats in order.

Jon


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: piecost on July 02, 2014, 12:55:43 PM
It looks stunning. I don't know if it is a subtle effect of using very thin sheet, but it looks a lot more realistic than more solid foam models. I look forward to the video of it dog fighting with your FW190...


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Don McLellan on July 02, 2014, 03:06:57 PM
Wow!!  Looks perfect!


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Monz on July 02, 2014, 03:21:07 PM
Fantastic Stephen! Look forward to a few vids.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on July 02, 2014, 04:26:19 PM
 8) 8) 8) :D


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Graiskye on July 02, 2014, 04:26:45 PM
 Stunning build Prosper. You are a recent addition to the forum since the last time I posted, and your builds are first rate. Very original and innovative as well.
I loved the way you built the Tempest, the wing was awesome, need to know... is this a plan that you drew up on your own, working from a plan in your head, or what? Just what is the story on this great design ?
 Thanks for sharing bud, look forward to your future builds.
-G.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 03, 2014, 01:26:59 PM
Thanks for the kind words (and emoticons) folks - remember the 'camera flattery' factor, and remember that it has to fly to be worth anything - solid scale modellers could probably churn out much better than this in a week or two! Talking of solid models, I made a stand for this 'hollow' model, as I did for the Fw 190 preceeding it. Both of them have aerial masts sticking out underneath (haven't added this detail yet), and also a stand gets the model out of 'ground clutter', up where I feel it's a bit safer.

When you do something like transposing the wing camouflage colours, you know you're never going to do that again. Lesson learnt. So how come I got the underwing serials the wrong way round? I made a stencil, dragged the airbrush out, blasted away, did the other wing, took a look and immediately saw that the letters are supposed to be outboard, not inboard. Never even occurred to me to check - just knew that one had to be upside down relative to the other. Oh well, the next 'un'll be better finished. I was thinking, if this one flies at all I might build a couple or three more, be a Tempest specialist. Then I recalled that I also want to be an Fw 190 specialist, a Bf 109 specialist, a Boulton Paul Defiant specialist. . .have they discovered that elixir of youth yet?

Graiskye, yes, this is all out of my tortured mind. It's been an evolutionary process really, starting with a model that is a topic in the "Outdoor Peanut" forum on HPA - it's called "1/24 Percival Mew Gull" or somesuch. I would hate to reread that - shucks, I was sooo naive back then in 2012 ;D. Then I made an Andreasson BA4-B, then a 109, then an Fw 190. I spend far more time pondering and mulling and researching than building. If I'm doing some household chore or fixing the roof, I'm probably thinking about how to do X or Y in my model. Still masses to learn, but it seems that a protocol may be emerging, a methodology that may (stress MAY) work for modelling many or most stressed-skin originals, s'long as you're happy with heavy models.

piecost, your comment re. foam models interests me. I've never seen one close up, nor do I know how heavy they are. There must be some way that people seal the surface prior to painting I suppose. I don't know if a foam model can carry sharp trailing edges, or cutouts such as cartridge ejector chutes for example. In fact I know nothing about them. If they can be made nearly as light as the same-sized stick-and-tissue rubber model, and if they're quite easy to build, then I may look into the subject.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: billdennis747 on July 03, 2014, 01:45:07 PM
Stephen
There have been some very good foam models indeed and with similar appearance to your finishes, although the details would have to be from different materials. The two big drawbacks are that they usually feature single surface wings (I'm sure you would be hollowing) and the sheer horribleness of working with foam. I think you have to like the material you are working with.
Bill


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: billdennis747 on July 03, 2014, 01:48:30 PM
Here is an example
https://www.google.co.uk/search?q=foam+models+david+deadman&source=lnms&tbm=isch&sa=X&ei=7pa1U_v4CYSlPdiVgcgJ&ved=0CAYQ_AUoAQ&biw=1366&bih=673#facrc=_&imgdii=_&imgrc=1J_vaQ0OYgZV_M%253A%3BA2Ml60-fjNjkzM%3Bhttp%253A%252F%252Fwww.freeflightsupplies.co.uk%252Fimages%252Fkate.jpg%3Bhttp%253A%252F%252Fwww.freeflightsupplies.co.uk%252Fpublications.htm%3B300%3B166

and there is a book by David Deadman, Pete Smart et al


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 03, 2014, 02:47:46 PM
Thanks for the link and the insights Bill. There's even a foam Tempest II on that site you link to - an interesting comparison. The image is very small but I think I can see that it has an enlarged 2-D tailplane and a 2-D fin/rudder, and the wing looks thin. This could be because foam is heavy, or perhaps it's a pistachio - no idea. What's foam anyway, is it polystyrene foam (EPS), polythene foam, polyurethane foam? I expect I should buy the book but that's a step too far for my impecuniosity, to invent or mangle a word.

. . .and the sheer horribleness of working with foam. I think you have to like the material you are working with.

Quite. I have to say I'm very prejudiced - I'll happily rush up an EPS form in order to mould balsa wood, but I wouldn't allow a speck of EPS to appear in a flying balsa model (yet I'll allow solid plastic such as the toothbrush-handle nav lights on this model). A bit daft, I know. I'd prefer an all-foam model over a balsa model with foam parts.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: piecost on July 03, 2014, 03:03:32 PM
Stephen,

I should have been more clear with my comment about more solid models; I was thinking of some of the R/C ARTF type models which look distinctively toy like even when the shape is correct.

As Bill posted, excellent results have achieved using blue foam (Floormate 200). These were typically indoor rubber models; often multi-engined subjects. I got the impression that they could be built as light or lighter than traditional construction models and that the thin curved plate wings made them more efficient than stick and tissue models.

I have dabbled with blue foam, but am not an expert. The material can be made as thin as you want, using a hotwire or by sanding, but is much less stiff than balsa. A thin trailing edge is easily damaged. Cutouts in the skin would invite cracks, but I guess a tissue patch could be applied locally. Hollow wings could be made. But, I don't think that the foam would withstand the rough and tumble of outdoor use. A balsa leading edge would be vital.

Foam models tend to feature a soft matt looking finish, but I have seen models covered in tissue. Applying the tissue without wrinkles or warping the structure sounds tricky. It is straightforward to hotwire sheets to less than 1mm thick, so perhaps your method of finishing balsa could be adapted. A surface finish being applied to the block of foam before the sheet is hotwired off.

I have played with 0.5mm and 1mm depron; it bends into single curvature stress skinned wings really well using a root and tip rib to hold the shape. I made a wing for a foam He162 in this way, using masking tape to cover the leading edge joint. I could not get the thin depron to negociate the leading edge radius without creasing. See attached photo. This material is rather heavy, IIRC about the same mass/area as paper. The thin depron has a relatively thick skin compared to the thicker material; it is in effect a sandwhich material. Thicker depron can be molded into double curvature shapes such as fuselage shells.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on July 04, 2014, 08:39:53 PM
Thats a work of art Stephen. Do you seriously mean that you're going to fly it :D ;D It certainly proves your point re using the thin sheet balsa to better represent the monococque originals.
Goood luck with it.

John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 05, 2014, 04:38:12 AM
Ah! Thanks for the foam info piecost. If it weighs about the same as paper, in a usable form, then that puts it right in the same neck of the woods as the material I'm using here. If double-curved shells can be moulded then that's intriguing. I saw a couple of "thin curved plate" peanut P-51s on the search results page that Bill linked to above; for me that puts them in the no-cal category (losing calories from the flying surfaces is presumably a better duration strategy than losing them from the fuselage, if it's one or the other?), so excellent results should be mandatory!

I think I know what you mean re. ARTF models. The only model shop in my ambit is primarily a helicopter shop, but handles fixed-wing ARTFs too: the first and only time I've seen one of these up close was when a customer brought in a large DHC2 Beaver. Yes it was a "3-D" model, with aerofoil sections and box fuselage, but on a scale appearance ranking it scored about zero. IIRC the large box with it was declaring that it was "supa-scale" "realistic" or whatever. I can't recall whether it actually had large rubber bands criss-crossed over the wings, but that was the impression it gave. I was wondering what was the point, when the customer started talking about his aerobatic exploits with the Beaver - that summed up the essential daftness of the game IMO. However, where owner/builders appreciate scaleness then of course these large models quickly end up looking indistinguishable from the full-size original.

John, I'm not going to fly it - I'm going to wind it up and throw it: whether it flies is presumably up to itself :D. My neighbour's field is now mown - the second crop normally leaves a harsh stubble because of seed stalks, but this year it looks not too bad although really short (yipes). There'll be cattle on the field soon, and the best option for a forced landing will probably be a recent cowpat. . . At the moment we have a spell of wet and windy weather, which we imported specially for the start of the Tour de France.

Here are some more pics from the last couple of days. I've tried to get some light-and-shade, and some close-ups, which highlight both the best and worst unmercifully! One thing I've done with this model is to make every join where there are actual joins or at least panel lines, on the original. If the join is less-than-marvellous (or botched completely :( ) then doing this looks better than having a poor join stretching across what should be a blank area. The absolute biggest let-down is the canopy - so nobody look at the canopy, right? ;D. Until I get this right there's no point going for cockpit detail. In fact I've done all I intend to with this model, until a crash or the passing of time puts it in line for a big overhaul, when I'll add more detail and some more painting.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 05, 2014, 04:39:32 AM
More. . .


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Monz on July 05, 2014, 05:12:17 AM
That's a proper miniature aircraft, not a model!


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: RalphS on July 05, 2014, 05:59:35 AM
Great detailing.  Looks right.  Good luck with the flying.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Don McLellan on July 05, 2014, 10:53:01 PM
Mouth watering Stephen, and I can hardly tell that the camo is mixed 'A' and 'B'.....only joking......  What ever you did to repair the paint is exceptionally well done! 

Wondering what is the wing loading (in grms/sq in)? 

Don


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: green-man on July 06, 2014, 02:56:54 AM
Hi Prosper.

A question about your beautiful model - where did you get the thin balsa sheet you've used?

Thanks.

Nick.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 06, 2014, 04:47:17 AM
Thanks Monique, thanks Ralph, I'll need that luck I think :).

Don, I forgot to say that as pictured, the monsterdel weighs 42.9g. About 3.5g of that is pendulum and associated mechanism.  HOWEVER that includes a motor of just 15" length. I intended to make a new motor of 20" but cut 120" of 1/16" rubber instead of the 160" needed, in a duhh moment. I'm hoping it can eventually carry 24" but that's a long motor relative to hook-to-peg; vibration may prevent this.

ALSO I made some 100-120 turn test flights last evening and immediately added noseweight. The paint job shifted the CG back. Without any noseweight the CG was about 33% m.a.c. but the model was very unstable in pitch - thank gawd I was flying it over a dense tangle of weeds and grasses, on one flight it reared vertical and bunted in from quite a height. On other flights it would either climb or dive with no attempt at correction. The tailplane on this model is very fat, and also for some reason it needs a lot of down-elevator for level flight. Since the tail is in download and there's lots of down elevator I'm thinking that the fat aerofoil, cambered the wrong way, is very inefficient and probably stalls as soon as it can. I added an arbitrary 1.2g of lead in the spinner cap which improved things a lot. I got the model flying until its ~100 turns ran out, but it would still stall when the turns ran out so I added another 0.7g noseweight and at dusk took it onto my neighbour's field of cropped grass and compacted dirt . The model could now fly (under power) near the stall and just mush along without losing it - but as the turns ran out it would still stall and nosedive in (luckily from only 3-4ft altitude - no sig. damage). Moral: 2 gram paint-job needs 2g noseweight. Hmmmm.

