On Oct 15, 10:21*pm, " wrote:
I am looking for the benefit of recent experience with modern ropes
for auto tows. *We fly off a 5000 foot grass runway (with a couple of
rocky spots). *What size and type of rope would be best?

1: Based on personal experience, I would advise against using
polypropylene rope for autotow: it's much too elastic. The amount of
rubber-banding during the initial part of an autotow launch of a 1000
ft - plus poly rope is very interesting, and adds one extra,
unnecessary safety consideration to the first phase of launch: having
the rope go completely slack d/t elasticity just about the time the
glider is 15 or 20 feet off the ground and barely above stall is, in
my judgment, not desirable.

2: After the initial poly experience, our club switched to low-stretch
1/4" dacron rope in 1500' lengths from David Bradley (now via Wings &
Wheels). We actually strength-tested our ropes at first; found that
new rope broke at about 1700-1800 lb as claimed, and as the rope
abraded through being (inevitably) dragged along our highly abrasive
5000-ft runway, during a season the 100 yards of rope nearest the tow
vehicle decreased to about 1100-1200 lb (and became the actual weak
link). Your experience will be better on grass, though the fibers will
pick up sand, that will chafe internally and decrease strength.

3. Jacketed rope would wear longer but is more time-consuming to
splice in attaching hardware or when it does break, and is more
expensive. We did not try it.

4: I replaced the 1/4" steel cables of my hangar door with 1/4"
Spectra, and am very pleased with its durability and the lack of steel
slivers We are thinking of changing from Dacron to Spectra next time
we buy rope.

I would expect tow-vehicle traction to be a concern on a grass runway,
as grass is slippery. This would be mitigated with a (slow) running
start: use about 50 ft of slack rope, drive the tow vehicle about
about 10-15 mph until the slack comes out. Withf 1500 ft of woven
rope, this does not jerk the glider; meanwhile, there's less tire spin
and it's much easier on the engine and transmission. (Only an
automatic transmission is safe and durable; staying in a single gear
through the tow is highly desirable, as shifts take the power out of
the tow at poor times.)