Servo torque help?

[Only Fine Helis]

Hey guys - 2 questions.

First, when a manufacturer recommends a certain minimum torque rating for a control surface on their plane, I'm assuming that they are talking about actual torque applied to the surface and not just rated servo torque?

For example, if they quote a required 200 oz/in of torque and you use a 200 oz/in rated servo with a 1 inch arm, you're good to go. However, if you need a 2 inch arm to get your desired throw, you now need a 400 oz/in servo to apply the same required 200oz/in to the control surface. So again, I'm assuming that they are discounting the needed servo arm length and are just talking about required torque to the control surface. Is this correct?

Secondly, I've been trying to find a formula or think about coming up with one to determine required torque to a control surface, but as you can imagine, it seems to be kind of tricky.

I know you would have to factor in the surface area of the control surface, the weight of the control surface, and the airspeed, but you would also have to consider the maximum g forces it would encounter to use as a factor into the weight.

Any thought?

Thanks!

 

[gyro]

I think you're posing an interesting question. I think ARF vendors specify the minimum rated servo specs, and understand that you may need a 1.5-2" arm for the surface to reach full deflection.

If you look at the specs over the last few years, they've definitely gone upward, and I think that reflects the growth in control surface size.

However, using a large arm definitely can limit your strength. On two of the planes I was flying yesterday, both with extremely oversized elevators, I noticed flutter at high speed. Both servos were connected at the 2" point on the arms. Ill be reducing them to 1.75 to see if it helps. On one, the EF 104 Extra, the servo was rated at over 550 oz/in, but as you mentioned, a 2" arm negates much of that strength.

 

[njswede]

As for a formula, this works as long as the geometry is fairly linear:

t[SUB]control[/SUB] = t[SUB]servo[/SUB] * l[SUB]servo[/SUB] / l[SUB]control[/SUB]

Where
t[SUB]control[/SUB] is the torque at the control.
t[SUB]servo[/SUB] is the torque at the servo
l[SUB]servo[/SUB] is the length of the lever at the servo (the length of the horn)
l[SUB]control[/SUB] is the length of the lever at the control horn, measured from where the pushrod connects to the pivot point of the control surface hinge.