On Jul 15, 7:00*pm, Don Johnstone wrote:
At 23:52 15 July 2009, bildan wrote:

A misconception seems to be that a tension controlled winch goes
instantly *from 0 to 100% tension at the beginning of the takeoff
roll. *In fact, tension ramps up smoothly and only becomes constant
during the main climb. *That's why it's called a "tension controlled
winch" rather than a constant tension winch.

The steady tension make it far easier for the pilot to maintain good
airspeed control. *There is much less effect from thermals and gusts.
It's a quieter, smoother easier launch. *Because the "area under the
curve" is greater with constant tension in the main climb, more net
energy gets transferred to the glider than in the case of varying
tension so launches are higher with no increase in maximum climb
angle.

How can you possibly know that without measuring the "tension" at the
release hook. You certainly cannot measure the increased tension caused by
a thermal at the winch as the inertia of the steel cable is the main cause
of the increase in tension experienced at the release hook. *This
"tension spike" cannot be measured at the winch. From a point at which
the cable is at 45 degrees to the ground the "tension" imparted to the
glider, through the cable release, is increasingly provided by the weight
of the cable, and therefore measuring tension at the winch has no meaning
whatsoever, in fact the tension there should be almost 0.

First, we're not using steel cable, we're using UHMWPE (Specra/
Dyneema). Lightweight "plastic" rope only weighs about 1 pound per
hundred feet and has very little inertial at all. Tension measured at
the winch is within a percent or so of that at the glider and has
essentially no effect on tension measurement.


Tension through the ground roll and rotation phase follows essentially
the same profile as with an excellent conventional winch launch.
There are limits set to preclude too rapid acceleration and the
tension cannot reach the weak link or rope breaking levels. *The
automation make one launch pretty much like the next with little
variation due to the winch operator.

You may get good results by serendipidy but you certainly won't get them
through control of tension.

Gee, I must be dreaming But we seem to be getting exactly what I'm
telling you we're getting - with tension control. This isn't theory,
we're actually doing it. There must be some sort of magic effect no
one knows about. Or maybe the laws of physics are different over here.