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). My club, Southern Eagles (aka Sufferin' Eagles) started with 4000 feet of dacron rope and a truck wheel mounted on a steel bar the went into a hitch receiver on the back of a very old Ford LTD with an automatic trans. One end of the rope goes to an anchor (another vehicle or big metal stake in the ground). The rope goes over the pulley and to the glider. The car pulls from the middle of the rope, thus giving a 2:1 mechanical advantage. We would just put the old LTD in low gear and leave it there. Launching a 2 seater, the car would get up to maybe 28 miles per hour at most, then slow down from there. We were usually going about 18 mph when the glider was at the top of the launch. We got hundreds of launches off that dacron and that was on asphalt pavement. I think this is the best autolaunch system. The car is going slow so it's easily controllable and you can do this on less than smooth surfaces. Acceleration at the glider end of the rope is very fast, pretty much just like a winch. Simple, simple, simple. We sometimes got our single seat gliders to 2000 agl with a bit of headwind. Email me at if you want details.

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.