At 00:30 15 July 2009, Don Johnstone wrote:
At 22:34 14 July 2009, Gary Emerson wrote:
Just exacly where are you making these tensions measurements????

Are you making them at the weak link??

Are you making these measurements at the winch end of things?

I'd really like to know...


That is the question that is puzzling me too. The tension sensed by the
winch will, and should be constantly changing during the launch,
measuring
that would be, how can I put it, useless.
The tension at the glider release hook will be relatively constant but
measuring that introduces the problem of the addional weight of the
device
that does it and a reliable transmission of data to the ground.

That is the bit that the proponents of constant tension winch launching
seem to gloss over. How do you measure tension in a long and thrashing
cable that is being wound into a winch at up to 70 knots as the glider
lifts off? There are gadgets called running line tensiometers that are
used in underground cable laying, but they don't work at the sort of
speeds required.

The other approach is to mount a load cell at the glider end and transmit
the data back to the winch in some way, such as a radio link.

Even if you can directly measure tension in the cable, you have to bear in
mind that the glider has to pull against that tension and that changes in
its pitch angle due to pilot inputs or gusts will temporarily increase or
reduce the tension. You would therefore have to damp the system or it
would tend to hunt. I know as a glider pilot and winch driver myself, that
the less you can change the pitch angle of the glider and the throttle
setting of the winch, the steadier the launch becomes. If the pilot is
trying to control the airspeed with the stick at the same time that the
winch is trying to sense and maintain a constant tension, there is a risk
that they will end up chasing each other, especially when separated by a
long, sagging (even Dyneema sags slightly) and slightly elastic cable.

In practice the Skylaunch approach of limiting the power setting as
appropriate to the glider type, headwind component and weak link strength
seems to work well. If the pilot pulls back harder, or the glider runs
into a thermal, it slows the winch engine down by just the right amount.
Easing forward or hitting sink allows the engine to speed up. Since we
replaced Tost winches with Skylaunches, broken weak links have almost
become a thing of the past and the vast majority of the launches are
correctly speeded given correct pilot inputs. The pilot can control the
airspeed by changing the pitch angle.

Derek Copeland