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Aviation History
1942
1942 - 0783.PDF
APRIL gra, 1942 FLIGHT 353 LIFT, LOAD and LANDING Functions and Functioning of the Fowler Flap An American High-lift Device With Properties Similar to Those of the Better-known British Types yiLTHOUGH the principle of the Fowler flap was con- r\ ceived as far back as 1916, it was not until the *• * advent of the Lockheed 14 in 1937 that this device for improving the take-off and decreasing the landing speed of aircraft really came into prominence. Articles on the Fowler flap have been written before,, including one by its originator, Harlan D. Fowler, in Flight of April 17th, 1941, but the rising speed of military aircraft and their increasingly heavy wing- loading has -brought the subject of auxiliary high-lift devices well to the front at the pre sent time. It has to be remembered that the 400 m.p.h. fighter would be very severely limited in its usefulness if it could only .Operate from a spacious, smooth-surfaced air port, and the same applies, of course, to the weight-lifting bomber. In effect, the higher top speeds and increased loads must be ob tained by extending the speed-range and not by shifting the existing speed-range higher up the scale; the landing speed must be kept down to reasonable practical limits if superior performance in the air is to be attained with out-a hampering sacrifice of safety in taking- off and landing. aluminium alloy extrusions) must have sufficiently hard flange surfaces to prevent the rollers from bedding in. These considerations by no means exhaust the list of structural problems involved, but they are sufficient to illustrate the amount of development and experimenta tion which was necessary before an essentially simple idea could achieve practical application. One important feature of the Fowler flap has not yet Curved track on which the Flap moves Wing spar to which it is anchored Carrier truck with rollers Flap in closed position for normal Flight Trailing edge oF wing Flap in Fully lowered position Structural Problems This, then, is the function of the Fowler flap; it provides high lift with increased drag for slow landings, high lift with minimum drag for quick take-off, and, when not in operation, lies snugly in a recess under the trailing-edge of the wing, forming part of the normal wing section, and thus offers no impediment to high speed. Simple enough in its actual operation in that it merely slides rearwards and downwards on curved tracks or guides, this type of flap presented certain structural problems to be solved before its application became a thoroughly practical proposition. This is easily appre ciated when one remembers the comparative thinness of lew-trailing-edge of a wing section which had to be still further reduced to provide accommodation for the flap in normal flight and yet remain rigid. The tracks on which the flap slides and their surrounding parts were a struc tural problem in themselves; the former had to pass through the whittled-down trailing-edge to be firmly secured to a specially strengthened wing beam, and the rollers, which carry tfce weight of the flap below flying speed and provide easy fore and aft movement, obviously call for extreme care in fitting. When the aircraft is in flight, of course, the air pressure holds the rollers firmly against the track flanges: in fact, the loads so imposed on them are such that the tracks (generally made from A diagrammatic sketch showing the movement of the Fowler flap along its curved tracks, on each of which it is supported by a group of rollers mounted close to its leading-edge. Maintaining complete rigidity of the structure under all conditions presented a difficult problem. FOWLER FLAP In up position FOWLER FLAP Extended and lowered GOUGE FLAP No slot YOUNGMAN FLAP <^^ Split trailing-edge with slot The action of the Fowle: flap is here compared with that of the Gouge flap (Short) and the Youngman (Fairey) which have been developed along parallel lines and have similar lifting properties. The arrows indicate the airflow when the flaps are down. been mentioned. The tracks are so arranged that when the flap is extended it leaves a gap between its own leading-edge and the trailing-edge of the wing. This gap acts on the same principle as the Handley-Page slot, so that, in addition to increasing the drag for landing and increasing the lift by presenting an extended wing area (thus reducing the effective wing-loading) and a greater incidence, it also maintains a laminer flow and low-pressure area above itself, adding still further to the total lift. Slot Effect When taking-off, the flap is extended part way, when it increases the lift without causing excessive drag. Inci dentally, the Fowler flap has a natural tendency to retract itself in flight. This is because the accelerated velocity of the air stream through the slot sets up a low-pressure zone about the leading-edge sufficiently pronounced to over come skin friction drag and exert a forward pull on the whole flap. Recent N.A.C.A. tests have shown that the fitting of a 40 per cent, wing-chord Fowler flap to a normal wing has approximately doubled its lifting power when the flap was extended and deflected 30 deg. downwards. Actual lift coefficients obtained in the tests varied from • 2.3 for a 20 per cent, chord Fowler flap to 2.9 for a 40 per cent, chord flap of this type. The British equivalent to the Fowler flap is the Gouge flap designed by Mr. Arthur Gouge, general manager of Short Brothers. It differs from the Fowler in that there is no slot effect, and is much neater as there are no unsightly projecting rails. The Handley Page flap does not slide, but obtains its effect from the variable slot between it and the wing. In the Fairey flap designed by Mr. Youngman (also the in ventor of the Youngman dinghy) the flap forms an auxiliary surface which drops below the level of the main wing, in which position its attitude can be altered. Each type has special advantages. ^
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