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Aviation History
1956
1956 - 0247.PDF
FLIGHT, 2 March 1556 245 ENTERPRISE IN AIRSCREWS . . . Command's Hurricanes and Spitfires in 1940. This workwas done by D.H. engineers at R.A.F. stations in between sorties: 1,051 aircraft were converted between June 25thand August 16th, an average of 20 a day. It was a few days after Dunkirk that an engineer officer at an R.A.F. station hadtelephoned Hatfield to ask whether a Spitfire could be con- verted to constant-speed, as R.A.F. fighter pilots were findingthat they were being outflown by MelO9s with their V.D.M. multi-pitch airscrews. The Spitfire conversion was completeda few nights later, the aircraft tested and found far superior —particularly in its rate of climb and ceiling—and work beganin earnest. Production of constant-speed-units was stepped up; D.H. engineers, in their brief appearances at Stag Laneduring the next few weeks of day and night work in the field, are remembered best for the length of their beards and thebleariness of their eyes. Airscrew development 1939-45 provides a story typical ofthe way in which war accelerates technical progress. A brief account of the main achievements is an essential preface to thepost-war story. Until 1940 very little original airscrew development workhad been undertaken, the main effon being devoted entirely to the production of Hamilton designs. Some of these were,of course, modified to British requirements—several of them quite extensively (a new size of hub was provided for theHercules engines in the new range of 1939 heavy bombers). The nucleus of a technical department had been formed; atthe outbreak of war there were quite a few technicians engaged on vibration and aerodynamic problems. It was, in fact, vibra-tion analysis that led to the formation of the original technical department, which was to become the pioneer of strain-gaug-ing techniques in Britain. Taking American experience as its background, the D.H. vibration department was soon tohave no small influence on aviation in this country. The vibration measurement of airscrews in flight—a strain-gaugedunit first flew in December 1941—was invaluable to engine manufacturers : two examples of changes made as a result ofthis experience were the firing order of the Griffon and the vibration dampers of the Hercules. The services and experi-ence of the D.H. vibration department soon came to be drawn upon by the industry as a whole. On the aerodynamics sidethe R.83 report published by the company in 1941 became acceptedas an important standard for the calculation of airscrew perform-ance. Great strides were being madein the design of Dural blades, par- ticularly as more knowledge ofstress concentrations were gained by vibration analysis. For instance,the 1939 thickness ratio at 0.7 radius was 8.2 per cent, comparedwith 7.4 per cent in 1944. Like- wise, root thicknesses, even moreimportant because of engine cool- ing considerations, dropped from67 per cent in 1939 to 36 per cent in 1944. Today 20 per cent is A late summer's day in 1940 draws to an end, and a Spitfire is serviced for the next day's Luftwaffe onslaught. Conversion of the Spitfire's D.H. airscrew to constant-speeding had been given priority in the weeks before the Battle of Britain. regarded as quite satisfactory from the stress point of view.The effect on airscrew weight is obvious. The first four-bladers were flown in 1941 and in due coursewent into service on the Napier-Sabre-engined Typhoon. Oil-slinging difficulties were experienced at first, necessitatingthe improvement of blade-root seals, and modified four- bladers eventually saw fairly wide service on Typhoons,Tempests, and special P.R. Mosquitoes. They were scheduled for installation on the Stirling, Halifax and Wellingtons due The wartime scene in one of the D.H. airscrew factories, which built lOOfiOO new units during the war. Peak out- put rose to 1J00 a week in 1944.
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