Tow release servo - how much is enough?

[tewatson]

Building out my 1/3 scale Antares (finally...only had it for three years) and the tow release servo is the last one to mount. Using the new JR NX-8931 on the retract and have another on hand, but is 400+ oz/in overkill for the tow release? I know for that one time when you have to cut loose under extreme tension, you don't want any hiccups, although this still seems like killing a fly with a sledgehammer. What say ye?

Tom

[BobM]

I tend to use a very high torque metal geared servo on my tow releases for all my sailplanes, JR 8711's and JR 8911's. My sailplanes are 1/3 scale and 40% scale. ranging from about 25 lbs to about 50 or so lbs.

My servo choice more than likely exceeds the needs for the tow release, but I have not yet had any problems releasing from tow (the key word is not yet lol).

From what I understand it is important to have a higher torque metal geared servo that is reliable and also not really slow (some of the retract servos that have been used can have a pretty slow speed compared to other servos, they work, but it can be challenging waiting for the servo to release at times). I've seen a wide range of servos from all the major manufacturers used for the tow release (Airtronics, Futaba, Hitec, JR, Spektrum, etc). Probably anything from about 200 to 250 oz/in or higher, metal geared, should work well.

The JR NX-8931 servos are fantastic and powerful. This should work really well if you have one already.

I know many others here have considerable experience and can also share their thoughts and suggestions for the tow release.

Hope that may help.

Bob

[tewatson]

Thanks Bob.
Seriously considering the NX-8935 also. Same servo geared for speed, with 200 oz/in of torque. Might be a better overall solution.

Tom

[Steve P]

I use the 8411 in my Antares tow release. More than sufficient MG offering, A bit less spendy.

The 8931 is a superb retract choice! Have that in my new DG-1001

[tewatson]

Steve,
Good idea...may just go with that. The 8411HV is actually higher torque than the 8935, plus it's a bit more compact.

Tom

[Mac Powell]

Tom:
Here is a reference to some testing of tow releases done in 2008. http://www.rcgroups.com/forums/showthread.php?t=884523

It turns out that the type tow release in the nose of Antares has a 10:1 mechanical advantage.
If you estimate the worst case pull on the tow line at 50 pounds then the release force will be about 5 pounds or 80 oz.
For a 1 inch servo arm the torque will be about 80 ounce inches.
If the servo arm is 1/2 inch then the torque will be about 40 ounce inches.

Mac

[Mac Powell]

Tom:
If you mount the TR servo on the bulkhead that holds the canopy
holder upper make sure that the servo arm is vertical.
Mounted horizontally it is hard to keep the arm from rubbing on the fiberglass gismo.
Mac

[lenb]

Great information Mac.

[Mac Powell]

Thanks Len.

The situation at the other end of the tow line is quite different.
Most tow planes have a pin type tow release.
My guess is that the pin type falls some where between the slot type and the crossbar.
That would suggest a mechanical advantage of around 4:1.
With 50 pounds of tension on the tow line the release force would be 200 oz.

The tow release servo installed in my Bidule is an MKS 777 rated at 400 once inches.

[tewatson]

Went with the JR 8411HV and some aluminum brackets.

Tom

[BobM]

Looks good Tom, you shouldn't have any problems with that at all................very nice installation as well.

Bob