ORIGINAL: Rotaryphile

Hi Da Rock: My tab-equipped rudder that slammed hard over when the servo linkage disconnected (because I had neglected to adequately tighten the output arm screw) measured about 18'' high, with average chord of about 7'', on an 84 inch span full aerobatic bipe. It was activated by a servo with only about 60 ounch-inch torque, which was utterly inadequate at its full speed of about 85 MPH, providing solid rudder authority only up to about 40 MPH airspeed. The tab had a chord of about 1/2'', spanning the entire length of the trailing edge, and needed only about half as much movement as the rudder to do a good job, without trying to take the bit into its mouth and overpower the servo. With the tab connected, the relatively weak servo was able to slam the rudder over past 45 degrees nearly instantly, at 85 MPH airspeed

The rudder had no area ahead of the hinge line - I gave up on that idea several years previously. after discovering that boost (or balance) tabs did a much better job. I also found that that balance area ahead of the hinge line seemed to introduce non-linearities into the rudder's hinge line moment that sometimes resulted in a low frequency oscillation of the rudder, while not supplying all that much assist at extreme rudder movement.

That same model's ailerons used only a single servo to obtain twice the roll rate that previously was supplied by two of the same servos.
After experimenting with passive boost (or balance?) tabs for years, I have found that the tabs can reduce the needed servo torque by a factor of up to a maximum of about 85% (80% may be safer), before the control surface may tend to hesitate a bit before going to full travel, particularly with rudders. In other words, relatively small and light servo can do a job that would require a super-torque servo, and such servos were not available when I designed and built that airplane.

I have not, as yet, tried true servo tabs, where the servo moves only the tab, and the mass-balanced control surface is free to weathervane, actuated by the tab only, as in the DC-9, which used a combination of servo tabs and passive boost (or balance) tabs on its elevators and ailerons. Such a setup might easily multiply the effective servo torque by 100 or more, enabling a normal model-type servo to control truly gargantuan models, and probably even human-carrying aircraft, quite handily. Some homebuilt full-sized aircraft use normal model servos to provide electric trim, with good results.