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Old Nov 27, 2015, 11:42 AM
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Angle of attack in low speed

Hi to all
We all know that sailplane, in critical speed, can roll on the side … which first enter in the stall.

But imagine this situation…
Fly straight DLG very slow…..
THEN….., move ailerons- just little bit …to make turn (roll) to the left…(right aileron goes down little bit, left goes up).

Model will start to turn (roll) to the left ……(and possible it will slide on that left side and lost some altitude because model was to slow. …)

Question is:
Why right wing don’t fall in the stall when we apply down aileron on that wing, in critical speed but it still produce lift for roll (turn)?

So… in this situation…., at low speed, looks that right wing STILL create enough lift to made regular turn (roll)- but in theory, right wing should be stall first because of larger AOA (aileron moves down) compared with left wing which has lower angle of attack (aileron goes up).
If right wing stall first it should turn model in the opposite direction of initial turn (roll) – in this case to the right, not to the left.

No matter how slow I fly, I never saw DLG to stop roll in desire direction and fall in the stall on opposite direction.

Is that because DLG are with low weight, so even at very low speed, when aileron is moved down, it gives more effect of producing lift and not give effect of tip stall …. in a critical Angle of attack?

Very greatful for answer and thinking...

Best to all , Boris
This is a question about Angle of attack of wings in the turn (roll) in low speed , without rudder input.
Yes, we all know that rudder is essential for coordinate turn, but UP question is about wings -AoA and stall.
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Old Nov 27, 2015, 11:57 AM
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Your airplane is not flying slow enough to exhibit the behavior you describe. It can indeed stall to the left in a right turn situation. I've flown full-scale sailplanes that do exactly that.

Until we get on-board instrumentation for things like airspeed, we will never really fly our airplanes to the limit of their performance. When I was a flight test engineer, lo these many, many moons (years, actually) ago, I got my nose rubbed in the true limits of performance many times. U-2 spyplanes fly at the limits of their performance, with the inside wing in a turn exhibiting stall buffet, and the outside wing exhibiting Mach buffet, a difference of perhaps two knots between wing tips. Angle of attack at the stall can have the same fine line between flying and stalling. I got some time in an aerobatic airplane, and the instructor had me flying 45-degree bank turns for several hours, deliberately keeping the wing at the very edge of stall - the goal was 360 degrees of turn, both ways, feeling a stall on-set buffet all the way round. No, I couldn't quite do it.

And that full-scale sailplane I mentioned? It was a Schweizer 2-33, widely regarded as spin proof. But, get it slow in a turn and try to rudder a turn, and watch out! Imagine a low approach to the runway, and the student pilot finds himself low just before turning base to final. He might try a rudder-only turn, the keep the inside wing from hitting the ground. The result is that as he holds the inside wing up, it stalls and the airplane rolls abruptly to the inside in a spin. We tried it (at an appropriate height) and got spins all the time.

When in a landing approach, flaps full down, I have seen my airplane roll opposite to aileron input as one wing stalls, That's why I use rudder-only with flaps full down.

So: I believe your airplane, if it could be flown slow enough, would exhibit the behavior you describe (aileron-down wing stalling). If, you could fly it slow enough, and that's hard without airspeed indication.

Is this any help to you? Am I addressing your question?

Yours, Greg
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Old Nov 27, 2015, 12:42 PM
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The airplane couldn't be flown slowly enough by human hands. Our toys have to have a margin of error to maintain stable flight due to air density changes, small vertical air movements, etc. When you lower a flapperon slightly you're just eating into this margin.

If the air was a perfect fluid medium, you'd not need much if any margin but it isn't and you do.
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Old Nov 28, 2015, 04:30 AM
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Thanks for the answers ...
I understand.

It is good to fly DLG models , with high speed on launch, to fast run all over the sky when it need... and in the same time, same model to good control in so low speed with ability for tight turns in thermal bubble, :_)

Thanks again, Boris
Greg, when You describe landing approach with full flaps down -did You mean on DLG model or other type of RC sailplane (or full size) ? Thanks
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Old Nov 28, 2015, 12:27 PM
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I've had this happen to me and seen it in other models. But it seems to happen more strongly with a bigger model with a higher aspect ratio than what is seen with the smaller span models used for hand or discus style launched models.

If you could fly a little closer to the stall point then you would see it happen with a greater effect and more often. But you'll need to slow it down more than you have been. And when flying that slow and that close to the stall sometimes the sink rate will increase. So it's not a trim setting you would use a lot.
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Old Nov 28, 2015, 12:49 PM
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Although the following Dr. Drela AVL plot is for a steady state 30 degree banked turn, and for local Cl not angle of attack, it does show that the planform of a typical DLG with full-span flaperons may be causing any stall to start at the root area, rather than the tip. A good planform provides stall margin at the tips to avoid tip stall issues while thermalling.

If the root is stalling first, as it probably should with a properly designed planform, the roll and yaw moment from the stalled area will be low, and overcome by the un-stalled tip added lift with additional camber. If that is what is happening, the tip area will still provide a roll moment in the desired direction that overcomes any lift loss caused by the start of a stall towards the root by the down going flaperon.

It would seem that good DLG washout and planform shapes, coupled with the full-span flaperons, provide tip stall margin when flying slowly. You are far more likely to see the tip stall and turn towards the down going aileron with a higher taper ratio wing with tip ailerons instead of full-span flaperons.

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Old Nov 28, 2015, 07:00 PM
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Our DLG's have a very light wing loading and this can make a true stall very gentle. All DLG's will stall. If you cant get it stall you don't have enough elevator throw.
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