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Oct 09, 2012, 09:09 AM
Crash master...
Lazy's Avatar

Two section aileron deflection angle?

Hi gang
Short and simple question: what deflection angle must be on splitted ailerons (like in picture).

Question is: inboard section deflect more, than outboard, or outboard section deflect more?

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Oct 09, 2012, 09:30 AM
Registered User
kcaldwel's Avatar
The outboard aileron should deflect more than the inboard for roll commands. From Dr. Drela from the Supra build thread (sailplane), but I expect it would be a good starting point for any airplane:

"When setting up the aileron-stick servo motions, the flap motion should never be less than about 35-40% of the aileron motion. This gives the least added drag due to roll rate.

In practice you want to use somewhat more than this 35-40%. Increasing it to 60%, say, will give only a miniscule roll-drag penalty. But this larger flap deflection will increase the resistance to tip stall in a tight, slow, sustained turn. The limiting case of 100% flap motion is equivalent to full-span flaperons, which gives the most tip stall resistance."

Oct 09, 2012, 09:44 AM
Crash master...
Lazy's Avatar
Dear Kevin
We talking abort ailerons, ONLY ailerons, not a aileron + flap combination.
Deflected outboard section more, than inboard, on low speed you have a risk lost lift on wing tip, stall-spin-crash.
On high speed you pay penalty for more drag on wing tip.
Oct 09, 2012, 10:13 AM
Registered User
kcaldwel's Avatar
They are exactly the same as flap/aileron system. In sailplanes they are all used as ailerons as well. You just aren't using the inboard section as flaps, although I'm not sure why you wouldn't do that as well to assist with landing.

When the airplane is rolling, the induced aAoA change from the rolling motion is much greater at the tip than near the root. You want the outboard aileron to deflect more for a better lift distribution while rolling.

Deflecting the inboard and outboard the same will make the airplane more tip stall resistant, as was stated in my quote from Mark Drela, but it is less efficient. For a power airplane where efficiency doesn't matter much and with a lot of taper like your design, the safest way to start would be to deflect them the same angle, like full span ailerons.

Last edited by kcaldwel; Oct 09, 2012 at 10:20 AM.
Oct 09, 2012, 10:52 AM
Crash master...
Lazy's Avatar
Dear Kevin
Lot of inconsistency...

Deflecting the inboard and outboard the same will make the airplane more tip stall resistant
Yes, but - why? You argumentation?
If I need a 3 degree slow left roll - why do I deflect both surfaces?
When I deflect aileron - where I have much drag - on tip or inboard?

For a power airplane where efficiency doesn't matter...
Interesting point of view

would be to deflect them the same angle, like full span ailerons.
Again - WHY? Where is odd and even?
Oct 09, 2012, 11:13 AM
Sink stinks
Montag DP's Avatar
I don't think you have clarified the question well enough. Are you going for minimum drag, maximum roll rate, least chance of tip stall, or what?
Oct 09, 2012, 11:24 AM
Registered User
kcaldwel's Avatar
I see you clipped the "much" from my efficiency comment. For most power RC models efficiency really isn't much of a consideration. They generally have a great excess of power, are not asked to lift heavy loads or fly for maximum endurance. Efficiency is not much of a consideration, unlike a sailplane.

More from Dr. Drela:

