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.

Which is why I suggested we think about metrics. If there were not big
changes in tension at the Winch end you would have a REAL problem. Go
check it - on our 2km run the glider has not moved before the winch is
1-1.5s into power up - until then there is a lot of tension going into
the cable at the winch and zero happening at the glider end.

We have had a ham fisted learner winch driver snap the cable neatly
(1880KN breaking strain cable) at the drum without even moving the chute
at the launch point.

So - The place it has to be reasonably gentle (smooth curve) is at the
glider hook.

Now we get into a whole new debate about the relative advantage of
UHMDPE (Spectra / Dyneema etc.) and steel cable.

Steel has much more elasticity and inertia to absorb those transient
loads. The spring steel wire acts like a spring storing energy and
smoothing loads. Inconveniently this means that when it breaks under
tension you get a backlash that can result in big overwinds and injuries.

The plastic rope is - counter-intutively given the name, far less
elastic and very light. Result is that winches so equipped may need to
have smoother delivery. And (I am not a good enough engineer here) it is
possible that the tension at the winch on these is similar to that
experienced by the aircraft hook. Would depend on hysteresis and all
sorts of other things that are probably best measured rather than
calculated / guessed.

Any one with facts?