On Thursday, 27 June 2013 16:28:40 UTC+1, Bill D wrote:
On Wednesday, June 26, 2013 2:40:30 PM UTC-6, Martin Gregorie wrote:



the past I've used a stopwatch and timed consistent launches as taking 5 secs from first movement until the glider started to rotate into full climb, which I assumed meant the glider was accelerating through 50 kts.



This corresponds to roughly 0.5G on average.



That's about the same result I got from UK videos. The German "windenstart" videos seem to average 1.0G (19kts/sec)acceleration. I've yet to read of an accident that can be unequivocally attributed to excessive acceleration.

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Having been winch launching since 1969 I find myself disagreeing that very rapid accelaration at the beginning of the ground run is necessarily a good thing. Being off the ground in 2-3 seconds means (approximately) going from 0-60mph in that time and which gives a rate of increase of energy into the system such that if a wing does drop then and catch then there is little chance of the pilot preventing what should have been a release +/- groundloop turning into a cartwheel/flick tragedy. 1 g acceleration means that the glider is pulled off the ground so fast that it is unlikely to drop a wing but increases the chance of disaster if it does.

We lost the pilot of a Nimbus 3 to a cartwheel accident at our club last year. The accident report not yet published so I am not saying this was a factor there but it is self evident that if a glider pilot can release the cable in time before a wingtip hits the ground then there would be zero chance of a cartwheel accident

Furthermore excessive acceleration can cause some glider types to rotate too fast in that rate of rotation can increse the stall speed at this critical time.

The BGA advice for powerful winches is 3 seconds to the intended maximum intended (not necessarily full) thottle position for the glider and winch type - with the engine rpm following a bit behind that - and that feels right to me.

See the BGA winch operations slide show at:

http://www.gliding.co.uk/bgainfo/saf.../winchops.ppsx

John Galloway