Aileron Differential
 

Gents,

With respect to aileron differential between a high wing plane and a low wing plane, do the same aerodynamic rules apply? In otherwords, would the high wing plane need more down than up and the low wing plane need more up than down?

Thanks,

JC

RE: Aileron Differential
 

It's the same regardless of where the wing is located. It doesn't change with placement as you're indicating.

Why it's there....

In even travel ailerons the down traveling aileron adds camber to the wing to lift that side. Lift creates drag so the wing that's going up tends to drag back. Keep that in mind for a second.....

The opposite occurs on the other side. Raising the aileron reduces the lift. Less lift also means less drag. (well, up to a point. With enough travel the aileron will start to act like a drag tab but that's a LOT of travel... beyond 25 or so degrees... but I digress).

So when you put the two together you have the rising wing dragging back and the lowering wing trying to slip ahead. That's the opposite effect that you're looking for if you're trying to turn away from the high wing in the case of a normal turn entry. This is why this effect is called ADVERSE yaw. It's more prevelant with higher aspect ratios like on gliders and with higher lift models like flat bottom winged slow flying trainer types. It has more effect at higher lift coefficients, EI: slow flying near the stall. So if your model design will spend a lot of time in that region then it's a good thing to have.

To help counter this we add in differential aileron throw. More up than down to minimize the added drag on the lifting side and tries to add "tab" or high displacement drag on the dropping side.

I suspect that you can see now that this won't change for wing location. It's also only really needed in models for special cases as mentioned.

RE: Aileron Differential
 

More down than up aileron travel is very seldom used on either high or low wing airplanes. High wing airplanes frequently display a greater need for aileron differential (more up than down) than do their low wing brethren, simply because the high wing configuration tends to produce more yaw/roll coupling, and high wing models are seldom used for precision aerobatics, where aileron differential can produce unwelcome roll/pitch coupling - a downside of aileron differential.

RE: Aileron Differential
 

I never use aileron differential -- in inverted flight the differential is in the wrong direction, & it actually introduces adverse yaw in that case. I prefer to use rudder input to counter adverse yaw.

RE: Aileron Differential
 

In addition to the good explanations above, there's also another secondary reason. If your plane is prone, for whatever reason, to tip stalling, you can do two things: 1) move both aileron neutral positions up or 2) use differential, so the high wing doesn't have too great a tip AOA during roll. This is also independent of a plane having a high or low wing.

Inverted flight neutralizes many trim and set-up issues, so britbrat is right... at least until the radio manufacturers figure out that automatic inverted flight mix.

RE: Aileron Differential
 

To deepen the discussion a bit...

Why do some 'neutral' aerobatic planes (0 wing incidence, 0 tail incidence, center-hinged surfaces, etc.) also exhibit the need for differential? When fine tuning an aircraft for competition, you can climb high and establish a perfectly vertical power-off downline, and then roll continuously. Many planes, even with exactly equal throws measured up and down, you can see a slight barrel to the roll, which differential fixes. Obviously, a vertical line is used because in a horizontal roll you must input rudder to keep it axial, so it's hard to base setup changes on that, and a vertical downline is chosen to reduce any impact thrustline may have on how axial the roll is. It is a very small amount - almost imperceptible - but it's noticable sometimes if you're looking to really fine tune your setup.

P.S. The only time I've seen a relatively conventional plane require more down aileron than up is in the case of a top skin hinge, where the up deflection opens up a large gap in the wing creating drag and the down aileron closes the gap resulting in less drag. (i.e. FiberClassics/Composite-ARF... the screeching noise ;)

RE: Aileron Differential
 

You will get such a wobble due to adverse yaw (described earlier) and the vertical CG location with respect to the Aerodynamic Center.

RE: Aileron Differential
 

But if the plane is flying a perfect vertical downline, and the incidence is 0, etc. adverse yaw would not be a factor since the wing is not generating a net lift (the lift - and therefore drag - associated with each aileron would be equal and opposite).

And, if the vertical CG was not aligned with the aerodynamic center, as you put it, there would by a pitch or yaw present when flying the vertical downline without rolling, right? But it only appears when the ailerons are deflected. Otherwise, it tracks a virtually perfect downline.

