Servo tow hook

Ken Bauer brought a four servo model equipped with his first servo tow hook to the Maxmen contest in February, 1998. He presented the model and described the hook at an Electronic Timers Symposium which was organised as part of the event. The pictures show that version of the hook.

Tow hook body Assembled hook

The body of the hook. Photo by Ken Bauer.

Assembled hook, showing the servo and latch. Photo by Ken Bauer.

The hook's main structure was formed from a single sheet of 1.5 mm alloy. A strain gauge, which sensed the tension in the towline, was bonded to the inside of the knee just above the hook where it would be most sensitive to bending movements as the line tension changed. The strip of alloy extending down from the middle of the main hook body limited the hook's extension and prevented extreme loads from damaging it. The timer monitored line tension and controlled the latch servo which was mounted on the main hook body. The latch was opened and the bunt sequence started if the line tension remained above 12 lb (5.5 kg) for 0.2 seconds and then dropped to zero within the next 0.4 seconds. This combination prevented gusts from causing a premature unlatch and also allowed a launch attempt to be abandoned. This part of the system worked well.

This hook was fixed rigidly to the fuselage with the timer driving three other servos to control the rudder, wing wiggler and stabiliser. Other forces monitored by the strain gauge were used to control circling on the line. Ken considered that this feature was unsuccessful and reverted to using a swinging hook to control circle tow. Full details of how Ken developed this hook can be found in Story of an F1A Strain Gauge Towhook, page 105, 1999 NFFS Symposium.

Ken's swinging hook is the direct precursor of Victor Stamov's strain gauge hook. Here's how he describes its development:

Randy Weiler, who lives close to me and is a good friend, had long been one of Victor's close associates. Randy saw what I was doing and convinced Victor that this would be the next big breakthrough and that Victor should immediately start working together with me to develop the hook... So, I shared everything I had learned with Victor and the collaboration was successful. I had the electronics and system problems worked out but lacked the mechanical design. Victor of course contributed the mechanical machining, and Rene Limburger also helped by doing computer strain analysis of the metal. The actual strain element design was from Rene.
Stamov tow hook

Victor Stamov's strain gauge hook.
The strain gauge is the black strip visible on the right hand hook.
Photo by John Harte.

The only other successful servo tow hook, which also swings to control circle tow, is Allard van Wallene's Flexhook, which was first described in 2005.