On Monday, December 31, 2018 at 4:57:48 PM UTC-6, John Foster wrote:
On Monday, December 31, 2018 at 2:34:54 PM UTC-7, WB wrote:
On Sunday, December 30, 2018 at 11:02:46 PM UTC-6, John Foster wrote:
I'm seriously looking at a reverse auto tow set-up to cut the costs down when starting up the club and am playing around with design concepts in my head. As far as rope goes, I am really impressed with Dyneema as a rope material, and from what I can gather, Samson Amsteel Blue Dyneema has a breaking strength of 1400 lbs for a rope only 7/64" diameter. That's about the thickness of a string! I can get a spool of it 3280 feet in length. Our runway is about 4,800, so we'd probably need to splice some additional length to be able to utilize the full length of the runway and maximize the height of the launch. Would need to use a weak link on each end though. For this application, would a single axle wheel like a motorcycle wheel rim work better for the pulley, or would something like the Cotswold design work better? The rope wouldn't have the extra mass that the piano wire did that they used in Cotswold.

Forget the "reverse auto tow" and you don't need Dyneema to start (although it has definite advantages).
A few questions come to mind: how long was your runway? We are limited to 4800ft, at an altitude of 3100 (higher density altitude in the summer). Our runway is about 3 miles upwind from the ridge/mountain range, with elevation gradually climbing about 500ft to the base of the mountains over that 3 miles. I'm anxious to maximize the altitude we would get from a launch, thus the idea of a reverse auto tow where you could use the full length of the runway for rope. Plus, with the reverse tow and pulley, you don't waste runway length with static unused rope to the starting location. Longer rope = higher launch, everything else being equal.

We were at about 600 msl and used 4000ft of a 5000ft runway. We set up the launch point about 1000ft down the runway so gliders could land and roll up to be launched again. The reverse pulley will get you more run, but it takes a lot longer to accelerate to launch speed when using a 1:1 system straight line or reverse pulley launch. The 2:1 pulley launch gets you in the air very quickly.

The amount of usable rope, and thus climb, will be limited by the angle of the rope to the pulley. Once the rope gets to about a 70 degree angle relative to the ground, you are pretty much done climbing. After that, you are being pulled down more than forward, so you are just loading up the wings. With the moving pulley, the pulley is moving with the glider, so you can climb longer before you reach that 70 degree angle. Depending on wind, acceleration of the tow vehicle, and skill of the tow driver and glider pilot at ground launch (very important, that) the reverse pulley may very well get you some higher than the 2:1 pulley launch, but the difference won't be as much as you might expect. We stopped doing auto tows when our winch became operational. It was a beast of a winch with a 455 cubic inch GM engine driving the drums. A winch has the same line length advantages as the reverse pulley tow, yet we did not get significantly higher launches with the winch than with the 2:1 pulley launch.

There is nothing new under the sun when it comes to ground launching, auto tow, winch, whatever. It's pretty much all been tried. Derek Piggot wrote a book that is a great resource for learning about ground launch in it's many forms. Unfortunately, it appears to be out of print. I will hunt down my copy and send to you if you'd like. I am jammed up with some serious family issues at the moment, so it may be a few days before I can get to it. Feel free to ping me if you don't hear from me in a week or so.