My medicine for stalling at the transition into glide would be down elevator balanced by upthrust. Seems odd, the wing incidence relative to 0deg tailplane is only +2-3deg, quite normal, yet with a 25% CG there's down elevator for a straight glide and an indication for upthrust for powered flight! That's a big old 4-blade carpet-beater in the nose though, the equivalent of two seven-inch props of sizeable area. Something's going on with propwash over the broad-chord, near-symmetrical section inner wing perhaps. This may take some working out. If it runs out of turns 6 or 8ft up and stalls it could disintegrate. I don't want that to happen (gulp).

Anyway, say a 24" long motor, and a CG at 25% m.a.c., that's a wing loading of 0.095g/cm2 = 0.61g/in2.

Nick, the sheet is 1/32" c-grain sheet covered in aliphatic resin and then sanded down to the required thickness, about 0.3mm for most jobs. There's more on this in my "1/24 Fw 190" thread which is either here or in the 'completed builds' subforum.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: tom arnold on July 06, 2014, 01:53:13 PM
I hate to say this but what your flights are describing is the result of the horizontal stabilizer being too small for a low speed rubber model. I had a Spit XIV that I wanted to see circling above my head but it drove me nuts until I enlarged the stab by a goodly amount. The 2 tip-offs are the fact you need down-elevator to glide (meaning an up-lift on the stab). A properly balanced aircraft with the CG at 25-33% range and with a positive angle of incidence of 2-3 degrees will normally need up-elevator (or a down-load on the stab). Why the Tempest needs the down-elevator, I don't really comprehend but my Spit required the same thing. The Spit appeared to fly fine while the prop blast was flowing over the tail, but when the motor stopped, the prop blast stopped, those big blades out in front created a wind dam of sorts, and the Spit dropped its nose and fell out of the sky. I don't think it was a stall, it was just that the stab was too small to be effective and keep the tail down. The pitch stability was just very speed dependent.

I kept moving the CG forward until it was almost even with the leading edge before it would finally fly AND with some up-elevator. Needless to say the flights were pretty brief. I bit the bullet and enlarged the horizontal stab such that it was 20-25% of the wing area and the model calmed down immensely. Attached is a photo of the final size which could easily be criticized for being too big but it worked. Don DeLoach of the Flying Aces Club here in the colonies wrote a great article on stab size for scale models and a very effective rule-of-thumb (requires some calculation). I don't know if you have access to it. In the article he admits that what the numbers show will work sometimes looks silly on a model but you dial it back until you hit a happy point that looks OK and flies OK. On my Spit, I tack-glued sheet balsa to the stab to experiment with more area. I realize UK competitions are strict about scale sizes so this may not be a path you want to follow.

I am incredibly impressed with your final wing loading for a balsa sheeted model. Wow! That is outstanding----I have stick and tissue models that are heavier than that! (They fly very well on my shop wall, too.) Any way you look at it, your Tempest is a fabulous model.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 06, 2014, 02:48:16 PM
. . .Why the Tempest needs the down-elevator, I don't really comprehend but my Spit required the same thing. The Spit appeared to fly fine while the prop blast was flowing over the tail, but when the motor stopped, the prop blast stopped, those big blades out in front created a wind dam of sorts, and the Spit dropped its nose and fell out of the sky. . . .The pitch stability was just very speed dependent.

Yes that sounds pretty much identical Tom - the launch is absolutely critical, it can just dive away or zoom-climb, even with the well-forward CG. I can't grasp the need for down elev. either, but I imagine that the tail must be providing the necessary download or the thing wouldn't fly at all. My guess is that the tail is either partially stalled all the time and/or stalls completely at the least excuse. The root section is 14% thickness-to-chord which is a lot for such a small flying surface. However, the confounding factors are that 1) when under power it can tootle along near the stall, hanging in there; 2) under power its stall-recovery is actually pretty good, regardless of whatever the decalage looks like with all that down elevator; 3) I've successfully flown planes with smaller tails per wing area.

I guess it's to do with propwash - either the accelerated airflow, or the turbulence causing better flow-attachment, or some interaction between wing and tail that changes markedly when the propwash dies.

I don't mind whatever competition rules dictate so much, I'm just a scale pedant. I'll stick with it till I've tried everything, then (if the model is still intact) have a rethink.

I am incredibly impressed with your final wing loading for a balsa sheeted model.
It may just indicate that I've not built it strong enough to survive! The Tempest is a good candidate here, having a great big bat-wing on a fairly dainty fuselage, and yet the big round engine allows plenty of space for rubber.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on July 06, 2014, 04:36:15 PM
 ;D Interesting, I think from memory my Tempest was pretty erratic at first.  Since I repaired it the rubber is a lot further forward, but I dont remember taking out that much nose weight. ( guessing the cofg is further forward now.) I'm off to work now, but will try to remember to check it and stab size when I get home. I know the section is thinner than scale.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Yak 52 on July 07, 2014, 03:44:55 AM
Great pics. The light reflections are what make it so realistic.  8)

Just a couple of thoughts:

I guess it's to do with propwash - either the accelerated airflow, or the turbulence causing better flow-attachment, or some interaction between wing and tail that changes markedly when the propwash dies.

Accelerated airflow actually slightly reduces tail surface effectiveness (counter-intuitive perhaps as one tends to think of the prop blast over surfaces at low speed in take off/taxi etc but at flying speed the true airspeed over the tail is reduced by propwash.) The prop may have a useful turbulating effect which helps flow stay attached on the stab.

Another thing to consider - the stopped prop will act a bit like a canard surface forward of the CG ie de-stabilising so when it stops the tail loads will increase/change. In other words you may have less tail volume in the glide.

I think you are pushing the limits of tail stall with the small area and poor section and so it won't take much to 'push it over the edge' so to speak. That said I don't think your tail area is unviable, rather that with said 14% section :o you will have separation and hysteresis problems that will make trimming inconsistent.


The tailplane on this model is very fat, and also for some reason it needs a lot of down-elevator for level flight. Since the tail is in download and there's lots of down elevator I'm thinking that the fat aerofoil, cambered the wrong way, is very inefficient and probably stalls as soon as it can.

Is it positively cambered? (ie in the coventional way like a wing?)
A symmetrical section with less elevator might just cope with the required loads.
At this tail volume you will have a download on the tail regardless.

Jon


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Pete Fardell on July 07, 2014, 04:46:15 AM
Wow, what a model! Beautiful!


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on July 07, 2014, 06:34:17 AM
 ;D Just checked my Tempest and the Cofg is around 30%, definitely Less than 33% but I may have added a few percent to the tailplane.  :-[


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 07, 2014, 12:01:36 PM
Thanks Sparkle, that sounds about right. Probably those few extra percent are all that's needed!

Cheers Pete, I appreciate it. Not so beau at the mo (see below) - hey! is that poetry??

Jon, no the tailplane is symmetrical but I meant that  having to produce download to balance wing moment and a 25% CG, the AoA is presumably negative, and yet there's down elevator, so the camber is 'the wrong way round' if you see what I mean, which is mystifying. The area is actually 16% of wing area IIRC, but the moment arm is short. The Fw 190D has a forward CG and a bit of up elevator, so the airflow round the foil is presumably near optimal. What you say re. the tailplane reinforces the conclusions I'd stumbled to - however can you explain why propwash reduces true airspeed over the tail - as you say it is counterintuitive! I'd considered the fin area of the spinning prop but hadn't thought that it has a canard area too - of course it must. However, would this be the same whether spinning or still? Textbooks seem to mention the destabilising effect of a spinning prop (fin area equal to projected side area or somesuch).

Today's instalment:

Yesterday I thought I'd tackle the pitch stability issue by starting from scratch, with the CG well aft (33%) and downthrust. With 60 turns wound the model stuck its nose up, did a smart torque-roll just 3 or 4ft up and hit upside-down. This broke the pendulum rear pivot. Fixing this took some deft keyhole surgery this a.m. but meanwhile I had taken the opportunity to fly the model with fixed ailerons. This showed as expected that the model could stay aloft but clearly on a tightrope, and most times it would slide off within a second or two. With fixed ailerons I got an unequivocal demo of the power of the 'servo-tab' rudder set up. It works very well. I guess it's rare to have a scale rudder that a) needs to float in the first place and b) has a hyewge trim tab, but interesting anyway.

When reinstalled, the pendulum wasn't quite as free as previously, but because it's so hidden away I couldn't see why, or do much about it. This a.m. was pretty breezy but I wanted to try the aileron freedom and thought that the model had to learn about wind sometime anyway. I flew it on my patch, CG right forward again and downthrust too. I got a couple of 120 turn flights that proved (to me) that active ailerons are better than fixed ones for models like this. One flight demonstrated perfectly the transition from flyer to brick as the turns ran out, so I got my camera to see if I could film a repeat flight for analysis.

That's how I happened to video this flight. I couldn't pick up the model against the long grass but then it reared up into wind, the small bank corrections are apparent in the original vid. I think it hits wind shear just as turns are running out (200 turns), then it hits something else. There's an area of trees, shrubs and high hedges right across the wind and just 15 metres or so from where it stalls. See how the stall is quite docile under power.

http://youtu.be/F2LVLsLxirM

When I arrived on the 'accident scene' I thought for a second that I'd got away with it (the pic shows it before I'd even touched it), but of course that was wishful thinking. The sound of the crunch was much harsher than comes over on the video. I unhooked the prop from the fence and couls see that it and the nose area were undamaged.

The stbd wing had broken from its cross-fuselage braces in download (dang! I only ever think of upload when designing these things - same happened to my Fw 190D, failed in download :( ). The balance bar has snapped off the rudder. It was only when I got indoors that I spotted that it was the port L.E. near the tip that had really copped it. This must have been what hit first - it hit one of the fencing stakes leaning against the fence, and swung the model into the fence top rail.

The good news is first that the spinner cap is big and scarlet, so when it pinged off into the weeds it couldn't hide from me (I've spent hours searching for black or grey spinner caps!). Second, I managed to 'melt off' the PVA wing/fus. fixing with warm water and patience, and third, vitally, with the wing free I could test the aileron pushrods, which still worked fine despite having come adrift from the pendulum linkage. That means the bellcranks and pushrods buried in the wings are OK.

The great news is that the thing isn't a ball of matchwood splinters :).

The crushing of the L.E. seems only to go back to the second spar, and encouragingly, the skin did not choose to split along a scribed panel line (pic 2 you can see the panel line running through the blue of the roundel).

