Building a Scale Tow Release

Every Schweizer sailplane I’ve seen has a distinctive tow release under the nose. The latch is unique in that it will automatically release should the sailplane pass the tow plane. Looking at these photos (both 1-23’s) it should be fairly obvious how the release works:

 

 

 

 Tom’s plans show the common commercial release available to modelers installed in the nose. This is a good functional release, inexpensive and easy to install – and since Schweizer’s have a hole in the nose for the pitot static, it doesn’t detract from the scale appearance. But this just didn’t seem right to me, so I decided to build a scale release…

 

The first task was to scale the photo above for use as a 1:4 scale template. I knew the overall length of the “hook” was about 4” – so I scaled the photo to yield a 1” long hook when printed on plain paper. I then physically cut the hook out of the printed image and laid it on a piece of 1/8” Al as a template. A bench vice, hacksaw and a file were then used to cut the hook. A Dremel with a drum sander bit was used for final clean up.

 

The full scale hook mates with a square hole in the catch piece. Since milling a tiny square hole would be a chore, I decided to machine a round pin on the end of my hook and drill a matching hole in the catch piece. Now there are 3 ways you can do this:

 

1. Drill a hole in the hook and insert/epoxy a short piece of wire as the pin.
2. File the end round to make the pin
3. Place the piece in a 4-jaw chuck on a lathe and machine the end round.

 

I chouse option #3, but any of the above would do fine. The catch piece was made from 1/8” Al.

 

To hold these items in place I needed side plates that would make a sandwich with the center keel (F18). In retrospect I should have made these from brass (1/16” or possibly thinner). Brass would have provided a better bearing surface – Aluminum to Aluminum surfaces can gall and bind if not properly designed. That said, I made side plates out of 1/8” Al and was careful to round edges and provide clearance to avoid galling and binding issues. A portion of these plates must protrude through the nose tripplers – so I milled that portion down to 1/16” and milled a similar clearance in the tripplers. Using thinner material would have made this portion easier or even unnecessary. The following photos should make everything clear:

 

Basic release mechanism assembled and laid over plans. All hardware is 2-56.

Cutting the center keel for clearance.

Sandwich:

Release in assembled fuse. Side frames were epoxied and bolted. Locktite used on all pivot nuts.

 

The next task was to add a servo to activate the release. After much deliberation, I decided to mount the servo behind the instrument panel and use a cable, run through a loop on F1 to pull the release lever open. A spring from an old transmitter provides return force to close the latch. And here it is:

 

For those wishing to duplicate a catch like this on their build I have good news and great news. The good news is it isn’t that hard and can be done with simple tools. The great news is Tom is looking into having these releases produced to offer with all of his SGS kits.

 

 

 

 

Comments

[Tom Martin]

Nicely done Mark! That's what I'm talking about!