Control surface force

[mtnmnstr]

Has anybody done control surface study on how much force it takes to move a control surface?

[Ollie]

It depends on the pivot axis, control surface MAC chord, control surface area, air speed, control surface defection in degrees and control linking.

If the pivot is thru the 25% MAC chord then the control power is nearly zero!

[phat23]

Here's a calculator I've used. Seems to be pretty accurate.
http://atlas.csd.net/%7Ecgadd//eflight/calcs_servo.htm

Mike

[downunder]

Although not an actual study you get a feel for the force involved when flying CL. Basically the control surfaces are moved by increasing tension on one line while reducing tension on the other by the same amount. Moving the handle up increases the tension on the up line which then rotates the bellcrank that drives the pushrods. If the model flies too fast then much more load is needed to move the controls. Under certain circumstances it's possible to get to a point (I've been there) where all the load goes through one line and none through the other. That's where it's impossible to make the model turn any tighter. So how high is that load? I've been to the point where I was afraid the line taking the tension would snap and it's breaking strain is around 50 pounds. There's around a 4:1 gearing from the bellcrank to the control pushrods but this is split between the flaps and elevators (in what proportion it's hard to say) but suffice to say the bellcrank is applying a ~200 pound force split between the two control surfaces. If the pushrod is in compression it'll bend which is why we use CF pushrods.

The point where all control is through one line is called hitting the Netzeband Wall. For RC this would be when the servo is no longer capable of giving full deflection. As Ollie indicated, using aerodynamic balancing can reduce control forces needed down to zero. Or in the case of one model I was asked to test fly, down to negative (the elevator was hinged at 50% chord). That was an exciting flight!

[JetPlaneFlyer]

Downunder,
When I flew control line (many years ago) I'd find myself with all the tension on one line quite often as I tried to turn the model to avoid the fast approaching ground (not always successfully)... However I always put this down to the control surfaces or linkages just hitting their mechanical travel limit rather than any aerodynamic force issues. In fact I'm sure that this was the reason in most cases because I could actually see that the elevator was deflected at its limit.

Of course I never flown 'serious' CL competition type models, it was all just fun flying. Maybe you need to fly very high speed models to get the aerodynamic effect?

Steve

[Julez]

Look at bottom of page:
http://home.germany.net/100-173822/schwerp.htm

[mtnmnstr]

Thanks...

Danke

[John O'Sullivan]

One of the long time rules of thumb was to use at least one inch-ounce of servo torque for each square inch of control surface area.
Worked well, although much depends in the speed of the model. I see people use HS55s on quite large surfaces on 3D models without failure.

John

[Rodney]

By using boost tabs (about 10% of moveable area) I have flown very large models with old style servos putting out only about 32 inch/oz of force. Like Ollie says above, properly designed, it takes little force to do the job.

[rebell]

Design is sometimes depicted by scale.

Not only the size and speed but also the weight of a plane play a roll in control surface force. Also, the closer the total weight of the plane to the CG position, or the further away the control surface from the centre the less force is needed for the control.

[Work in Progress]

Control surface cross-section, and sharpness / thickness of trailing edge can also have a dramatic effect. A rounded trailing edge is worse than a knife edge or a square-edge cutoff.