I may yet uncover something sinister, and Im not sure how I'll tackle the L.E. yet but on the whole this looks quite fixable. For now though, I have to accept that fixing new ridge tiles to my house roof takes precedent.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on July 07, 2014, 04:36:59 PM
 ;D It's very brave flying such a well detailed model without the usual free flight cheats! Flight has promise! despite landing!


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Yak 52 on July 08, 2014, 04:37:18 PM
....can you explain why propwash reduces true airspeed over the tail - as you say it is counterintuitive!

Stephen, I'm sorry I got that slightly wrong. The fuselage and wing wakes decrease the airspeed (dynamic pressure) at the tail. The propwash increases it slightly but not enough to restore true airspeed. (Thanks piecost for the correction  :))

What I was referring to was the effect of propwash on longitudinal (pitch) stability. This works because any gust changing the angle of attack creates a restorative pitching moment from the tailplane. The tailplane bathed in prop wash is less sensitive to gusts, ie the aoa changes less. So propwash over the tail decreases effective tail volume and reduces pitch stability. (What confuses however is the fact that propwash does increase elevator effectiveness but this is irrelevant to us in free flight with a 'stick fixed' or locked down model.)
I've scanned a couple of relevant bits from Darrol Stinton.


I'd considered the fin area of the spinning prop but hadn't thought that it has a canard area too - of course it must. However, would this be the same whether spinning or still? Textbooks seem to mention the destabilising effect of a spinning prop (fin area equal to projected side area or somesuch).

I'm not sure to be honest. Generally power on is destabilising but with such large prop blade areas there could still be significant effects with a stopped or free wheeling prop?

Sorry to hear about the damage :gulp:

Jon


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: lincoln on July 08, 2014, 07:50:25 PM
I have a Seversky with a floating rudder and a trim tab that works. Wouldn't fly before that.
snip

With fixed ailerons I got an unequivocal demo of the power of the 'servo-tab' rudder set up. It works very well. I guess it's rare to have a scale rudder that a) needs to float in the first place and b) has a hyewge trim tab, but interesting anyway.

snip

Stephen.



Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Don McLellan on July 08, 2014, 09:34:18 PM
Hi Stephen,

I've heard that 'crunch' before.  Once again exceptionally bad luck.

 :'(


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 09, 2014, 01:27:38 PM
My fault Don, not bad luck. That was too many turns in a confined space for a basically untrimmed model. I bet you heard that 'crunch' when your lovely Stinson trimotor hit the big rock in your garden? ;)

In fact the Tempest's not looking bad, given that it flew straight into a fence in a shallow dive! I was puzzled as to how the nose appeared undamaged and later found that a prop blade was split along its length, which explains where much of the energy was dissipated - I've been aiming at making more springy prop blades and having them as shock-absorbers, so I guess it did its job. With some thin CA it was repaired in a few seconds :).

Hi lincoln - how big is the Seversky tab relative to the rudder? I'm wondering if even a small tab with a large deflection could work, or whether the tab needs to span a considerable part of the T.E. as is the case with the Tempest.

. . .The fuselage and wing wakes decrease the airspeed (dynamic pressure) at the tail. The propwash increases it slightly but not enough to restore true airspeed.
Thanks Jon, that makes sense. Darrol Stinton reminds us of the greater elevator effectiveness in propwash - I think it's not quite irrelevant to a 'locked down' model: this alone could explain the stall as the turns run out, and the great drag of the four-bladed potato-masher could explain the abrupt drop, as suggested by Tom Arnold from experience with his Spitfire XIV. I think I'll try as much of a forward CG as requires no elevator deflection (or even a bit of up elevator for camber to suit the downwash). That oughter minimise the change caused by propwash.

Stephen.



Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Yak 52 on July 09, 2014, 01:59:51 PM
I think it's not quite irrelevant to a 'locked down' model: this alone could explain the stall as the turns run out, and the great drag of the four-bladed potato-masher could explain the abrupt drop, as suggested by Tom Arnold from experience with his Spitfire XIV.

Yes perhaps, I was musing along the same lines myself  :) I think the four blader drag is a significant issue too.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Balsa Ace on July 12, 2014, 06:25:07 PM
Your Tempest is Top Notch.

Scott


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 13, 2014, 04:53:01 AM
Cheers Scott, it's kind of you to say so. Looks like it might be more 'top shelf' for a while though. . .;D  Er - did you have anything to do with Sea Furys?

Hi folks, I repaired the wing of the Tempest - I went for the simplest seeming option which was just to tease the sheet back into alignment and re-glue it to the front spar then zip it all together with thin CA, sand it as fair as possible, then smear CA over the whole area and sand again. Once embarked upon, the whole op. took just a few minutes. There was a flak hole which I cut around and let a square of sheet into. I sealed that with CA too but the edges needed some deeper filling so I used an acrylic filler.

I started flying the model, and realised that my analysis of the flight where it hit the fence was wrong. 'Model dips and lopes along then suddenly zoom-climbs, stalls and dives' I attributed to wind gusts and windshear. Now I was seeing that same pattern again and again in light air, even launched downwind. I decided I was launching too slowly, so the model would dip to accelerate then rear up at its optimal speed, the top of the climb would coincide with turns running low, the propwash over elevators decreasing, and the model would dive away. These flights were mostly 200 to 350 turns. I guessed that if I launched harder the model would climb straight off, reaching a higher stall causing a longer dive.

I went right through a CG range from 20% to 33% but the model would still do this. To give what is surely an inadequate hz stab more of a chance, I replaced the 1/4" motor with 3/16" and cut the prop blades down, also twisted the blades to a finer tip-pitch. This moderated things but the model would now land with loads of turns unused. Cutting the blades back some more made no difference (I've seen this before - I wonder if the lower Re of a narrow-chord blade = a thicker boundary layer so the air doesn't notice the cut-back!).

Then I tried a 7/32" motor in the search for that Goldilocks 'just right' setup, and wished I hadn't cut the prop back :).

With the original motor/prop I had also removed the rudder. The model flies better with no rudder at all than any floating rudder, balanced or unbalanced. This is partly a cheat because removing that weight means less nose weight needed: a lighter 'plane with less rotational inertia.

Here's a video of various clips spanning several days (the pendulum clip is from a few weeks back).

http://youtu.be/lZhXFvh78lE

In a hard hit yesterday the bellcrank in one wing came adrift - this means more surgery.

I plan now to put the model on the shelf and ponder what to do with the pitch instability. The Tempest has a very clumsy (thick) hz. stab on a short arm behind a broad-chord wingroot. It's probably in messy air and although the wing section is near-symmetrical I think the broad chord might engender more movement of pitching moment than the tail can cope with - just an idea. The Fw 190D model I made prior to this one has a smaller wing and a higher wing loading, yet I have a feeling I'll get the 190 comfortably past 30 sec when it's set up right. I just don't have that feeling with this Tempest (best flight so far 17 sec), unless I can get it stable in pitch.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on July 13, 2014, 05:14:27 AM
 ;D To my untrained german eye it looks as if the Englisher Flukzoid is haffing a stall?  ;)


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 13, 2014, 06:18:17 AM
Hi Sparkle, yes it is stalling, but that's the least of my worries. The elevators are so sensitive that if you frown at them from above, the model will dive in; if you frown at them from beneath, the model will stall ;). The little trim tabs are useful in this respect.

I like the way you get into character for your builds - das is wunderbar, real scale modelling dedication!

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: DHnut on July 13, 2014, 07:32:16 AM
Stephen,
             Your challenge in getting a consistant flight is giving me some food for thought because the glide on my big KK Tempest is fine but I am now concerned about getting the power trim sorted. I have used one of the Checz 4 bladers and while they are lower pitch there is still the destabilising effect.
I had a Stahl Barracuda trimmed using a four bladed prop and found it was very power sensitive with 600 turns of .150 thou Tan giving about 30 seconds but when I added a lot more a the Nationals (850) it spiraled into the left. The prop was based on the McHard blade used in thre WW2 Book printed by Model Builder that has the blade area clpse to the hub.At least the Barracuda has reasonable tail area and a high set tail out iod the downwash from the wing.
If you are using the scale section then this may be probmatic as even large radio models have found that there have been control issues with scale sections on the Tempest. It was after all designed for high speed. I hope you are able to sort it out becasue it looks so good in the air.    
      Ricky


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 13, 2014, 07:58:50 AM
Thanks Ricky, that's interesting info about the large radio models. Adds to my sense of gloom ;D. It did cross my mind to try it with two blades but the four-blader does look the part especially as it's scale diameter.  My model glides fine, like yours - and when there's bags of power it seems just about stable too - goes wobbly when there's a transition! I still can't see why it wants down elevator even with a forward CG, when the wing incidence relative to the tailplane is a pretty conventional 3 deg.

I did have plenty of blade area close to the hub but removed area here in preference to removing it near the tip. I think my next wheeze might be a tailplane of the same area but much thinner - in fact I'd just try a flat sheet to start with, could run it up in a trice.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Ex Member on July 13, 2014, 08:08:33 AM
Hi,

I wonder if your active ailerons are causing the instability?  From the video i would just add more nose weight or add down elevator, but i have never trimmed a model as complicated as this with your auto ailerons.  I haven't read your entire thread, so you have to excuse my ignorance, but they have a significant movement from what i saw in the start of your video and whilst they are battling away to try and keep things from rolling around, no adjustment is made to combat the overall lift that each wing is producing and the shift in the centre of lift on each wing that the ailerons are producing.  Why not just restrict their movement, ( as opposed to locking them altogether) And see what happens?  Having said that feel free to ignore this, it is pure guesswork and it's not my beautifully crafted model which will bite the dust when it goes wrong!

Cheers

Andrew


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 13, 2014, 10:37:44 AM
Hi Andrew, that's made me think (ouch). It sounds a bit chicken-and-egg but I suppose from the ailerons' point of view it's gusts that are changing the overall lift each wing is producing and the consequent change in centre of lift - the ailerons are trying to oppose that change. Thus if a model can maintain its pitch stability in gusts, it should also be able to maintain it with ailerons reducing the effects of those gusts. It's probably not that simple because gusts change lift by altering local airspeed and AoA whereas ailerons do it by altering camber, but I'm guessing that they're not part of the problem. They do have plenty of movement but full deflection is only reached at 40deg bank angle - the video clip shows me waggling the fus. to swing the pendulum from stop to stop. Inasmuch as one can assume they're helping at all (I believe they are :) ), they're normally doing their stuff at much lower deflections. But to borrow your words, that's all pure guesswork on my part!

I think the pitch problem is down to the tailplane. I've added noseweight until the model's quite hard to launch, because you're worried that the grip needed on the rear fus. might make something break. . .that didn't solve matters. . .with more down elevator the tailplane will stall very easily - it's probably part-stalled all the time IMO. I'm trying to think of fixes with minimal scale impact. Yeow that word impact is making me cringe :).

