Making Tow releases

by Barry Cole

I am no expert, and the following are just some thoughts, observations, and information from my experiences over the last few years.

On full scale sailplanes there are often two tow releases, one for winch launching and one for towing. One popular misconception is that the likely looking hole in the very front of the fuselage is the aerotowing point. I think that as often as not this is an air vent for the cockpit and a position for a pitot tube.

The Fox for example, often towed from the front on a model, is actually towed from a point some way back, and under the fuz. The LS4, tows from a point on the landing wheel mechanism.

On other types who knows, certainly not me, but if you are building a model, take the time to find out the correct position, even if you decide not to use it.

I was amazed about the LS4, and the owner of the one that I was getting details from told me that the position was a little unusual, and did need a little care. The rather rearward position did not evidently have much effect on pitch, but did need the use of "considerable" rudder, especially while on the ground, and until the speed built up.

My first scale model (an ASK23), came pre-owned and with a tow release fitted. This was a slot and pin type, with the slot in the side of the fuz.

	now.. where does this go?

This worked well, except that if the tow line snatched on roll out, the glider would turn 45 deg right, in a split second.

I learned my aerotowing with this plane and when I got my next scale glider, another pre-owned job. I wanted to fit the same type of release. The problem was that my standard way of balancing the beast is to fill the nose with lead shot, and epoxy resin. So how was I to stop the epoxy pouring through the slot.

The answer was simple, I needed a liquid tight slot.. See drawings attached.(Click to enlarge)

The insert was turned up, and fitted with a little 5 min epoxy. Then my balance weight can be cast round it. Once this is done, the unit will never come loose, and the line load is spread over a larger area.

I have fitted them, with a very short tube and rod, and the servo right up at the front, and with a longer tube and rod, and the servo close to the retract position. The tube can be bent a little to fit the shape of the fuz, as long as the rod is bent as well. There is so little movement of the rod, that it works OK. The rod, should be piano wire, and as thick as will operate. I have used sizes between 1 and 2mm dia.

	click images to enlarge

I normally use a standard size servo, (cheapest option), but a mini servo has been used on a 4mtr ASW 22.. The secret here , whatever the servo, is to keep the servo arm as short as possible, to get the maximum pull on the rod. I normally use a cut down disc, and drill a hole right next to the boss, and there is still more than enough movement.

I started by making the inserts out of brass. Easy to turn, but expensive. And the tube was soldered in.

Then from aluminium. Still easy to turn, cheaper, but lighter. The tube was loctited in. Then it seemed crazy to make it light, in a position where I was adding lead.

The Mk3 is steel, not so much fun to turn, but better value for money, and works well.

My personal opinion is that you should fit a loop type of release in preference to any other type, unless there is a very good reason not to. It is very reliable in operation, and does not require a special attachment. I put any loop in mine, string, nylon, wire, whatever.

	Make sure the servo fixing is strong, I have seen a servo ripped from it’s mounting while the release was being tested. The last thing you want in your moment of panic, is for the release to be a non-release.

If you need any more info, or the bit made, give me a ring or send an e-mail.

A couple of things to add:-

When I got the original ASK23, it was in my pre-aerotowing days, and I left the servo unplugged and unused. I treated myself to the Mx MC4000 transmitter, and set it up for the ASK, but still without the release.

Then one day I decided to have a go (White Horse club event, a few years back). On the Saturday evening I realised that I needed to set the release servo, so I delved into the 4000 programming and within a few second had managed to wipe out the whole programme. No problem I thought I can do it again. However there were two problems, one I could not for the life of me even remember how to start, and two it was pitch black outside and I cannot get the glider together in the workshop.

Best have an early night and get up on the Sunday morning, when all would be light and I was sure that I would remember at least how to start.

The day dawned bright, and I dressed eager to get on. Downstairs, and into the kitchen, straight into an inch (25mm) of ice cold water.

	A pipe had developed a pin hole leak. Mend the leak, clean up the water, and then off to the programming. Now the brain had recovered and the reprogramming was over in 15min, including the tow release. Such is life aeromodelling.

The rest of the day could be the subject of another article, but suffice to say we came home happy and unscathed, and hooked on towing.

When towing you do not want to have the rudder coupled to the ailerons, so if possible use the same switch to work the release and the rudder coupling, or as my power flying buddy Brian tells me, learn to fly properly and not use rudder coupling. One day Brian, one day. In my defence here, the "Wright Flier" had coupled wing warping and rudder. If it was good enough for them, it is OK with me.

One thing that amazes me when watching gliders being towed, is the number of pilots who do not seem to want to use the controls until the glider is off the ground. Maybe this comes from slope flying, but it does not suit towing. It is important to keep the wings level during the tow, to stop a wing tip dragging and slewing the glider round.

In many cases, if there is a little wind, it is possible to "fly" the wings level even before the glider is moving. This is a very good thing to practice, as large stick movements are required, which is not normally the case once the beast is off the ground.

Happy towing…