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SAFE Winch Launching and automatic gearboxes
At 21:00 19 July 2009, Don Johnstone wrote:
At 20:00 19 July 2009, Del C wrote: On a very still day the gliders up to DG1000Ts were being accelerated by the Skylaunch winch from zero to lift off speed (50-60mph) in about 3 seconds, which is little slower than Bill claims for his US superwinch. Any faster than this can be dangerous for some types of glider, because it can cause excessive pitch up rates. Excessive acceleration can also bang down the tails of those gliders that set on their nose wheels or skids pretty hard onto the ground, with a risk of structural damage! Derek Copeland Not if you hold the tail down to prevent that happening. In theory the faster the acceleration to flying speed the safer the launch, having effective controls makes all the difference. Being towed down the field on the ground accelerating slowly is a recipe for disaster. Whilst I agree with Don that the ground run acceleration should be reasonably brisk to give quick aileron control, if it is too rapid some types of gliders pitch up dangerously quickly, due to the rotational couple between the cable pull line to the belly hook and the centre of gravity. This risks a flick spin on rotation, or making recovery from a low launch failure almost impossible. This doesn't affect Grob G103s, which I believe is the type Don instructs in, due to its weight (it's not nicknamed the concrete swan for nothing), low set wings and hence low centre of gravity, but it does affect lighter high winged gliders such as the K6 or K8. We do hold the tail down for some of the more delicate vintage types, but we often don't have enough helpers at the launch point to do this additional task on a routine basis. OK if you have lots of ATC Cadets hanging around with nothing much else to do! Derek Copeland |
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