Both glider and towplane need strong servo releases. At least 90-100 in
oz in 1/3 scale sailplanes. Not only that, you need a tow release
mechanism that works under load, with no bends in the pushrod
controlling it. Set it up so that the servo pulls to release, not push.
This sounds obvious, but I have seen guys set theirs up the other way.
The aluminum cam wheel pivot "hook" type is the best type for this in
the sailplane. Other types. As in the pushrod and cross bar type can
actually bind more under load. In a big towplane with lots of power, the
F shaped milled aluminum top mounted tow release works well for
convenience, but you should have at least 100 in oz servo on it. The
only time you need to release the towplane other than unhooking up the
towline at the end of the day, is when there is an emergency. Plan for
it.
Let's look at when you really need a release quickly. When towing under
normal conditions a 30 in oz servo is fine, there is practically no load
on the system. But when trouble occurs, that towplane going down fast
and your still hooked, you better have all you can get in servo power
and mechanical advantage. Relying on the tow pilot and tug release is
not enough. I have seen tow releases fail to work even with strong
servos and good tow releases installed under high load. Not fun. Plan
for the worst case, and that is almost good enough.
With small light sailplanes and tugs, space and common sense should
dictate protocol.
The pull test is a great idea. How much? If I can pull the loop on my
1/3 scale anything as hard as I can with one hand and it still releases,
it is just about enough.
Someone is sure to mention using a "weak link" loop in the sailplane.
While good in theory, when you're going down fast waiting for your weak
link to fail, call me. Actually using line for loops that will break
before the tow line rips the release from the nose of your glider makes
sense, but it won't be practical to rely on it as a fail safe if it is
strong enough to pull your sailplane up in the first place.
JD
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