Winching: Steel vs. Plasma

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