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Author Topic: Hawker Tempest II, 1/24 scale rubber power.  (Read 9716 times)
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OZPAF
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« Reply #75 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 Cheesy Grin It certainly proves your point re using the thin sheet balsa to better represent the monococque originals.
Goood luck with it.

John
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Prosper
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« Reply #76 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 Cheesy. 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 Sad ) 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? Grin. 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.
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Prosper
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« Reply #77 on: July 05, 2014, 04:39:32 AM »

More. . .
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Monz
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« Reply #78 on: July 05, 2014, 05:12:17 AM »

That's a proper miniature aircraft, not a model!
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RalphS
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« Reply #79 on: July 05, 2014, 05:59:35 AM »

Great detailing.  Looks right.  Good luck with the flying.
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Don McLellan
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« Reply #80 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
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green-man
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« Reply #81 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.
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Prosper
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« Reply #82 on: July 06, 2014, 04:47:17 AM »

Thanks Monique, thanks Ralph, I'll need that luck I think Smiley.

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.
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tom arnold
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« Reply #83 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.
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Prosper
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« Reply #84 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.
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sparkle
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« Reply #85 on: July 06, 2014, 04:36:15 PM »

 Grin 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.
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« Reply #86 on: July 07, 2014, 03:44:55 AM »

Great pics. The light reflections are what make it so realistic.  Cool

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 Shocked 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
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Pete Fardell
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« Reply #87 on: July 07, 2014, 04:46:15 AM »

Wow, what a model! Beautiful!
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« Reply #88 on: July 07, 2014, 06:34:17 AM »

 Grin 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.  Embarrassed
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Prosper
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« Reply #89 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 Sad ). 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 Smiley.

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.
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« Reply #90 on: July 07, 2014, 04:36:59 PM »

 Grin It's very brave flying such a well detailed model without the usual free flight cheats! Flight has promise! despite landing!
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« Reply #91 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  Smiley)

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
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« Reply #92 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.

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Don McLellan
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« Reply #93 on: July 08, 2014, 09:34:18 PM »

Hi Stephen,

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

 :'(
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« Reply #94 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? Wink

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 Smiley.

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.

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« Reply #95 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  Smiley I think the four blader drag is a significant issue too.
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« Reply #96 on: July 12, 2014, 06:25:07 PM »

Your Tempest is Top Notch.

Scott
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« Reply #97 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. . .Grin  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 Smiley.

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.
Attached files Thumbnail(s):
Re: Hawker Tempest II, 1/24 scale rubber power.
Re: Hawker Tempest II, 1/24 scale rubber power.
Re: Hawker Tempest II, 1/24 scale rubber power.
Re: Hawker Tempest II, 1/24 scale rubber power.
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sparkle
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« Reply #98 on: July 13, 2014, 05:14:27 AM »

 Grin To my untrained german eye it looks as if the Englisher Flukzoid is haffing a stall?  Wink
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Prosper
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« Reply #99 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 Wink. 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.
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