Tracy Tillman http://ozreport.com/emailer.php?toName=Cloud9SA&code=436p6s756439534120617420616s6p20646s7420636s6q <<email>> writes:

I have copied/pasted section I(B) from USHPA's aerotowing guidelines below. Pagen and Bryden provide similar advice in their "Towing Aloft" book.

USHPA AEROTOWING GUIDELINES

These guidelines are established by the USHPA Towing Committee for the sole purpose of enhancing the safety of towing hang gliders aloft with an airborne tug)

I - EQUIPMENT

B - BRIDLES Requirements: 1) Bridle lines should be from 3/16 to 5/16 inches in diameter. 2) Bridles should have no metal parts.

Recommendations: 3) V bridles should be from 6 to 7 feet long. 4) Bridles should be constructed of hollow braid materials so loops can be formed without knots.

Discussion: 1) Thinner lines tend to whip around more during release and can thus entangle the towline. Thicker lines are harder to cut with a hook knife. 2) Metal parts can injure the pilot upon release and will promote entanglement of the bridle with the towline. 3) Longer V bridles present more of a problem with stowing after release, while shorter bridles increase the forces on the harness and may hamper control. 4) Knots in the bridle promote entanglement with the towline.

We have found best results using 5/16" hollow braid polypro for longer (primary) V-bridle line. As far as I know, we have never had problems with the thicker bridle line whipping and catching on the tow rope ring. Thus, that is what we ask our club pilots to use. Thinner spectra line is ok for short (secondary or pro-tow) bridle line, as short line seems to have less tendency to whip and catch on the tow rope ring, but more tendency to do so if used for a long primary V-bridle line.

For tandems, I will use a 5/16" or 3/8" hollow braid polypro V-bridle line, and we use a round metal ring at the end of the specta tow rope, which is 300 feet long for tandem and first solos. We have not experienced any problem with the long V-bridle line catching on the ring, because we use the relatively thick bridle line.

However, sometimes visiting pilots will use thin spectra for their long V-bridle line. To help reduce the chance of the thin spectra catching on our standard 200 foot long spectra tow rope, we use an eyeloop-shaped ring (rather than a round metal ring)--which we have to make, rather than buy. The eyeloop ring is made from a shaped aluminum tube (batten/rib tube can work) and is lighter and therefore safer than a steel ring--because if the tow rope breaks, the tow rope end ring can snap back and hit the pilot in the face. (We have not had that happen, but it is another reason for pilots to wear full-face helmets and face shields.)

http://ozreport.com/pub/images/TowlineEnd.pdf

Above is a picture and explanation of how to construct an eyeloop tow rope end, from the "Towing Aloft" book. We have tried using plastic tube, but it did not work as well as aluminum tube. We make our tow rope end as shown, with the addition of a single piece of heat shrink tubing shrunk over the open ends of the aluminum tube.

The eyeloop tow end seems to work well to reduce the greater likelihood of thinner spectra bridle line whipping and catching on the tow rope end. Pilots should be reminded that a weaklink that is too long may whip and catch on any tow rope end, no matter what the shape is (eyeloop, ring, carabiner, etc). We feel that snap carabiners as tow rope ends may be OK for use at comps for quick launch cycles, because most comp pilots use short pro-tow bridles--but they may not be best for use with long bridles, which tend to whip more.

Discussion and anecdotal information about best practices is helpful, but it would be better for someone to collect and analyze real data, such as by running a reliability/design of experiments study. A the end of the day, it is all about safety for pilots, and liability and legal defensibility for aerotow operators. As such, we will continue to follow the published recommendations provided in the USHPA aerotowing guidelines and "Towing Aloft" book, which have worked very well for us here at Cloud 9.

More coming re experimentation.