CHAPTER 2 RADIO INSTALLATION AND CHECKOUT

Shown here is a typical radio installation in a Sig Astro-Hog. Notice that balsa pushrods are used for the elevator and rudder, and flexible cable pushrods are used for the throttle and nose wheel steering. The antenna has been carefully routed to exit just aft of the wing opening. The remaining length of antenna is taped along the fuselage bottom.

SERVOS

As with all of your radio components, we recommend that you follow the radio manufacturer's instructions when it comes time to mount your servos. The methods described below are fairly standardized and should work well with any system.

SERVO PLACEMENT: Positioning the servos in the fuselage will involve considerable planning on your part. Think ahead about the locations of the various pushrods and how they will effect the rest of the installation. For example, the choice of which side of the fuselage the rudder pushrod will exit is determined by the position of the throttle control arm on the engine to be used! If the throttle arm is on the right side (most common), the throttle servo should be mounted on the right side of the fuselage. The rudder servo should in turn be mounted on the left side so that there is plenty of room for the nosewheel cable pushrod. Since the rudder pushrod will exit the left side of the fuselage, the elevator pushrod should exit on the opposite side. In most cases, the elevator servo can be centered between the rudder and throttle servo.

SERVO REVERSING: Almost all modern radios have a feature called "servo reversing" which allows you to reverse the rotational direction of the servo by flipping a switch built into the transmitter. With servo reversing, you don't have to worry about which side of the servo output arm to attach your pushrod - simply hook it up exactly the way you want. If the control surface, say the rudder, goes to the right when you move the rudder stick on the transmitter to the left, just flip the switch and it will work the correct way.

HOW TO MOUNT THE SERVOS: When mounting servos in the airframe you should remember that they are electrical components and must be isolated from engine vibration. Rubber grommets are supplied with all servos and should be used on all of the servo mounting screws. We recommend that you use the plastic servo trays supplied with the radio system to mount your servos. Not only are the trays convenient, they also provide extra protection from vibration, since they too are mounted on rubber grommets. Screw the servo tray to hardwood mounting rails that are epoxied across the inside of the fuselage. We recommend that the hardwood rails be made of at least 3/8" square or 3/8" x 1/2" spruce, basswood, or pine. Do not make the rails out of balsa! Try to keep the rails parallel to each other so the tray won't be warped when bolted down. Glue scrap pieces of balsa to the fuselage sides around the ends of the servo rails so that they can never come loose in flight. If you don't have enough room for the tray, or you want the servos arranged differently, the servos can be mounted directly on the hardwood rails.

In very small planes (.15-size engine or smaller) or gliders, where mounting space is normally limited, double-coated servo mounting tape (soft foam tape) is sometimes used to stick the servos directly onto the fuselage side - no trays or rails are used. For the best bond to the fuselage, coat the balsa where the servo will be seated with a thin film of epoxy. Although this method is quick and easy, it is generally accepted that using servo trays and hardwood rails is more secure in the long run and also protects the servos from engine vibrations better than using servo mounting tape.

ABOVE: Most radio manufacturers supply several types of servo arms with their sets. The servo wheel or x-style servo arm work well in most applications.

SERVO OUTPUT ARMS: The holes in most serve output arms are 1/16" diameter to accept the pin in a R/C link. Occasionally however, you might find that the holes are a little undersize and fit too tight, causing a slight bind. This puts an extra load on the servos anc can cause servo centering problems. Ideally, the holes should be snug, but not too tight. If necessary, drill the holes out with a #51 or #50 size drill bit, no larger! An overly large hole will cause control surface slop and flutter.

These are some of the more popular servo tray configurations available. One or more of these should be suitable to virtually any servo installation.

The servo reversing switches for this transmitter (same as in Chapter 1) are located under the lower front panel.