Water performance of a four-engined flying boat with step fairings of length 3, 6 and 9 times the step depth

Abstract

Tests were made to investigate the hydrodynamic qualities of the Sunderland flying boat, when fitted with step fairings of mean gradient 1 : 3, 1 : 6 and 1 : 9. Attitude and acce]eration measurements were made during take-offs, landings and constant-speed taxying runs. Water pressure measurements were made at various stations over the forebody and afterbody hull bottoms with and without the step fairings of 1 : 6 and 1 : 9 ratio. The fairings have no perceptib]e effect on water moments and water drag of the flying boat in steady conditions, although there appears to be a small reduction of the hump speed of 3 to 5 knots with the 6 and 9 : 1 step fairings. The 6 and 9 : 1 step fairings, however, introduce a bouncing type of porpoise in taxying runs at high speeds and high attitudes, although there is no evidence of the normal single- and two-step stability limits being affected. This bounce porpoise was not encountered during any take-off or landing with the 6 : 1 fairing, but was severe in landings with the 9 : 1 fairing whenever the datum attitude on touch-down was greater than 3 deg. The bounce porpoise is associated with a fluctuating water flow over the forebody and over the afterbody behind the fairing, and pressure and suctions of the order of 5 lb/sq in. and -- 2 lb/sq in. respectively were recorded on the afterbody. On the forebody, all pressures were positive. This bounce porpoising takes the form of violent pitching, combined with violent heaving in which the flying boat apparently bounces off the water once per complete cycle at less than stalling speed. Ordinary single- and two-step porpoising is accompanied by fluctuating water pressures on the forebody only. Zero pressures were recorded on the afterbody stations of the hull, with and without the fairings, for all stable hydroplaning conditions during take-off, landing and steady runs. The aft step was just immersed in some very high-attitude runs at high speed, but no recorders were located at the actual step. This bounce form of instability is undoubtedly due to afterbody interference with the wake from the forebody. The water flow from the forebody re-attaches itself periodically to the afterbody because of the presence of the step fairing. This probably occurs on the step fairings, but measurements were not obtained in these tests. The greater the fairing, the greater the re-attachment seems to be and, therefore, the more severe and the more frequent the attendant instability. A further programme of tests will be made to investigate the water flow conditions with the various fairings and means of making these efficient hydrodynamically.