Flight control systems

Mechanical

Mechanical flight control systems are the most basic designs. They were used in early aircraft and currently in small aeroplanes where the aerodynamic forces are not excessive. The flight control systems uses a collection of mechanical parts such as rods, cables, pulleys and sometimes chains to transmit the forces of the cockpit controls to the control surfaces. The Cessna Skyhawk is a typical example.

Since an increase in control surface area in bigger and faster aircraft leads to a large increase in the forces needed to move them, complicated mechanical arrangements are used to extract maximum mechanical advantage in order to make the forces required bearable to the pilots. This arrangement is found on bigger or higher performance propeller aircraft such as the Fokker 50.

Some mechanical flight control systems use servo tabs that provide aerodynamic assistance to reduce complexity. Servo tabs are small surfaces hinged to the control surfaces. The mechanisms move these tabs, aerodynamic forces in turn move the control surfaces reducing the amount of mechanical forces needed. This arrangement was used in early piston-engined transport aircraft and in early jet transports such as the mostly mechanical Boeing 707.

Hydromechanical

The complexity and weight of a mechanical flight control systems increases considerably with size and performance of the airplane. Hydraulic power overcomes these limitations. With hydraulic flight control systems aircraft size and performance are limited by economics rather than a pilot's strength.

A hydraulic flight control systems has 2 parts:

The mechanical circuit

The hydraulic circuit

The mechanical circuit links the cockpit controls with the hydraulic circuits. Like the mechanical flight control systems, it is made of rods, cables, pulleys, and sometimes chains.

The hydraulic circuit has hydraulic pumps, pipes, valves and actuators. The actuators are powered by the hydraulic pressure generated by the pumps in the hydraulic circuit. The actuators convert hydraulic pressure into control surface movements. The servo valves control the movement of the actuators.

The pilot's movement of a control causes the mechanical circuit to open the matching servo valves in the hydraulic circuit. The hydraulic circuit powers the actuators which then move the control surfaces.

This arrangement is found in older jet transports and high performance aircraft. Examples include the Antonov An-225 and the Lockheed SR-71.

ARTIFICIAL FEEL DEVICES

In mechanical flight control systems, the aerodynamic forces on the control surfaces are transmitted through the mechanisms and can be felt by the pilot. This gives tactile feedback of airspeed and aids flight safety.

Hydromechanical flight control systems lack this "feel". The aerodynamic forces are only felt by the actuators. Artificial feel devices are fitted to the mechanical circuit of the hydromechanical flight control systems to simulate this "feel". They increase resistance with airspeed and vice-versa. The pilots feel as if they are flying an aircraft with a mechanical flight control systems.

Fly-by-wire

Mechanical and hydraulic flight control systems are heavy and require careful routing of flight control cables through the airplane using systems of pulley and cranks. Both systems often require redundant backup, which further increases weight. Furthermore, both have limited ability to compensate for changing aerodynamic conditions.

By using computers and electrical linkages, designers can save weight and improve reliability. Electronic fly-by-wire systems can respond more flexibly to changing aerodynamic conditions, by tailoring flight control surface movements so that airplane response to control inputs is consistent for all flight conditions. Electronic systems require less maintenance, whereas mechanical and hydraulic systems require lubrication, tension adjustments, leak checks, fluid changes, etc. Furthermore putting circuitry between pilot and aircraft can enhance safety; for example the control system can prevent a stall, or can stop the pilot from overstressing the airframe.

A fly-by-wire system literally replaces physical control of the aircraft with an electrical interface. The pilot's commands are converted to electronic signals, and flight control computers determine how best to move the actuators at each control surface to provide the desired response. Those actuators initially are usually hydraulic, but electric actuators have been investigated.

The main concern with fly-by-wire systems is reliability. While traditional mechanical or hydraulic control systems usually fail gradually, the loss of all flight control computers will immediately render the airplane uncontrollable. For this reason, most fly-by-wire systems incorporated redundant computers and some kind of mechanical or hydraulic backup. This may seem to negate some advantages of fly-by-wire, but the redundant systems can be simpler, lighter, and offer only limited capability since they are for emergency use only.