School of Thought #1:

Any deflection below the wing only produces drag, and therefore contributes nothing to roll rate.

School of Thought #2:

Downward deflection should be equal to upward deflection for the best roll rate.

Which is it? We had a big discussion about this the other day at the field- no consensus was reached. I don't know, and while I could look it up, I thought some of you guys might have some insight...

Steve

Only produces drag??? Whoever came up with that wasn't thinking at all. What do flaps do? LIFT!

It is however true that down aileron produces more drag than up aileron. This is because the down aileron is going into the high pressure, while up aileron into low.

So, both of the theories are both semi accurate, however the one where down aileron does nothing was definetly not thought through.

This is where differential comes in. In computer radios, differential is a mix in which the down aileron travels less than the up aileron. This helps stop adverse yaw, which is when the plane yaws towards the down aileron due to its higher drag.

Hope that helps

Alex

Steve, this is kind of tough to understand and explaining things is not my strong suit but I'll give it my best. Any time an airfoil is asked to generate lift, a by-product of that function is a corresponding increase in induced drag. This will happen with both a change of angle of attack or an increase in camber. Lets for the sake of this discussion assume the AOA is 1 degree and we are in level flight. We will now assign a drag number of "3" to each wing. When we deflect the aileron {up or down} what we are really doing is changing the camber of the airfoil. Now lets say we deflect the ailerons 10 degrees. On the side that drops the camber will increase, generating more lift with an increase in drag of 4 to a total of 7, on the side that raises we have reduced the lift with a corresponding reduction in drag of 4 to -1. Now we know you can't have negative drag but what has happen is that we have asked the airfoil to generate down force not lift. The difference between the two {6 not 8} is the drag that causes adverse yaw. If you set the differential so the aileron does not go down, the negative number on the up side will yaw the plane in the direction of the roll. Now that would be ok for a Piper Cub and is the reason differential is used, but if you fly aerobatics rolls wouldn't be true and inverted flight would be a real challenge. I hope this makes some sense

Okay, I've got a grip on all of that.

So, how far to set the differential? IOW, given max deflection available on the "up" side, how much "down" to program in for optimum roll rate/least adverse yaw?

I realize an absolute answer is not possible; I'm just looking for ballpark figures here.

Steve

I have found that 35% differential is a great sweet spot to start in.

This was brought up in another thread recently. Depending on what kind of plane you fly, you might not want differential at all. This would be the case if you fly your plane inverted as much as you do up right.

If you think about it, the aileron that goes up when the plane is up right goes down when the plane is inverted. It would actually cause more adverse yaw to roll a plane with differential when inverted. And contra to common belief, aileron differential does NOT produce a more axial roll. To produce an axial roll, you would need a differential that would reverse itself when the plane is inverted.

Steve, with my high wingers {CG cub, Kangke Monocoupe} I use 50%, My Space Walker likes 25% On my Hurricane, Laser, and Extra I don't use any.

Okay. I'm about ready to button up a Contender. I'll try it first with none (equal throw both sides), and go from there.

Thanks for all the replies.

Steve