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Aviation History
1952
1952 - 0647.PDF
FLIGHT, 14 March 1952 291 AIRSCREW 1% DE-ICING 1 Application of Internal Electrical Heating to the Rotol Composite-Blade Design THE composite airscrew blade developed by Rotol, Ltd., represents a marked advance over previous practice in that it enables an airscrew of a high propulsive effi ciency to incorporate an effective internal de-icing system. The standard Rotol airscrews developed for the Viscount and Bristol 175 employ blades of solid duralumin, but there appears to be no valid reason why this internal de-icing installation should not be applicable to any type of solid blade. As seen in the accom panying illustration, the leading edge of the duralumin blade is finished to a profile around which a space is left when the leading edge sheath is put on. Within this space are accommodated elec tric heating elements, one on each side of the blade leading edge. The elements are flat strips, varying in width between one and two inches, of woven glass-cloth in which are interwoven lengths of electrical resistance wire. Seven-stranded ribbon wire—a phos phor-bronze of between 36 and 41 s.w.g.—is used, and its single length is so arranged in the glass-cloth as to extend back and forth several times between the blade root and to within about four inches of the blade tip. The woven element is impregnated with raw neoprene and cured in an autoclave to form a compact, firm unit; it is then dipped in neoprene and thereafter bonded with a Bostik cement to the inside faces of the leading-edge sheath. This sheath is formed from 0.048m beryllium-copper sheet, through-riveted to the duralu min blade; countersunk rivets are used, the heads being finally ground flush and filled. The root of the sheath is secured by a through-bolt of large diameter. The final blade-assembly is then ground smooth so as to maintain sectional contour fidelity. The spinner is likewise equipped with resistance heating; a disc element is cemented to the inside of the spinner nose, while further elements, shaped like the gores of a parachute, are arranged over the remainder of the spinner interior. Three-phase power at 208 volts is supplied to the airscrew from an engine-driven alternator. The current is first led to three rows of brushes in a housing usually attached to a convenient point on the front of the engine reduction-gear casing. The brushes then transfer the current to three slip-rings in an assembly secured to the rear face of the spinner back-plate. These slip-rings are con nected to groups of terminals, each group consisting of three bolts— one for each current phase—and each feeding one blade; current for the spinner is drawn from an appropriate-phase terminal in each of three of the groups. The spinner current is taken through spring-loaded plungers in order to obviate the necessity of dis connecting wiring when the spinner is removed. The arrangement cf the heating element in the leading edge of the blade; the long input-cable is necessitated by the 110-deg. pitch range. In operation, the heating current is cyclically controlled by a time-switch, and is applied alternately to the engine air intake and to the spinner and blades. The de-icing sequence resulting from the rapid heating and cooling is beneficial from the point of view of aerodynamic effici ency and, of course, in the economy of electrical consumption. The cycle time is variable, but will usually be of the order of 30 seconds "on" and 90 seconds "off." It may be found worth while making the cycle time controllable from the cockpit, in order to counter varying severities of icing. During the cooling period, ice is allowed to form over the nose of the spinner and the leading edges of the airscrew blades. Being a poor heat-conductor the ice layer acts as an insulator, with the result that the heating current causes a much more rapid rise in temperature than would occur on an ice-free surface. The ice in immediate contact with the heated surfaces melts, and the entire accretion is then removed by centrifugal force. A special advantage of cyclic heating is that it prevents super cooled water from coming into contact with a continuously heated surface, from which it would flow rearwards to freeze over the remainder of the blade. The current taken on an airscrew of the size fitted to the Viscount is 1 kW per blade and 2 kW for the spinner—a total of 24 kW for a four-engined aircraft; it should, however, be noted that this load is not continuous. The spinner current is particu larly heavy, since the most severe icing is usually experienced where centrifugal force has the least effect. Further, a build-up of ice on the spinner would, in many turboprop installations, tend to starve the compressor of air. In practice, it is stated, the composite-blade airscrew has been found to work very well, and to be resistant to all normal wear. The beryllium-copper leading edge sheath, although thin, is sufficiently hard to resist abrasion from particles of sand and similar air-suspended matter, while being no more vulnerable to impact—from, say, ice thrown from an adjacent airscrew—than sheaths of more conventional types. The airscrews used on the prototype Viscount 700 (which is at present engaged on de-icing trials) employ composite blades with resistance heating and, to this end, were designed with unusual straight leading edges; it is, however, now possible to form the sheaths with double curvature. FARNBOROUGH PRIZE-GIVING "T AM afraid our results are not quite up to last year's standard : A then we took first place out of 2,500 candidates; this year we have only a 'second'—out of 2,695." This comment upon a recent examination was made by Mr. R. D. Peggs, M.A., M.I.Mech.E., A.F.R.Ae.S., Principal of the R.A.E. Technical College, at the annual distribution of prizes and certificates on March 5th. It would, of course, be invidious to single out the achievement of any particular student, or even to attempt to list the total number of degrees, scholarships, medals and certificates which are gained by R.A.E. apprentices each year. It may, however, be mentioned that the prizes—distributed on this occasion by Mr. Kenneth Pickthorn, D.Litt., M.P., Parliamentary Secretary to the Ministry of Education—when neatly stacked, covered a large table; they were arranged between a pair of superbly finished bookends made by an apprentice, which, as is the tradition at Farnborough, were afterwards presented to Mr. Pickthorn. The Principal paid tribute to the late Mr. W. G. A. Perring, C.B., former Director of the R.A.E. and Chairman of the College s board of governors, whose memory is to be perpetuated by the institution of a Perring Memorial Prize, awarded this year for the first time. Mr. Peggs was also able to announce the award of an annual scholarship to a full-time course at the School of Gas Turbine Technology, made available to the college by Power Jets (Research and Development), Ltd., through the good offices of Mr. D. L. Brown, the S.G.T.T. Principal. Perhaps the only jarring note struck during an enjoyable after noon was the announcement by Mr. Peggs that severe limitation of lecture-room space would make it necessary to restrict the intake of new students for the next session. Though no doubt sympathetic, Mr. Pickthorn, as "the man from the Ministry," confined his address to happier topics. Although, as he put it, he was an "academician," he was able to recall the shooting-down of his B.E.2C by an Albatross as evidence of a former disparity of equipment between Great Britain and her enemies. It was certain, said Mr. Pickthorn, that Farnborough would not let such a state of affairs happen again. R.Ae.S. ASSOCIATE EXAMINATION NAMES of candidates successful in the associate fellowship examination of the Royal Aeronautical Society last Decem ber are announced as follows :— Part I.—I. F. Burns, Ilford; F. J. Butters, St.. Albans; S. G. Corps, St. Albans; V. N. Ferriman, Headington, Oxford; J. F. G. Hast, Hat field; M. E. King, Potter's Bar; D. W. Pickston, Reading; D. K. Ray, London, S.W.10; E. C. Turner, Hatfield; W. B. D. Wardle, Shcnfield; J. D. Watkins, Reigate. Part I (Abroad).—F/O. W. Raj, Jodhpur, India; A. V. Ranga Rao, Madras, India; Cpl. T. W. Thomas, M.E.A.F. Part II.—B. R. A. Burns, Coggershall; R. Boocock, London, S.W.18; M. P. Hanvey, Eastleigh; W. A. Page, Leicester; G. S. Pool, Hatfield; A. Syme, Bathgate, W. Lothian; F. I. V. Walker, Hull.
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