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.
In order to secure all the above described advantages, a combination is sometimes effected, thus: