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Aviation History
1973
1973 - 1667.PDF
FLIGHT International, 14 June 1973 921 PRIVATE FLIGHT By ANN WELCH LIKE A BIRD ON THE BREEZE if THE AGE-OLD DREAM of flying like the birds largely faded in the late 1930s after the solo migra- r tions across the world in genus Moth and Klemm. But each generation is free to dream, and to some the desire to fly on one's own individual wings is as real as it ever was; big jets and airborne automobiles are not real flying. In the late 1960s the dream received a surprising impetus from that source of most advanced aviation technology, Nasa; an unwitting space-age spin-off. From its thought processes and com puters came the unbelievably simple concept of a delta sailwing, intended to lower space capsules on to land as an alternative to parachuting them into the Pacific. It was not, of course, an invention; the sails of boats work only in a wind, the parachute has been around since da Vinci and kites since the Chinese. But Francis Bogallo, working on the space project, brought together the appropriate principles and, by marrying them to modern materials, opened the door to the cheapest flying machine possible; you can get airborne on £35 and a week end's intelligent work. Very soon the space-age delta sail- wing was cut down to size, modified to improve glide ratio and controlla bility, built of thin-walled aluminium tube and dacron or of bamboo and polythene, fitted with swing seats, or prone harness for less drag, and flown by latter-day dreamers from populous Californian sand dunes and wild mountains of Death Valley. The pilots ranged from aeronautical engineers with thousands of airborne hours to teenagers first flying in bare feet and crash hats. They came together, all these people, to fly and camp and talk into the night about new ideas, new materials, new technology. There are now several hundred variations on the Bogallo theme flying in the United States. All are sailwings, with the fabric taking up its aerofoil shape only as the air flows past it, and controlled by the pilot physically shifting his body weight. The struc ture of the basic Bogallo consists of three booms joined at the apex for ward and spread, usually, to an angle of 80°. A cross member fixes this angle which is slightly less than that to which the sail itself is cut, thus allowing the material to take up the characteristic double curves at the trailing edge. The more pronounced the curves, the greater the lateral stability but the less the control in roll, and vice versa. The length of the booms is usually somewhere between 16ft and 20ft and the weight of the whole aircraft about 351b-401b. Attached to and below the central boom is an A-frame which the pilot holds and pushes against to shift his weight as he needs. Also located here is the seat harness so that, once air borne, the pilot can be more comfort able—and safe—than just hanging from his armpits. Speed is adjusted by shifting the body weight forward or back, and roll by energetic lateral displacement. A pilot familiar with aeroplanes may, however, be ergono- mically confused since to move his weight aft, for example, in order to raise the nose, he pushes forward on the A-frame. Although longitudinal control of a sailwing is reasonably'positive, lateral control is less predictable, and it is for this reason that some variants have independently operated spoilers at the wing tips. One of the control prob lems with the Bogallo, and indeed with any very slow aircraft when it is flown close to the ground, is that it is disproportionately affected by wind gradient. For example, when running into wind to launch an 18ft delta, the higher leading-edge apex will be in a stronger wind than the trailing edge, which will tend to increase the angle of attack. If the increase is excessive the aircraft shoots into the air and may stall, although normally deposit ing the pilot back on earth with damage only to his dignity. Put with One of the most sophisticated hang-gliders, the VJ-23 has full controls. The very thick aerofoil is about I6in at the root and essentially symmetrical to a point where the top surface only continues aft to give a flapped/undercamber effect. Transported on a light trailer, the VJ-23 takes two men about lOmin to assemble for flight figures, if the wind is 15kt, and the aircraft will fly at 18kt, a wind gradient increment of 5kt at the apex could easily run the pilot out of con trol. Gusts, of course, aggravate the situation. Once airborne, if the sailwing is flown back and forth along the ridge it is easy to get into trouble if the aircraft is allowed to become banked towards the hill in a strong gradient; the stronger wind under the higher wing may prevent the turn being arrested or reversed. Turns at the ends of slope soaring beats are naturally made outwards from the hill when the gradient provides a righting or levelling tendency—the difficulty occurs if the pilot allows a turn to continue in an effort to get quickly back into a narrow belt of slope lift, or even when altering direction to follow an indent or bowl in the ridge itself. Since the glide ratio of most Bogallos is not better than 1:4 a pilot flying less than 50ft up needs better control in roll than can be obtained by body shift, if he is not to collide with the ridge from time to time. In attempts to achieve improved performance some builders have in creased the aspect ratio by opening - up the basic delta to an elongated diamond planform. The wide-angle Seagull and Kronkite are also fitted •with bowed leading-edge booms, intended to reduce the rate of sink, and tip spoilers to aid turning. Enthusiasm for the simple, cheap Bogallos shows no decline, because the pleasure in flying them is in the direct sensory experience of personal flight that they give. The sailwing does not easily lend itself to significant
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