Why is there a difference between aileron upwards and downwards deflection angles?

Question

I have noticed that on airliners the ailerons have different angles if deflecting upwards than deflecting downwards. When deflected at the maximum angle maximum angle downwards is lower than when deflected upwards. I suppose that this is due to the fact that when the wing has to be deflected upwards and consequently the aileron moves downwards, the force required is lower as there is already lift being produced by the wing whereas the opposite would happen when deflecting the wing downwards and more power is required.

Is this assumption true or are there any other technical reasons behind this deflection angle difference?

Answer 1

You're correct that the ailerons are deflected differently when they are deflected up or down. This is done in order to counter the adverse yaw effect which occurs when ailerons are deployed. Consider the situation when the ailerons are deployed.

Image from aerospaceweb.org

One aileron is deflected downward while the other is deflected upward. One the side with the downward-deflected aileron, lift increases as the deflection effectively increases the camber of that portion of the wing. The opposite happens in the other side.

However, the drag is also affected by aileron deflection- both induced and profile drags. For same deflection, the profile drag increase is same in both the wings. However, the induced drag on both sides are not equal, with a larger amount the wing with the down aileron (as the lift is more and induced drag is proportional to the square of lift).

One way of overcoming this is to deflect the ailerons differentially, i.e. deflect the down aileron by a lesser amount than the up aileron. The following table shows the aileron deflection in a number of aircraft, which shows the differential aileron deflection ($\delta_{A}$ is the aileron deflection, in degrees).

Table from Aircraft Design: A Systems Engineering Approach by Mohammad Sadraey

This adverse yaw can also be prevented by the use of,

• Frise ailerons

• Use of spoilers (in the wing up aileron)

• Cross coupled controls (rudder and ailerons).

There is another reason to use a differential aileron- to prevent tip stall at high angles of attack (stall speeds). The rolling couple on the aircraft is always the difference in lift between the two wings. At or near stalling speeds, it is better to reduce the lift in up-going aileron than increase it in down moving one and risk stalling it.

Answer 2

You are right regarding the control forces. Differential ailerons is the easiest way to reduce aileron forces at high angle of attack.

The mechanism is as you suspect. Key here is the free-floating angle of the control surface. Since the wing creates lift, its hinged flap will be pushed up by that lift. The equation for the floating angle $\eta_f$ is $$\eta_f = -\frac{c_{r0}+c_{r\alpha}\cdot\alpha}{c_{r\eta}}$$

with $c_{r\alpha}$ the hinge moment coefficient over angle of attack $\alpha$ and $c_{r\eta}$ the hinge moment coefficient over control deflection $\eta$. Both coefficients are normally negative and the absolute value of $c_{r\eta}$ is larger that that of $c_{r\alpha}$. The result is a trailing-edge-up floating position - the aerodynamic forces push the aileron trailing edge up.

This means that the up-moving aileron will do so with gusto while the down-moving side needs extra force to be moved. If you now change the gearing such that equal amounts of stick travel mean ever smaller deflection angles when the aileron moves trailing edge down, your leverage is improved. At the same time, the up-going aileron will help to push the control rods into the deflected state, so forces become smaller still, if both ailerons are connected with stiff pushrods.

This effect is greatest at high $\alpha$, as you can easily see from the equation above. It helps less in cruise when $\alpha$ is low, but then aileron movements will be small or even zero when a high-speed aileron takes over.

On modern airliners this direct link is missing, so they do not see the benefit in control force reduction. Nor do they need so since the ailerons are moved by hydraulic actuators. Here the main advantage is better stall resistance in slow flight. When the high lift devices are deployed, they induce extra suction on the outer wing and bring it close to stall. Without leading edge devices the outer wing would stall even before the full lift potential of the inner wing has been exhausted. Adding a large aileron deflection almost guarantees poor stall characteristics in the approach and landing configurations. This is the main reason for the reduced downward deflection in airliners.

Now to adverse yaw: Drag due to deflection is small and plays little part in the yawing moments unless one aileron has separated flow on its suction side (which means it is stalled). What really causes adverse yaw is the different tilt of the aerodynamic force vector on each wing. Remember that lift is acting perpendicularly to the local flow direction? The rolling wing sees a variation in angle of attack over wing span, with the angle of attack on the down-moving wing being increased from the rolling motion while the up-moving wing sees a reduced angle of attack. Consequently, the lift vector on the down-moving wing points forward while the one on the up-moving wing is slanted backwards. This is what causes adverse yaw.

Answer 3

I do agree fully with both previous answers, however we need to distinguish required relative defections for a bank order, from the maximum physical possible up and down deflections for the following reason:

Many modern aircrafts have a « droop » fonction which uses the ailerons to provide a flap effect when flaps are extended, in this case the droop function should not prevent the ailerons roll function to work properly, thus the aileron maximum deflection, (with respect to the rigging aileron neutral position in the clean configuration),might become larger for the downward aileron compared to the upward aileron.