History of aicraft piston engines

68b was one of the early successful designs later the

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Unformatted text preview: in propeller operation. At m y insistence, further " r o u t i n e " p ropeller tests on experimental propellers were m a d e in a specially constructed "bombproof" shelter. All metal propellers of t h a t time (1921-1922) f ailed, with m u r d e r o u s results to the engine. I n one case the whole assembly of crankshaft, rods, and pistons was pulled out and t h r o w n 20 feet from w h a t r emained of the engine and stand. S ubsequent metal propeller development involved careful attention to v ibration problems. T h e R e e d type, using a twisted a l u m i n u m plate as a b ase (fig. 68b), was one of the early successful designs. Later, the m anufacturers of metal propellers developed elaborate e q u i p m e n t and p rocedures for the measurement and suppression of blade vibration. A n excellent historical record of the development of variable-pitch p ropellers is given by K . M . Molson. T h e idea started with m a r i n e propellers as early as 1816. T h e n eed for pitch control in airplane propellers w as realized as early as 1912. Various designs of controllable-pitch p ropellers were tested, usually with disastrous results because of mechanical w eaknesses. V a r i a b l e pitch became essential with the advent of the highperformance airplane, the Boeing 247 and the Douglas D C - 3 being early e xamples. I m p o r t a n t propeller developments, with a p p r o x i m a t e dates are: 1921 A luminum blades, fixed pitch (Reed) 1923 Aluminum blades, adjustable pitch 1931 Hollow steel blades 1929 Controllable pitch, 2-position 1935 Automatic, constant speed 1938 Feathering 1945 Reversible and feathering 77 T he above dates indicate general use in at least some airplanes. Among t he first flights with controllable pitch were those at McCook Field with t he Heath propeller about 1921. T he first application of the constantspeed variable-pitch propeller was by H ele-Shaw a nd Beacham for a test flight in England in 1928. Both hydraulic and electric pitch control w ere used until after World War II. Now hydraulic control and aluminum b lades are standard on piston engines, with a few exceptions. Figure 68 shows examples. Reduction Gears T he Wrights, with their chain drive, were evidently aware that the optimum speed for engines is not usually that for propellers. Even before the Wrights, most experimental airplanes (Stringfellow, Maxim, Langley, and others) h ad belt- or gear-driven propellers, although the drive ratio for steam engines was usually up rather than down. D irect propeller drive, with the propeller mounted on the crankshaft, is attractive for its simplicity and reliability, and was used by most of the early fliers after The Wright's and up to the start of World War I. An exception was the early Renault air-cooled V-8 (fig. 17, p . 20), the propeller shaft of which was an extension of the camshaft (or vice-versa) and ran at half crankshaft s peed—a r atio which has been widely used since. O ther geared engines which appeared for use in World War I included t he RAF (a copy of t...
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