Flight tests of a falcon fitted with an irving flap

Abstract

It has been stated that flaps for landing a clean, heavily loaded aeroplane, should provide a range of settings over which the lift is constant but the drag variable within wide limits, with low operating forces. The Irving flap aims to do this and it was decided to test it in flight, to see how nearly it approached the ideal and to gain experience of the landing technique to be employed with such a flap. The flap was fitted right across the span of a standard Falcon inboard of the ailerons; the changes in gliding angle and trim due to the flap, together with its contribution to maximum lift were determined. Measurements were made of the operating force, and the linkage and chord ratio varied with a view to reducing this to a minimum. By means of a 'gate' the flap movement could be kept within the constant lift range, when making landings, and the effect of variable drag explored. Handling trials were made by a large number of service and firms' pilots. The lift due to the flap increased uniformly until the half open position and thereafter remained constant, whilst the drag increased steadily. The total gliding angle change was 3½°, and trim change was negligible. The aerodynamic hinge moment was not as low as calculation suggests is possible for this type of flap. Tests indicated that it would be reduced were the chord ratio of the upper to the lower member increased from 1 to 2. Nevertheless, the present flap was quickly and easily operated and enabled pilots to reach a given point at a given spee d, without sideslipping, S-turns or use of engine. Pilots who tested the flap, generally considered it a definite help in facilitating the landing approach, but suggested that much more drag could be used with advantage. It is feasible in the landing technique, to control the gliding angle with a rapidly adjustable flap, which gives variable drag at constant lift. With the present Irving mechanism, quick movements of the flap are possible for aeroplanes up to about 3,000 lb. weight. With the Falcon, the flap was sufficiently large to show its advantage over the non-variable flap, but not large enough to enable the pilot to take full advantage of the landing technique used. A modification to the flap suggested by Mr. W. E. Gray, considerably reduces the operating load, and makes it applicable to larger and more heavily loaded aeroplanes.