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Aviation History
1955
1955 - 1657.PDF
778 FLIGHT MAN-POWERED FLIGHT A Stimulating Review by Mr. B. S. Shenstone at the L.S.A.R.A. Meeting THE problem of flight with minimum power was the subjectof a thought-provoking lecture given in London on Saturdaylast, November 12th, at the second annual conference of the Low Speed Aerodynamics Research Association. The speaker wasB.E.A.'s chief engineer, Mr. B. S. Shenstone, and the title of his paper was The Problem of the Very Lightweight, Highly EfficientAeroplane. The people who wanted light and highly efficient aircraft, thelecturer began, were those who thought that a light aeroplane needing more than 50 h.p. was no longer light, those who wantedauxiliary power in their sailplanes, those who wanted a very light efficient glider that could be launched by the pilot and those whowanted to fly by manpower. The general problem was one of weight and drag. The payload, a man of standard weight of 165 lb,was constant, but everything else was fluid. Lilienthal had con- centrated on a lightweight structure made of cloth and osiers. Thisarrangement had weighed approximately 65 lb, giving an all-up weight of 230 lb, and his gliding angle was approximately 5.5:1.This indicated that the power needed to fly level would have been about 1 h.p. at 16 m.p.h. Almost 20 years ago there was a contest in Germany for aircraftoperated by manpower. Four of these designs, although unsuccess- ful, were worth mention because of the low weights and goodaerodynamics which had been achieved. These aircraft were the Bossi-Bonomi (Italy), the Haessler-Villinger (Germany), the See-hase (Germany) and an unnamed Russian design. In spite of the greater weights of these machines compared with Lilienthal's air-craft, the powers which they required were considerably less. Very little had been accomplished since the time of the Germancontest, the lecturer continued. What had been done was indirect work by Raspet on the reduction in drag of sailplanes, which hadresulted in much improved performance. Raspet's technique was extremely simple and was concerned with making the surfaces assmooth as possible, getting rid of leaks and eliminating inter- ference between surfaces. He had been able to show that a mini-mum drag coefficient of 0.01 and high span efficiency factors were attainable and these could be applied to the type of aircraft nowunder consideration. At 30 m.p.h. the Haessler-Villinger (77 lb empty weight) re-quired only 0.82 h.p. If Raspet's improved drag coefficient were applied to an aircraft of this weight, the minimum power requiredwould be reduced from 0.82 to 0.68 h.p. If the machine were not only cleaned up but also flown close to the ground (say 10ft) thepower needed was reduced to 0.58 h.p. at 30 m.p.h. because of the reduction of induced drag caused by ground effect. So far only the most straightforward and simple means forreducing drag had been considered, yet the estimated power required had dropped to 70 per cent of the originally achievedcondition. If the pilot exerted himself at the'rate of 1/10 h.p., which was possible indefinitely without fatigue, and if this powerwere applied to boundary-layer suction, the whole wing should achieve laminar boundary-layer conditions. This should reducethe power needed for flight by about 0.2 h.p. and with ground effect the necessary power could be reduced to about 0.38 h.p.These estimates had been based on the aerodynamics of the Tiny Mite sailplane used by Raspet, the minimum drag coefficient ofwhich occurred at a lift coefficient much lower than those under consideration. It was probable that more highly cambered aerofoilsections designed for high-angle-of-attack flying would give better results. Turning from the possible aerodynamic improvements to con-sider the aircraft structure, Mr. Shenstone emphasized that the The Haessler - Villinger man - powered aircraft, in flight and on the ground. Span was 443tt. development of sailplanes and light powered aircraft in the pasthad not led to particularly low structure-weights. The very scale of the low-powered aircraft under consideration revealed newstructural problems which in some respects were akin to those met in large flying models. Very light structures were practicablefor the speeds were low, the manoeuvre loads were low and the load factors might be as low as four. An interesting wing design was that of Seehase some 17 yearsago. He had been carrying out research on man-carrying kites for many years, and had applied this knowledge to man-powered air-craft. He based his wing (43ft span) on the use of only four ribs per side, which were attached to two magnesium tubes acting asspars. The only other structures were a pair of nose ribs between each of the main ribs, and the whole wing was covered withstretched doped silk. The whole machine, complete with control surfaces and machinery for driving it by manpower, weighed only79 Ib, making a gross weight of 244 lb. The main point was that the structural problem was probablyfar more difficult than the aerodynamic one, because it needed more patience. For a given speed the power required was directlyproportional to weight. Most of the weight, being that of the pilot, however, was outside our control. The structures beingconsidered were of the order of 75 lb, making a gross weight of the order of 240 lb, of which only one-third was under control.No matter what was done it was unlikely that this variable weight could be as much as halved, making a gross weight of 203 lb andlowering the power needed by 15 per cent or about 0.06 h.p. To be realistic there was not a great deal to be accomplished bydecreasing the weight much below that already achieved, but there might be much to be gained by improving the aerodynamics forthat same weight. Although the power required was directly proportional to theweig.it at a given speed, it was also inversely proportional to L/D. If we could improve L/D without a proportional weight increasewe were gaining. If the polar of the Haessler-Villinger cleaned up to Tiny Mite standards were studied it would seem that, at thelow speeds being considered (CL about 1.0) the total drag coefficient was about 0.03. Of that figure the induced-drag coefficient was0.0187, or 62.3 per cent of the total. To reduce the induced drag the span must be increased, which meant adding weight. Data on a large number of sailplanes indicated that the emptyweight varied linearly with span over a range from 30ft to 60ft, the effect of aspect ratio being only secondary. Calculations showedthat, if the span of the Haessler-Villinger were increased from 44.3 to 60ft, the gross weight would rise by 53 lb to 297.2 lb.If wing area were kept constant, approximately the same power was required; if, however, wing loading were kept constant, therewas a slight reduction in power of about 1/20 h.p. But, if laminar flow could be achieved and ground effect used, the power requiredmight come as low as 0.35 h.p. plus the power for laminarization. This showed how a proper use of weight could result in a lowerpower being required. Over-generalizing, we could say that, by making an aeroplane 30 per cent heavier, we had cut down thepower required by almost two-thirds. This meant that even a man could fly such a machine. If hewere launched at about 30 m.p.h., and if the drive and propeller efficiency were 75 per cent, he would need to exert 0.6 h.p. Fromdata collected by Oscar Ursinus and published in 1936-37, this meant that, using feet alone, a man could fly for 50 sec or half a mileor, using both hands and feet, he could fly indefinitely. If a two-seater man-powered aircraft were developed it couldhave a better power for its weight. From Wilkinson's work it was clear that, for a given span and aspect ratio, the two-seat gliderneeded an empty weight not more than 20 lb greater than the single-seater. Thus, if the aircraft described above were con-verted into a two-seater, the weight would increase by 20 plus 165 lb, making the gross weight 485 lb. Taking the same lowerwing loading as used for the Haessler-Villinger of 2.26 lb/sq ft and 60ft span, it wasfound that an all-out laminar plus ground-effect assumption re quired 0.5 h.p. Oneman, the galley-slave, exerting both legs andarms together, with the pilot working only hislegs, could produce a power of about 1 h.p.for an indefinite period
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