Aileron size ??
 

Simple questions often pose the hardest answer, and this is one of those times...

What dictates aileron size ?

Required roll rate

The size of the wing!

Remember that, a large portion of the wing is used to fit the flap!

I would say the size of the wing mainly. Outboard ailerons tend to be smaller due to te increased moment arm and inboard ailerons/spoilers tend to be larger dueto decreased moment effectiveness.

Ailerons are plain flaps used at low speeds for lateral control. I would have to say that they are sized on roll rate requirements, and lateral stability at low speed.

They lose effectiveness at high AoA, and high dynamic pressures on aircraft with swept wings. Aileron reversal occours at high dynamic pressures, e.g. the 747, 727, 707 airerons are locked at high speed to prevent aileron reversal.

Most jets use use a combination of spoilers and ailerons at low speeds, and spoilers only at high speed. Spoilers are most effective with flaps down.

From memory the MU-2 has no ailerons....just spoliers

:rolleyes:

The 747 has 2 ailerons per side, inboard and outboard. They are know as low speed (outboard) and high speed (inboard)ailerons, there are of different areas, the outboard being of a larger area. The outboard ailerons are automatically locked out when certain conditions are met, the reason being that the turning moment produced by these is far greater than the inboard ailerons and it would be almost impossible to fly as the inputs required to roll would be so small even the best pilot in the world wouldn't be able to fly it, as far as I am aware it is not to do with aileron reversal.

Aileron reversal is a very serious potential problem for most aircraft with swept wings which operate at high subsonic mach number and beyond. Due to the flexible nature of the wings, the elastic deformation at high speeds due to aileron deflection can be sufficient to cause reversal. The obvious way to solve it is increase bending stiffness but this has weight penalties, so as always there is a trade off. This has to be a significant reason to use inboard/outboard aileron configurations.

[This message has been edited by aerostude (edited 27 May 2001).]

I am told that the aileron size has to be sufficient to counteract the rolling moment of an asymmetric flap failure and still leave sufficient control for a "reasonable rate of roll" towards the side of the failed flap. I am also told this is an arbitrary requirement, but that's how it's decided in theory.