What servo torque for pivoting motor+prop?

[MTEXX]

Here's one for those that didn't sleep thru physics class.

I'm scratch building a large thrust-vectored park jet.
http://www.rcgroups.com/forums/showthread.php?t=997119

The motor assembly will pivot. Thrust line goes thru the pivot. Most likely, the pivot will be at the opposite side of the prop shaft.

I need to determine a ballpark torque for the TV servo.

What measurements do I need and formula should I use?

Cheers,
-MTEXX

[Brandano]

I hate seeing interesting questions unanswered, but effectively while I didn't sleep through my physics classes I promptly removed all the bits that seemed irrelevant and/or uninteresting from my memory to make space for more mundane stuff. So I started digging up for some meaningful math, and found this page: http://mb-soft.com/public/precess.html . As far as I can tell the proper answer is "depends". The moment of inertia of the spinning mass gets into it, and so does the rate of spin and the torque originally applied to it.

[MTEXX]

Thanks Brandono. Perhaps I'll just try some trial and error.

[Texas Buzzard]

I have been using a HiTec HS 56 MB servo for thrust vectoring on an 18 ounce Mig -29 (a Dave Powers kit). It has had about 35 flights on it with no servo problems. The motor is a GraysonHobbies GH 2212- 06 ( high kV + 6 x 4 prop).

As I see it there are no excess forces on the servo gear train while the plane is flying. The damaging forces could occur as you are landing - so cut the power before touchdown. I think that HS 56 servo delivers 25 ounce inches thrust.

[BMatthews]

There's no real torque needed to do this since you're only talking a few oz of weight. BUT.... there's always a "but" isn't there? :D When the fan is running there's going to be some thrust effect as well as the gyroscopic effect of the fan blade.

Your best bet would be to make the shrouded fan assembly up first and then with it running at full power push and pull on the housing angle actuator with a rod that has a scale attached to it. Take the reading it provides and double it and that's the minimum torque value for the servo. The reason you want to double it is that the torque values provided are the "stall" torque where the servo is not able to move. No way you want to come anywhere close to that point on something so critical. In fact I'd almost suggest triple the findings and shop for a servo with that torque.

You're asking for formulas. But it's impossible to determine anything like that since it's so dependent on the housing pivot position, weight of the housing and where the center of gravity of the housing and motor is located, any thrust anomolies that produce off center forces, and finally the mass, diameter and RPM of the fan and the gyroscopic forces created. Far easier to make that assembly first and then test what is needed to actually move it and pick the servos from the testing.

[Julez]

You don't need that much.
I have some small servos of this type, and they have more then plenty of torque for the job:
http://www.inolabservo.com/Products/Nano/HG-D202MG.html

See some pictures of the contraption currently working in my FunJet attached.