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