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Old 03-17-2003, 09:21 PM
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Location: Spring Hill, FL
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Default 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.