Types of helicopter flying control systems

6.4.1.1 Manual flying controls

This system uses a direct mechanical link, in the form of rods, bell cranks, and cables, between the pilot’s controls and the pitch change linkage. Exceptions are some of the Kaman helicopters where the pilot’s cyclic and collective controls are connected to servo tabs on the main rotor blades. Aerodynamic forces acting on the servo surface cause flap pitch changes on the rotor blades. Control systems that move a pitch change linkage have been found to be rather inflexible with some poor control characteristics especially at high AUM. They have been found to be unacceptable on helicopters with AUM in excess of about 10000 lb (4500 kg). These systems, however, are still fitted to light helicopters and to some heavier machines as a reversionary mode in the event of failure of the powered control system. Refinements can be fitted to improve the characteristics:

• Spring bias units to reduce steady state cyclic control forces.

• Spring arranged to provide a pre-load on the collective lever or a more complex arrangement of levers and weights attached to the rotor head to reduce collective pitch loads.

• A one-way hydraulic lock unit or damper to reduce fluctuating loads fed back from the rotor head to the pilot’s controls.

6.4.1.2 Powered flight controls

Most modern military helicopters are fitted with either simplex or duplex power control systems of varying complexity to suit the characteristics of the aircraft and its roles. The system is a remote position, closed loop servomechanism and to be satisfactory must meet the basic requirements of such a system. It must have a satisfactory performance, with good response and stability characteristics and it must be safe and reliable.

It is important that the performance of the system is such that the servos are capable of producing the necessary thrust to overcome the blade pitching moments under all conditions of flight otherwise it is possible that control of the rotor will be lost during some critical manoeuvre. Usually the required servo performance can be obtained by suitable design, since the thrust produced by a hydraulically operated servo is propor­tional to the cross-sectional area of the piston and the effective pressure of the hydraulic fluid. For precise control the response characteristics of the servo must be appropriate for all conditions of flight. The rate of movement required from a servo will depend to some degree on its function. Servos required to follow pilot’s inputs and stabilize the helicopter in the cyclic channels will be rapid; slower-acting servos can usually be tolerated in the yaw control and collective circuits. To obtain satisfactory control, the lag between a demanded input and the resulting movement must be small otherwise the pilot will complain of a lag in the response of the rotorcraft.

Since powered flying controls are basically high gain servo mechanisms, it is necessary to ensure that the system is a stable one. This is usually achieved by designing the system with a high natural frequency, the inertia of the moving parts being made small in relation to the thrust produced, and by connecting the servo unit output direct to the pitch change mechanism of the rotor blades. The aerodynamic loads from the rotor that have to be overcome by the servos are oscillatory in nature. Great care is therefore taken to ensure that these oscillatory loads are not fed back into the control system by making it as stiff (irreversible) as possible. For obvious reasons, powered flying control systems must be made as safe and as reliable as possible. A single system may be used with a manual reversion capability but with the increased AUM and higher speeds of modern aircraft, the trend has been to fit multiplex control systems for safety.

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