Aileron differential
 

I understand that one way to test aileron differential is to fly the model level heading into the wind or downwind, pull to a 45-degree
climb, and roll with aileron.
A) If the model rolls without yaw - Make no adjustments

B) If the model exits yawed in opposite direction of roll - Increase differential, Increase UP-THROW on aileron

C) If the modem exits yawed in direction of roll - Decrease differential, Decrease UP-Throw on aileron

Now this is my question, why only adjust the up aileron and not touch the downward deflected aileron? Is it to maintain the roll rate?
Is this a good test or is there another favorite that you guys like to use?

RE: Aileron differential
 

Up aileron creates adverse drag. The idea is to only drop them for added lift without the added drag. Never tried the set-up testing as you mention. I always go with just an axial roll as an indicator. If the model rolls on the central axis and stays level I'm set.

RE: Aileron differential
 

Flyboy,

You are correct in the adjustments required as a result of your roll/yaw findings. The reason the "up" aileron is suggested for adjustment is because increasing the amount of "down" aileron will increase the stall speed of that wing. The larger the "down" surface deflection on a surface, the higher the stall speed of the surface. That's one reason why flaps are on the inboard section of a wing. It causes the center section of the wing to stall first, not the tips. My technique is to adjust the "down" aileron to a lesser deflection rather than increasing the "up" aileron deflection. This accomplishes the same thing without increasing the stall speed of either wing. In flight this is usually not a problem, but at slow speeds on final it is a significant issue.

I haven't heard of the 45 degree profile, but it should work fine. I like to fly the plane directly away from me and do a roll to see exactly what the tail does during aileron deflection.

Paul

RE: Aileron differential
 

Sorry, Paul. Quite the opposite is true. Extending flaps or deflecting an aileron downward increases the wing area and camber, thereby increasing lift and decreasing the stall speed. The direct byproduct of lift is drag (both induced and parasite). In the case of flaps, the drag can be overcome by an increase in power. The drag of the downward deflecting aileron (adverse yaw), however, must be overcome by rudder.

SCW

RE: Aileron differential
 

SCW,

Of course you are correct. You would never guess that I'm a professional pilot with test pilot experience would you? That's what I get for trying to do this and listen to my employees in the background at the same time. What I meant to say is that reducing the deflection of the "down" aileron reduces the drag component on that wing, thus solving the adverse yaw condition. Increasing the "up" aileron accomplishes the same thing by adding drag to that wing, but has much less effect. Especially at slow speed and high angles of attack.

In fact the first thing we're taught in recovering a plane from a wing low, stalled condition is to NOT use aileron. Use rudder. The reason is that the low wing will receive "down" aileron and the increase in drag on that wing can induce a spin.

Thanks for the catch. I'll try to do better next time!

Paul

RE: Aileron differential
 

On the test for "differential or not" I quite agree with Charlie P.!
I've read more wacky tests for simple trimming than I ever could have believed. Many of them attributed to an organization or individual who should know (better).

If you adjust for best possible axial roll experience-----you've got it pure and simple!

I came from the time when we didn't have in-flight trims (save elevator) and we trimmed as you do a free-flight----without the turn of course.
Takes about 50 flights to get it "right" but once you are there it is a thing of beauty as it doesn't give you a 'different look' each day.

If yours requires some trim touch up each time you go out you simply aren't trimmed period.

RE: Aileron differential
 

I love this question - because the answer depends on what you are trying to fix.
on a heavily loaded slow model the answer will be extremely different -ascompared to a very lightly loaded aerobatic setup

If yo are a contest buff who is looking for the best appearing roll - yet another answer
Some wil swear that the model must have all forces in line to get a perfect axial roll - not so - I can show you rolls with a 526 AFS Zlin which has a widely seperated force setup and the rolls appear perfectly axial-
why?
because the rolls all have the spinner and aft fuselage in an apparantly straight line.
An optikal delusion---

unloaded rolls (vertical appear different than rolls with the wings loaded so that presents another setup-for best appearances.
basically the model which needs least AOA for a roll will always look the best -

RE: Aileron differential
 

I guess that's true, I never thought about a loaded wing. It does sound as if a vertical line would be unloaded, but difficult to see I would think. Maybe that's why from what I have read that by pulling to a 45 and rolling it would be a good way to test for an axial appearence. Especially if you are flying directly at yourself or away from yourself.

RE: Aileron differential
 

the up is more important because of the slower boundary layer on top of the wing due to the angle of attack,so to get good air you have to lift the control surface higher.Dan

RE: Aileron differential
 

You are kidding -------- right???

RE: Aileron differential
 

Friends as far as the aerodynamics that i studied go,aileron differential is there because the upward moving aileron moves into a low pressure zone and the downward one into a high pressure zone,so to get the same effect the upward one must move further . This will also aid that the drag will remain the same on both sides.

RE: Aileron differential
 

With our models, an awful lot of our "design" is actually development, and done after the airplane is flying. Aileron differential is one of those things that is most often discovered to be needed, not cleverly designed into our models ahead of time.

Why does it happen. Bottom line is that the wing going up also goes back, and the wing going down goes forward. Why? Because one side loses drag while the other side's drag increases.

It usually happens with cambered airfoils. It happens because the aileron going up actually creates a profile that has less drag than before, and the aileron going down simply increases the drag that side was already producing.

Creating lift creates drag. The wing with the up going aileron is creating less lift than before so creates less drag than before. The down going aileron is creating a profile with more lift than before and creates more drag than before.

We discover that our latest model yaws the wrong way when we try to turn with aileron/elevator and we wind up learning about adverse aileron-induced yaw. And we decide to fix it. What's the most obvious way? Change what's causing the problem, the aileron movement.