RE: Aileron Differential
 

Yep -if it is all in a ZERO lift attitude (vertical dive trimmed to establish vertical ) it would be 50/50.
One of those "ideal" worlds which few see.
I got a pair of George Hicks' TENSORS to play with -one for me n one for son Guy
these little toys take a bit of up trim in all ailerons - to get roll perfect.
When you look at the aileron linkage -it is apparant coupled ailerons do not move at same rate - due to off center attaching points.
So geometry of throws as well as AOA change the picture .
When we played pattern models for many years -there was a bunch of "voodoo" aerodynamics batted about - among them was differential in respect to position of wing to thrust line -- I could not make sense of it.

RE: Aileron Differential
 

Not true- remember lift and drag vs. AOA coefficients aren't the same curves. Why do you say that the wing is generating zero lift? And why, even if that happens to be the case, would deflecting ailerons from zero lift AOA have the same effect up and down? Your assumptions are only true if you have a symmetrical airfoil AND are flying at 0 deg. AOA (those things aren't necessarily true); then your drag would be equal on both sides with the same up and down aileron deflection. But with practically any other airfoil section, or if you are not flying a symmetrical airfoil at 0 deg. AOA, the drag with an up aileron deflection would not be the same with the same amount of down deflection.


No, if you are flying at equilibrium, you already have the surfaces in a position to balance everything. You stated an assumption that you were established in a downline- so somehow, you are flying in an equilibrium state. If you then deflect ailerons, the newly added roll forces will act at the AC of the aileron area airfoil sections, which will rotate, or orbit, around the CG. Irrespective of anything else, if the wing is not in line with the CG, you will get a kind of barrel roll effect; if they are in line, you will have a clean rotation around the CG, not withstanding the aforementioned adverse yaw wobble effects, if applicable.

RE: Aileron Differential
 

I like to fine-tune aileron differential for aerobatics by the same method Baron Johnson described. I get the model really moving directly away from me, then give it full aileron. As it rolls, I move the elevator control slightly up and down, and observe whether the roll looks more axial with a little elevator input. If the roll looks best with a little down elevator, it means that the airplane is barreling inside, so it needs a little positive differential (more up than down). I add a little positive differential, and repeat the same test. Sometimes a little negative differential is needed.

RE: Aileron Differential
 

mulligan, I was referring to a symmetrical airfoil (thought it went without saying with a competition aerobatic plane), and as I stated, zero incidence, zero up/down thrust, etc, implying zero AoA on the vertical downline. I'll buy the second part of your answer... Are you basically saying that the roll axis should go through the CG (vertically, for this argument) in order to theoretically eliminate this need for differential?

RE: Aileron Differential
 

I thought you might be talking symmetrical, but since I'm an engineer, I didn't assume ;) In any case, believe it or not, with everything you've stated, you can still be flying at a non-zero AOA (in fact, it's likely) depending on fuselage lift & drag, thrust line vs. CG, wing and stab ACs vs. CG, and other aerodynamic traits of the plane that may dictatate a need for the wing to be producing some lift, positive or negative, on a downline. You get the idea. The ideal aerobatic airplane, I guess, would be one that has all component ACs in line and all of its control surfaces aligned with their respective axis of revolution (i.e., no moment arms other than for the function of the surface).

On the AC vs. CG roll issue, by definition, the roll (all, really) axis goes through the CG. So more accurately stated, the line between the each side's AC should ideally pass through the CG's vertical location, intersecting the roll axis. That way, when the plane is rotating about the CG, the wings are not barrel-rolling around the CG, they are twisting around the CG. And adding differential does not affect this- it is a natrual trait of the aircraft. Adding differential would only help the separate, adverse yaw due to drag issue (by balancing the drag).

I didn't explain it well before, but in the longitudinal axis of the airplane, if the AC of the wing is well away from the CG, then aileron deflection would produce a slight off-axis moment around the CG, yielding a small wobble. I don't believe differential will cure this well, as the lift and drag forces would be on different axes.