I saw at your web page, that you winch from a concrete runway.
Doing so with steel cables reduced the lifetime by a factor 10
compared to launching on gras.
How is the abrasive behaviour of this cable?
Do you have a the manufacturers address?
Chris Hostettler
Melbourne
Australia

"Bill Daniels" wrote in message
...
Let me take this since Bob Johnson and I have been talking about his for a
long time.

"Plasma" is a US marketing name for Ultra High Molecular Weight Poly
Ethylene (UHMWPE). It is sold elsewhere as "Dyneema". (The use of the
word
"Spectra" makes it hard for search engines to find the stuff.) Spectra
has
a specific gravity of about .97 so it is very light. (it floats)

It is available as 12-strand braided cord in a range of diameters. 5mm
diameter seems a good choice for winch launching gliders since it has a
breaking strength of about 5500 pounds. For lighter single-seat gliders,
3mm might work very well. Since it is very strong, light, and has no
tendency whatever to tangle it seems the perfect winch line.

There are two drawbacks. The large one is cost which is three to four
times
as much as steel but Plasma promises to last much longer than steel. (The
price seems to be coming down fast) The other is a low melting point of
about 287F with strength starting to diminish above 150F. Both these can
be
managed by operational changes.

Splicing is very easy. Just a tuck splice like water ski rope works fine.

Bill Daniels

"CH" wrote in message
...
explain PLASMA mate,
I always thought plasma would be
the liquid part of blood or
highly ionised gas
is it kind of a rope?
nylon?
kevlar?
latex :-) ?
diameter?
how to repair?
....

Chris

"Bob Johnson" wrote in message
...
Further to Craig Freeman's post of July 28, photos of our new winch
are
available at our club web site at http://www.permiansoaring.us/

The winch was conceived and built by Craig in about three months of
spare time work, which is surely remarkable. Even more remarkable,
Craig
also wears additional hats as both club president and CFIG.

A short winch description includes a freshly shopped 454 cu.-in.
engine
turning a 48-inch by 6-inch drum at about 8.5/1 reduction. As near as
I
can learn, the stock 454 engine generates about 325 hp and 350 lb-ft
of
torque at 3500 rpm.

In June, we started our new winch operations using 3/16-in. stranded
steel cable. With 5000-ft. of line out, we were averaging about 1200
ft.
of launch height with our 1100-lb. Blanik, using the CG bridle. Craig
wanted to evaluate Plasma rope, so we re-equipped in July. Since then,
our launch heights have been running 1500-ft. or better. All of our
towing so far has been in light winds of 10-15 kt. Since spooling on
the
Plasma, we have also enjoyed zero line-handling hassle, as covered by
Craig and Bill Daniels in their recent ras posts.

The physics of the two types of launch cable together with winch
operation are kind of interesting. With steel, we were launching our
Blanik in about four seconds to a speed of 50 kt., at which point
climb
was initiated and maintained at about 55 kt. Horizontal acceleration
for
the period was about 0.7 G. Total power draw from the engine was
calculated at about 135 hp. With Plasma, the acceleration period was
reduced to about three seconds to reach the same airspeed. Required
engine power was reduced slightly, and acceleration was increased to
about 0.9 G.

During climb to release, which takes about 45 seconds on average, I'm
estimating engine power required is reduced to about 110 hp for both
steel and Plasma, including aerodynamic drag of the glider and
towline.

In my calculations, the force accelerating the glider at momentary
full
throttle was estimated at 1000-lb. I backed into this figure by
observing the regular acceleration-phase line breaks we were initially
experiencing using certified 1000-lb. poly rope for the weak link.
Since
switching to Plasma and 1320-lb. Tost weak links, any line breaks are
occurring in the release area of the launch, where they are supposed
to
take place. As Bill Daniels has previously mentioned, it might be
interesting to learn just how much force the glider is actually seeing
during acceleration and climb.

Bob Johnson