MODERN CONTROL SYSTEMS

In order to accomplish the stability and control objectives, various configurations of control systems are necessary. Generally, the type of flight control system is decided by the size and flight speed range of the airplane.

The conventional control system consists of direct mechanical linkages from the controls to the control surfaces. For the subsonic airplane, the principal means of producing proper control forces utilize aerodynamic bal­ance and various tab, spring, and bobweight devices. Balance and tab devices are capable of reducing control forces and will allow the use of the conventional control system on large airplanes to relatively high subsonic speeds.

When the airplane with a conventional control system is operated at transonic speeds, the great changes in the character of flow can produce great aberrations in control sur­face hinge moments and the contribution of tab devices. Shock wave formation and separation of flow at transonic speeds will limit the use of the conventional control system to subsonic speeds.

The – power-boosted control system employs a mechanical actuator in parallel with the mechanical linkages of a conventional control system. The principle of operation is to pro­vide a fixed, percentage of the required control forces thus reducing control forces at high speeds. The power-boosted control system requires a hydraulic actuator with a control valve which supplies boost force in fixed proportion to control force. Thus, the pilot is given an advantage by the boost ratio to assist in deflecting the control surface, e. g., with a boost ratio of 14, the actuator provides 14 lbs. of force for each 1 lb. of stick force.

The power-boosted control system has the obvious advantage of reducing control forces at high speeds. However, at transonic speeds, the changes in control forces due to shock waves and separation still take place but to a lesser degree. The "feedback” of hinge moments is reduced but the aberrations in stick forces may still exist.

The power-operated, irreversible control system consists of mechanical actuators controlled by the pilot. The control surface is deflected by the actuator and none of the hinge moments are fed back through the controls. In such a control system, the control position decides the deflection of the control surfaces regardless of the airloads and hinge moments. Since the power-operated control system has zero feed­back, control feel must be synthesized other­wise an infinite boost would exist.

The advantages of the power-operated con­trol system are most apparent in transonic and supersonic flight. In transonic flight, none of the erratic hinge moments are fed back to the pilot. Thus, no unusual or erratic control forces will be encountered in transonic flight. Supersonic flight generally requires the use of an all-movable horizontal surface to achieve the necessary control effectiveness. Such con­trol surfaces must then be actuated and posi­tively positioned by an irreversible device.

The most important item of an artificial feel system is the stick-centering spring or bungee. The bungee develops a stick force in proportion to stick displacement and thus provides feel for airspeed and maneuvers. A bobweight may be included in the feel system to develop a steady positive maneuvering stick force gradient which is independent of airspeed for ordinary maneuvers.

The gearing between the stick position and control surface deflection is not necessarily a linear relationship. The majority of powered control systems will employ a nonlinear gear­ing such that relatively greater stick deflection per surface deflection will occur at the neutral stick position. This sort of gearing is to advantage for airplanes which operate at flight conditions of high dynamic pressure. Since the airplane at high q is very sensitive to small deflections of the control surface, the nonlinear gearing provides higher stick force stability with less sensitive control movements than the system with a linear gearing. Figure 4-21 illustrates a typical linear and nonlinear control system gearing.

The second chart of figure 4-21 illustrates the typical control system stick force variation

with control surface deflection. While it is desirable to have a strong centering of the stick near the neutral position, the amount of force required to create an initial displacement must be reasonable. If the control system “break-out” forces are too high, precise control of the airplane at high speeds is difficult. As the solid friction of the control system con­tributes to the break-out forces, proper mainte­nance of the control system is essential. Any increase in control system friction can create unusual and undesirable control forces.

The trim of the powered control system is essentially any device to produce zero control force for a given control surface deflection. One system may trim off bungee force at a given stick position while another system may trim by returning the stick to neutral position.

Flight at high supersonic Mach numbers might require a great variety of devices in the longitudinal control system. The deteriora­tion of pitch damping with Mach number may require that dynamic stability be obtained synthetically by pitch dampers in the control system. The response of the airplane to longitudinal control may be adversely affected by flight at high dynamic pressures. In such conditions of flight stick forces must be ade­quate to prevent an induced oscillation. Stick forces must relate the transients of flight as well as the steady state conditions. Such a contribution to control system forces may be provided by a pitching acceleration bobweight and a control system viscous damper.