Tow Bridles

I love hang gliding. For me there is nothing that can compare to the joy of a quality cross country flight. Like many pilots I have discovered the easiest way of achieving this is by going towing. It’s been four years now since Rohan Holtkamp taught me how to ground tow. Since that time I’ve been lucky enough to enjoy a number of great cross-country flights, mostly at places like the Australian Nationals at Hay, the Forbes Flatlands, and the Birchip Flatter than the Flatlands and lands tow competitions.

Unlike hill launched hang gliding towing requires an investment in extra equipment. Central to that extra equipment is the tow bridal. This simple piece of equipment, pioneered by Donnel Hewitt in the 80s has been responsible for the dramatic increase in towing safety when modern towing is compared to that practiced in the early days of the sport.

Given the amazing reliability of modern hang glider designs it amazes me that pilots put up with sub-standard tow bridles. In the past four years I have witnessed numerous incidents and accidents caused by poorly designed and engineered tow bridles. These have included:

  • Releases

    • failing to release due to design faults
    • accidentally releasing prematurely
    • breaking before the weak link
  • Release lines

    • coming off the release mechanism
    • coming off the pilot
    • breaking
  • Bridles becoming detached from pilot or glider due to

    • the bridal snapping,
    • stitching coming undone
    • knots coming undone
    • hardware store quality quick links breaking
  • The release hitting the pilot in the face due to using bridal line with too much elasticity

All of these problems are simple mechanical ones. There is no need for any of them to occur and yet they have, do, and will continue to until pilots recognise that the tow bridal represents a piece of equipment as important as the gliders side wires.

After witnessing a number of incidents at the recent Australian nationals in Hay involving pilots in my team using substandard bridles I decided to try to do something about this problem. In the best Australian tradition, it was time to try to build a better mouse trap.

The first element that was required wast an ultra-reliable release mechanism. The key requirements for this release were:

  • Reliable release at both high and low tow tensions (see graph below)
  • Light weight
  • Effective pulley mechanism for tow bridal to allow smooth pitch control
  • No sharp edges to provide facial protection in the event of recoil
  • Affordable price

In the Hewitt bridal system the release can potentially be mounted at the apex of the bridal, on the glider, on the pilot or with dual releases on both pilot and glider. All these systems have been used. Dual releases are complex and leave the bridal to free fall to the ground. The disadvantage of a release mechanism mounted on either the glider or the pilot is that it requires the bridal line to feed successfully through a ring at the apex of the bridal for release to be effected. Although the Moyes bridal system successfully uses this method it was rejected on the basis of both high cost and the fact that several release failures have been reported when the bridal line has failed to successfully feed through the apex ring. Although I used a Moyes system for two years without problem I decided to design my mouse trap with the release at the apex of the bridal.

Metallic sailplane style releases were rejected as they are expensive, heavy, and prone not to release under low tension. The most proven release design is the three ring circus first used in skydiving parachute releases. To produce a thoroughly reliable release proved to be a little more difficult than I had expected. My initial prototypes using a rope secondary release loop demonstrated the importance of having both the correct geometry and a sufficiently soft and flexible secondary release loop to provide reliability. The final design will be familiar as it is similar to a number of releases currently available. It does however incorporate a number of subtle but important changes over what is currently available on the market. These changes are:

  • Heavy-duty 15 mm seat belt webbing primary loop for the greater strength and improved wear resistance
  • Soft flat nylon braid secondary release loop which reduces wear on the primary loop and also pulls through the grommet more easily than rope to facilitate low tension releases. This flat braid also increases the contact area with the pin which significantly decreases (to around half) the pin pull tension during high tension releases.
  • Both the primary and secondary release loops are stitched to the body of the release over 30mm with heavy duty nylon thread to ensure they do not accidentally pull out.
  • Elastic to retain the release pin at both ends providing optimal friction
  • A decent sized piece of velcro to facilitate sticking the release to the harness and actually having it stay put
  • A 5mm stainless steel wire D ring to reduce friction both prolonging bridle life and improving pitch smoothness
  • Light weight – 37g

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The common “Brand W” release failed at 220Kg, thus there are no results above this tension. You can see it requires far more force to pull the pin and effect a release when using “Brand W” as compared to the Dynamic Flight release. This gives the Dynamic Flight release a potential safety advantage. Many of the more experienced pilots will be aware that the forces required for a full tension release using a “Brand W” release are very high. With a Dynamic Flight release the required pull force is roughly halved for any given rope tension.

The next issue to be addressed was that of the bridal itself. Although Hewitt initially described a bridal with a 2 to 1 system which placed twice as much of the tow force upon the pilot as upon the glider (mimicking the force of gravity) experience has shown that a 1 to 1 bridal system which divides the tow the force equally between pilot and glider is better option as this reduces the tendency for the pilot to cause oscillations while on tow. For this reason a 1 to 1 system was adopted.

