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Aviation History
1969
1969 - 0045.PDF
FLIGHT International, 1 January 1969 33 On tow the only danger is that an inattentive tug pilot may initiate a gentle dive whereupon the sailplane will rapidly overtake the tug and the brakes or release will need to be used promptly. After casting off, the wheel is retracted and the cockpit falls silent. A high performance glider is like a beautiful woman, slender, perfectly proportioned, graceful in her every movement, but with no brains whatsoever and always needing respectful treatment. At low speeds the Diamant is delightfully docile, stalling at 28kt without any tendency to drop a wing. With flaps fully up the stall speed is still only 35kt but the nose can be kept permanently up at what feels like 20° with a sink of about 5kt, and still quite stable. At the other end of the speed range considerable caution is required since there is no stick-free stability above 90kt and the extreme flexibility of the wings permits pilot-induced oscillation to occur. Below 150kt these are more frightening than dangerous, but it does mean that when in trouble one cannot just let go and leave the machine to sort itself out because this aircraft will just go into a screaming bunt. The mechanism of the oscillation is rather obscure. Below 90kt, trimmed, fast there is stick-free stability so that the pilot's arm acts as a force servo working against a spring. The feed back loop is short so the gain-bandwidth product is high. Above 90kt the arm acts as a position servo with visual feed back, but the flexibility of the wings introduces a considerable phase lag so the pilot is suddenly looking at an entirely different set of equations. Here again the solution is practice but. the pilot must be warned that a 10 per cent change in speed can produce a very dramatic change in handling characteristics. JiAOONAt UNCTHVtlSE Above, the basic fuselage and wing sub-assemblies with respective cross-sections. Below, a Oiamant over the manufacturer's airfield near Lake Constance Flying at high speed through rough air the Diamant is a spectacular sight as the wings can oscillate in quite complex fashion and more than one mode can often be observed together with rapidly moving spanwise waves. Subjectively the motion is at first rather alarming but later rather exhilarating and the whip of the wings makes the air craft feel alive. The soaring characteristics are well adapted for British &*<*aSfr> "SSE-i*-*
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