Tow Release Mechanism as fitted to Flair Cub

  1 Mar 02 - More information added.

This is just a very short information segment on the tow release mechanism I made to fit to the Flair Piper Cub. This design is intended for towing a banner as the release is on the bottom of the aircraft. For models, the release is fitted as close to the Centre of Gravity as possible to minimise any effects of the towline load.

To tow a glider, it is essential that the release is fitted on the top of the model. The reason for this is that a banner will always hang down below the model, but a glider will want to fly above the tow plane. If a glider climbs above the model it is extremely likely that the rope will wrapped around the tailplane, pulling the tail of the model up. The tow plane will almost certainly run out of elevator authority causing it to nose down and probably dive into the ground.

The mechanism is simply a 16 swg pin attached to a servo disc with a screw terminal, with a 16 swg loop to hold the tow line in place on the pin. The servo can be connected to the gear channel, or any other suitable channel as required. I have included a couple of photographs showing the completed unit. For the Cub, the unit sits in the aperture I left in the fuselage bottom in place of the camera unit.

	A view of the tow release servo that would be inside the model. The release pin is 16 swg (1.6 mm) diameter piano wire and is attached to the servo by a screw block (in the 8 o'clock position). The pin runs through a reinforcing / guide block which also supports the wire hoop (that can be seen below)
	This is the front view showing the release pin and retaining hoop. The pin needs to sit at least at the same height as the retaining hoop and against it when under load to ensure that the tow line can't release unintentionally. The release servo is connected to an auxiliary channel of the radio gear, and the pin needs to retract flush (or below the surface) when operated..
	This is the tow release fitted to the underside of my Piper Cub, just behind the main undercarriage. It needs to be rigidly mounted to ensure that any shock loads do not pull it out. The primary retaining method here is pins at the rear and a screw at the front. The plastic catch is not used to hold the tow release (it is used for the clear glazed panel for normal flying).
	A similar view, but this time with the banner towline attached.
	This is the front of the banner I made. The banner is just a piece of polythene sheet cut to 12" (30 cm) wide and just over 12' (3.7 m) long. Plastic struts are taped (top to bottom) to the banner at approximately 18" (45 cm) spacing to stop the banner curling in flight. The leading edge stiffener of the banner is a 1/4" (6.3 mm) diameter dowel, but a carbon fibre rod would be durable. The main towline, approximately 50' (15 m) long, is polypropylene and passed around the dowel and knotted. To prevent the front of the banner tilting back and forward, I attached 2 additional line from the top and bottom of the dowel to the main towline as shown. I also fitted a line top to bottom with a lead fishing weight (with a hole through the centre) to keep the banner upright. This weight would be better fitted to the line from the bottom to the main line, but I couldn't be bothered to untie it. The message was drawn onto to banner using a marker pen for simplicity.

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