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Author Topic: Hawker Tempest II, 1/24 scale rubber power.  (Read 10692 times)
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OZPAF
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« Reply #125 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 Cheesy - 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
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Marco
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« Reply #126 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
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Prosper
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« Reply #127 on: July 28, 2014, 06:02:44 AM »

Buona mattina, Squadriglia Smiley.

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 Sad. 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 Smiley.

Stephen.
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sparkle
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« Reply #128 on: July 28, 2014, 06:07:33 AM »

 Sad Sad to see the temporary end to such an outstanding model.  Sad :'(
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Don McLellan
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« Reply #129 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. 
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OZPAF
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« Reply #130 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 Grin

Thanks  for all your detailed building and trimming discussions.

John
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Marco
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« Reply #131 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.... Smiley
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piecost
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« Reply #132 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...
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Prosper
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« Reply #133 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 Smiley. 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 Cheesy.

Stephen.
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tom arnold
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« Reply #134 on: August 04, 2014, 11:03:04 PM »

Tell us, what would you do different if you could do it again?
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Modelace
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« Reply #135 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.
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Pete Fardell
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« Reply #136 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!
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« Reply #137 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.
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billdennis747
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« Reply #138 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.
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Prosper
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« Reply #139 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' Cheesy. 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.



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sparkle
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« Reply #140 on: August 07, 2014, 04:11:53 AM »

 Grin 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!
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Marco
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« Reply #141 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
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dosco
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« Reply #142 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

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Prosper
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« Reply #143 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.
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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Marco
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« Reply #144 on: September 30, 2014, 02:52:37 PM »

Nice to see it flying again !
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OZPAF
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« Reply #145 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 Grin
John
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Prosper
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« Reply #146 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. . .Shocked 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 Smiley.

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.

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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OZPAF
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« Reply #147 on: April 14, 2015, 08:37:27 PM »

You seem to be getting on top of this pendulum control Stephen. Nice video.

John
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« Reply #148 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 Grin.

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 Smiley.

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.


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« Reply #149 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 Cheesy ).

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

Stephen.

Attached files Thumbnail(s):
Re: Hawker Tempest II, 1/24 scale rubber power.
Re: Hawker Tempest II, 1/24 scale rubber power.
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