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Graiskye on July 13, 2014, 06:51:55 PM
Stephen,did you ever try it with the ailerons locked ? Is it possible to fly it with the surfaces locked a'la a conventional model ?
 There are so many things affecting the trim, would make my mind boggle, yuour too be comended for getting as far as yo have.
Locking surfaces was just a thought I had(hey it happens...) hope you eventually solve her and she ends up in the sky for a minute or more.
 Best of.
 Mike.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on July 14, 2014, 09:21:35 PM
Lousy luck with the latest crash Stephen- you certainly need to be stubborn to be a pioneer Stephen ;D. So close and it looks magnificent when it’s behaving itself.

My money unfortunately is that the lovely 4 blader is the culprit, due to its area and canard effect as Jon has mentioned. It will be destabilizing in pitch under power and on the glide IMHO.

The increased elevator control under power would help to explain why the elevator is dead critical with a very low effective tail volume due to the propeller area.

You did mention reducing the propeller area but i think i would try to keep the diameter and reduce the area to that of an equivalent 2 blade propeller that would work.

I agree that the fat tail airfoil should be replaced with a thinner airfoil.

Your pendulum aileron control appears to be working well. They are controlling all that torque on a low wing model with minimal dihedral.. Yes it does wander a bit near the stall – which would be expected with the aileron movement causing incipient stalls. This should not be the case if the tail was still in control.

It looks like your flying field is surrounded by German trees, fences and obstacles :D.

Keep at it.

John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 15, 2014, 03:24:09 AM
Hi Mike, thanks for the encouragement. Yes I have tried it with locked ailerons and it didn't show too well in a series of five or ten short flights. The problem in trying to reach solid conclusions though, as you point out, is the number of variables, and that's without the wind! I have no way of saying, after a complete orbit - "Aha! That proves that these 'ere pendulum ailerons work" because the model could have made that circuit with them locked. If I can sort the pitch trim out perhaps I'll see some  conclusive patterns emerging. . .

John, thanks likewise :). I think that - like the wing leading-edge fix - once I bite the bullet fixing the bellcrank should be easy. I'll have to cut a hatch in the top wing surface, which will be hard to hide - that's what's making me hesitate!

I'm secretly quite smug that the prop has scale diameter - I'd be reluctant to change that - but I have noticed in the past that reducing blade area doesn't seem to make much diff. as compared to changing the pitch or diameter. I've also noticed how too fine a pitch is great for stall recovery (model can hang on prop at v. low airspeed because of the greater power output) but lousy for cruise performance (prop acting as brake). I think I'll reduce blade area a bit more, try to thin the blades some more, and reduce pitch as far as seems sensible. Since the model produced some duration (relatively speaking!) on only 3/16" rubber, perhaps I can get by with a lighter motor.

Then I'll put a temporary sheet-balsa tailplane in of the same weight.

Yes there's a cordon of anti-aircraft defences all around, but I think they may be United Nations rather than German, because they attack Focke-Wulfs and Messerschmitts too - they even attack civvie stuff like my PA15 Vagabond :(. A big problem with the active control setup is that the model can achieve a stable (in roll) state to left or right. . .if the Tempest loses airspeed near the stall or in a gust it can wander off in a new direction. . .the precipitate dives in a couple of those video clips were down to turbulence and windshear around the trees - however in those clips the model was a few metres short of flying into the tree so perhaps windshear saved it from a worse fate :).

Regards,
Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on July 15, 2014, 09:19:37 PM
Quote
I'm secretly quite smug that the prop has scale diameter - I'd be reluctant to change that - but I have noticed in the past that reducing blade area doesn't seem to make much diff. as compared to changing the pitch or diameter. I've also noticed how too fine a pitch is great for stall recovery (model can hang on prop at v. low airspeed because of the greater power output) but lousy for cruise performance (prop acting as brake).

This I feel would be true largely if the motor torque was the issue. However it looks like your pendulum ailerons have that largely under control - its the actual area of the propeller that is causing the problem i suspect - a combination of the blade area and pitch.

Thus I'm hoping that your area and pitch reductions will show some improvement.

John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 17, 2014, 05:23:39 AM
Update:

I gave the propeller blades a much narrower profile, thinned them accordingly, and twisted the pitch finer.

I cut a hatch in the top wing skin to get at the bellcrank and reglued that - I hadn't fixed it well initially, which was why such a light and unstressed part could have broken free in a hard bump which didn't do any other damage.

I started with 3/16" rubber then moved to 7/32". The truth was that I noticed very little difference in the model's behaviour - still did the same thing, namely tooling along then suddenly rearing up then diving in a long straight dive. I'm still left wondering if the reduced Re of a narrow-chord blade causes more drag than the lower aspect-ratio and greater surface area of the wide blade? The model showed no more zest than before, and there was no manifestly evident jump in RPM. . .I don't know what you make of that (OZPAF) John, or any other prop experts. . .it's all a big mystery to me. . .I know static motor runs can be very misleading but I should have done some static runs before and after modifying the blades to measure RPM change.

Next I soaked off the tailplane. This had been fixed with PVA with a view to just this contingency. I ran up a 1/16" balsa sheet tailplane and increased its area using strict TLAR methodology. When I measured its area it turned out to be 20% of wing area. It weighs the same as the fabricated, scale one.

The blinkin' great 'ole in the blinkin' fuselage left by the scale tailplane tells a story, I think.

All these mods/repairs were quick to achieve, but by the time the tail was rigged the wind was 13mph according to a local weather station, with stronger gusts, but a few 100-turn flights were all I needed to see that the model was transformed - hardly surprising, but quite something to witness. I immediately felt confident to remove the noseweight, including spinner cap, leading to a 2.5g weight saving, instanter. An overscale hz stab confers a double benefit: stability+reduced weight. Reduced noseweight = reduced pitch inertia; rearward-moving CG = less-loaded, more sensitive tailplane. It seems like some kind of virtuous circle.

This a.m. in what seemed no wind I chucked the model with 490 turns (the un-rounded number is a result of multiples of the gearing of my winder). The model flew off in a way I'd not seen before. A big stall brought it nearly to ground but it just recovered and climbed again. Now I could see there was a very slight wind, drifting the model back into the trees. It stalled its way through what must have been around 20 sec, covering a lot of ground crosswind before smacking into a neighbour's tree. Over the hedge I could see it bizarrely hanging from its prop from the treetrunk (a young birch maybe 20cm diameter, perfectly smooth trunk).

I went cap-in-hand and said "can I 'ave my 'plane back Mister?" and was amazed to find the Tempest completely undamaged! It had flown without its spinner cap, and the freewheel thingy had stabbed the trunk deep enough to suspend the whole model, which was dangling on its braided rubber. I wish I'd had my camera. I must admit I thought the model would be smashed, with severe internal ruptures and dislocations. The motor thrust button has twisted in the noseblock, that's the only evidence of a crash that I've found.

Phew ;).

Now I have to figger out where to go from here.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on July 17, 2014, 05:51:20 AM
Quote
I don't know what you make of that (OZPAF) John, or any other prop experts. . .it's all a big mystery to me. . .

Me too Stephen –D. I guess I should slink to the back of the class now :D. I'm a little disappointed you didn’t see more of an improvement – however tongue in check I’m afraid – them blades still look large in area. The reduced pitch would also reduce the effective area.- however. The Re no effects on the prop drag and thrust could be interesting I admit. As an empirical rule – I would suggest that a significant drop in thrust would accompany an increase in drag – for similar conditions of power and pitch.

However I’m out of my tree here(trees have a lot to do with this saga it seems ;D) – John Barker (Hepcat) is a much better bet in this area I would think.

It’s good to see that it has improved with the larger tail. Perhaps it can be pared down closer to scale when combined with an even lower blade area.

John failed eggspurt.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on July 17, 2014, 07:27:09 AM
 :D Hi Stephen, glad to hear that you are making some progress. Some models seem to need a little cheating! Keep up the good work!  :D :) ;D


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Don McLellan on July 17, 2014, 10:17:06 AM
Great stuff Stephen; one of the all time best threads on HP. 

However, I'm still amazed that you can take a knife to such a beautiful model.  Did the 1:1 have a hatch where you made the opening for servicing the bellcrank?  ;D


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on July 18, 2014, 01:42:26 AM
Ammunition bays perhaps?
John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Yak 52 on July 18, 2014, 03:14:15 AM
Very interesting Stephen - progress of a sort, although perhaps leading you away from your preferred scale direction  :-\

What would be good next is to try the new tail with the scale tailplane area, which should give some indication of the effect of it's thickness?

Jon


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 18, 2014, 06:13:04 AM
Well there was no hatch there on the fullsize, but serves me right for not fixing the bellcrank properly. If/when this model has a full overhaul I can probably 'invisibleize' the hatch. I always get this crossover point where a model, beautiful or not in one's own humble or not opinion, becomes a flying machine - then it's a test vehicle and the nicks, scratches and patches that accrue are part of an inevitable process. If a model of mine has ever got as far as a complete overhaul I'm amazed at how doggy and ragged it's become without me even noticing, because I'm treating it as a prototype flying machine. I think if I had the airspace I'd be more of a non-scale duration modeller really.

I guess I should slink to the back of the class now :D.

No, I think you're on the right track John, I reduced blade area a bit more and twisted the blades a bit more and there's a new zip to the model IMO. I don't mean in airspeed, but in its directional stability and gust-handling. I think I just wasn't following your bold vision far enough! I may keep on reducing the blade chord millimetre by millimetre while retaining the same blade length, just out of curiosity, and make new ones to a new profile if any optimum chord/twist seems to emerge from the experiment.

Jon, great minds think alike. Unfortunately I've already muddied the water by changing the prop a large amount, but yes I'm going to creep back towards scale area, that's why I marked a dotted line on the slab tailplane. I suspect thickness might be the culprit since I've flown models with smaller tail volumes than this, but on the other hand maybe John's right and all it needs is a smaller prop.  The prop blades are already reminiscent of scale blades of the Rotol type fitted to late generation British piston aircraft - it's hard to believe that the air bothers to notice them, but they seem to be doing something!
 
Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on July 18, 2014, 08:20:33 PM
Good news Stephen. With  luck it could lead to a near scale tail area of thin profile with the skinnier prop.Keeping the prop blades as thin as structurally posible will help to keep Mr Reynold's at bay with these narrow chords.

John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 21, 2014, 04:00:33 AM
Current status: 16" loop of 7/32" rubber; rudder in place but unbalanced and free to float approx 25deg either side; spinner cap fitted to balance rudder weight; temporary flat-plate tailplane approx 1.17 scale area; CG 30% m.a.c.; approx 0.7g added to pendulum bob to increase aileron authority: flying weight 42.2g.

I've uploaded two separate clips rather than compiling them with "windows movie maker" 'cos that program compresses clips drastically, then Utube mashes them some more. Perhaps this way there might be some useful resolution left. . .

http://youtu.be/SQpj_JD-gOc

http://youtu.be/om3A9oTvQ7Y

The first clip is with 490 turns and the turns run out maybe a second before landing. This gives an averaged RPM of over 1800 - considerably higher than I'm used to, but in theory the current motor might just shove the model past 30sec. The 2nd flight is with about 600 turns and the wind is blowing quite a bit. The zooms are partly wind induced probably, but I don't want to add more downthrust so I might try to control power stalls by increasing blade pitch or by reverting to 3/16" rubber. At some point I still plan to reduce blade area further just for the heck of it. If I go too far new blades aren't hard to make. Although the 2nd flight looks harum-scarum and no stabler than earlier test flights I uploaded, I can say there's a real qualitative difference - the earlier flights I felt it was on a tightrope whereas here despite a gusty wind I was more worried that it would fly into the trees than get right out of control. That's down to paring the blade area right down and the temporary hz stab. Kudos to OZPAF John for nagging me about the prop :).

