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Flight Control Systems - Description

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. The rudder surface is typically the first surface to require powered control, where the critical flight condition is an engine out, rejected takeoff occurring just prior to wheels leaving the runway. In some airplanes, a boost system or a hydraulic assist system is used to supplement pilot rudder forces. An assist system is usually less expensive and less weight than a full powered control system. Crosswind landings also result in high rudder hinge moments and must also be considered.

Generally speaking, flight control systems are simply mechanisms that can be broken down into basic components (see Mechanisms modules for the basic components) plus four bar linkages and cables. For example, the system shown in Figure 3 is consists of four bar linkages connected in series until they reach the flight control hydraulic servoactuator. The reversible system shown in Figure 1 consists of 2 four bar linkages separated by a cable system. Therefore, any flight control system can be evaluated and analyzed as discussed in the Mechanism and Four Bar Linkage modules. In fact, the basic principles of a four bar linkage apply to any basic mechanism and are key to understanding general mechanism characteristics.

Beyond the mechanism fundamentals, flight control systems can also include hydraulic system and hydraulic actuators, electromechanical actuators (including power screws), cable stretch, structural stiffness, springs, load variation with flight condition and surface deflection, and dynamic effects.

As stated above, the purpose of a flight control system is to transfer motion and force from the flight compartment to the surface or actuator. To design a flight control system, the required movement of the surface or actuator servo plus the required load must be known. Input motion is limited by human factor considerations and maximum human force capability.

The approach to design a mechanical flight control system follows the same process and considerations covered in Mechanisms – Design Considerations. A design example for a reversible flight control system is presented in Flight Control Systems – Design Example. Additional considerations for flight control system design can be found in Flight Control Systems – Design Considerations, Flight Control Systems – Design Requirements, Flight Control Systems – Reversible Systems, Flight Control Systems – Irreversible Systems.