SAFE Winch Launching

Generally speaking, the only way to break a weak link or a cable at the
start of a winch launch is to open the throttle wide before all of the
slack has been taken out of the cable, so that you get a sudden snatch.

I have launched on both stranded steel and UHMWPE cable on a conventional
Tost winch with a changing automatic gearbox, and couldn't honestly feel
any real difference, except that you don't get any jolts from cleated
repairs going through the rollers with the synthetic cable. These
generally occured near the top of the launch, where we were most likely to
break cables with the Tosts, and are more down to the way we repair broken
steel cables, rather than the cables themselves.

I personally much prefer synthetic cables, because they give smoother and
higher launches, but the club management said no, due to the much higher
cost of the stuff!

Dyneema is definitely safer in that it has less tendency to spring back
after a break and no sharp ends to cut anybody who is unlucky or stupid
enough to stand in the way. My club has had a couple of minor injuries
that way.

Derek Copeland


At 06:55 15 July 2009, Bruce wrote:
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?