Servo torque (physics question)
 

David

I agree with your analogy, but my point was the slop remains the same irregardless of the arm length. The relationship to arm length is the other story. The thing is were not talking about inches in our application. I suspect the differences are barely notable in our scenario.

Servo torque (physics question)
 

Hi Michael,

Yes, that's probably true, although, if the arms get particularly short in a small model to allow free movement of the arms in the fuselage, the slop / arm length might become critical.

What I do know is, I always put the pushrod as far out as possible to minimize the effects of slop, because, well, I might as well, if the arm length is there to be used!

-David C.

Servo torque (physics question)
 

Longer arms with the same physical slop at the clevis will result in less surface deflection - the important metric here. The physical amount of slop at the clevis is not nearly as important as the surface freedom of movement due to that slop.

Servo torque (physics question)
 

I don't think I have heard anyone mention yet the fact that the mechanical advantage increases with the 1/cos() of the angle through which the servo arm turns. At the farthest most travel limits of the servo arm, the pushrod is moving less and less actual distance per degree of servo arm rotation. The region that is most linear is at 90 degrees to the pushrod. Once the servo arm has rotated 60 degrees, the mechanical advantage has increased from 1.00 to 2.00. When it has rotated 70 degrees, the mecahnical advantage goes up to 2.92. At 80 degrees, it is 5.75!

Servo torque (physics question)
 

The best reason to try and use all the servo throw possible, next in line for me would be that it allows use of all the resolution the system is capable of. (These swap back and forth depending on the type plane I am working on)

Why some setups restrict you to short servo arm travel when just a few minor changes make major differences.

Ed M.

Servo torque (physics question)
 

Exactly - but remember that the advantage works in the opposite direction on the surface end! So if the servo arm and surface arm are the same length, this advantage is cancelled out entirely.

Hence, use the spreadsheet. It takes all of this into consideration and displays all of this information graphically.

Thinking again
 

Using the same example: minimum 75 oz of torque needed at the surface, 100 oz/in servo, desired 50 degree surface deflection is met with ATV at 100%:

Say I want to increase the speed of the control surface so I move the horn in on the control arm to .75" with the servo arm at 1" for a mechanical disadvantage (we all did this before at one time right?).

If I did my math right, a 1" servo horn at 100 oz coupled with the .75 control horn will net 75 oz/in. So far I am meeting the torque requirements of the surface. But I will have to dial down my ATV to reduce the now increased surface deflection.

I can see that I lose resolution by dialling down my endpoints (ATV). Also I guess with less throw I won't be getting the full range of available mechanical advantage (re: what mtthomps posted)?

Did I just answer my own question or are there any other negatives of working at a mechanical disadvantage (even when the torque requirements are met)?

Servo torque (physics question)
 

Basically yes you did answer your own question.

However I would allways go for 100% ATV to get max force at the control surface, having too much is better than too little.

To increase speed, I buy new servos.

RE: Servo torque (physics question)
 

I thought this would reduce the amount of torque since the servo arm force vector is no longer 90 degrees to the pushrod.

RE: Servo torque (physics question)
 

Yes, but if you do the maths with sins and cosins etc, you will see that you have to be more than about 20 degrees to get any kind of measurable change, and even more degrees to get a noticable difference.

You have bigger angles due to the section of the wing than you will get going to a different hole on the control arm. It is not an issue.