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Section 1: Introduction
	1	Scope
	1.2	What R/C Systems Do
	1.3	Mechanical Connections

How R/C Works -
Thinking Inside the R/C Box

1 Scope:
In this tutorial, the inner workings of standard Radio Control ("R/C") systems will be explained from a very simple point of view. It will concentrate on how information gets from the transmitter joysticks to the servos that ultimately move in the same manner as the controls. You don't need an engineering background to understand all of this. In the interest of keeping it simple I will not elaborate too much upon the radio frequency transmission and reception, but rather upon the control system itself as if the transmitter and receiver were connected together. The examples presented throughout the tutorial will explain model aircraft for the most part, but the same concepts can apply to anything you wish to control using a R/C system. Your feedback is appreciated! Please e-mail me at technoinfo@veetail.com.

**Figure 1: Typical R/C System**

1.2 What R/C Systems Do:
There are usually several "channels" that the R/C system can control. Each channel controls a different function such as elevator, aileron, rudder, engine throttle, or landing gear on a model airplane. Channels are separated on the transmitter by physical motion in different directions when moving the transmitter joysticks, or by separate switches or knobs. For example, moving the left side joystick from side to side will control the rudder while moving it up and down will control the engine throttle. Separate channels control each of these functions even though they are both controlled by the same joystick. R/C systems most commonly used for aircraft, cars, boats, and other models are composed of three basic items including the transmitter, receiver, and servos. The hand-held transmitter typically has one or two joysticks that are used to control certain functions on a model of some type.

You might have noticed that some R/C radios use the term "proportional" control. What this means is that the movement of the model control surface, such as the elevator or rudder, will move proportionately with the joystick movement. If the rudder joystick is moved slowly from side to side then the rudder will do the same. If it is moved only slightly, then the rudder will also move only slightly. In short, the term "proportional control" means that the position of the control surface will always reflect the position of the transmitter control.

1.3 Mechanical Connections:
We now know that a control surface will move proportionately to the transmitter command, but how does it physically move? There must be something inside the model that provides a force on the control surface in order to get it to move. That "something" must also position the control surface in a precise manner if the control surface is to move in locked step with the transmitter command regardless of other external forces such as wind resistance. This is the job of the R/C servo. A servo is a small, rectangular box shaped device with a rotating arm on top. The servo body is held in place by mounting hardware that holds it firmly to the model frame. The servo arm is the part that moves proportionately to the transmitter command and it can be connected to control surfaces using certain mechanical linkages. Servos typically have high torque, more than enough to move a control surface or hold them in place.

The control surfaces are held to the model frame using hinges that allow the surface to move easily and freely about a single axis. For example, an aircraft rudder would only be allowed to pivot about the end of the fin the same way that a door can only pivot about the doorjamb that it is affixed to. Like the door, the rudder would be constrained in all directions except for its single pivoting axis. An arm of sorts, called a "control horn", is permanently affixed to the control surface close to the hinged axis so that it protrudes away from the control surface. Applying force to the control horn in a direction that is perpendicular to the hinged axis and parallel to the control surface will cause the control surface to pivot. If the control horn is held fast, then the control surface will be constrained in all directions and will no longer be allowed to move at all.

**Figure 2: Typical R/C Servo**

A typical R/C servo installation uses a stiff wire linkage to mechanically connect the servo arm to the control horn. When the servo arm rotates, it pushes or pulls on the linkage and that same force is applied to the control horn. The force on the control horn is in a direction that is perpendicular to the hinged axis and parallel to the control surface. Sound familiar? The end result is mechanical motion of the control surface that is directly proportional to the transmitter command.

**Figure 3: Typical Servo Linkages**

General Table
of Contents Sections
Introduction - A Brief Radio Synopsis - The Pulse Stream - Decoding the Pulse Stream - The Servo -

How R/C Works - Thinking Inside the R/C Box...