The Aeroplane Speaks. Fifth Edition

This tendency is offset by increasing the angle of incidence (and consequently the lift) of the side tending to fall; and it is always advisable, if practical considerations allow it, to also decrease the angle upon the other side. In that way it is not necessary to depart so far from the normal angle of incidence at which the lift-drift ratio is highest. Wash-in is the term applied to the increased angle. Wash-out is the term applied to the decreased angle. Both lateral and directional stability may be improved by washing out the angle of incidence on both sides of the surface, thus: The decreased angle decreases the drift and therefore the effect of gusts upon the wing-tips, which is just where they have the most effect upon the aeroplane, owing to the distance from the turning axis. The wash-out also renders the ailerons (lateral controlling services) more effective, as, in order to operate them, it is not then necessary to give them such a large angle of incidence as would otherwise be required. The less the angle of incidence of the ailerons, the better their lift-drift ratio, i.e., their efficiency. You will note that, while the aileron attached to the surface with washed-out angle is operated to the same extent as the aileron illustrated above it, its angle of incidence is considerably less. Its efficiency is therefore greater. The advantages of the wash-in must, of course, be paid for in some loss of lift, as the lift decreases with the decreased angle.