Original flap development was motivated by three desired benefits. 1) slower flying speeds, hence shorter takeoff and landing runs; 2) reduction of angle of attack near minimum flying speed; 3) increase of drag, or control of drag, in order to steepen glide angle in approach and reduce floating tendencies. Currently, because of large aircraft noise problems, the emphasis under the third item has changed. We are trying to reduce flap drag in order to reduce thrust requirements and hence the noise.
AMO Smith