What is differential aileron?
Questions about differential aileron sometimes pop up in my forum and this simple aileron set up can cause some confusion for beginners to the hobby, who have heard the term but don't fully understand if, or why, they might need it.
Differential aileron simply means that the ailerons move more in one direction than the other, with the greater deflection being upwards. The reason why ailerons are sometimes set up this way is to counteract any adverse yaw when the airplane is turning.
Adverse yaw is a term used when the nose of the plane isn't pointing quite in the direction it should be pointing in, and during a turn it's normally caused by the tail end of the plane dropping as the plane banks round. This drop of the tail end can occur because a downward deflected aileron (ie the aileron on the outer wing during the turn) causes more drag than the upward deflected aileron, and this drag tries to pull the airplane in the opposite direction of the turn; for example, if the plane is in a banked turn to the right, there's increased drag on the left hand wing (because of the down aileron) which pulls the plane to the left and hence forcing the tail to drop.
Above - WWI warbirds are classic 'tail droppers'; this picture shows the difference
between a turn with adverse yaw (left) and a tidier turn without (right). The blue arrow represents the drag on the outer wing which forces the tail downwards.
The correct fix for this common issue is to apply a small amount of rudder during the turn, in co-ordination with the aileron deflection. It's how real planes are flown, and is known as 'co-ordinated flying'. The deflected rudder forces the nose of the plane round in the direction it's turning, thus acting against the drag and so no tail-drop is seen.
Rudder can be applied manually during the turn (by using the rudder stick!) but a very easy and convenient fix if you have a computer-based transmitter with channel mixing capability is to mix rudder with aileron, so a small amount of rudder is automatically applied when you move the aileron stick.
Setting up some differential aileron is the other fix, particularly if you have an airplane that has a strong tenancy to drop its tail during a turn - some planes do it more than others - and especially if you're flying a plane without rudder control (ie aileron & elevator only).
Most (or many) computer based Tx's offer an aileron differential programming option if each aileron has its own servo - refer to your transmitter instruction manual for the 'How to...' part.
If you don't have such a Tx and have just one servo operating both ailerons, you'll need to use a circular servo horn (disc) and connect the control rods to holes that are either in front of or behind the lateral centerline of the horn. Whether the holes are in front of or behind the centerline depends on whether your plane is a high winger with servo in the underside of the wing, or a low winger with servo in the topside.
The accurate way of positioning the holes in the disc is to divide the disc in to three equal size areas, the lines separating each one being at 120 degrees to each other (if you're a heli flier, you'll recognise the 120 degree swashplate servo pattern).
The simple illustration below shows why using offset holes causes aileron differential...
As you can see, the aileron differential is achieved because although the fore and aft movements of the servo disc are equal (the blue arrows are the same length) the actual fore/aft travel is different; the red arrows show that the forward travel that the control rod goes through is less because much of the disc movement is actually sideways. This particular set up, with less forward travel, would work for a low wing plane with the servo mounted in the top side of the wing.
So there you have it, differential aileron explained, I hope! Personally I've only ever used the single servo set up as shown once - my computer Tx lets me do it the lazy way these days...
