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Aviation History
1941
1941 - 2020.PDF
132 FLIGHT SEPTEMBER 4TH, 1941. AIRSCREW DEVELOPMENTS New Fou.r-blad.ers and Counter-rotating Models Announced THE De Havilland Aircraft Co., Ltd., has obtainedAir Ministry permission to announce the existenceof two airscrew developments which have been pro- ceeding in secrecy for a long time. One is the De Havillandall-metal four-bladed airscrew, and the other is the counter- rotating airscrew. Both have full constant-speeding andfeathering action. Both developments have arisen out of the phenomenaladvance in the supercharging of big engines, increasing their operating altitude to such a degree that three bladescannot provide enough blade area to work effectively in the rarer atmosphere, except if the airscrew be made solarge that it cannot be accommodated without unduly tall undercarriage legs. The first of the De Holland four^bladed series was thesize to suit engines of 2,000 h.p. and higher, the biggest engine the R.A.F. is using at present. This airscrew hasa diameter of 16ft. Other sizes are being built covering the range of operational horsepowers. The four-bladerembodies all the structural and mechanical principles that have proved their merit in millions of hours of flying in earlier De Havilland variable-pitch airscrews, the develop-ment and production of which has been undertaken by the company in this country since 1934. An important feature is the use of metal blades. Thesemake possible the thin blade sections necessary for obtain- ing the last ounce of efficiency, and also the relativelysmall blade roots which permit the use of a correspondingly small hub, with reduced weight and cleaner spinner pro-filing. Furthermore, in forced landings with undercarriage retracted, the strong, malleable duralumin blades can berelied upon to bend backward and form skis, protecting the main body of the engine and the whole airframe, andlocalising the damage. As for the blades themselves, 80 per cent, of those which are damaged in crashes or bygunfire (including those " holed " by bullets) are repairable. The De Havilland counter-rotating double airscrew hasthree blades in each plane, and here again the familiar De Havilland actuating principles and construction, includingthe metal blades, are preserved. The airscrew is being made in a range of sizes, and its development proceeds inpace with the development of engine reduction gearing. The new D.H. four-bladed airscrew,and (below) a larger view of the very neat hub. The U.S.A.-U.S.S.R. Ferry REPORTS from Washington indicate that American aircraftwill be ferried to Russia by two routes. One of these will be by an extension of the South Atlantic-African route north-ward from Cairo into southern Russia, and the other will be across Canada, over the narrow Bering Sea and across Siberia.It would appear that the chain of aerodromes extending across Canada to Alaska, whose construction was commenced earlierthis year, will be immediately useful. This route was described in our article, " The Territory of Alaska," in Flight of May1st. Perhaps Whitehorse, Anchorage, Fairbanks and Nome will be in the news shortly. Our guess is that fighters and thoseaircraft made on the West Coast will be sent this way, while bombers trom the eastern factories will probably go over theSouth Atlantic. A Electric Thickness Gauge NEW type of electric gauge for measuring the wall thick-ness of the hollow part of the light alloy blades of air- screws is described by Canadian Aviation. This instrumenthas been developed by the General Electric Company at Schenectady and is capable of measuring the thickness of anynon-magnetic metal even if only one side of it is accessible. Thicknesses of up to ijin. can be measured with an accuracyof about five per cent. The device consists of a bridge circuit, voltage amplifying equipment and an indicating instrument.When one of the two inductances of the bridge circuit is pi't in contact with the metal to be measured, the impedance isaltered by eddy currents and the balance of the bridge dis- turbed. This effect is proportional to the thickness of t:iemetal.
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