On Jul 15, 12:55*am, Bruce wrote:
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?

As you point out, the mfg, spec's call for about 1.5% elongation at
failure for UHMWPE (Spectra/Dyneema) That is much stiffer than
steel. And yes, any roughness or variability in winch power output
does get passed to the glider. That said, rope launches seem much
smoother than with steel cable.

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.

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.

By the way, tension control is nothing new. I recall an article in
Sailplane and Gliding from the 1960's on the subject. Soaring
Magazine had an article on an auto tow system using a tensiometer made
from a hydraulic master cylinder and a pressure gauge from about the
same time. Many of today's hang glider winches and auto tow systems
use tension control.