The choice of rope for the bridle was the next question. The requirements were as follows:

  • Minimal elasticity to reduce or negate the possibility of the release recoiling back into the pilots face in the event of a high tension weak link break or release
  • Sufficient strength to perform the job
  • Good wear resistance
  • Ability to be easily and reliably knotted or stitched
  • Affordable price

Ropes come in an almost endless variety. A basic rope consists of many discrete fibres and may be constructed either as a twist or a braid. An outer sheath may be added to provide wear resistance and protect the core fibres. Some of the fibres used to construct ropes included cotton, hemp, polypropylene, polyethylene, Nylon, Spectra (Dyneema), Kevlar (Aramid), and Vectran. The critical feature for a bridal is low stretch which means low elasticity and tendency to recoil.

Any rope will stretch to a certain degree. Even a steel cable stretches. The amount of stretch depends upon both the construction of the rope and the fibres used. A twisted rope will stretch more than a braided rope constructed from the same fibre. For this reason only ropes of braided construction were considered.

Cotton, hemp, polypropylene, and polyethylene were all rejected on the basis of excessive stretch, insufficient strength, unsuitable construction, and poor wear resistance/longevity.

The remaining synthetic fibres can be rated in terms low of elasticity roughly as follows: Nylon 10%; pre-stretched Nylon (including starter cord) 5-7%; Spectra 1-2%; Vectran 0.5-1%; Kevlar <0.5%. These figures indicate the amount of stretch that can be expected at the breaking load of each type of rope.

Practical experience has decisively indicated nylon ropes are excessively elastic when used for tow bridles.

Kevlar was rejected because of its poor wear qualities also because its strength rapidly decays on exposure to ultraviolet light. This left the choice between Spectra or Vectran.

The second choice was which diameter of rope to choose. 3 mm, 4 mm, and 5 mm ropes have breaking strains, depending on the manufacturer, approximately 300 kg, 600 kg, and 1000 kg respectively. Obviously any of these ropes provides strength well in excess of our requirements given that a brand-new 5 mm polypropylene tow rope breaks at around 200-250 kg to if it has any knots in it.

As pointed out above the greater the breaking load of the rope the less it will stretch at a given load. To cut a long story short a quick program of trial and error testing was employed. The testing method employed was simple. A bridal was constructed using the test rope. The ends normally attached to the pilot and glider were secured to a steel post with steel cable and shackles to insure no elasticity was added to the system. The release was then set with a standard 8 strand weak link (hopefully the maximum that anyone would consider towing with, except if you live in WA!). The snap hook was then attached to the tow bar of my trusty Toyota Hilux. With an observer placed to one side the car was then driven off to stretch the bridal until the weak link broke. The amount of recoil was observed. This testing showed that either the 5 mm Vectran or Spectra rope were by far the best choice. In this test neither recoiled more than 1 meter – less than half way back to the theoretical pilots face. We use spectra because it is 30% cheaper than Vectran which shaves a good $10 off the final price. For interest the commonly used 4 mm pre-stretch nylon bridal sends the release clanging into the pole at high speed when tested in this manner – this is in line with practical experience.

The options for forming the loops required at the end of the bridle were stitching, swaging or knots. Testing showed stitching gave by far the best results so this has been employed. Both knots and swages fail at loads far below the ultimate load of the rope.

The release line chosen was 3mm nylon for its good handling qualities. A loop was formed with four inches of heavy-duty nylon stitching. Under testing this was capable of reliably sustaining loads in excess 150 kg which is 25 times the required strength for a 100kg tension release

Quick links provide an easy and reliable way of attaching the bridal to both the pilot and the glider. Their screw gate design ensures that they cannot snag on anything as happened in a recent accident. High quality 5 mm stainless-steel quick links with a breaking strain of 350 kg were chosen. These offer significant advantages over cheaper hardware store items in that they will neither bend, break, or rust – in fact they should last a lifetime.

One optional refinement to the usual design is the addition of a lower stainless steel release attached to the pilots body. After releasing from the tow the pilot can use this to release the bridal rope which goes under the base tube. This allows the entire bridal and release line to be gathered, rolled up and placed in the pilots harness. The only extra rope trailing in the breeze causing extra wind resistance is the short length going from a the keel the pilots harness. This also removes the possibility of the bridal snagging on landing – a significant safety improvement.

Yes quality has a price and these tow bridles cost a little more, but you can rest assured you are buying the best bridle on the market which will still be providing you with reliable tows after lesser quality bridles have been pensioned off.

Safe flying

James Freeman

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