At 09:50 06 January 2011, Bruce Hoult wrote:
On Jan 6, 9:50=A0pm, Doug Greenwell wrote:
A winch launch is very different because (a) the angle between the
cable
and the direction of motion of the glider is large, and therefore
unlike
=
a
tow the downwards component of the cable tension is no longer
negligible,
and (b) the motion is not steady. =A0

In this case the lift is greater than the weight because it is
partially
counteracting the cable tension and weight. =A0The precise balance
depend=
s
on pilot and winch driver technique. =A0Even so, it is still the
forward
component of the cable tension force that is doing the work required
to
raise the glider to its release height. =A0

It's pretty easy to show that in the early part of full climb on a
winch launch when the cable is horizontal and neglecting cable weight:

tension in the cable =3D glider weight * tan(climb angle)

lift required from the wings =3D glider weight / cos(climb angle)

The latter is identical to the lift required in a turn with the same
bank angle as the winch launch climb angle.

Thus at 45 degrees the tension is the same as the weight and the lift
is 1.4
At 60 degrees the tension is 1.73 times the weight and the lift is
twice.

When using a tension-controlled winch, what tensions are actually
used?


I don't know. There's a winch design group over on Yahoo who seem to
debate the question of how you control a winch on a regular basis. This
gets into the whole question of fast vs slow acceleration at the start and
the danger or not of over-rotation ... at which point a completely new
discussion starts!

Doug