It needs a lot of flying room - flight No.1 it lands only a few feet from that oak tree :-0. I lost the rudder 'servo' tab - never can get in the habit of glancing over a whole model before walking away from where it landed - perhaps a new tab can be set to coax it into a tighter orbit, but then this increases the power of the fin, decreasing spiral stability. I tried the model with fixed rudder and spiral instability was quite evident.

In both flights the ship shows signs of entering the Spiral of Doom (that was a great Indiana Jones movie) when the turns run out. I'm doubtful whether the ailerons could correct this because centrifugal effects aside, the pendulum loses power proportionally to the steepness of the dive. Maybe underpowering it a bit might give a more 'indoor' flight profile where it's still under some power when it lands, and the spiral is averted.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: DHnut on July 21, 2014, 05:14:00 AM
Stephen,
             Both flights look a lot better. I am sure the reduced blade area is one of the reasons. The idea of using 3/16" rubber I am sure is worth a try as it should give a powered glide and reduce the drag to an acceptable level. The model has a really nice sit in the air and the ailerons do seem to keep the spiral stability in the positive. It almost seems as if there is a critical airspeed where drag related effects take over and are difficult if not impossible to trim out. Could this be another Re related consequence? I watch with interest because I think the lessons are applicable to my FW190D as well. I have just reduced the prop diameter on that as well and the one flight in Sydney was promising.
         Ricky


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 21, 2014, 05:33:41 AM
. . .It almost seems as if there is a critical airspeed where drag related effects take over and are difficult if not impossible to trim out. Could this be another Re related consequence? . . .I have just reduced the prop diameter on that as well and the one flight in Sydney was promising.

Probably! Reynolds gets into all of this. I'm surprised that with all these keen aeromodellers out there there's not one who has or has ever had access to a wind tunnel with the ability to visualize flow! So many questions could be answered so quickly. . .

Glad to hear about the promising flight. I'm going to cut down the prop on my Fw 190D too when I drag it out this autumn/winter. It's a quick fix stability-wise but smaller is inevitably less efficient, and a larger but slower revving prop may be no more destabilizing than a small, whizzing prop I'd have thought.  If one accepts a big enough tailplane and enough dihedral then one can mount a huge two-blader and get real duration - scale v. duration, no free lunch etc etc :).

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on July 21, 2014, 07:04:24 AM
 :) love your videos!  8)


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: DavidJP on July 21, 2014, 09:11:11 AM
Yes - me too!  Remembering my well spent youth persevering with the KK low-wingers notwithstanding now even more than 60 years on I am always impressed by any low wing fighter type flying for more that a few seconds and in what resembles a steady flight path!.. So well done. 

So far I have only been able to test glide the Sea Fury due to weather and opportunity. It did not fall out of the sky and landed the right way up. It was breezy. But there maybe hope. Thanks for the incentive to try harder.

 


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: RalphS on July 21, 2014, 11:45:09 AM
As far as I could tell from the videos it looks like it is flying.  Well done.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 21, 2014, 02:32:42 PM
Thanks fellas. The videos are a bit short and comic I know, but I take the time to shove 'em on the toob because I love watching other peoples' vids, however short (hey! the long ones can get boring!) and imagine that others also like FF vids even of trim flights :).

David, if your Sea Fury has some over-scale dihedral I'm sure there's hope - what can possibly go wrong? :).

Ralph, thanks. I think I know what you mean - Bill used almost the identical phrase I think earlier in the thread: "well it is flying" (sorry to paraphrase, Bill). I think you both mean that it's aerodynamic rather than ballistic??

I'm off flying for a few days from this evening, so thought I'd take a chance and wind it up. I put on 840 turns and though there was still a bit of wind, and I hadn't done anything to dampen power stalls, I threw it anyway. There are a couple of stills I 'grabbed' from the clip because they show aileron deflection in the useful direction.

http://youtu.be/Df0837KPJU4  note: the MVI xxxxx TRIMMED title doesn't mean the model's trimmed, just that I've trimmed off the junk video before launch with my AVI editor :D.

If it hadn't lost so much height/energy through stall recovery (the depth of these doesn't show well on the clip - it was pretty helter-skelter!) it might have done better, but that puts it through 30sec anyway with somewhat less than full turns, where I take full turns to be 75% of the theoretical breaking turn.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Monz on July 21, 2014, 02:42:56 PM
Faaantastic!!


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: RalphS on July 21, 2014, 03:17:34 PM
There are a couple of stills I 'grabbed' from the clip because they show aileron deflection in the useful direction.
Stephen.

If Rupert Moore had come up with your system in the 1940's he would have patented it.  Excellent grabs.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on July 22, 2014, 10:16:55 AM
Thats looking a lot better Stephen. It has enough stabiliy to handle the wind and the slight power stalling. The pendulum ailerons are working very well - quite noticeably picking up the wing in a couple of places.
I know you don't like it :D - but a little downthrust would just about cure the slight stall.
I'm glad its showing its promise after all your work.
It made 30secs + as well.
John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Marco on July 23, 2014, 01:10:21 PM
Stephen, I had the same problem while trying to trim my Koutny G55. Ok in glide testing, ok with few turns in, but when full turns were put in the model stalled when the power commenced to reduce. Only way to cure has been to build a bigger stab and  to trim the model for a steeper glide and to reduce the downthrust. Moving th CG further to the front helped as well.  However this set up is reducing the performance of the model, as it adds too much weight and is penalising the glide. I am going to build a new , two-blades prop - you instead are succeeding to trim your Tempest with the current four blades prop : congratulations, most deserved. 
I am learning a lot from your thread

Marco


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on July 28, 2014, 06:02:44 AM
Buona mattina, Squadriglia :).

Marco, I found your G55 story on the 'cookup' forum - I hardly ever look at that forum so I missed it before. That's a beautiful model! I liked the episode where you had to reclaim your model from someone's back garden; very drole. Anyway, it sounds like you've managed to trim your Fiat, or that you're getting close to succeeding. As you mention, I think prop torque is a big factor, as well as blade pitch and area. I don't know what your three-blader weighs but I took a look at my 3-blade Fw 190 prop which has a 146mm disc diameter (~5 3/4") and lots of blade area, this weighs 2.4g including the large hub which acts as the main part of the spinner (i.e. a lot of the total weight is concentrated near the centre of the prop). Compare that with a Peck 6" 2-blade plastic prop at about 2g, much less area and a lot (?most?) of its weight well outboard.

Apart from torque due to the spinning mass, I presume there must be an 'aerodynamic torque' which makes the model want to rotate against the drag of the prop. This makes achieving low blade drag essential for trimming as well as performance. Trouble is, I don't know how to go about achieving this apart from trial-and-error!

I always go for a forward CG with these hard-to-trim model types, and I agree that a forward CG brings a big weight penalty with it :(. If I were you I'd stick with the three-blader, and if possible try reducing the pitch. This will increase stability but impair the glide and reduce motor run - but will need less power so perhaps a thinner motor could be used. Looking at your pics that appears to be a big prop with a coarse pitch on a long nose, with a fairly small vertical tail to balance it, which suggests directional instability to me - but there's plenty of dihedral to keep the model flying! OZPAF John and Yak Jon have pointed out the canard effect of a prop which can cause pitch instability too: a smaller prop or finer pitch may help pitch stability too. Good luck!

Well folks my Tempest II has taken a big old smash. The first chance I've had to fly it for some days. Not sure how it came about; I thought I'd discerned some slight binding in the aileron system at one time or another, but whenever I examined it closely indoors I could never find anything wrong. . .could have been that, or the flopping rudder, or the aileron bias. . . What is certain is that the right wing was almost sheared off. The field was grassy but it found a patch of sun-baked mud with just a few tufts of grass, and hit under high power. I'd wound on full turns and everything was fine initially. The model crashed too far away for my 640x480 rez pocket camera to record the control deflections.

The repair will be difficult and time-consuming: I'm a bit bored with working on the Tempest TBH, so it's going on the shelf for now, until my enthusiasm is rekindled - my mind is on other projects just now :).

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on July 28, 2014, 06:07:33 AM
 :( Sad to see the temporary end to such an outstanding model.  :( :'(


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Don McLellan on July 28, 2014, 11:33:27 AM
Agree, very sad.

Just make sure that it is slightly visible when you put it on the shelf so you don't completely forget about it.  It is way too beautiful not to repair. 


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on July 28, 2014, 07:42:37 PM
That's sad news - it's not a fitting end to such a nice model and all your carefully considered efforts to trim it.

Agree with Don - put it on a podium position on your shelves ;D

Thanks  for all your detailed building and trimming discussions.

John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Marco on July 31, 2014, 04:21:28 PM
Stephen
sorry to hear about the Tempest - but in Rome we say :"when a Pope is dead, just create a new one...". Time for a new subject ! I learned a lot from your trimming considerations. In fact few weeks ago the G55 succeeded in landing on the only stone in the filed and two blades were broken beyond repair; so I was going to carve a conventional two blades prop. However, reading your advices, I will give it another try and will attempt with a much reduced pitch to see the difference.

Thanks again, and keep doing your outstanding models

Marco

ps :...and congratulations for the italian.... :)


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: piecost on July 31, 2014, 05:29:22 PM
Stephen

Sorry to hear about the Tempest.

But, can you gives us a hint about your next project?

We are dying to know...


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on August 01, 2014, 04:20:37 AM
Thanks folks. The Tempest will re-emerge - scarred and heavier no doubt! There's too much to learn about the pendulum setup to disregard it entirely. I've been very pleased in general by the way the aliphatic/balsa material and structure has withstood some hefty whacks, but clearly I need to beef up the wing/fuselage union even more in future designs!

piecost, like all of us I expect, I have several model projects buzzing round in my head at any one time :). Dunno what will emerge. I'm rather concerned about my flying space - it doesn't seem big enough even for the smallish low-duration models I make. I'm thinking I ought to go for smaller models, or more stable, predictable flyers or. . .projects that are sufficiently Heath-Robinson (U.S. Rube Goldberg) that they're unlikely to fly at all :D.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: tom arnold on August 04, 2014, 11:03:04 PM
Tell us, what would you do different if you could do it again?


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Modelace on August 05, 2014, 01:32:23 AM
The model is clearly tail-heavy and lacks sufficient dihedral. If it were mine, that's what I would fix.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Pete Fardell on August 05, 2014, 04:55:11 AM
Well yes, plenty of non-scale dihedral would certainly be my cop out too, but it is the scale fidelity, pendulum ailerons etc. which made this model so interesting to less brave souls like me!


