Is there a formula, or other method, to determine the servo torque needed for control surfaces? There are so many servos- and the cost escalates accordingly-that it's difficult to know what is needed. For example, I'm scratch building a twin engine jet (Su-27) with all moving stabs (elevons). One suggestion was to use two- JR 8611 at 220oz. each, on each stab. That seems extraordinary. Since I scratch build mostly, very few plans indicate what servos are required. Any thoughts? Bill S.

Try this spreadsheet
http://www.geistware.com/rcmodeling/...calculator.xls

Very interesting spreadsheet!

Is the model utilized an approximated one? If so, what is the accuracy?

I was thinking about a servo related issues. Imagine a servo whose arms are attached to ailerons (i.e., tipical trainer setup). What is the required torque now? We have two surfaces to move, instead of one... but I was thinking that perhaps in this case the required torque is LESSER than the required torque for a single servo attacched to one aileron. In fact, since ailerons move in opposite directions, when one pushes the servo arm in one direction, the other aileron will push the other arm in the other direction... so the servo in this case will not see any torque!

What do you think about that?

Oooops!!!! The loads don't cancel -- up aileron imposes a load and down aileron also imposes a load. They are additive, although not necessarily of the same magnitude. It doesn't matter if they are driven off opposite sides of the servo -- the servo shaft rotates in the same direction to deflect each surface & "sees" the resistance from each flight surface.