# 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.

Executive editor About me: I’m a publishing professional who has a passion for aviation and RC, and I love creating issues, books and a website that help RC pilots to enjoy this sport even more. I admire scale aircraft and enjoy the convenience of flying smaller electrics.

#### 12 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. Robert Lundstrom says:

What about sport jets that have a “Delta” wing planform with no rudder control. I take it that some aileron differential is stil needed?

4. Steve says:

When you say “down” you mean that the aileron is physically in the down position…correct?

• Debra Cleghorn says:

Exactly right!

• Mark Scheibenzuber says:

how can I set up ailerons with one servo per aileron so I can use them as flaps by using a aux switch to activate them I have a tactic 650 radio can anyone help! Thanks Mark

5. Dave Fitzgerald says:

Seems to me this would make inverted flight dificult. Am I wrong ?

6. mike stroup says:

Your assesment is correct for slower Non-Aerobatic models.

In the context of an IMAC/3D or Pattern plane, the use of differential is to negate the positive stability of a plane.

Typically, our aerobatic planes have a CG just forward of the “NP” (Neutral Point)…. this is so the planes are stable and fly nice.
The consequence of this is the plane will not hold level inverted flight. But that is OK.. we have all learned to hold a little down elevator during inverted flight.
Another consequence is rolls tend to be “Barreled” a bit. This can be exagerated if one holds a little up elevator during a roll…
One can prove to oneself that the adverse yaw is not the mechanism by doing a vertical downline roll and observe the model will still barrel a little even though both wings are not lifting at all…

The purpose of Aileron differential is to negate the “Up Trim” needed for level upright flight during a roll… (Or in the case of a truly neutrally pitch stable plane such as my Compy SX) to compensate other asymetries such as a skin mounted aileron hinge.

Regardless of the reason for differential, there are widely published processes for determining the exact amounts needed to achieve an axial roll…

7. Nigel Rollason says:

All Correct.
With aerobatic models and adequate power, litle differential is needed. you can always tune it out.
Yes, a model requiring aileron differential will normally be more difficult to fly inverted……If not impossible, with a flat bottom winged trainer like a “Piper Cub” or “Arising Star”.
The full size Tiger Moth.has a LOT of aileron differential and the S2A Pitts special has about 60%up and 40% down.
You can always mix in about 25% rudder with aileron to achieva similar effect……….Just like they have to do on full size aircraft !!
Adverse yaw has killed a lot of full size pilots………and countless models when aircraft are flown too slow at high angles of attack…..The plane just stops flying and falls out of the sky !!!!
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8. Tony says:

What a great article, this has helped me understand the reasoning for having it in all my aircraft. Cheers.

9. Charles Lee says:

As a new flyer I found the article very interesting but I’m still unsure if I need to apply ail diff as the model is TX adjusted to 16mm up aileron & 14mm down. Any thoughts welcome.

10. Dave Sanderson says:

Hi Debra,
Great info, thanks.
Need your help, flying a fairly slow electric delta, although very stable, its roll rate is very slow, and loses a lot of height, would aileron differential help it to roll faster.
Best regards,
Dave