Flight Tests of a Balanced Split Flap with Particular Reference to Rapid Operation

The flight path of a small parasol monoplane equipped with a special type of balanced split flap was determined for a series of glides during which the time taken to deflect or retract the flap was varied from 1 to 15 seconds in order to study the effect of the time taken to complete a flap movement on the motion of an airplane between the start of a flap movement and the attainment of steady flight with the new flap setting. For flap movements accompanied by a change of lift characteristics, and consequently of velocity, there is an appreciable delay in obtaining a desired change in glide angle even though the flap is operated instantaneously. Immediate control of the glide path is obtained only when the speed is maintained during the flap movement. When the speed is changed, the deviation from the desired path during the transition increases in proportion to the rapidity with which the flap is moved so that, with a high-lift flap, abrupt retraction at speeds less than the minimum speed with the flap retracted may be dangerous if practiced close to the ground.