# Aileron Differential: Why it’s so important and how to set it up

For years, depending on the model setup, modelers often used offset servo output arms and bellcranks to achieve differential aileron movement. Today, however, using separate aileron servos and the aileron differential program menu in your computer radio has greatly simplified the task. But before we take a closer look, let’s first check out the mechanics of our model during a turn or a roll to understand why aileron differential is so important.

AERODYNAMICS
Typically, most models are set up with equal amounts of elevator (pitch up and down) and rudder (yaw left and right) control surface movements. But when it comes to ailerons, equal amounts of up and down (roll left and right movement), can cause the model to yaw in the wrong direction. Here’s why: When the ailerons are at their neutral positions, the lift and drag produced by each wing panel is equal and the model tracks straight ahead. But when a model has ailerons that move in equal amounts both up and down, the amount of drag (and lift) created by the wing panel with the down aileron becomes greater than the one with the up aileron. The panel with the aileron pointing downward moves up because it creates more lift. The opposite panel goes down (less lift) and causes the model to back toward the up aileron. But here’s the rub! Because of the increased drag caused by the upward motion, that down aileron wing panel also slows down; this causes the model’s nose to yaw in the opposite direction of the roll. The model yaws nose right in a left-hand bank/turn. This condition is known as adverse yaw. Without aileron differential, most airplanes require a certain amount of coordinated rudder to prevent, or at least minimize, adverse yaw while the model is banking through a turn. For sport and scale planes, this can be done manually or with a program mix-however, it won’t work in all types of flight conditions.

HIGH-PERFORMANCE PLANES This adverse yaw thing is also an important consideration while flying aerobatic planes. Aerobatic pilots need to set up their models to react in pure yaw, roll and pitch motions. During a roll (whether it’s executed on a horizontal or vertical line), the model must roll axially without its nose yawing or wandering off the straight line of flight. Aileron differential helps keep the model’s tracking straight.

The model skids through turns.
The tail drops during a turn.
The nose swings out of the turn.
It’s very difficult to roll your model in a straight line.

Even with high-speed jets and race planes, correcting adverse yaw with aileron differential is much better than relying only on coordinated rudder mixing. If speed is the ultimate goal, then minimizing drag is key. Less rudder deflection equals less drag. Fine-tuning your model for maximum performance is easier if you know what to look for and how to correct it. If you can’t use coordinated rudder to correct adverse yaw, then aileron differential is the way to go. Using your radio’s programming is the easiest way to get the job done.

HOW TO USE A PROGRAM MENU
>  Install dual aileron servos. One connected to the aileron receiver port and the other in the Aux.1 port. Make sure the aileron servo moves in the proper direction.
> Activate the flaperon wing type or, depending on your radio system, the dual aileron function.  Install and connect the ailerons and control linkages.
> Start with 30% to 40% differential (down aileron 30 or 40% less than up).
> If differential mix is backwards (more down than up), reverse the servo connections by switching the aileron and Aux. 1 servo leads.
>Adjust the differential percentage after flying the model. Land the model before making adjustments and test fly again.

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#### 11 Responses to “Aileron Differential: Why it’s so important and how to set it up”

1. Mick says:

I was struggling with this subject on my first large plane, H9 Taylorcraft. Thanks for the timely write up, I think I have everything I need to know about it now.

2. Nigel Rollason says:

Aileron Differential is rarely discussed in the modelling press.
Most ARTF’s specify equal amounts of aileron movement, but adverse yaw can bite badly and easily wreck the model.
ALWAYS use at least 60% up and 40 % down when setting up ANY model.
You can always dial it out IF there is too much steering with the aileron ……….you don’t often get a second chance when a model yaws badly on takeoff or landing when the wing is at a high angle of attack when flying slowly.
Due to the adverse yaw, the airflow over the wing changes and the fuselage creates extra drag with the result that the the plane just stops flying and falls out of the sky ……….
Read the article on adverse yaw on our club website ” http://www.edrcc.com

Cheers,
Nigel

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3. Bob B. says:

I believe that differential aileron only is a positive asset on models that are always flown “right side up”. Putting differential aileron into a model that flies inverted will only make the adverse yaw problem worse when the model is in the inverted position. Now, when the ailerons are deflected, the aileron that is deflecting “downward” into the air stream is moving further downward than the aileron that is deflecting “upward” into the airstream. This causes even more adverse yaw than there would have been if the ailerons only moved equal amounts.

For models that were intended to be flown upright all the time, differential aileron is a positive attribute to use. For fully aerobatic models with symetric airfoils, I believe that it is actually a detriment.

• Steve Hollis says:

It is my understanding that, aerodynamically speaking, the attitude of the wing is unimportant. That is: The wing doesn’t “know” whether it is upright, inverted or vertical; the aerodynamics are the same.

• George Ch. says:

I am afraid this is not accurate. Aerodynamically speaking there is indeed a difference as Bob originally implied and is actually a huge one.

Check this out, assuming we are taking a Left roll with full aileron deflection (ie stick fully Left) and we have a 40-60% differential dialed in :
1. Upright flight then: Left aileron 60% Up + Right aileron 40% Down
2. The plane rolls to the inverted position
3. Inverted flight:
Left aileron (still) 60% (deflected BUT) Downwards!
whilst
Right aileron (still) 40% (deflected BUT) Upwards!

See what I mean? In the inverted position you have actually amplified the adverse yaw effect.

4. Ron says:

I would like to know the amount of rudder to mix with the ailerons to smooth out the turns. Is it a certain percentage since it depends on the speed, the length of the fuselage and the area of the rudder? Perhaps this is just as important as differential?

• George Ch. says:

You fly the plane Ron, any plane, so your fingers will be the only ones to be “certain” about their inputs! You already introduced three variables yourself. Can’t get more uncertain!

5. Russ says:

How about a review of how to set up mechanical differential for those of us using non-computer-based transmitters?

6. gustyk says:

Very interesting. I will definitely test it out. Thank you!

7. George Jung says:

It is my understanding that the aerodynamic forces acting on a wing are the same regardless of attitude. That is, aerodynamically speaking, the wing doesn’t “know” whether it is upright, inverted or vertical; differential is still required.

8. Don says:

That’s a great explanation. I had read of AIL differential but didn’t understand it. Now it makes sense. The how-to setup was especially helpful. Thanks for the info.