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Aviation History
1929
1929 - 0408.PDF
SUPPLEMENT TO FLIGHT 10 FEBRUARY 28, 1929 THE AIRCRAFT ENGINEER inn m 6oa soa rv'40Q-LL D - o • MILE S PE R H ... . § ... . 3 ... . P . . sooa a . 8 ; LU O I ^) 2000. aI 1- I00Q 0 C VELOCITY OF SOUND WAVES. Q z2 • o • 2 • Ctzoo. LJ a ; RESISTANC E 10 0 F T 8 8 8 N ( Assumed limit" T.HP Single engine -^ / /^ O y s c / >1 / 0ra - I "Or e <& 9• ir e Assumed limit' TH.P ^Tandem engines ^.^— (Tandc ngj ne ^ • P 3eaj W nq nee a lane) j Besat! ) 01234.56769 10 Fig. 1. taken as sufficient justification to exterpolate to an assumed limit of 600 m.p.h. Around and beyond this figure certain fundamental conditions are modified in such a manner as to preclude the prospect of appreciable improvement in this direction. While one would hesitate to prescribe an arbitrary limit to the resources of research, invention and development, it has already been experimentally determined that the normal laws of resistance increase at a much higher rate as the velocity of sound waves is approached. An attempt, therefore, to advance to any extent beyond the region *of the normal drag ratio would appear to introduce a condition where the gain would be insufficient compensation for the effort expended. It is now proposed to review and summarise the progress that has been made up to the present time by an analysis of the performance of recent examples of high-speed machines, and to consider in what direction and to what extent future machines may be exacted to materialise. The resistance of a modern twin-float racing seaplane at a velocity of 100 ft. per second may be assessed at a minimum figure of 45 lbs. for a monoplane and 49 lbs. for a biplane having the same engine installation. The individual drag of the various components, including their " interference effects," may be considered according to the following approximation :— Monoplane. Biplane. Main plane unit ... Body Tail unit Two floats Struts and wire bracing Total An average T.H.P. of 825 for each machine indicates that speeds of 317 and 309 m.p.h. should be obtained. Assuming that the present association of the various units is retained as a basis for future development, the following analysis of resistance at 100 ft. per second may be regarded as approaching the minimum ultimately obtainable by successive stages of refinement in design. Main plane unit BodyTail unit Floats Struts and wire bracing Total Lbs. 12-3 5-5 2-2 7-2 7-8 .. 35-0 Lbs.14 7 2 8 12 45 •5 •2 •8 •2 •3 •0 Lbs.15-5 8-0 2-8 8-2 14-5 49-0 A reduction of nearly 23 ]>er cent, from the present figure of 45 lbs. is therefore indicated, corresponding to a speed of 344 m.p.h.. with the same available T.H.P. There appears to be little evidence, at the present time, to justify an assumption of any further appreciable reduction in resistance, and consequent improvement in speed, except in proportion to increased power output. A comparative limitation of this nature cannot, however, be applied to the possible development of the aircraft engine. The advent and successful application of supercharging, together with the continued advance in the manufacture of high-grade materials, affords considerable opportunity for increasing the output from the high standard already obtained. In com- parison with specialised automobile practice, the brake horse-power per unit volume of the aircraft racing engine is considerably less than the best available per unit volume of racing car engines. As an example, the engine of one of the foremost racing cars in America is said to develop 143 b.h.p- for a capacity of H litres. Admittedly the speed is higlv— 6,500 r.p.m. The capacity of a typical engine for high-speed aircraft may be taken as 24 litres, and an average output at, say, 3,600 r.p.m. to be in the neighbourhood of 1,000 b.h.p. Assuming, for the moment, that the standard set by the ' Miller " engine represents the comparative limit, pro r#Ui, 1626
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