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Maxout on August 05, 2014, 07:33:02 AM
The model is clearly tail-heavy and lacks sufficient dihedral. If it were mine, that's what I would fix.

No, it is not tail heavy, and the scale dihedral issue was successfully addressed. I watched the videos and I know better than to spout an ininformed dismissal like that. This is Stephen's thread, and he was asked what he would do differently, not you.

Would I build a model like that? Not on your life! But it's not my model, and AS3X is still banned from freeflight competition, even though it's just an electronic version of pendulum control.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: billdennis747 on August 05, 2014, 07:51:14 AM
The model is clearly tail-heavy and lacks sufficient dihedral. If it were mine, that's what I would fix.
Well, in the video I just watched, it was flying pretty realistically and stably for nearly 20 seconds, and in my book, that´s a success, so clearly it isn´t and doesn´t. That flight would quite probably win our up-coming Nationals.
I think some people have a knack for prolonged trimming efforts with difficult subjects. I don´t but I admire those who do. One that sticks in my mind as clearly having insufficient dihedral was Andy Hewitt´s Gee Bee racer - the one that looked like a barrel. No dihedral at all.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on August 05, 2014, 09:44:07 AM
Thanks for the interesting observations, chaps; much food for thought.  I've thought about Tom's question: on the face of it Tom, I'd say, 'design a stronger wing/fuselage junction for the next one'. However, on reflection I think the answer is - 'make a pendulum-aileron Supermarine Spiteful in the first place' :D. The Spiteful has a big tailplane. I think the Tempest II is a difficult customer, due to the absurdly thick tailplane section, added to the not over-generous area and fairly short moment-arm.

The Chilton DW1 has a very similar layout to the Tempest including dihedral and wing thickness, and flies soundly with a slightly smaller tailplane area - that's the insight that led me to progress with this build, and for the Tempest I chose to try pendulum ailerons as a substitute for the large trouser-fairings the Chilton has round its mainwheels (I think the trousers act as fences boosting dihedral effect). But the Chilton model I made had a considerably lower wing-loading and correspondingly lower inertias than the Tempest, making it more stable.

I'm keen to get to the bottom of this pendulum malarkey, because if it can be shown to bring a model with minimal dihedral safely to earth much more often than not then it opens up a new range of types to model.

I think that "tail heavy" might be a term that has real meaning if the decalage is fixed. You can alter CG to balance a set decalage or alter decalage to maintain a set CG. In the latter case you can place the CG for best pitch stability then alter decalage to match. That's important for models with small tails - moving the CG forward is instinctively the wise thing to do, but doing so decreases pitch sensitivity, making a model more likely to overcorrect, and brings the tail closer to stalling, which is definitely bad news. I've had the CG of this model at 20% and found that the fully scale tailplane couldn't recover it from a dive before it hit the ground. I've also had the CG at 33% and found that the scale tailplane simply didn't have enough leverage. If I remember my history the Sopwith Camel was called "tail heavy"; it had no trimming device and so the pilot had to exert forward pressure on the stick at all times. If it had had pitch trim it wouldn't have been called tail heavy even though the CG hadn't moved.

The video linked to shows, IMO, that a bit of down elevator is needed, or a bit of downthrust, or both.

Stephen.





Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: sparkle on August 07, 2014, 04:11:53 AM
 ;D hi Stephen, a Spiteful might be a good option. Under modelled, large tailplane, long nose, not sure how the wing is affected, but Spitfires Ive done have been very stable. A big fin and low dihedral can be a problem. I look forward to whatever comes from your construction company!


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Marco on September 24, 2014, 02:48:55 PM
Stephen, just to let you know that, as I wrote some months ago, I learned a lot from your trim efforts and I built a new prop for my G55 with a much reduced P/D, capitalising on your experience and suggestions. The cgaracter of the model changed dramatically and now its temper has been finally tamed ! Thanks a lot for sharing your experience !
Marco


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: dosco on September 24, 2014, 03:11:31 PM
I'm keen to get to the bottom of this pendulum malarkey, because if it can be shown to bring a model with minimal dihedral safely to earth much more often than not then it opens up a new range of types to model.

I think that "tail heavy" might be a term that has real meaning if the decalage is fixed. You can alter CG to balance a set decalage or alter decalage to maintain a set CG. In the latter case you can place the CG for best pitch stability then alter decalage to match. That's important for models with small tails - moving the CG forward is instinctively the wise thing to do, but doing so decreases pitch sensitivity, making a model more likely to overcorrect, and brings the tail closer to stalling, which is definitely bad news. I've had the CG of this model at 20% and found that the fully scale tailplane couldn't recover it from a dive before it hit the ground. I've also had the CG at 33% and found that the scale tailplane simply didn't have enough leverage. If I remember my history the Sopwith Camel was called "tail heavy"; it had no trimming device and so the pilot had to exert forward pressure on the stick at all times. If it had had pitch trim it wouldn't have been called tail heavy even though the CG hadn't moved.

Interesting to note your distinction between static and dynamic stability ... has anyone done the stability and control modeling/computations for an aircraft stabilized using this method? It might be interesting.

Tapio had once sent me an interesting paper on stability considerations for a wakefield ... perhaps he has access to some articles covering this aspect?

Cheers-
Dave



Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on September 30, 2014, 07:55:37 AM
Thanks Marco, glad it's working out.

Dosco, unhappily I struggle with arithmetic, let alone mathematics, so I won't be the one doing those calculations. Flying outdoors it's hard to find the 'sweet spot' between dynamic and static, although it's easy to infer that it's there, because as described above you can have a model that flies very steadily in pitch, but equally steadily flies up to the stall or into a terminal dive - OTOH with CG further back it will dance around seeming to shrug off turbulence - until it loses it entirely and slams in. Mind you, flying indoors it ought to be impossible to find the sweet spot because assuming a good launch, that same model will be on rails at all times!

Tempest II, Take 2.

I finally got round to fixing this model. It was quite finicky work and doesn't come out too neat, esp under the belly of the plane. I increased pendulum weight and leverage in accord to what I'd done in a Fairey Battle model. I reduced the area of the temporary 2D tailplane slightly, but it's still overscale.

Heavy, highly-levered pendulums seem to be good. Types with pendulum-space in the nose (like the Hawker Tempest) can take this because they need noseweight anyway, and the only place further forward to put it is the spinner cap, so you only lose a centimetre or so of balance leverage.  It's bad news though for models where the pendulum will only fit further aft.

Anyways in early tests in very calm air a picture began to emerge, namely that with rudder fixed, basic trimming is easier (I found this with the Battle) but the model can go into an incipient spiral dive gently enough that the pendulum remains on the model's vertical axis through the whole show, and has no corrective effect. In that case the big vertical tail added to the lack of dihedral are lethal. Flying it in wind the pendulum is nudged and jerked into responding, but in smooth air the insidious spiral can develop. Luckily my space has been limited recently to my own handkerchief so this has only happened on low flights and hasn't wiped anything out. [N.B. to be accurate, or 'disambiguate' as the word seems to be, the whole point is that the pendulum is not "nudged and jerked", due to its inertia. The model is nudged and jerked around the pendulum.]

So I went back to the beginning. The Tempest now has an overbalanced rudder, and since more weight seems to be the way of it, I made the swinging weight bigger. When the model banks the overbalanced rudder deflects to yaw the model in the opposite direction, thus picking the lower wing up. There is a 'servo tab' to control rudder bias and this works very well despite its size.

In the vid the model is power-stalling a bit because I increased turns from 200 to 700 without adjusting elevators or thrustline. The model falls off from a stall but recovers okay. So the model is now about back where it was before it crashed. The problem is the waywardness of the model - it can wander anywhere. I've seen this tendency in all three pendulum models I've built. I was flying the model in a brief interlude when the cattle had vacated the larger field I have to fly in. These aren't docile moo-cows that chew the cud and make scathing comments about men who fly toy planes; these are young bullocks who charge around (remarkably fast) looking for thrills. So even if they're off in a far corner it's unsafe to fly on the field - I can look after meself but they'd eat the model! The cattle are a fixture now unless the field gets too wet for them, so I have no space for large or wayward models.

http://youtu.be/JpX9oqmOfj8

The pictures show how the skin can split along the grain in a crash - the bracketed example was easy to fix. At the trailing edge root the skin had folded in on itself as the wing swept back. The underlying structure was smashed so the whole area was renewed. Next pic shows the end of the video flight - how a model will unerringly find a tree if ever it can - it was turning into the trunk but hit just short, like a Kamikaze.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Marco on September 30, 2014, 02:52:37 PM
Nice to see it flying again !


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on September 30, 2014, 09:36:17 PM
Thanks for the video and report Stephen. It appears that the Tempest is trying to fly in a loose turn so perhaps it is just a matter of further fine adjustments to the rudder stops/servo tab to achieve a more steady turn.
That 4 blade prop looks neat at the beginning of the vid.
I suggest you arm the Tempest to protect the model and yourself from those rampart young bullocks ;D
John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on April 14, 2015, 03:01:35 PM
Next off the shelf is this Tempest II.

It may seem hasty or premature (life is short, non?), but I dug it out a week or so back in order to incorporate some of the features of the pendulum test model I made recently. My tardy discovery that even light pendulums can exert adequate leverage on unbalanced ailerons, admits the possibility of locating a pendulum at the longitudinal balance point of even an overweight scale model. In fact, I even ventured some calculations which show that a pendulum at the CG yields a lighter AUW than a pendulum located in the nose - this surprised me no little. I think it will only apply to a few types with a very similar layout to the Tempest.

The model as pictured on the grass weighs 43g including 2g noseweight and 7g rubber. Paint and canopy will add most of 3g more I guess, and will demand extra noseweight too. The tailplane is the 2D temporary one, now cut down to scale plan area.

I had to do some in-depth reconstruction; the whole belly area is new between the pink lines in the photo. Plenty of cosmetic work is still required. I stripped off the paintwork and unfortunately this took some of the aliphatic coating with it in one or two places - not sure how to rectify that.

First trim flights with the model as pictured were encouraging; in light-and-variable wind it flew small circles readily. It had good stall recovery but marginal pitch stability. Then I coarsened the pitch of the new prop blades, in the fashion of my Fw 190D model. This seems to have improved pitch stability but I need to fly it more to verify this. I'm also pretty sure that the CG pendulum has improved its behaviour a lot. It's flying with a fixed rudder now. I now plan to prioritise flying it over re-finishing it, since the grass is a-growin' and the sun is warm!

http://youtu.be/t0mViZCdtOk

The video flight is 500 turns, made this evening when the wind died off. The clatter of the motor in the near-miss is because the spinner rubs against the plastic nosecone it fits over. Shoddy construction. . .:o I should have made a new, matched unit rather than bodging the existing one. Still, I'm very pleased with its showing so far - famous last words but it actually seems better behaved than the non-scale test model :).

Stephen.

Note - in reply #143  above I say "The good news is that heavy, highly-levered pendulums seem to help"

I would like to recant on that comment! Since writing that, I've found to my satisfaction that light pendulums give the ailerons adequate authority, so if heavier pendulums helped it must have been to do with CG movement or some inertia effect which I haven't as yet divined.



Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on April 14, 2015, 08:37:27 PM
You seem to be getting on top of this pendulum control Stephen. Nice video.

John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on April 15, 2015, 01:45:34 PM
Quote
You seem to be getting on top of this pendulum control Stephen.

It'd be nice to think so John. In fact, when I walked out to fly the Tempest this lunchtime, I saw the strange sight of quite a few ducks, standing in a row ;D.

Seriously, I'm only a small way through all the things I want to try on the non-scale test model, but a few cardinal facts seemed to emerge right from the start - the possibility for light pendulums positioned at the CG; the non-necessity for balanced ailerons, and the apparent ability of the ailerons to cope with poor yaw/roll coupling arising from low dihedral and large fin area. This being so, and with the year marching on at a lick, I thought I'd better get on with some scale stuff :).

The Tempest has done well today - at lunchtime it made a nice 24sec flight on 750 turns in a fresh little breeze. It needs room though; the flight made a figure 8. Then late afternoon I sneaked in another flight (lighter breeze): put 980 turns on it. A while back I made an annular wedge to sit behind the nosecone and set the thrust line - but since it was neither attached to the nosecone or the aeroplane it was a complete pain to handle, so I sidelined it. For the 980-turn flight I tapped one elevator down, so slightly that I couldn't see if I'd actually moved it, and left the rudder where it was.

http://youtu.be/9FSZoV3iGc8

The 'interesting' arrival in the second clip, after the turns run out is a worry, but a likely explanation is that if the motor runs out, the last few turns of now clumpy, lumpy rubber can swipe the aileron lever on the pendulum. I knew the risk but hadn't modified the setup to remove it. That would explain the side-to-side rocking followed by complete control loss. Anyway, we'll see. I realise the model is unpainted and hood-less, but it's already exceeded its previous best and this motor should take 1,200 turns without complaining. The model might carry a longer motor.

When it's grounded due to weather or crashing I intend to fit the scale (very thick) tailplane, make a new rudder and fix the old wing fillets. I'll be very pleased if it can fly with the scale tail.

Stephen.




Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on April 21, 2015, 03:19:47 AM
I'll be very pleased if it can fly with the scale tail.

A tail of woe.

This model won't fly with its scale tailplane. From thinking that 45-50 seconds was within reach based on flights last week, it's now right back to its precarious condition of last year - exactly the same behaviour: it either rears up suddenly and dives in after stalling, with no recovery, or goes into an aggressive bunt or steepening dive. It has no stall recovery. Just as before, it needs lots of down elevator. Considering the forward CG I found this odd, and went to the lengths of resetting and refairing the tailplane at zero incidence relative to the wing. This ought to require up elevator which would provide the best profile for the tail in downwash. In fact it needed only the faintest degree of up elevator and showed little improvement anyway. Big moves to the CG likewise modified the behaviour but the underlying instability was still there. Bearing in mind that the model was beginning to fly really well with a flat-plate tailplane of the same area, I imagine the the scale tailplane is in a semi-stalled condition all the time due to its fatness, perhaps exacerbated by a position in the wake of the wing (the Tempest tailplane is a reasonable 17% of wing area but is on a very short arm).

I suppose the best compromise is a slightly bigger and somewhat thinner tailplane, but the model is shoved to the back of the hangar for the foreseeable.

The Fairey Battle also has a very thick tailplane, and I now recall a similar (though much milder) instability in my model of last year. I might experiment with a flat-plate tailplane on the model, which still has a few landings left in it before it turns completely to splinters (it's largely held together with sticky tape :D ).

Meanwhile, forward the Spiteful, which has a huge hz. tail of a Supa-slim 8% thickness-to-chord!

Stephen.



Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on April 21, 2015, 04:03:36 AM
Ah what a pain- it looks like Mr Reynolds wins at last ;D Your Spiteful should prove it one way or the other, Stephen.

So I guess its back to the podium position on the shelf for your Tempest.
John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: DHnut on April 21, 2015, 06:13:08 AM
This is facinating. Last weekend I finally managed to fly the 1/24 Hurricane in the Doug McHard series. It has been converted to CO2 from rubber and in the process has become some 10 grams lighter balacing at 25% with no ballast. The weight all up is now 45 grams with an empty tank. The glide was defintely better and the behaviour under power was transformed with a nice initial climb to the left ( once a bit more tab had been added ) followed by a good transition to the glide. I suspect the tank being right behind the Telco motor is helping control the power burst by moving the CG forward when charged. It may be a little under elevated as the glide is fast and a little steep. The tail was built as per the plan with a 1/16" frame capped with 1/16" strips sanded to a symetrical section. I have not measured the thickness but would estimate I have last 1/16" in sanding making it 1/8" thick. The thick wing will clearly not help the glide and a tab was needed to keep the inside wing up. When it was rubber powered the weight was 55 + grams and the model was clearly in need of more rubber and the weight was climbing all the time. I wonder if you are reaching a point of the wing loading becoming to great for the model. By the way the model has a full airbrushed finish and will now be completed with markings and a spinner.
The use of a pendulum clearly has possibilities and I wondered if some form of damping may be beneficial ( dont ask how it is to be achieved ) to reduce the overshoot of the control surfaces. I look forward to hearing of the further adventures with pendulums.
Ricky
     


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Pat D on April 21, 2015, 06:57:34 AM
This is facinating. Last weekend I finally managed to fly the 1/24 Hurricane in the Doug McHard series. It has been converted to CO2 from rubber
     

Ricky

would love to see a photo of this CO2 hurri - what size motor did you use?


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on April 21, 2015, 12:01:43 PM
Quote
- it looks like Mr Reynolds wins at last ;D
Yeahhh? I'm just lulling him into a false sense of security.

Hi Ricky, thanks for the useful comparison. I measured the root thickness of the Tempest tailplane and it's 8.3mm. That's a bit more than 1/8" but it's interesting to hear that a reasonably thick tailplane can work - you don't know your Hurricane tailplane area as a percentage of wing area by any chance?

Wow - I didn't realise CO2 conferred such a weight advantage :o.  Mind you, I don't think the Tempest wing loading is the problem - the model as flying in the video clips in replies no.146 and 148  was 43g which isn't toooo bad for 486cm2 wing area. The wing is plenty thinner than the Hurricane wing (I'm assuming yours is scale thickness) and has an almost symmetrical section which should reduce pitching moments. I expect the problem is a result of some particular quirk of the Tempest II layout, perhaps combined with the prop.

It's good to know that the Hurricane will fly at 1/24 scale as it's always high on my list.

Stephen.



Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on November 30, 2019, 11:49:31 AM
A new Tempest model - only this Tempest II is a Tempest V. . .

I got some way with this second project way back in 2016 or somesuch but shelved it - can't recall why. Now it's about ready for initial flight tests, after a recent fever of building work. There's a lot of finishing and painting undone, but no point in doing that until I see whether it can fly with a scale tail (it was this - the thickness more than the area I think - that killed off the Tempest II model).

The main improvement I felt confident of achieving was to build a lighter tail. The tail of the first model was a separate unit, so it needed a weight of timber to attach it to the rear fuselage. What's more there was provision for a balanced, floating rudder. This proved unnecessary, but carried its legacy round as dead weight.

The reason I felt a Mk.V might just work with a scale tailplane is as follows: the Mk.V has a slightly shorter nose than the Mk II, meaning more noseweight would be needed if the tail weighed the same. I hoped that if the whole tail of a new model was lighter, it might balance with the same amount of noseweight despite the shorter nose. This lighter tail, plus noseweight slightly closer to the CG, would reduce pitch and yaw inertia overall. That means the tailplane (and fin) would have less work to do, so it could afford to be smaller.

Furthermore the shorter nose reduces the destabilising effect of the prop, further reducing the workload of the tail. These effects are small, I'm sure, but perhaps enough to make the difference needed.

Well, the new model has come out several grams lighter than the original one. The model is built using the same methods as the first one, but has less internal structure, and the pendulum system is much lighter.

I took no photos during construction except the two of the outer wing panel shown here. I took those as a memo to myself, showing how I installed the whole pendulum system in the lower wing and then attached the top wing skin, aileron surface and all. This is a bold move because it means that once the top wing skin is fixed there is no access whatsoever to the pendulum system, other than the pendulum itself on the fuselage centreline. Everything has to work perfectly first time, without any testing having been possible. Once the wing panel is complete, the ailerons are carefully cut free of the wing and the aileron can (one fervently hopes) be separated from its actuator for final finishing and painting. This idea worked, and was worth the fairly extreme anxiety engendered, because it's considerably more simple to make an installation this way, and because if the wing has any washout (this wing doesn't) the aileron T.E. will automatically have the correct washout built in. At this scale, fitting a pendulum system after the wing is built is fiddly, no question, and making an aileron with the exact T.E. washout to match the wing panel is tricky too. Hawkeyed observers will spot that the aileron skin is obviously separate from the wing skin - the grain is in a different direction. . .that's quite right, but the wing skin blanks were made back in 2016 or thereabouts, and I only had the 'pre-installation' idea a few months ago. The aileron skins were tacked to the wing skins during assembly, and the aileron cut free later.

Even less-than hawkeyed observers will spot that spar 5 goes all wobbly inboard of the aileron. None of the spars had had their final gluing and shaping at the time the pic was taken. The white dome is the landing light reflector. the little block fixed behind spar 5 further out is just a mounting block for the pitot tube.

The propeller blades are from the Tempest II model. The model as pictured weighs 39g. That includes noseweight to balance the model at an admittedly rather far-aft point; and a 22" long motor of 7/32" rubber (two loops of 1/16" and one of 3/32") which weighs 6.5g. Looking back thru the thread I believe that the original model in the same state only minus canopy, weighed about 43 grams.

I'm estimating paint at 2g and that will require more noseweight to balance. We'll see. When test chucks might take place I dunno: the weather where I am has been lousy. An almost wholly wretched, anti-model-flying summer has been followed by maybe? the wettest autumn on record, and cold too. Everything is waterlogged. There has actually been some sunshine today, hence the outdoor photos. The one of the tail was sposed to show the internal structure (as in: there isn't much) but the sun didn't co-operate just then.

The two whole-model photos are intended to illustrate what a handsome beast the Tempest was, even the chinny one.

Stephen.



Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on November 30, 2019, 07:16:11 PM
Fascinating monocoque artistry as per your usual efforts Stephen. Good luck with this one - tell Reynolds to take a holiday :) That is quite a motion reduction on your aileron pendulum linkage.

John


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Don McLellan on November 30, 2019, 08:27:11 PM
Beautiful and exceptionally interesting work Stephen!  Is that a 'rest' for the wire leading out to the aileron? 


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: MKelly on November 30, 2019, 10:17:22 PM
That really captures the Tempest's shape, and you're going to be quite a bit lighter than my leaden West-Wings-derived Tempest.  Very much look forward to seeing how it flies compared to your Tempest II.

