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Flaperon differential

Why more up than down?

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David Ramsden31/08/2020 18:07:16
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Two questions if I may:
1) Why are flaps and ailerons usually set to travel further up than down?
2) With full length flaperons, where only small deflection is called for, is equal up and down movement worth a try?
Thanks!
David

Frank Skilbeck31/08/2020 18:26:39
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David, if you are using flapperons to aid the elevator, i.e up elevator moves both ailerons down, and vice versa, then equal movement is what you would probably dial in.

Ailerons differential where the ailerons move up more than down it to counter adverse yaw, where the down going aileron creates more drag than the up going one and also acts against the turn. You often see this effect in models dragging their tail in a turn. Full size planes tried various options to counter this, e.g. Frize Ailerons, to add drag to the upgoing aileron. You can also counter this by co-ordinated rudder in the turn.

For aerobatic models then equal up and down aileron is usually specified other wise you'd have increased adverse yaw when inverted.

Dedicated flaps usually go down a lot further than up.

David Ramsden31/08/2020 19:44:52
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Thanks Frank. If you want the same behaviour when inverted then I guess you have to put up with adverse yaw. Is it the case then, that aerobatic pilots must use aileron and rudder together in every turn?
Secondly, if dedicated flaps go down more than up and aerobatic ailerons go up and down equally, then it sounds like - with full length flaperons - equal up/down IS worth trying is it not?
David

MattyB31/08/2020 19:59:24
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David, it’s pretty complex - there are a lot of variables (wing section, aspect ratio, flight speed, AoA etc) that contribute to how much adverse yaw you will observe. The best way is to fly the model on recommended settings, optimise the control balance and CG to your tastes, then look at tuning the differential to optimise the roll accuracy. Don’t forget though that any high aspect ratio design with outboard ailerons (i.e. most gliders) will always need plenty of differential.

David Ramsden31/08/2020 20:33:34
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Thank you MattyB. There are no recommended settings because it's an own-design that I'm building. It has high aspect ratio wing (16-1) with a semi symmetrical HQ3-12 airfoil, and full length flaperons. I have given it a bit less than half the dihedral that a rudder/elevator glider would have.
My instinct is to initially set it up with modest equal up/down movement on the flaperons, launch it off the slope, and see how responsive it is to rudder first. If neccessary use tiny amounts of flaperon to initiate turns until I've got some height, then start testing the flaperons properly.
David

MattyB31/08/2020 21:19:09
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At such a high aspect ratio you will definitely need aileron differential, probably quite a lot. I would start with twice as much up as down; remember if you have too much the worst that can happen is an untidy turn where it speeds up and tightens slightly. Too little and it could conceivably lead to a stall and spin if you didn’t correct with elevator and rudder.

Personally with an a/r that high I would think you would be much better with a 4 surface wing and separate flaps inboard. All high performance F3X models are configured this way to give optimal handling at both ends of the speed range.

Edited By MattyB on 31/08/2020 21:20:14

David Ramsden31/08/2020 22:28:08
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Thank you for the warning MattyB. That's what I'll do then - twice as much up as down - at least initially.

Experimentation is all part of the fun but at the same time I'm quite cautious.

My thinking behind full length flaperons is that I've never really understood why people try to get different parts of a wing to behave differently at the same time (i.e. flap down / aileron up or vice versa). Surely it could be more efficient to give the whole length of a wing one 'instruction' rather than two?

Edited By David Ramsden on 31/08/2020 22:28:58

David Ramsden01/09/2020 00:35:03
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I found this thread which is an interesting read **LINK**

Fortunately, my full length flaperons are tapered. They are also quite flexible and I was thinking of full length film hinges. Also. the horns are going on the inner ends so hopefully the tips will deflect less (than the inner ends).
Ever hopeful!

Thanks again Frank and Matty

MattyB01/09/2020 01:15:30
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Posted by David Ramsden on 31/08/2020 22:28:08:

...My thinking behind full length flaperons is that I've never really understood why people try to get different parts of a wing to behave differently at the same time (i.e. flap down / aileron up or vice versa). Surely it could be more efficient to give the whole length of a wing one 'instruction' rather than two?

Short answer - because what constitutes aerodynamic efficiency varies dramatically depending on what you want the wing to do at that moment (I.e high lift at low speed vs. cruise at high speed etc). Efficiency is not the only goal either - we need the model to handle well too, particularly when close to the ground during takeoffs and landings.

Case in point - slowing up a high a/r model for landing. On a 2 surface flaperon wing I could drop both down to 30 degrees, increasing AoA. However, in doing so I would decrease aileron authority whilst increasing the likelihood of a tip stall. Why the latter? If I’m turning right the RH tip is flying slower than the left; if I then try and “pick up” the RH tip with left aileron I increase the AoA of the slower moving wing even further*. If that pushes it beyond the AoA it will stall then we are looking at a bag of bits!

Compare that to a model with inner flaps - I can deploy them to extreme angles whilst reflexing the tips slightly (crow braking), washing out the wing. This enables me to fly slowly (lots of lift from the inner panels) whilst also guaranteeing I have plenty of aileron authority and that the tips will stall after the centre section as they are at far lower AoA.

* - This is a bit of an oversimplification; there are some other factors going on too, but this is accurate enough to give an understanding of why tips can stall before the root on a high a/r wing.

