TOWING GLIDING WAIRARAPA’S ADVENTURES
WITH DYNEEMA / DYNEX 75 / SPECTRA by Jim Bidenell
During 1999, Gliding Wairarapa became interested in replacing the single strand 3.2 mm high tensile steel wire that had been in use on the winches since flying operations began in 1989. The wire was dangerous and at times a break under tension would drive loops many inches into the ground. I often worried what it could do to a human body. Wire was also conductive and would sometimes come in contact with the farm’s electric fence system with shocking results for the club members retrieving the winch line. On one occasion a pilot during a launch drifted off course and carried the wire over adjacent 11,000 volt lines. The winch driver at the time coolly summed up the situation and ‘flew’ the parachute and winch wire safely back over the power lines without causing any damage. A strike from a charged cloud was also a possibility because at times a launch above 3000 ft would occur and even a small cumulonimbus cloud can pack a wallop. Enquiries led the club to a Kevlar based product called Dyneema and we purchased a one kilometre length 5 mm in diameter. Its breaking strain is about two tonnes and its weight is one tenth that of steel wire of an equal breaking strain. This one km length was added to the wire on the winch. The club pilots were amazed at the way the glider climbed away during the launch. This was largely due to the much lighter weight of the Kevlar cable. With wire we must have been adding an extra 200 kg of weight to the aircraft when we were already filling the cockpits with two bulky pilots. There was also a large reduction of noise in the aircraft due to the fact that the Dyneema blocked all the vibrations that had previously travelled along the wire from the winch. 38
June 2009
We did experience many over-speed launches and a number of slow ones. Also the odd pilot would hang in there with the nose above the horizon verging on a stall. Why we did not have a spin off a launch I will never know. About a year later the club purchased another kilometre of Dyneema, 6 mm diameter this time. The extra bulk filled the drum and at times a loop would form after a launch which tended to add to the winch driver’s woes. The original drum was built for wire and was only five inches wide, so thoughts turned to designing a new drum. A rear wheel rim from a Ford tractor was selected. The dimensions were 12 inches wide and 36 inches in diameter. Observation of the Dyneema when it was being recovered after a launch showed that the braid became semi rigid and that loops would rise about 250 mm off the drum, so it was reasoned that the drum would need rims about a foot high to stop the loops from escaping and breaking the cord. At this stage two agriculture wheels from a defunct silage wagon were used to lift the winch another 18 inches to accommodate the five foot diameter drum. A larger parachute was added to increase the recovery load on the cord. A cable pay-on system has also been installed on the winch. It consists of an oscillating arm that moves the cable across the width of the drum. This has solved the cable stacking problem that had generated loops and tangles in the past. Evenly laid and tightly packed cable makes a 50 kph return to the glider launch end possible but a 100 kg weak link must be placed between the car and the winch line and the winch automatic transmission must be disengaged for speeds over 20 kph. Kevlar, unless it is protected from UV radiation, will eventually fail. During 2007 the Club started to experience a number of cable breaks so we purchased a 200 metre length of 4mm Dynex
