Basic power flying control systems
A basic power control system consists of a pilot valve/main servo arrangement. The pilot’s input is transmitted by mechanical means to the pilot valve. Hydraulic fluid pressure controlled by this valve causes the body of the servo to move in the required direction and this movement acts on the rotor blade through the normal pitch change mechanism. As with all servo mechanisms, there must be some feedback to stop the movement at the required position so that the input at the rotor head is proportional to the pilot’s input in the cockpit. Typically this is achieved by arranging for movement of the servo body to cancel the pilot valve displacement. The force required to move the pilot valve is small with the servo providing the power to move the rotor blades against aerodynamic and inertia loads. Thus this basic system provides a large power amplification.
The basic powered flying control system reduces the pilot’s workload but has no force feel, apart from inherent friction in the control runs. Thus the pilot is unable to release the controls to carry out other tasks, as they may move under gravitational or vibratory forces. In addition, there is no cue to tell the pilot how far the control has been moved. There is, therefore, a requirement to provide control retention and control centring force cues, which ideally can be trimmed over the full range of control movements required for flight. Below is a list of additional devices that may be fitted to tailor the control characteristics to the pilot’s needs, to the aircraft and its roles.
(1) Friction control device. A friction control device is the simplest addition to a powered flight control circuit. It normally employs an adjustable sliding friction device acting on the control or control rods. The major drawback of this device is that the force required to overcome static friction (stiction) is usually greater than that required to overcome the sliding friction with the control in motion. This tends to lead to jerky control movements and possible overcontrolling, although the device does provide control retention. With wear and the ingress of dirt and oil, this system is prone to binding and the generation of nonlinearities in the force required to move the control over its full range, all of which is not conducive to the smooth and precise control of a helicopter. Friction devices are common on the collective pitch control where small and rapid movements are not normally required.
(2) Spring feel and clutch systems. The spring feel and clutch system provides a synthetic force gradient about the trim position, control centring, and an instantaneous trimming capability. The clutch is usually disengaged by a trim release button on the pilot’s control. Fail-safe operation tends to vary from aircraft to aircraft, some leaving the controls free or at a set trim position. One problem with this type of system is the potential for stick jump. If, while holding a control force against the spring, the clutch release is operated, the force resisting the pilot drops to zero faster than the pilot can relax his applied force so the stick will jump. Very light forces do not present a problem in this respect but experience shows that forces in excess of 1 daN tend to produce a rapid jerk of the control when the clutch is released.
(3) Cyclic ‘beeper trim’ system. This system meets the requirements for positive control retention, force cues and the ability to trim the control over its full range of movement. In most aircraft the system can be disengaged by the use of the trim release button which releases an electromagnetic clutch between the motor and the spring box. The trim rate has to be tailored to the control gearing, the aircraft’s stability characteristics, control force gradients and the mission of the aircraft. For example, for a given aircraft and gearing, a slow trim rate would be ideal for instrument flight, giving almost vernier adjustments, but this rate would be unsuitable for more aggressive manoeuvring particularly if high force gradients were used. In this latter case, aggressive manoeuvres would require the pilot to make large control displacements leading to high control forces unless the trim rate was fast enough to relieve them quickly.
(4) Viscous damper. Rate damping, in the form of a viscous damper is used in a number of control systems to limit the rate of application of a control for stress reasons or to prevent a pilot induced oscillation in some mode. The characteristics produced by a viscous damper can enhance or detract from controllability depending on the combined effects of the damper, friction, breakout force and force gradient. From a pilot’s point of view a rate damper gives him a sense of how fast the control is being moved and tends to remove some of the jerkiness from control movements. A damper may also be used to prevent stick jump in a spring feel and clutch system.