Flight Control Systems Overview

They are generally fabricated from carbon fibre or corrosion resistant steel wire used fro aircraft control. All steel cables used in aircraft mechanical transmission are constructed with 7 strands which are helically twisted and each strand consists of a number of wires. The size of the cables is based on the circumscribed circle, the number of strand and number of wires per strand.

They are used to change the direction of a cable in an aircraft's control system.

They are commonly used for adjusting the tension of the control cables.

It is used to transmit and permit change in the direction of the force. Bell crank

It used between bell crank to torque arm to transmit the force and motion from one member to the other. Push pull rods are also called control rods.

It is used at the base of a control column or control stick to impart force and motion to a cable system.

On large or high performance aircraft, the control surfaces have high pressure exerted on them by the airflow hence it is difficult for the pilot to move the control manually. The hydraulic actuators are used within the linkages to assist the pilot in moving the control surface to reduce the pilot from fatigue and improve system performance.

The types and complexity of control mechanisms used depend on the aircraft size, the speed it flies and the mission of the aircraft. A small or low speed aircraft may have cockpit controls connected directly to the control surface by cables or push-rods. Some aircraft have both cable and a pushrod system. The force exerted by the pilot is transferred through them to the control surfaces

A hollow shaft that converts linear motion (cable or push-pull tube) into rotary motion.

Devices that prevent cables from jumping out of pulleys, ensuring smooth cable movement.

They guide control cables through structural members, reducing vibration and maintaining alignment.

A device that keeps tension on cables constant, even with temperature changes.

A system that uses steel cables directly connected to control surfaces, providing a direct link between cockpit controls and the aircraft's movement. Effective in low-speed aircraft.

A steering system that utilizes push-pull rods, bell cranks, and torque tubes to connect cockpit controls to control surfaces. Offers more mechanical advantage than direct cable systems.

A system that uses cables and pulleys to transmit control forces. This system typically uses a series of pulleys to change the direction of the cable and offers mechanical advantage.

A system that uses chains and sprockets to transfer control forces. This system often leverages gears for mechanical advantage and is commonly seen in larger aircraft.

Cables are lighter than push-pull rods, offering a weight advantage.

Cables transmit pull forces only, while push-pull rods can transmit both push and pull forces.

Cable systems have fewer connections, leading to less mechanical play and more precise control surface movement.

Cables provide warning signs before breaking, such as stretching or wire breakage, making them a safer option.

Hydraulic actuators assist the pilot in moving control surfaces, providing power boost for easier control.

The mechanical circuit links the cockpit controls to the hydraulic circuits, using rods, cables, pulleys, and sometimes chains.

The hydraulic circuit includes pumps, reservoirs, filters, pipes, valves, and actuators, delivering hydraulic power to the actuators.

Servo valves control the movement of hydraulic actuators, directing high-pressure fluid to control surfaces.

Fly-by-wire (FBW) systems replace traditional mechanical linkages with electrical signals, using computers to control flight surfaces. This system utilizes wires to connect cockpit controls to electrohydraulic valves, which direct hydraulic fluid to actuators, ultimately moving the aircraft's control surfaces.

FBW systems are lighter than conventional mechanical systems, reducing weight and improving fuel efficiency. They also eliminate the need for mechanical adjustments, reducing maintenance time and cost.

FBW systems are more reliable than their mechanical counterparts. They can adapt to changing flight conditions and provide enhanced control precision. However, a failure of the computer system can lead to complete loss of control.

The cockpit controls in an FBW system are electronic devices that convert the pilot's inputs into electrical signals. These signals are sent to the flight control computer which processes them and then sends out signals to activate the hydraulic systems.

A backup system is crucial for FBW due to the potential risk of complete control loss resulting from a computer failure. These backup systems can be mechanical, hydraulic or a combination of both, providing a safety net for the aircraft.

In power-assisted flight control systems, the hydraulic actuator is bypassed in the event of hydraulic system failure, allowing the pilot's input to directly control the control surface.

A control lever connects the pilot's control column (e.g., elevator) to the control surface in power-assisted flight control systems. When the pilot moves the control column, the lever pivots, moving the control surface accordingly.

In power-operated flight control systems, the pilot only controls the control lever, which in turn activates the servo unit connected directly to the control surface. This reduces the pilot's effort needed to move the control column.

The pilot does not directly feel air load effects on the aircraft in power-operated flight control systems. An artificial feel unit simulates these effects, providing feedback to the pilot.

An artificial feel unit helps the pilot feel the air load on the aircraft, providing control feedback. Two main types are used:

Spring feel provides feedback at lower speeds and higher deflection control demands, simulating the air load effect on the control surfaces.

Q feel dominates at higher speeds and low deflections, providing a sense of air pressure on the control surfaces.

Fly-by-wire (FBW) systems use electronic signals to control the aircraft, instead of mechanical linkages. This offers precise control, safety features, and reduced weight.