SERVO STRENGTH
To properly match your servos for your airplane, you have to select the proper size and strength of the servo. The maximum amount of strength (force) your servo is able to produce is the torque rating of the servo. As an example, a servo rated with approximately 40 ounce inches (oz.-in.) of torque equates to having 2.5 pounds of force applied one inch away from the output shaft of the servo.
As a general rule, the heavier your model is and the faster it flies, the stronger your servos need to be. For gliders and small, lightly loaded sport flyers, servos rated with 20 to 25 oz.-in. of torque will provide adequate control. For standard size powered airplanes with spans of four to five feet, flying at moderate airspeeds, 40 to 60 oz.-in. will perform well. As performance and speeds increase, as with aerobatic and warbird aircraft (in the 6-foot and larger span ranges), 60 to 90 oz.-in. is a good place to start. With giant scale models (80-inch spans and larger), the range begins to approach 90 to 120 oz.-in. of torque. Really big, giant-scale, unlimited 3D aerobatic aircraft require servos in the 150 to 200 oz.-in. torque range (and higher). Plus, many of these 3D aircraft will use multiple servos for each individual control surface. This practice is called "ganging" the servos.
With today’s really giant airplanes (40 to 50 percent scale), it is common to see multiple servos grouped together to command a single control surface. When you use multiple servos in this way, it is important to do it correctly. Mechanically, all the control horns should be positioned close to the centerline of the servo, with the linkage’s pivot point directly over the hinge line. Each control horn should be positioned so that the distance from the hinge line to the pivot point is the same. Most high-quality kits and ARF models are designed to address each of these points and provide the proper starting point.
The setup starts with choosing the proper programming setup from your radio menu. Then, after you have installed and connected all the servos properly, it’s time to start programming. Using ailerons as an example, with two or more servos per control surface, start by attaching the first servo linkage, but not the others. Assign the right aileron servo 1 to the AR1 channel, and the left aileron servo 1 to the AL1 channel. Now properly center the servo arms and adjust the linkages for maximum deflection up and down, and check that both ailerons move precisely to their full deflections. For the best servo resolution and torque, output adjust the travel for all your servos to their maximum value, usually 125 to 150 percent. Now attach and adjust the linkage for the second servo, again checking that the linkage length is correct, and that the arm is parallel to the first aileron servo arm. Both arms should also be parallel to the hinge line. Move the aileron slowly from neutral to the maximum-deflection points, and, if needed, adjust the adjustable travel volume (ATV) for the AR2 channel until the linkage is perfectly centered at the control-horn pivot point. Adjust the settings so that the maximum-deflection amounts are the same up and down. Now repeat the process for the AL2 channel. The goal is to have each pair of servos work in unison without binding. If small adjustments are required, do them mechanically, do not use the subtrims.
Servo synchronizers provide another way to gang servos. The JR Matchbox and the Futaba MSA-10 Servo Sync are great for programming multiple servos to function together. If your radio doesn’t have the wing type needed, the amount of programmable mixes required, or the number of available channels on a receiver, the synchronizers will provide enough channels and mix functions to set up multiple servos to a single channel, as well as handle ganged servos.
Several brands of digital servos also feature the ability to program the servos individually, instead of using the radio’s menu. Servo programmers let you customized each servo’s travel direction, servo speed, neutral point, and endpoints. With some digital servos, you can also program in overload protection and resolution mode settings. Hitec offers excellent programmers for their digital servos. Just connect the servo you want to program, then navigate to the appropriate menu program you wish to adjust (e.g., endpoints). Use the control knob to move the servo arm to the point you wish and then press the button; that position is now the servo’s new endpoint. Apply the same procedure for the other endpoint and the center position. Reversing servos is also easy: Navigate to the set rotation menu and pick either clockwise (CW) or counterclockwise (CCW), and then close the screen; the servo now moves in the new direction.