"Deflected ailerons deform the load distribution away from the ideal near-elliptical shape, and hence increase induced drag. Partially slaving the flaps to the ailerons can alleviate this load distribution deformation, and thus mitigate the ailerons' CDi penalty. The question is what's the optimum amount of ail-> flap mixing.
The lowest-drag aileron system is wing-warping as used by the Wright Brothers -- the wing is linearly twisted from tip to tip. When such a twisted wing reaches its steady roll rate, the load distribution returns to its optimum level-flight shape, and the drag penalty is zero.
With a finite number of hinged control surfaces such a linear twist cannot be achieved. But it can be approximated as close as possible if each surface's deflection is made proportional to its distance from the aircraft's centerline, measured at the surface midpoint.
If the four control surfaces have equal span, we then have:
Surface mid-span loc. deflection
L.Aile. -3/4 -100%
R.Aile. -3/4 -100%
L.Flap. -1/4 -33%
R.Flap. -1/4 -33%
So for this wing the flap motion should be 33% of the aileron motion. Using AVL I've verified that this mixing ratio produces very nearly the smallest induced drag penalty. If the flap span differs from the aileron span, the table above can be adjusted accordingly.
Longer flaps will have larger travel and vice versa.
BTW, this "distance-proportional deflection rule" strongly argues against stopping the ailerons short of the tip. The resulting unhinged tip portion should in fact have the largest deflection.
The "distance-proportional deflection rule" can be fudged if there is a tip stall problem in a sustained turn, where some opposite aileron must is held. By increasing flap travel over its "optimum" amount, the flaps can carry a greater share of the roll power, which reduces the required downward deflection of the inside aileron, and thus delays tip stall. So if your TD glider has insufficient tip stall margin, I suggest increasing the flap mixing and you should see some improvement.
The extreme case would be 100% flap mixing, which mimics full-span flaperons. Flaperons give excellent tip stall resistance, as is obvious to anyone who flies a DLG with a good 2-servo wing. A 4-servo TD wing with decent planform should not need to go to this extreme."

Oct 09, 2012, 12:02 PM
Registered User
I see, thanks for posting that. Now, if the aileron and flap are deflected a different amount, a v-shaped gap opens up between them. That must generate a vortex there. Is there any data about how much drag this will create and is there any merit in trying to keep the gap minimized by having aileron and flap deflections equal?
Oct 09, 2012, 12:35 PM
Grad student in aeronautics
The reason there would be a vortex there is because of the sudden change in the lift distribution, which an aileron or flap in isolation would have as well. I'm sure reducing the sudden changes in the lift distribution using the "distance-proportional deflection rule" helps.
Oct 09, 2012, 12:59 PM
An itch?. Scratch build.
eflightray's Avatar
I vaguely remember someone, (some model), using a full span strip aileron, where the inner end was fixed, (didn't move), and the servo operated out towards the wing tip. The ailerons would progressively twist over their length with maximum deflection at the tip.
Oct 09, 2012, 01:53 PM
Ascended Master
Sparky Paul's Avatar
The outboard aileron all by itself will be more efficient in terms of control power than the combination.
The center of lift for the outboard surface has a longer moment arm than the combined operation, which makes it more effective.
There's really nothing to be gained by deflecting the inner surface with the outer surface. It adds complexity.
Oct 09, 2012, 01:57 PM
Crash master...
Lazy's Avatar
Paul, problem not in complexity.
In depends of all, I build split ailerons, because each one a 2 meter long...
Overall wing span - 9 m.
Oct 09, 2012, 04:21 PM
B for Bruce
BMatthews's Avatar
The wing looks strongly like it is intended for serious aerobatics and likely some 3D side to the flight tasking. As such you should just simply make both the inside and outside deflect by the same angles. The outer portion will work best at regular and fast flying speeds and the inboard section is needed for post stalled control using the prop blast to provide the reactionary airspeed over the inner portion of the ailerons. I see no advantage to an aerobatic model to having different angular motions for a split style aileron such as this. So you may as well keep it simple by just moving both inner and outer parts through the same sort of angles.
Oct 09, 2012, 04:25 PM
Crash master...
Lazy's Avatar
Wing draw - it's ONLY example of splitted surface. Is not a my wing...
9 meter span, 18 AR, 40 kg TOW, motoglider - that a my design.
Oct 09, 2012, 05:59 PM
Registered User
kcaldwel's Avatar
Originally Posted by jkettu View Post
I see, thanks for posting that. Now, if the aileron and flap are deflected a different amount, a v-shaped gap opens up between them. That must generate a vortex there. Is there any data about how much drag this will create and is there any merit in trying to keep the gap minimized by having aileron and flap deflections equal?
As Dan said, I think most of the vortex created is going to be because of the step in the lift coefficient, not because of the gap at the trailing edge between the flaps and ailerons. I suspect the drag from that gasp is very small, and for a sailplane, much less than the difference in the higher induced drag from having a poor lift distribution with the same deflection all along the span.

You can seal that gap with end plates, at the expense of slightly higher drag even when the control surfaces are not deflected (see photo attached). I'm not at all sure which might result in less overall energy loss - it probably depends on the flight profile and the control deflections used.


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