Mike


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on December 01, 2019, 06:12:16 AM
Hi fellas, thanks for the comments. Yes the reduction is considerable, and gives enough leverage that a 0.3g pendulum bob moves the ailerons such that, if I blow like billyo over the outer wing, the aileron isn't budged, whether deflected fully up or fully down. One difficulty with the Tempest is that the bottom surface of the aileron has a much greater chord than the top (a deep-Frise aileron?). This means - see diagram - that the aileron has only to pivot down a small amount before the upgoing nose of the aileron, far ahead of the hinge line, starts interfering with the actuator. In the diagram I guess it looks like there's quite a ot of room available but remember this is a 2D sketch and the real actuator has depth and width. This is an example of how all installations are different, and an example of why I sweated having chosen to 'pre-install' the setup and hide it away forever without even having tested it.

The 'rest' Don mentions is another example, because it's the first time I've used 'em (well spotted Don). I've been reluctant to use these in the past because of the friction created, but in this case the weight of the spanwise bar running from pendulum to crank can bow the long crank arm slightly. The crank is made of 0.38mm piano wire. The 'rest' is a balsa strip with a 0.28mm piano wire slide, just visible in the pic I think. In reality the crank arm barely kisses the slide, and there's no noticeable friction.

I'd actually meant to post a snap of the pendulum itself yesterday but forgot. Pic 1 is the pendulum and hanger which links the spanwise crossbars to the pin halfway down the pendulum. Pic 2 shows the pocket on the front spar into which the pendulum assy slides, and 4 shows the lot together. Note that there's a vertical slot in the hanger and in pic 4 the pendulum pin is right at the top - the hanger and crossbars are literally hanging from the pin. This is unintended (the hanger was nicked from a previous project, not tailormade). The slot should extend above and below the pin so that the pin exerts only sideways forces on the hanger (friction again) but since the system works I've ignored that for now.

The system - excluding the ailerons - weighs just under 1g. I made a very brief video clip of the ailerons working, which I meant to append to a flight video, but since it mightn't fly for some long time and I'm gassing about the pendulum anyway, I've uploaded it today:

https://youtu.be/4O6SGN07wok

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on December 12, 2019, 04:24:58 AM
I had a nerve-wracking morning last Saturday, flying the model.

It soon became clear that the model has no static pitch stability worth a damn. Man that model took some thumps. Sometimes the particular noise of the impact made me sure the nose must be smashed or the fuselage split or the wing wrenched off, and I could hardly bear to go up to the model - but nope, it remained undamaged. Most of the nosedives were onto bouncy grass, but not all, and once the model dived into bare mud (soft, thankfully). So the airframe's strength and resilience is encouraging. All four blades had chunks bitten out of them by the chin intake, but no more blade breakages, which was encouraging as well.

The tailplane situation was very disappointing though. There was just no elevator position that gave any balance, it was either zooming up to a full stop and flipping into a vertical power dive to the ground, or diving in right from launch. I taped 2g of lead inside the chin and incidentally blanked off the chin intake, which I'd forgotten to do previously.

The pitch behaviour with more noseweight was possibly better, but it certainly wasn't adequate. A few more hefty thumps. The pic shows a typical 'after landing' scene, on bouncy grass this time. I taped yet more weight to the nose and in a couple of lulls in the breeze got an indication of improved flights. But the noseweight is now over 6g - that's putting the model into lead-sled territory even without paint and final details. I'll continue experiments when weather allows.

Stephen.




Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: ZK-AUD on December 12, 2019, 04:54:27 AM
Stephen I love these all sheet builds of yours and this one is no exception.  This one has the added interest of your pendulum system.  I wonder whether the next time you try it you should make up a thin sleeve to slip over the existing tailplane to increase the area. This will tell you the story and if that is what it needs you can work out a permanent alteration


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Indoorflyer on December 12, 2019, 11:38:25 AM
I know you're not fond of "Tail volume calculations", but it would be interesting to know where the calculated c.g. position is, based on scale stab area :)


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on December 13, 2019, 06:45:42 AM
Ta Mike, that's not a bad idea. In this case the tailplane isn't fixed yet so it's about as easy to make another one as to make ultra-light sleeves which accomodate elevator adjustment. One thing that's niggled me is that I think the Bentley drawings may have the tailplane outline slightly wrong - in the few photos and video I've seen which allow a fair 'plan-view approximation', the outline looks more symmetrical and a bit rounder-tipped than the drawings suggest. Since the last model I've found info giving the true tailplane span and area (15% of wing area). I'll try adding more-more noseweight first, but I honestly think a bigger tailplane will be necessary. The model would be several grams lighter probably, and better-behaved. Just look wrong.

Quote from: Indoorflyer
. . .not fond of "Tail volume calculations",
;D Indoor, I'm glad you've noted my negative feelings towards 'tail volume' calcs, but I don't really think you've grasped the true depth of my contempt for them! It's deep! Really deep!

I'm sure this very matter was discussed on Mike's Lysander thread just recently. It's possible that once you're up to Mike's Lysander's 50" wingspan these calculations are of some use.

Stephen


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: tom arnold on December 13, 2019, 08:33:48 PM
I have really stood in amazement at the sheet balsa skin you can work with as it is really hard as I found in my exuberant youth. Since I have a Tempest on my "To-Do" list and inspired by your build, I thought I would do my usual calculations for my efforts. I found that the scale CG drop line touches the back edge of the tire (aircraft being level) and that is logical for any tail wheel aircraft. That is also at the 25.6% point of the centerline chord. As for an enlarged stab (which I do) of 126% I can move the CG back to 34% of the centerline chord. Of course, to a modeler's eye you can notice the enlarged stab but it is interesting to see the comparison between stab area and CG movement. Good luck in any case on your flights.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Crabby on December 14, 2019, 03:03:14 AM
I was going to ask if tail ballast would be out of the question. A larger stab would do that. Whatever, I love the idea of the pendulum control, and can't wait to see it in action once the perplexing tail issue is sorted out.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on December 14, 2019, 03:25:22 AM
Thanks Tom.  ". . .interesting to see the comparison between stab area and CG movement. " Yes, that's what's keeping me engaged with this project at the mo. I want to keep the root of the tailplane very close to the original otherwise it will mean re-making the fixed, paper fairings which in the original held the rudder actuator and the tail position lights. I can't recall the CG I used on my first Tempest, after putting a temporary flat-plate tail on it. It's probably written in this thread somewhere but this model's a bit different in terms of leverages so stability will be different too.

Quote from: tom arnold
. . .as it is really hard as I found in my exuberant youth.
Ah! But were you using sheet that was coated with some glue or other? That's the thing that makes it workable, and amenable to forming using moisture and heat. I know I've posted this picture before (perhaps more than once ;) ) but I like it so here goes again. . .

Stephen. P.S. thanks Crabby, you posted while I was writing. I don't know about tail ballast - I'll be hoping to save weight at the nose, which amounts to the same thing maybe?



Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: 3view on December 14, 2019, 12:45:26 PM
Stephen,

Thanks for sharing your build, very entertaining.

Re. pendulums and getting a pendulum to work.

I'm wondering if a pendulum equipped model should not have aileron differential, i.e. more up than down?  Maybe try to avoid the balanced turn? It could be worth trying more down than up to keep the model side-slipping in the turn to let the bob-weight see some gravity?

Definitely not a case of Straighten up and fly right!

Steve



 


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on December 15, 2019, 04:26:05 AM
Hi Steve, I think I see what you're getting at but let me work this through:

Differential aileron is intended to reduce adverse yaw, and if Frise ailerons are used this reduction is greater yet. Without yaw there can be no sideslip and without sideslip wing dihedral doesn't work. Thus it makes sense to allow sideslip, and to set up a pendulum system to allow sideslip. Is that your point? Please forgive me if I've misunderstood.

Against this is that sideslip = height loss,* so is best avoided, esp. since the less dihedral the longer the slip before the low wing picks up. My intention from early days investigating pendulum ailerons has been to act fast and hard - a low-friction system with minimal play, that moves the ailerons a lot per pendulum movement. The idea is that the ailerons should be acting correctively even as a roll is taking place, and before a sideslip has set in (inertia means there's a finite time before the sink and slip develop). I can't say whether I've ever achieved this though, because I don't know what's going on in flight.

A sideslip can be defined as too much bank angle per turn rate, and a skid (flat turn) as too little bank per turn rate. The first means a lack of the centrifugal force necessary to provide a balanced turn, and the second an excess of centrifugal force. Centrifugal force is the only thing the pendulum reacts to**; at all other times it simply points to the centre of the Earth. In a sideslip the pendulum droops to the inside of the turn, which moves the ailerons to pick the lower wing up. In a skid the pendulum is flung to the outside of the turn, which shifts the ailerons to increase bank angle. The pendulum is always seeking a balanced turn.

Whether it can achieve a balanced turn I can't say; it's working with or against the prop torque and the fixed trim of a freeflight model as well as the basic aerodynamic properties of any given layout, and of course it's being bullied by wind and turbulence. I've sometimes seen my pendulum models perform nice flat turns - which shouldn't be possible.

I worried a lot about the vertical tail of a Fairey Battle model I made as well as the first Tempest model. They both have lots of fin area. This prevents sideslip and reduces dihedral effectiveness, so with both these models I went to lengths to install floating, mass-balanced rudders with servo tabs [ground adjustable only]. This effort proved unnecessary, and introduced a 'woozy' look to the flight pattern.

*This may not be apparent: a model with inadequate dihedral might be climbing nicely under power - but it would be climbing more if it wasn't sideslipping. It might be losing height imperceptibly more in the glide. Excess dihedral is not just a stability enhancer but a duration enhancer.

**Yes, I know there are other things that will upset the pendulum - jus' trying to keep it simple.

Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: 3view on December 15, 2019, 05:40:23 PM
Stephen,

Thanks for your detailed reply.  Very interesting, your ideas of correcting the roll before sideslip develops and the resultant height lost in a sideslip.

Steve


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on December 15, 2019, 07:22:31 PM
It really looks as Mr Reynolds doesn't want to take a holiday :) it's nice to know I suppose that your sheet construction can take a lot of punishment though. I'm afraid that the thinner and slightly larger tail plane will be necessary.

It's interesting to see how small a tail can work when it and the wing are both thin. I use a 13.4% tail on my small CLG's - the tail being 4.5% thick on an average(thinner at the tip) and the wing around 6% at the root , 3.5% mid span and thinner at the tip.

Merry Christmas

John



Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: Prosper on December 23, 2019, 08:16:12 AM
Thanks for the info John. Yes, a thin wing is greatly beneficial. I'm pretty much reconciled to a larger and thinner tail, but the weather has been so unflyable for so long that there's little incentive to 'crack on'.

Merry Christmas, hope you're not affected by the extraordinary bushfires around Sydney.
Stephen.


Title: Re: Hawker Tempest II, 1/24 scale rubber power.
Post by: OZPAF on December 26, 2019, 12:31:51 AM
Thanks Stephen - nothing life threatening at the moment - the nearest large one is about 45k away in a straight line. it does limit RC flying to days with out a Total Fireban warning, due to the LiPo. Also very smoky!

John