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-   -   servo: oz/in to pounds (https://www.rcuniverse.com/forum/questions-answers-154/622748-servo-oz-pounds.html)

gprime 03-14-2003 04:57 AM

servo: oz/in to pounds
I need some help converting oz/in or kg/cm to pounds that I or rather the servo can lift? Spisficaly if I have 173.58 ounces an inch or 12.5 killigrams a centimeter how many pounds can the servo lift or pull?

mglavin 03-14-2003 06:52 AM

servo: oz/in to pounds
You can do the math, 16ozs. to a pound. 173 / 16 = 10.8lbs.

The thing is these ratings are for torque which is defined as a twisting FORCE. The specific rating is based upon a known lifting FORCE at a given distance (inches). So this means that a servo rated at 173oz-in of torque will do so at 1" from center. If you change the arm length the TORQUE remains constant but the FORCE changes with the arm length.

Force x Distance = Torque

173 ozs of FORCE x 1 inch length = 173oz-in of TORQUE.

173oz-in / 1.5" arm = 115.33ozs. FORCE

173oz-in / .5 arm = 346 ozs. of FORCE

RCaeroguy 03-14-2003 11:22 AM

servo: oz/in to pounds
Just to reiterate: it's oz. in. not oz./in. Torque is not force per length. It's force times length. Ounce Inches, Newton Meters, pound feet (or foot pounds), etc....

FlyBoyDavid 03-14-2003 02:00 PM

servo: oz/in to pounds
For general conversions go here.


mglavin 03-14-2003 03:58 PM

servo: oz/in to pounds

Originally posted by RCAeroguy
Just to reiterate: it's oz. in. not oz./in. Torque is not force per length. It's force times length. Ounce Inches, Newton Meters, pound feet (or foot pounds), etc....
Your right, I noting as such is/was confusing. I removed the slash and in lieu of a hyphen. But as you noted its FORCE times LENGTH.
And its typically noted without the hyphen as well. Just looks kind a funny to me.

oz.in. or oz-in...

CafeenMan 03-17-2003 06:19 AM

servo: oz/in to pounds
What I've always been curious about is how to approximate the actual load that a control surface places on a servo without going into astro-physics.

Is there some type of simple equation that takes into account control size, deflection and flight velocity? I'm pretty sure I generally use servos that are much stronger than needed and if I could save some weight using smaller servos it would be great.

greenboot 03-17-2003 08:42 PM

servo: oz/in to pounds
Some other factors to consider are; factor of safety, dynamic loading, gear strength, and stiffness. A simple calculation based only on aerodynamic hinge moment will likely indicate you need undersized servos.

A simple relationship you might have been asking about is this: an 80 oz-in servo produces 80 ounces (5 lbs) of force in a pushrod. That's assuming the rod is attached 1 inch from the center. If it only 1/2 inch out, the force is 160 ounces. Put it two inches out and you get 40 oz.

You can easily generate high pushrod forces by putting the rod on an "inside" hole. Of course the rod won't move nearly as far.


CafeenMan 03-17-2003 09:21 PM

servo: oz/in to pounds
Tom - Thanks. That doesn't really answer my question though. I know how to do the math as far as force goes, but I don't know how to do the math for what amount of force is required for the application.

Years back I lost a couple plane due to sloppy pushrods. I lost two to elevator flutter and one to a pushrod that collapsed under flight loads. I'm a much better (and smarter) builder now so I don't expect poorly constructed pushrods to ever plague me again.

On my Stik (.46 now, but designed for a .30) I'm using Futaba 3002's on all flight surfaces (one each aileron). They're listed (depending on whom you believe) between 48 and 52 oz in of torque. They're plenty, but I wonder how much I need. I mean could I use 25 oz in servos? The ailerons will deflect 45 degrees in each direction on high rate and the plane rolls faster than I can count them. These servos seem to be more than adequate.

There has to be some formula somewhere to figure out a reasonable approximation. The pushrods are 5/53" carbon fiber tubes with 2-56 threaded rod epoxied in the ends. When the plane had a Webra Speed .32 I put it in several terminal velocity power dives from a high altitude with no flutter or noticeable control surface blow-back.

There's a great flyer in our club who routinely uses Hitec HS-81's on his flight surfaces. He even has a plane with a .52 four stroke using them. He said they're 20-something oz-in. Now he is the master of throttle management and flies his planes through manuevers using piloting skill rather than horse power.

But still I wonder how you know for sure that a servo is adequate without being over-kill before installing it. I think I could take several ounces out of my planes by using smaller servos and an appropriately smaller battery pack.

greenboot 03-18-2003 07:31 PM

servo: oz/in to pounds
I see what you're asking but I don't think there is an easy answer. In a vertical dive, the ailerons are in trail and there is theoritically zero servo torque. But something must keep them for fluttering. I believe the stiffness of the gears and linkage is a major factor.

No doubt a large plane flown slowly can use smaller servos. Especially if it is not rigged for "3D" throws. If a servo is momentairly overloaded, it will move slowly and control would be sluggish. Not a big deal if you are flying cautiously. The big risk is from broken gears if flutter should develop. A tail slide manuever could also develop large, rapid servo loads.


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