Eric Greenwell wrote:
Bill Daniels wrote:

Good show!

Dyneema/Spectra winch cable allows for launches to approximately 50%
of the original cable length. Calculations show that there is really
no upper limit to the amount of Dyneema that can be used.

90 seconds to 5200 feet AGL on a winch beats the hell out of 20
minutes on aero tow. That's especially true when the typical cost is
$10 for the launch.


That sounds a bit frightening - 3500 fpm! What are the stresses on the
glider to get that kind of climb rate? How far away is disaster if the
pilot twitches a bit on the controls or hits some wind shear or a thermal?

Hi Eric

I would expect that the aircraft were flown within their maximum winch speeds,
which limits the lifting load on the wings to ~2G (this is with a high bending
moment at the root because there is no relief on the wing)

The limiting structural element on most airframes on winch launch is the
elevator down load.

The 3500fpm comes from flying with a very steep angle to the ground - the
aircraft maintains a 12 to 14 degree AOA, but as vertical speed builds it
rotates relative to the ground to maintain the AOA. Do the trigonometry. The
aircraft is never at extreme speed, G load or angle of attack. It is a common
misconception that normal winch launches involve dangerous loads on the
airframe. When the airspeed exceeds the design speed, it is probable that the
loads on some part of the airframe can exceed design limits. This is
particularly true on the "top" of the launch.

The wind shear part is a possible cause for concern, which is why one uses the
specified weak link.

For what it is worth my personal best is 2300" off a 2km wire solo in a low wing
loading twin seater (Bergfalke III) into a 25 - 30 km/h wind. The winch driver
was almost at idle for the top part of the launch. The key to a high launch in
these conditions is a winch driver who knows to keep the power down once the
glider has rotated. (of course a tension controlled winch does this automatically)

A long field like this has no part of the launch where a cable / weak link break
would make it unsafe to land ahead, or execute some form of circuit. It is
generally the "no good options" window on short runways that is cause for
concern on a winch launch.