Fall 2006 Issue

Airborne Radio

Servos—Installation and Rigging Tips

By Del Schier, K1UHF

A special flat-wing servo for thin sail plane wings.

Inside a servo with a ball bearing
but plastic gears.

Standard servo arms supplied.

Smaller full-scale airplanes have controls linked to the airplane’s control surfaces by rods or cables to mechanically control the airplane with just the muscle power of the pilot. Bigger airplanes have hydraulic servo control systems to amplify the pilot’s strength in order to control the airplane. Modern fighter planes and airliners now have fly-by-wire controls where the pilot’s controls are linked electronically to hydraulic pumps or electric motors. Fly-by-wire systems are used in conjunction with a flight computer to assist in safely stabilizing and controlling the airplane. The fly-by-wire system is very similar to radio control, except in the latter case radio is used instead of wires—call it fly-by-radio, hi. There are computer functions in the better RC (radio control) transmitters, but they do not stabilize or fly the airplane. Gyros are used in some RC airplanes and most RC helicopters to stabilize the flight.

An RC servo is an electro mechanical device that uses an electronic control circuit to proportionally position the output shaft by controlling a DC motor connected through gear reduction. A servo has feedback from a potentiometer on the output shaft that “tells” the amplifier the position of the shaft. The amplifier generates an error signal by comparing the potentiometer position to the transmitter pulse position and then quickly runs the motor the correct direction and amount so the relative position of the servo’s output shaft is matched. All but the very cheapest RC systems provide fully proportional control of the airplane. Unlike a real airplane, there is no force feedback on the control sticks. Some computer game controllers have this feature, and I would like to fly a model airplane with feedback to the transmitter sticks.

Early RC systems were not proportional. The controls just flipped one way or another and back to center. Today’s proportional control is very precise and effective, but it depends to a great extent on the quality of the installation and setup.

For RC servos to work properly, they must be chosen for the proper airplane, for proper mounting, and for power output (torque). The airplane’s specs should be appropriate or you may choose what you prefer. Choosing servos may be a bit bewildering, as there may be dozens of suitable choices from the hundreds of makes and model servos available. One manufacturer, Hitec, makes 46 different models, and there are about a dozen manufacturers. Servos range in size from that of a postage stamp to half the size of a brick. They have generic sizes, such as sub-micro, micro, sub-mini, mini, standard, giant scale, etc. Check the dimensions and torque specs to determine if they will fit an airplane. Special wing servos are made to fit flat in a thin wing, and other styles would be better to mount in the fuselage.

The torque outputs range from less than 10 inch/ounce of torque to over 350. Servos vary in speed and centering accuracy, digital servos having the claim for better performance. High-performance servos generally are needed for high-performance aircraft and helis. Servos come with your choice of plastic or metal gears and may or may not have ball bearings. Generally, they all operate from a 4.8-volt battery pack and have the best performance if operated closer to 6 volts.

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