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Aviation History
1980
1980 - 1195.PDF
Manpowered flight: what next? By Ian TEN years ago, the manpowered flight prize offered by industrialist Henry Kremer was a challenge worthy of much time and effort. Many designs appeared, involving thousands of manhours and comparable amounts, of money. But nothing performed well' enough to even attempt the Kremer prize until Paul MacCready appeared on the scene. Serious attempts at manpowered flight started in the 1920s. The Ger man designer Lippisch flew a flapping wing aircraft in 1929. It was similar to a conventional glider of the period and indeed made several successful flights from a shock cord launch. The flapping wings probably only assisted the aircraft, but flights of up to 300yd were achieved. Even though the con cept of flapping wings seems archaic, Lippisch was still advocating their use as late as 1960. The first prize for manpowered flight was arranged in 1933 by Oskar Ursinus of Flugsport. A total of DM500 was offered for the first flight of 1km around two markers 400m apart. The prize was never claimed, but Helmut Haessler and Franz Vil- linger of Junkers made a 790yd attempt in their Mufli machine for which they received a consolation prize. A similar prize was offered in Italy and resulted in a 980yd attempt by Enea Bossi and Vittorio Bonpim The Second World War put a stop to further attempts, and the chal lenge was not taken up again until the mid-1950s. It was about this time that a possibly significant change of direction took place. D. Perkins, a civil servant at Cardington balloon establishment, built an inflatable wing of 27ft span, with a wing area 250ft2 and weight of 381b. Four pneumatic airframes were constructed, and the last one, Reluctant Phoenix, first flew in a hangar on July 18, 1966 and made 97 ground-effect flights. Three of the best known aircraft of this period were the SUMP AC (Southampton University Man Powered Aircraft), the Puffins, built at Hatfield UK and the Linnets built at Nihon University, Japan. All of these aircraft were of similar design, incorporating large span, high aspect- ratio wings and a conventional tail- plane and fin. Although these air craft flew successfully, their duration was short due to pilot exhaustion. Flights varying between a few hun dred and over 1,000yd were achieved, but turns were made difficult by the large wingspan and conventional fly ing controls. The advantage of flying in ground effect is big for a man powered aircraft, but this inevitably means the aircraft cannot bank more than a few degrees. Once the aircraft is turning, the inboard wing will be travelling considerably slower than the outboard wing which can result in tip stalling. Pilot exhaustion and control are the two main problems that faced the conventional designers, and Paul MacCready achieved out standing success by using radical materials and radical thinking. The success or failure of man powered aircraft revolves around power requirements. Structural en gineering is sufficiently advanced to produce most shapes that the aero- dynamicist requires for conventional aircraft, but the needs of a man powered design are such that there is an extremely delicate trade off be tween structure and aerodynamics. Most of the manpowered aircraft designed so far would fly excellently with moderate weight reduction. To achieve this, new materials or new design concepts are called for. As a powerplant, the human being is limited. Even a highly trained athlete produces only marginally more power than an untrained per-' son. An athlete's training usually increases endurance and stamina, rather than just power output be cause of the way in which the human body develops power from food and oxygen. This warrants careful study by the manpowered aircraft designer. Muscle power is developed in two ways—aerobically and anaerobically. Aerobic power is produced by the oxidation of glucose in the muscles. The blood carries oxygen from the lungs for this purpose, but the whole system is limited by breathing rate and blood flow. Anaerobic power is produced without oxygen, but is limited in quantity. The two forms of energy production can be exemplified by comparing sprinters with long distance runners. A sprinter is usually well muscled and completes his event with little stress on the cardiac and pulmonary systems. A long-distance runner is usually lightly built, but has an extremely well developed oxygen transportation system—a strong heart and lungs. This is the kind of per formance that is required of the man powered aircraft pilot. He may use his sprint capability for take-off, but must settle back to a fairly low power for cruise. An average man can pro duce about 0-3 h.p. continuously with an endurance of about one hour. At this power value, most of the energy is being produced aerobically although there is always some anaerobic metabolism involved and exhaustion will occur eventually. If a manpowered aircraft is built with a power requirement at, or below, 0-3 h.p. it can be flown for a long period by an average fit man and several hours by a trained pilot. This is what Paul MacCready did. Gossamer Albatross maintained the high aspect-ratio wing and large span of previous designs, but used carbon- fibre reinforced plastic (CFRP) to produce a light, strong structure. Total weight was kept even lower by external wire bracing. Earlier de signers had avoided external wire bracing due to the high drag, but at the speed MacCready's aircraft were designed to fly, it was not significant. Control of the aircraft was achieved with a canard and wing-warping. MacCready did away with the com- „ plicated structure of previous aircraft. This not only reduces weight, but makes modifications and repairs much easier. MacCready would be the first to admit that he used a good deal of trial and error in developing his air craft, but this would have taken years if every change required a great deal of work. As it turned out, even serious crash damage could be re paired in a few hours. Rear Admiral Nick Goodhart, designer of the New bury Manflier, has moved in the oppo-
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