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Aviation History
1950
1950 - 0348.PDF
FLIGHT. 16 February 1950 A build-up of ice around the Dart main air intake and a thin-lipped ridge on the de-icing air scoops, some nine inches in length—from around the intake went into the compressor without ill-effect. The missing piece DE-ICING THE VISCOUNT Hot-Gas Thermal Method Preferred : A Report on the Recent Trial Flights COMPLETION of a series of flights by the Vickers-Armstrongs Viscount in severe icing conditions wasrecently reported (Flight, January 26th). Now it is possible to give a fuller report of these investigations, which were carried out over the Atlantic Ocean from Shannon Airport and which entailed some ten hours of flying in icing conditions. The Vickers-Armstrongs Company's exhaustive de-icing trials with the earlier Viking aircraft will be recalled (see Flight, March 13th, 1947). Prefacing the report, the Company states that as a basic principle it believes that an aircraft should be " iceworthy '' even if the de-icing system itself is rendered inoperative, thus great care has been taken to ensure that the controls are relatively unaffected, even though there be considerable ice formation on the aircraft. Vickers, it seems, are confident of the comparative immunity from icing of the Rolls-Royce Dart turboprop with centrifugal-type com- pressor. Had justification been required, it was provided during the recent trials. After consideration of the various types of airframe de- icing systems, the conclusion was reached that the hot-gas thermal-duct method was superior in almost all respects. Accordingly, the Viscount wing was designed to provide the necessary gap for the transfer of hot gases along the leading This rough build-up of ice around the tailplane leading edge hadno effect at all on the control characteristics of the aircraft. Slight ice run-back in the form of narrow chordwise streakssometimes occurred about four minutes after heat was turned on. edge. A very considerable quantity of heat is required to prevent ice formation under the most severe conditions likely to be encountered, and in the case of the Viscount l$ million B.T.U.s per hour—the equivalent of about 600 h.p.—must be available. The Viscount mainplane carries a double-skin leading edge extending as far back as 0.15 of the the chord, and the gap between the skins measures -tin. Intermediate spacers' —not corrugations—separate the skins. The nose portion of the inner skin forms the curved wall of a D-shaped duct running along the leading edge and, as may be seen in the diagram (p. 237), hot air flows forward through a slot in the duct to the gap between the leading-edge skins and then back into the wing, whence it is exhausted to atmo- sphere through surface louvres. A similar method is em- ployed for the tailplane and fin. When the de-icing system for the prototype Viscount was considered, it was resolved to avoid at all costs the use of combustion heaters, and in any case it is logical in an aircraft powered by gas turbines to turn to the jet pipe for the supply of heat. A thermostatic control valve is employed to bring about the mixing of exhaust gases from the jet pipe with cold air; the mixture is then passed at appropriate temperature to the surface heating ducts. Later marks of Darts developed for the production Viscount 700, and a general increase in the overall size and payload of the aircraft have, to quote the manufacturers, "brought in their own train certain developments which ^Pv *_i *?CS
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