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I have very little practical knowledge of winching and I
still do not understand what the theoretical factors are that determine max launch height. Neglecting wind speed and wind gradient for the moment, as well as safety concerns relating to rope breaks down low, what is the technique to max out height given the changing climb and tow line angles and geometry? Derek Piggot writes in his book "Ground Launches , 1996, pg 34.): “The optimum speed on the launch depends to some extent on the wind strength with a slightly higher speed giving the best launch height in calm conditions. Best L/D speed is close to the optimum, with the minimum cruising speed or speed for minimum sink being the lowest safe speed.” Derek Piggott, Ground Launches , 1996, pg 34. If the speed for best L/D is close to optimum, what is optimum? The American Soaring Handbook, Vol 3, Ground Launch, 1980, pg 32 says the following about auto tow “The factor which determines the towing speed of the automobile is the indicated air speed (IAS) of the sailplane. It has been shown that the launching forces on the sailplane are at a minimum just below the IAS for maximum L/D of the sailplane” Just how far below the best L/D IAS is the point of minimum stress on the airframe? Once we have derived the factors to max out launch height, how do you then balance efficiency versus climb rate after that point in the launch when you have enough altitude for a safe recovery from a low rope break? Pete -- Peter D. Brown http://home.gci.net/~pdb/ http://groups.yahoo.com/group/akmtnsoaring/ |
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Pete Brown wrote:
Derek Piggot writes in his book "Ground Launches , 1996, pg 34.): “The optimum speed on the launch depends to some extent on the wind strength with a slightly higher speed giving the best launch height in calm conditions. Best L/D speed is close to the optimum, with the minimum cruising speed or speed for minimum sink being the lowest safe speed.” Derek Piggott, Ground Launches , 1996, pg 34. If the speed for best L/D is close to optimum, what is optimum? The optimum is the best glide speed at the wing effective load factor based on the takeoff weight of the glider and the cable tension. For example, for an ASK-21 with a best L/D speed of 49 knots at a weight of 1300 lbs, if the cable tension is 1900 lbs, the effective load factor is 1950/1300 or 1.5, the optimum launch speed is 49*SQRT(1.5) or 60 knots IAS. The American Soaring Handbook, Vol 3, Ground Launch, 1980, pg 32 says the following about auto tow “The factor which determines the towing speed of the automobile is the indicated air speed (IAS) of the sailplane. It has been shown that the launching forces on the sailplane are at a minimum just below the IAS for maximum L/D of the sailplane” Just how far below the best L/D IAS is the point of minimum stress on the airframe? What they appear to be discussing here is a rule of thumb for automobile speed based on the non-adjusted best glide IAS of the glider. Given a nominal climb angle of 45 degrees, if the automobile is driving at 50 mph (44 knots), the glider will be flying at 62 knots TAS. So, in this case, once the glider has rotated to climb angle you'd want to drive 6 or 7 MPH slower than the gliders best L/D speed at sea level density altitude. If the density altitude is higher, you'd want to drive faster. Once we have derived the factors to max out launch height, how do you then balance efficiency versus climb rate after that point in the launch when you have enough altitude for a safe recovery from a low rope break? Assuming the winch (or automobile) is providing a constant cable tension, the max launch height will be pretty close to optimal if the pilot maintains the optimum launch speed IAS as calculated above. This should result in a climb angle for most of the launch of about 45 degrees. Marc |
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