Servo installation seems pretty straightforward but there are a few rules that should be followed. The servo should always be in line with the control rod and as close to 90° as possible. The brass grommets are always installed from the bottom of the servo, the flange of the grommet should be against the mounting surface. A pencil or an awl is useful to make installation of the grommets easier.

Control horns should always be 90° to the servo. Modern control horns have many teeth on the spline and can be turned one tooth at a time until proper alignment is achieved. You should get as close as you can mechanically and if you have a computer radio the sub trim function can be used to fine tune this alignment.

For airplanes up to 60 size 2-56 pushrods are fine for anything bigger than 60 size or airplanes with large control surfaces 4-40 pushrods should be used.

A good quality clevis should be used with safety measures taken to insure that the clevis does not open in flight. Metal clevises have clips for safety. A piece of fuel tubing approximately 1/4" long can be slipped over a nylon clevis for safety.

Remember also that the pushrod should be 90° to the servo arm and 90° to the control horn. If you use metal clevises make sure that the control arm is nylon and not metal as found on some engines you do not want any metal to metal contact.

EZ connectors have their place but be careful where you use them they can loosen with vibration.

Control horn installation is critical to flight control performance. The alignment of the control horn is very important. The pushrod attachment holes in the control horn should be located over the hinge line of the control surface. The distance from the control surface to the pushrd should be the same as the distance from the center of the servo to the pushrod.

Throttle pushrod setup is one that is important but usually forgotten. It should also follow the 90° rule. At half throttle on the transmitter the servo horn should be at 90°, the carburetor at half throttle, and the carburetor control horn at 90°.

Start with the manufacturers recommendations on all control surface throws. This should be done mechanically by moving the pushrod in or out on the surface control horn or on the servo arm. Moving the pushrod in towards the center of the servo arm will result in less surface throw. Moving the pushrod closer to the surface on the control horn will result in more surface throw. This can be fine tuned with the ATV (adjustable travel volume) function of most transmitters.

You can use low rates for the manufacturers recommended throws and set the high rates 10 or 15 percent higher for the test flight. Make your test flight on low rates but you can always switch to high rates if more control authority is needed. If your transmitter has exponential set about 25 percent on the high rate side to soften the feel around center stick. Once the airplane has been trimmed for level flight you should mechanically adjust the control rods and return the trims to neutral. This may take two or three flights to get it just right.

You can at this point start to fine tune the airplane for what feels best to you. May be the roll rate isn't quite fast enough so you might want to increase the aileron throw. Adjust your high rate to reflect a 10 percent increase in aileron throw that way you can try the new setting but your not stuck with it. If the high rate feels good then go ahead and adjust the low rate to the new setting. Continue with this procedure until the airplane feels right to you and you are comfortable and satisfied with it's performance.

Once you are satisfied with the setup it might be best to disable the dual rates. Some people use dual rates for a landing setup others for aerobatics but if you forget to flip the switch you could loose an airplane.

Trimming an airplane is an on going process and can be taken much further than the suggestions found here. If you are interested in learning more about trimming your R/C airplane check out the trim chart located in the training document section of our web site.