Edited By MattyB on 01/09/2020 01:23:01

Frank Skilbeck01/09/2020 08:00:50
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Agree with everything Matty has noted, you've called them flapperons but if the aim is to introduce a bit of camber for thermalling and maybe reflex for speed, then you only need a very small deflection. You may however want to use them as spoilerons for landing where you raise them to reduce lift and you would need more up travel for this, and as you apply spoileron it's good if you can reduce aileron differential at the same time to maintain roll authority, check your radio you may find if you make raising the ailerons, spoiler control then it will do this automatically. So for camber adjustment, speed and thermal, you use the flap control but for landing you'd use spoiler control (always the throttle stick on my gliders so it can be applied progressively, but that's pilot choice)

David Ramsden01/09/2020 10:55:34
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Thank you again MattyB and Frank. Those are really useful explanations and tips.

So based on all that, what I think I'll do is set up the flaperon flap action with a very very small amount of down ("to introduce a bit of camber" when I want it) and a moderate amount of up ("to reduce lift for landing" ) and the aileron action to 'moderate' with twice as much up as down and with much more movement (both ways) than the flap action.

I am aware that I may need to programme in some elevator compensation for the flap actions. I'll cross that bridge when I get to it.

I haven't actually fitted the flaps or covered the model yet so I could, as a further precaution, cut the flaperon length so the outer 10% of each wing has no control surface at all. Good idea?

This has been really helpful THANK YOU.

David

Edited By David Ramsden on 01/09/2020 10:57:08

Peter Jenkins01/09/2020 11:45:24
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One further point to bear in mind David is that for a long span wing the inner wing tip in a turn is travelling significantly slower than the outer tip so there will be an imbalance in lift resulting from the different local airspeed that the wing experiences. In full size gliding you sometimes had to add a touch of aileron to stop the bank increasing too much. That is also why in full size gliding on windy days you aim to be wings level for the final approach to be higher than on low wind days as the wind gradient will cause a differential in airspeed between the low and the high part of the wing and can cause the need for full aileron to roll wings level sometimes with full rudder to yaw the wing and try and reduce this imbalance in local airspeed.

That effect shouldn't be an issue with model gliders unless you are turning close to the ground on a high wind day.

Simon Chaddock01/09/2020 12:43:35
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David

Flaperons that are long, thin, full span, a bit flexible and with their horn near the root?

Sounds to me like a good way to induce control surface flutter particularly when flying at speed.

If it happens you may loose the fluttering surface or even break the wing.

David Ramsden01/09/2020 13:59:56
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Thank you Peter. I'll watch out for the inside wing dropping and turns tightening as you describe.

Thank you too Simon. Control surface flutter is not something I've come across before. I'll watch out for that and modify the set up if I need to. Forewarned is forearmed!

The more I think about it, the more convinced I am that the outer 10% should be non-moving.

Edited By David Ramsden on 01/09/2020 14:00:30

Denis Watkins01/09/2020 16:10:37
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Posted by David Ramsden on 01/09/2020 13:59:56:

Thank you Peter. I'll watch out for the inside wing dropping and turns tightening as you describe.

Thank you too Simon. Control surface flutter is not something I've come across before. I'll watch out for that and modify the set up if I need to. Forewarned is forearmed!

The more I think about it, the more convinced I am that the outer 10% should be non-moving.

Edited By David Ramsden on 01/09/2020 14:00:30

David,

Don't loose sight of the fact that most aileron effectiveness is produced at the outboard end of each wing

So the missing outboard 10% you propose would " soften " the aileron effect.

 

Edited By Denis Watkins on 01/09/2020 16:11:12

Frank Skilbeck01/09/2020 16:47:53
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And if you can arrange it so the up going aileron has more drag then this will help the turn in that direction. Looking at my scale gliders (Vintage and glass ship) the ailerons are all outboard, some going all the way to the tip

David Ramsden02/09/2020 00:50:13
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Thanks Denis and Frank.

Having looked at the wings this evening I think I'll just leave the outer 5cm non-moving. That's about 4% of each flaperon length. Hopefully that, along with the winglets will reduce any tendency to tip stall. I may incorporate a bit of washout too.

I'm quite up for modifications after early test flights so nothing is cast in stone. In fact, after all the advice I've received it sounds like modifications are very likely to be needed - especially if I get control surface flutter.

In a month or two I'll post the outcome of the test flights if you are interested.

Jonathan M02/09/2020 07:18:28
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Interesting thread, from which I've learnt a bit more still.

Stopping the ailerons just short of the wing-tips would also have the benefit of protecting them a bit more on a rough landing.... not that anyone ever has any of those!!

David Ramsden03/09/2020 00:09:53
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Posted by Jonathan M on 02/09/2020 07:18:28:

Interesting thread, from which I've learnt a bit more still.

Stopping the ailerons just short of the wing-tips would also have the benefit of protecting them a bit more on a rough landing.... not that anyone ever has any of those!!

Perish the thought!
No. You're right of course. I originally designed it so each flaperon went up to a winglet which protected the end from catching on vegetation. The extra 4cm will given even more protection.

David

MattyB03/09/2020 10:43:35
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David, can you post a picture of this model and a few details (section, AUW, intended usage)? Might help us to understand better what the best configuration really is.

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