Hi all,

Seeking any advice. I don't normally give thought to these things for my "normal" size models; in this case it might be justified.

I'm planning, as a future project, to scratch-build a really large glider, and don't want to risk using undersize (w/respect to torque) servos, yet at the same time don't want to play it overly safe by using r/c sailboat winches

How can I realistically estimate pressure forces acting on control surfaces, e.g. flaps, in order to ensure that the servos can handle it?

As a starting point, figured I could take the product of dynamic pressure and projected control surface area, multiply by sin of deflection angle to get the perpendicular component, then multiply by distance from mid-chord of control surface to hinge axis to get a torque value.

Thought that would be conservative, since center of pressure should be somewhere on hinge side of mid-chord.

Would like to get an idea of pressure loads during level flight (speed est'd from CL and weight) and terminal velocity (from CD and weight).

Better method or data sources anyone?

Should servos be selected based on expected worst-case torque, or some multiple of normal operating torque??

Mike Garton's soaring column in Model Aviation a couple of months ago had lots of useful information on the subject. Analog servo accuracy suffers a lot under heavy loads but digital servos do much better under heavy loads. for torque requirements go to
http://www.multiplexrc.com/calcservo.htm

You may also want to consider nonaerodynamic loads, such as a dragging rudder in a ground loop or a dragging flap on landing.

Here is a formula for making reasonable estimates. You need to know maximum speed in mph, maximum surface deflection in degrees, control surface length and width.

torque req't (oz.in.)= (length)(square of width)(square of speed)(deflection angle)/430,000

Ex: assume elevator length 20", width 1.5", deflection angle 25 degrees, max speed 75 mph; torque req't = 20x1.5x1.5x75x75x25/430000 = 14.7 oz in, which doesn't sound like much, but if you widen to 2" it almost doubles to 26.16 oz. in., and if you keep the 2" width and go for full 30 degree deflection at 100 mph it becomes 55.8 oz.in.

These calculations assume zero friction in linkage, but with reasonable care you should not have more than 1-2 oz of friction in pushrod & hinges.

What this tells us is that width of control surface involves a big penalty and speed same thing. If you don't fly too fast, or don't bang your stick at full speed, you don't need a lot of torque; but wild maneuvers at high speed take lots of torque and obviously put a big load on your bird.

I don't remember exactly which website I got this formula off of, but I guess I probably found it by searching Yahoo on "servo torque requirements" or some such phrase. I think the Multiplex website has a java calculator, which may use the same formula, or maybe not.

A general rule of thumb is 1 oz/in of torque for each sq. in. of control surface.
I thnk I heard that from John Beech, the master builder and maker of Model Sport Video.
I would imagine high load surfaces would require a bit more and low load surfaces a bit less.
Just a thought.

Jeff