At 11:49 20 July 2009, brian whatcott wrote:

There is another factor worth considering in the matter of launches.

An automatic gearbox / transmission is intended to transfer as much
horse power as possible from engine to axle.

Maximizing HP in this way involves a LARGE torque at slow speed,
tapering to a low torque at high speed.

You have mentioned that the objective of a glider launch is holding the
tension constant. This is a different objective altogether!
This calls for a ramped HP, so that either a ramped throttle application

to achieve flight speed is needed, at which point that HP is held - or
as a possible alternative: running an engine at constant (max) HP, but
throwing away excess power during the ramp to flight speed.

Brian W

I don't think it's quite as simple as that. You need a lot of torque and
power to accelerate the glider up to flying speed, then a slight reduction
to allow for the safety climb and rotation and then lots of torque and
power again for the full climb. As the glider nears the top of the launch,
the climb flattens out and you need less power and torque. The cable speed
reaches a maximum as the glider starts the rotation and then drops quite a
lot as the glider enters the full climb, due to the water-skier effect, and
then continues to drop thoughout the rest of the launch.

The water-skier effect means in effect that the vertical speed is being
added to the horizontal speed, so that the maximum winch launching
airspeed would be vastly exceeeded if the cable speed is not reduced in
some way. BTW, the water-skier effect is used by water skiers themselves,
only in the horizontal axis, to gain speed for jumps and other tricks
while the ski boat continues on at a constant speed.

Oddly enough, the Skylaunch throttle stop system and automatic gearbox
seems to automatically compensate for all the above effects, except that
you have to back the throttle off near the top of the launch to avoid
overspeeding the glider.

Derek Copeland