This article describes a tow release for scale towplanes that uses the same design principal as the Tost mechanism used on full sized gliders and tugs. I now recognise that it's practically impossible to guarantee the reliability of all scale sailplane releases. So, the towplane release is the 'last resort' in a sticky situation and must be 110% reliable. Reliability can easily be achieved with a 'Pin and Hole' device coupled with a strong (expensive) servo. However the Tost style design provides a much-improved action and only requires a standard servo.

I was made aware of this design by Neal Francis (a R/C car racing rival of mine and keen observer of scale aerotowing). Neal borrowed a Tost unit from one of his gliding pals and perused its inner workings. I then adopted the principal in a new Frisch Wilga I was building at the time.

The photographs show a version built by Steve Davis for a 1/3 scale Frisch Wilga that incorporates the servo mount and a pull rod. The pull rod exits the Wilga fuselage and enables the line loop to be attached or removed independently of the pilot working the Tx switch. Like a quick release, release, if you get my drift ….!!

The photograph above shows the release with one side plate removed to reveal the action.

The device is spring-loaded to the closed position and uses a piece of closed loop cable to connect to the servo. This makes Tx set-up much less critical than it would be if we used a push rod. In the closed position, the three pivots that join the lever, links and latch are perfectly aligned. This has the effect of locking the device. The towline forces are directed into the link pivots and the release cannot be forced open by snatches on the towline. Nice…

As the servo begins to pull the lever towards the open position, the three pivots move out of line. Any tension on the towline is directed into the links and now helps to push the latch open. Most importantly, it requires only a very small force to initiate opening.

On the Full size Tost release, the middle pivot joining the lever to the links actually goes slightly over centre. This causes towline tension to force the device closed. The downside of this would be the little extra effort required of the servo to initiate opening.

The cutout in the latch is angled upwards so that when the release begins to open, tension in the line actually helps to push open the device and there is no possibility of the line snagging on the latch.

The picture below shows the layout of the latch with the housing removed.

This top view (below) shows the towline loop locked in place.