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Aviation History
1951
1951 - 0462.PDF
290 FLIGHT, 9 March 1951 POWER versus WEIGHT . i M. Maurice Roy's Louis Bleriot Lecture to the Royal Aeronautical Society: Past History and Present Practice " Flight" photograph M. AlauriceNjoy—a portrait at the readingiyf^ihe pap A we recorded last week, the fourth Louis Bleriotlecture was delivered before the Royal AeronauticalSociety in London on February 23rd. The author was M. Maurice Roy, and an English version of his paper— which is summarized below—was read for him by Com- mandant G. de Fraget. M. Roy is director of the Office National d'Etudes et des Recherches Aeronautiques; we published a note on his distinguished career on page 249 last week. The lecturer aroused some considerable surprise by observingthat in France, Ader, a Frenchman who conceived and used a steam engine weighing only 6.5 lb/b.h.p., was credited with the firstflight of a piloted aircraft—in October, 1897. Having thus effec- tively secured the attention of his audience, the lecturer then wenton to make a power/weight comparison between the 1910 Bleriot and a modern fighter, concluding that the increase in performancewas a result of the power/weight ratio having been multiplied by 12 to 15. For commercial aircraft the prime reason for progresswas neither operational needs nor passengers' demands for speed; rather was it the need for the best power plants with increasedperformances. Turning to aerodynamics, the lecturer discussed the per-formance—in terms of polar curves of Q. against CD—-of the 1910 Bleriot Mk XI, the transatlantic Breguet of 1930, a typical moderntransport aircraft and an experimental transonic type. From this it was shown that, whilst great progress was made between 1910and 1930 and between then and 1950, compressibility would introduce difficulties at speeds up to about M=i.5- The lecturer then referred to weight-saving in the airframe andobserved that, despite the constant increase in performance and size of aircraft during the past 40 years, the structure-weight ofnearly all types, even including seaplanes, had been kept down to between 35 and 40 per cent, and this was about the value of thecross-channel Bleriot. It could be assumed that the relative weight of transonic and supersonic airframes of current size and weightwould remain at about 0.35 to 0.40, and there was little hope of reducing it appreciably. Dealing with the trend of wing loading, power loading, and thelimit of performance, the lecturer presented a series of statistical charts which, he said, should not be taken too literally but regardedrather as indications of the general trend of aeronautical progress. In terms of the general picture, reference was made to the empiricallaw put forward by Harold Adams in 1948: this could be expressed as: "In the race towards higher speeds the fighter metaphoricallytows the commercial aircraft, with a time-lead which grows slowly but which is of the order of six to eight years." There seemed nowto be some slackening in the "tow" and this was probably due to the uncertainty, both in the case of the bomber and the airliner, asto whether the turbo-jet would have a successful rival in the turboprop, or in the compound-engine (foreshadowed, the lecturerpointed out, by Rateau's work as early as 1916). This depended on how much the maximum speed of the so-called strategic bombercould be reduced for the benefit of range, but a bomber could escape the threat of ever faster intercepters only by vying withthem in speed, and this made it appear that, after a short period of transition, Adams' empirical law would again prove true, exceptperhaps for a small class of very long-range aircraft. The lecturer next turned his attention to what he described as"power-plant effectiveness," and stated that when trying to com- pare the turbojet with the typical piston engine improved bycompounding, or with the turboprop, a criterion had to be found which took into account lightness, compactness and overallefficiency, the speed of flight being a fundamental variable in such a comparison. He chose the criterion put forward for the first time(so far as was known) by L. Laming, and which had been given the name of "effectiveness in flight." This total criterion, E, was theratio of the net thrust, F, of the machine streamlined by fairings to the total propulsive weight, Wpr. The total propulsive weight, Wpt, was the sum of the actual weight WV of the engine, fairings included, and the weight W"prof the fuel and tanks needed to develop thrust F over a distance A, with no wind, under ideal operating conditions of constant speed Vand constant height z. By definition, E = F/(W', r+W"Pr).Under the particular conditions denned, and with a lift/drag ratio /=CL/CD of the airframe, the relative propulsive weightdenoted by w vr was the ratio of wpr to the initial all-up weight W,that is to say w B=WK/W= r/(/E), and therefore was equal to thereciprocal of the product of the lift/drag ratio of the airframe, and the effectiveness of the power-plant. Whereas / was a function ofV, of z and of the instantaneous total weight, E was a function of A, z and V. A propulsive system could conveniently be characterized bycurves of E against V for given values of A and 2. With V as abscissa, Wpr could be calculated and, subtractingfrom unity the relative weight w s of the airframe (i.e., about 0.35to 0.4), it was also possible to plot a simple diagram showing the relative useful load that could be carried over a certain distance Aat speed V and altitude z. Such a diagram was shown in Fig. 1. Fig. I. Diagram expressing relative useful load as a derived function of speed for a fixed height and a fixed range. Along these lines, and drawing out the basic difference betweenpiston or turboprop engines and turbojet units: (i) the first group was defined by a specific weight w per unit of effective power(in lb/b.h.p.) by an airscrew efficiency r\ and by a specific con- sumption n per unit of effective power and per unit of time(lb/b.h.p./hr), n including an additional 6 to 9 per cent for the weight of the tanks, (ii) The second group was defined by a specificweight p per unit of effective thrust (lb/lb) and by a specific consumption y per unit of effective thrust and per unit of time(lb/lb/hr). Expressing A and V in miles and in m.p.h. respectively, the formulae for E in terms of other quantities could be given as:— Aircraft with Airscrews Jet Aircraft p_ 375 r, V_ As a first approximation, it could be assumed that, for A and 2 fixed, r], to, fi and p were independent of V, the only variable, whereas y increased linearly with V for a jet, so that y = a+bV, where a and b were positive constants. It was clearly to be seen from the preceding formulae that E was, USEFUL LOAD ALL-UP WEIGH Fig. 2. Power-plant effectiveness ex- pressed as hyper- bolic branches derived as func- tions of speed for a fixed height and a fixed range.
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