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Aviation History
1956
1956 - 0248.PDF
PLIGHT Thrustful line-up: the Merlin-engined D.H. Hornet was for a decade the fastest (470 m.p.h.) airscrew-driven aircraft in service. It had what was surely the cleanest piston-engine installation of all time. ENTERPRISE IN AIRSCREWS . . . to be powered by Hercules engines with turbo-blowers, which in the event never materialized. The first D H. counter-rotater was completed in September 1941 and flown in 1943 on the R.R.-Vulture-engined Tornado. It then seemed that the capacity of single-disc airscrews was going to be inadequate to meet the requirements of the bigger engines than envisaged. This view has, of course, changed since, and although counter-rotating airscrews are still in production and giving efficient service (for example on the Griffon-engined Shackleton) their weight and complexity are to their disadvantage from the development angle. But it is interesting to recall the original way in which the complicated design was tackled in 1941. A new type of pitch-change mechanism was evolved by the D.H. designers, since the existing Hydromatic type could not be applied. Known as the rack-and-pinion design, it comprised a moving cylinder with racks which engaged with pinions on the blade roots of the front airscrew. The racks continued to die rear airscrew through a "translation unit", so that axial movement of the front racks was conveyed to the rear racks while at the same time a change in direction of rotation was achieved. There were many development difficulties, mainly with the translation unit, and although a smaller version developed for the Merlin was successful, counter-rotaters did not enter large-scale production. A con- temporary rumour, incidentally, that crash landing a metal %counter-rotater would result in the engine being torn out by the blades locking was scotched in August 1944, when a Spitfire being tested with a Griffon airscrew made a wheels- up landing at Hatfield and survived comparatively undam- aged. It is interesting also that an experiment with a counter-rotater whose halves ran at slightly different r.p.m. (to avoid the blades passing when the same reduction-gear teeth were in mesh) was discarded owing to the resulting optical pattern of a steady rotating star which bothered pilots. The rack-and-pinion design, however, was extended to single-disc airscrews, mainly for production reasons: the type could be made in about two-thirds of the man-hours required for the Hydromatic airscrew, and in 1942 difficulty in obtain-ing specialized American machine tools had hindered the expansion of Hydromatic production. The Merlin-enginedLancaster was one of the types for which the rack-and-pinion airscrew was intended, but production was postponed whenlarge quantities of American-built Hydromatics suitable for the Lancaster became available under lease-lend. Productionwas later resumed for the Mosquito, but the rack and pinion eventually gave way in favour of the Hydromatic. War-time development of reversing airscrews gave verypromising results and prepared the way for their early appli- cation to civil aircraft after the war. The first tests wereaimed at assessing the value of the airscrew as an airbrake; a promising pre-war experiment had been carried out with ade Havilland Don trainer (Gipsyking) whose airscrew blades were allowed to windmill in fine pitch for braking in fast dives.A similar test made with a Spitfire in 1940 was promising enough to encourage D.H. to extend the trials to includeactual reverse-pitch landing. The pitch settings were altered and landing tests made at Hatfield; there was, however, atendency for the aircraft to swing violently when the throttle was opened in reverse pitch. The expectation that this wouldnot occur on a tricycle-undercarriage aircraft was confirmed during subsequent tests with an Albemarle. The way wasclear for development of a reversing airscrew for the first de Havilland post-war aircraft, the Dove, and it was this unitthat gained the first British type-approval for equipment of this kind (March 1947). At the end of the war the de Havilland propeller divisionwas still a pan of the parent Aircraft Company, but it was clear that its size and specialized responsibilities called forthe formation of a separate company. Thus de Havilland Propellers, Ltd., was incorporated on April 27th, 1946, withthe main headquarters at Hatfield as the centre of design, development and flight-testing, and with the main produc-tion plant at Lostock in Lancashire. By then D.H. had air- screws prepared for the abundant crop of new British post-warairliners—de Havilland Dove, Bristol Wayfarer/Freighter, Avro Tudor, Handley Page Hermes, Airspeed Ambassador,Vickers Viking, Percival Prince, Miles Marathon, etc., and procured production orders for the majority of these types.Although not the first to get a turbine propeller flying— Rotel, always the keenest competitor to de Havilland, wereon the famous Trent-Meteor in 1945—DM. were the first to secure an A.RB. type-approval. This was in December1945, when a 13ft Hydromatic airscrew successfully completed a 100-hour type-test on the h.p. Bristol Theseus. Installed in place of the outboard Merlins of a Lincoln,they flew for the first time on February 17th, 1947, an historic date in the development of de Havilland turbine propellers.It is in this field that almost the entire post-war airscrew design and development effort has been conducted: radicalnew thinking was required to master the special control prob- lems of the turbine, and long-term planning was essential toensure that the airscrew kept abreast of the turbine, which was—as it still is—always pressing on at a startling pace.Experimental facilities at Hatfield were expanded, and the end of 1946 saw the completion of twin 22ft test tunnels, each200ft in length, for the full-scale testing of airscrew and powerplant. The foremost problem was to provide governors to handlethe rapid power build-up that characterizes the turbine, while at the same time introducing a measure of anticipation intothe system to allow for the initial delays in response to throttle. Higher-capacity governors, with anything up to double therate of oil flow, were necessary to deal with the required high rates of pitch-change. The airscrew designer found himselfworking more and more closely with the engine man, tackling the handling and controlling problems of the more sensitiveturbine together, and at an early stage. Reversing, partial-
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