The general aviation maintenance technician gets the chance to see how different aircraft manufacturers solve the same problem in different ways. Every aircraft has to be controlled in flight and the laws of aerodynamics generally dictate the way the aircraft is moved in pitch, roll and yaw. Conventional controls are elevator for pitch, ailerons for roll and rudder for yaw. Some aircraft use a stabilator in place of an elevator for pitch control.
The interesting part of the controls is how the input from the pilot is transferred to the control surfaces. The controls must be similar in each type of aircraft to allow a pilot to move freely between each and still be able to make the aircraft move at his will.
One solution is to use flexible stranded wires called control cables that are routed from the pilot control yoke or control stick to the proper control using pulleys as necessary to change direction of the cables. The system must be "closed" to allow the control surfaces to move in two directions. To roll the aircraft right, the control wheel is turned to the right or the stick is moved to the right. The wheel or stick movement pulls on the proper cables, the right aileron moves up to spoil some lift, the left aileron moves down to increase lift and the aircraft rolls to the right. The second control cable for each aileron allows the system to a complete circuit and also allows the pilot to feel the control pressures. The control cables normally do not connect directly to the control surface. The cables are attached to a bellcrank which pivots and changes direction of the applied force. The change of direction in force is transferred to the control surface by use of a push-pull rod. It sounds complicated, but in reality is very simple and reliable. Part of the general aviation maintenance techinicians job is to ensure the control cables tension is correct, there is no chafing of the cables, there is no corrosion on the cables, and the rigging of the controls is correct.
The elevator or stabilator control system operates the same way as the ailerons. When the control yoke or stick is pulled back, the elevator moves up and the nose of the aircraft pitches up. The ailerons and elevators can be moved at the same time to allow the pilot to control the pitch and roll in any way desired.
The rudder is normally controlled by the pilot's feet and the control cables serve the same purpose as the ailerons and elevator. When the right pedal is pushed forward, the rudder moves to the right and the nose of the aircraft yaws right.
During maneuvers, especially during landing in a gusty crosswind, all of the controls are in a constant movement dance to bring the aircraft to a smooth completion of the flight. Good thing the general aviation maintenance technician has everything rigged correctly.
Another method of transferring the pilot control input to the control surfaces is by push-pull tubes with a swiveling "rod-end" attached to the ends of the tube. There may be several rods throughout the aircraft depending on the size of the aircraft. If there is a change of direction necessary, a bellcrank is used with another rod attached to the other side of the bellcrank. Using a push-pull type of control system has the advantage of not requiring a "return system" as the control rods can either push or pull the control surface. A control cable is not very efficient when you try to push on it. Its almost like trying to push a car with a chain. Push-pull tubes must also be checked for corrosion, chafing and rigging just like control cables. It is important to keep the rod-ends properly lubricated to prevent seizing and possible breakage.
Another type control on an aircraft is a trim tab. The trim tab changes the aerodynamics of the primary flight control to compensate for changes that occur during a flight. As fuel is used the weight and balance of the aircraft may change and in order to fly level the force of the elevator must be changed. To reduce pilot fatigue, the trim tab can be moved to allow the aircraft to fly level again without any input from the pilot. Quite often the trim tab actuator is a jackscrew that will move the tab to the proper position determined by the pilot. The jackscrew can be moved by control cables, torque tubes similar to push-pull tubes or electrical servos.
All of this sounds so complicated when written down, but the systems are really very simple and elegant in design. The aerodynamics of flight is complicated but the engineers at the different aircraft manufacturers obviously have a good understanding because these aircraft fly so well and can be controlled so easily. The general aviation maintenance technician keeps the systems operating correctly and ensures that the pilot has control of the aircraft.