Chris Nicholas wrote:
16 gauge piano wire sounds remarkably thin, unless it was a special high
tensile type. Cotsold GC and Essex GC in the UK used 13 gauge (easier
than 11 gauge to handle, join, etc. but prone to breaks) or later 11
gauge (rarely broke, but harder to tie knots, and needed larger pulley
diameter for reverse pulley). The large diameters required led to
Cotswold going for a non-rotating "pulley" made up from lots of small
rollers round its rim (but it had no guillotine). Essex used two large
rollers, lots of inertia, but it enabled a flat anvil to be between them
so that a spring-loaded chisel could be released to cut the cable in
emergency.

Before reverse pulley, Essex used 13 gauge for straight autotow. It
often broke. Theoretically, the weak link should be weaker than the
cable, but we were using uncalibrated polypropylene rope of uncertain
breaking strain.

Starting over, I would be looking at Dyneema stronger than the highest
rated weak link needed, and the main cable should rarely if ever break.
For reverse pulley, I would look at the Cotswold type but incorporate a
flat part of the "pulley" with an anvil in line with the pivot, like the
Essex set up. The spring loaded chisel would go through the hollow pivot
shaft.

For the benefit of those who have not seen a pulley system, it needs to
pivot about a horizontal axle, and to swing to some extent, to equalise
the angles and allow the cable to run true from the glider (which might
be to one side of the runway, e.g. in a cross wind), into the top of the
pulley, and out from the bottom to the tow vehicle.

Chris N.

When using steel wire and wheels/pulleys, the diameter of the
wheels/pulleys should be 60 times the diameter of the wire to prevent
work hardening. That's why 7/7 wire rope works with much small
diameter rollers and guides on winches.

See http://tinyurl.com/c3pd5

Frank Whiteley