Mechanical control systems

Early aircraft and small modern types use a direct mechanical linkage between the control surface and the pilot’s control stick. The linkage normally consists of an arrangement of multi-stranded wires and pulleys. Figure 10.22 shows the complex system used on an executive jet. The rudder actuating wire may just be seen under the tailplane on the Auster shown in Fig. 10.5. Alternatively, push-pull rods and twisting torque-tubes may be used, and are in some ways preferred, since they produce a stiffer system, less prone to vibration problems.

As the speed and size of aircraft increased, so did the control forces required, and some considerable ingenuity went into devising means of reducing these loads. The position of the hinge line can be arranged so that the resultant force acts just behind it, thus producing only a small moment. A typical arrange­ment, used on many aircraft up to the 1950s, is seen in Fig. 10.5. The top of the rudder projects forward, in front of the hinge line, thereby moving the centre of pressure of the rudder forwards, towards the hinge line.

Unfortunately, the position of the resultant force changes with angle of attack, speed, and deflection angle, so that it is difficult to devise an arrange­ment that produces small forces under all conditions. It is particularly import­ant that the resultant force should not be in front of the hinge line, as this would cause the control surface to be unstable, and run away in the direction of the ever-increasing force.

In addition to such aerodynamic balancing, the control surface mass should also be balanced so that gravity forces do not pull it down in level flight, and inertia does not cause it to move relative to the aircraft during manoeuvres. A rather crude external form of mass balancing may be seen in Fig. 10.17. As described later, masses may also be added to the control surfaces to alter the natural frequency of oscillation.