On Wed, 24 Mar 2004 20:40:02 -0700, "Bill Daniels"
wrote:

Andreas will be airborne at 65 km/h but won't really begin his climb until
over 102 km/h. At 100 km/h he is still accelerating at 10 meters per second
per second with a line pull of 615 kg. At that instant, he is demanding 169
kW or 227 HP at the glider. Since, as you pointed out, the winch engine is
also accelerating the drum and cable as well as overcoming cable friction
with the runway the real power demand at the engine is much higher.

It's difficult to forecast the worst case power demand so I've always
advocated a large power margin to insure the power demand can always be met.

I'm offering German Daimler-Benz 280 hp...
Typical acceleration is about 3 seconds till liftoff (which happens
around 80 kp/h I'd estimate since the tail wheel prohibits the AoA of
the wing, and once off the ground the speed is immediately in the
green range. Precise speeds are hard to tell because due to the quick
acceleration the airspeed indicator is lagging behind.

Our winch operation typically looks like this:
The thrust lever is moved smoothly withing three seconds to a desired
engine RPM (which depends upon the wind condition and is typically
between 2.600 and 2.800 RPM for a DG505 or ASK-21).
Advancing the throttle quicker greatly enhances the risk of killing
the wek link during initial acceleration.

Shortly after the liftoff of the glider the acceleration is already
done (and the glider at a safe speed of 90-110 kp/h) and the glider
immediately starts its climb, reaching full climb angle at about 150
to 200 ft.

We sometimes even fly with pretty strong tailwinds of up to 20 kp/h
tailwind component (which should give a good comparation to the TAS
effects on high-elevation airfields). Launch height and ground run
distance are significantly reduced of course, but reaching a safe
speed for the glider is never a problem.

Launching with a Dynema cable (PU) on the winch of the other club on
our airfield didn't show much difference concerning acceleration and
tow heights (the Dynema cable is offers less than 10 percent better
launch height).
The advantages of using a "plastic" cable are rather in the handling -
splicing a Dynema cable is pure joy (although it requires the same
time (15 minutes) than doing a complete splice on a steel cable).


Bye
Andreas