Flight Control Systems

Flight Control Systems The purpose of a flight control system is transfer motion/force input from a pilot to a flight control surface. In a traditional aircraft, flight control systems are broken down by axis of control: pitch, roll and yaw. Flight control systems can be either reversible or irreversible. A reversible system is a flight control system where movement applied to the control surface moves the control in the flight compartment. A simple example is shown in Figure 1. Reversible flight control systems are used on smaller aircraft where the hinge moment (surface) loads are small enough that a mechanical linkage system is adequate.

Figure 1 Simple Reversible Flight Control System

Another reversible flight control system is shown in Figure 2. This is a 2D representation of a system that shows some typical components in a reversible flight control system. The systems shown in Figures 1 and 2 both contain a cable system, however, reversible systems can also be designed using pushrods and bellcranks without cables.

Figure 2 Reversible Flight Control System

An irreversible system is a flight control system that utilizes powered controls so that movement of the surface will not move the control in the flight compartment. An example of an irreversible flight control system is shown in Figure 3. Figure 3 shows a mechanical system connected to a hydraulic actuator. The linkage positions the servo within the actuator that controls which side of the actuator sees high-pressure fluid and which side is ported to return.

Figure 3 Irreversible Flight Control System

Irreversible systems are required when the maximum pilots input force is not sufficient to drive the surface loads. In irreversible systems, the mechanical linkage will drive a hydraulic power control unit (PCU). The linkage controls the spool position in a servo that applies hydraulic pressure to a hydraulic actuator. For traditional aircraft, the transition from reversible to irreversible occurs in the 25,000 – 30,000 gross weight range of the airplane.