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Aviation History
1935
1935 -2- 0564.PDF
5io FLIGHT. NOVEMBER 14, 1935. PREVENTING ICE FORMATION The Thermal, Mechanical and Chemical Methods of Combating the Danger : A Resume of the Paper Read by Mr. B. Lockspeiser before the R.Ae.S. NO one can deny that the formation of ice on aero-planes constitutes a very real danger now that blindand general bad-weather flying is the order of the day. On November 4th Mr. B. Lockspeiser, M.A., in a paper read before the Royal Aeronautical Society, described the work done at Farnborough and the system which has been developed there for the prevention of accretion. This system has, incidentally, now been taken up by the Dunlop Rubber Company, and was described in Flight of July 4th. Mr. Lockspeiser, before proceeding to deal with methods ofprevention, dealt briefly with the meteorological conditions under which rapid ice formation was possible. These condi-tions were, incidentally, enlarged upon in an article in Flight of February 21 this year. From the mass of evidencecollected in recent years, he said, as to the particular meteoro- logical conditions and the associated ice deposits, the followingbroad conclusions might be summarised: — (1) In the main ice formed only at leading edges whenceit built forward and outwards. At times the whole wing surface might become covered with a light frost or glassyice film and icicles might appear at the trailing edge. (2) Ice dangerous to aircraft formed only when visiblemoisture was encountered in the form of rain, mist, cloud or fog. *~" (3) The heaviest rate of accretion of ice was due to rainfalling from a wanner stratum of air on an aeroplane flying in a colder region whose temperature was below freezing point.The air temperature at the height of flight was usually, under these conditions, not more than two or three degrees belowo°C. Clear ice, hard and glassy, was invariably formed under these conditions. (4) Super-cooled droplets of water might exist in mist,cloud or fog down to — 2O°C. On contact with leading edges instantaneous freezing took place. Because the moisturecontent of air masses decreased with lowering of temperature, air temperatures just below the freezing point were mostconductive to rapid ice accretion. Ice formed under these conditions was usually white and opaque, granular in struc-ture. Clear ice might also be deposited under these condi- •tions. (5) Both clear and opaque ice adhered strongly to the sur-faces on which they were deposited in flight. The adhesion of clear ice was, in general, greater than that of the opaquevariety. (6) The deposition of clear ice often gave rise to irregularflatfish shapes of increased frontal area at the leading edge. The sketch on the left shows the method suggested byMr. Lockspeiser for leading the anti-freezing mixture from the airscrew hub. On the right the Anticer is shown indiagrammatic form. The device is now being marketed by the Dunlop Company. This remarkable picture was secured by Flight's chiefphotographer, who went " ice-hunting " in a Hawker Hart piloted by Fit. Lt. Bulman. It shows ice that has formedon the fuel tank vent pipe. Another photograph appears on page 511. (7) White opaque ice tended usually to build forward intocrescent shapes, less dangerous immediately than the shapes associated with clear ice, but sufficiently so to impair per-formance and lead to forced landings eventually. (8) The temperature range favourable to the most rapidformation of either clear or opaque ice was within a few degrees below o°C.The three possible methods whereby the accretion of ice on aeroplanes might be prevented were thermal, mechanical and chemical.Although the ice deposit was normally confined to leading edges, it was found that if the nose of a wing alone was main-tained about o°C. the water was blown back and freezed in ridges parallel to the chord. Theodorsen and Clay, who hadmade a full-scale investigation of this method, provided a slot, running along the span in the neighbourhood of maximum wingthickness, to catch the water and means for draining the slot in flight. There were obvious aerodynamic objections to sucha course, and there was little doubt that the successful appli- cation of the thermal method of preventing ice accretion in-volved the provision of heat for raising the temperature of the entire wing. ,;v Thermal Methods How much heat, he asked, would be required for this pur-pose? The amount, of course, depended on wing section and air speed. From wind tunnel tests 'on a R.A.F. 48 section.M. Scott estimated that a monoplane of 40ft. span, with an average chord 6.7ft., at an air speed of 180 m.p.h., required144 h.p. to maintain the wing 20°C. higher than the surrounding air. This figure was comparable with those deduced fromother experimental work. It was clear that the heat was such that we must look either to the jacket or to the exhaust htatfor the source of supply. The evidence also pointed to the use of a direct rather dunan indirect use of these sources. From time to time schemes had been forward for leading warmed air from muffs round theexhaust pipe to the interior of the wing. The fundamental difficulty here was that the necessary rates of heat transfer, mall stages, from the exhaust gases to the wing, could only be obtained by the piping of large surface areas involving weightconsiderations and constructional difficulties such as to render these schemes entirely impracticable. The thermal method was
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