6. Flight control system

6.1. General

A flight control system consists of the flight control surfaces ( see part 4), the respective cockpit controls, connecting linkage, and necessary operating mechanisms to control aircraft in flight.

Generally the cockpit controls are arranged like this: control wheel (yoke) for roll which moves the ailerons; control column for pitch which moves the elevators; rudder pedals for yaw which moves the rudder. Some light aircraft use a control stick for both roll and pitch; the rudder pedals for yaw. Flight control systems (FCS) are classified as follows: mechanical FCS; hydromechanical FCS (powered flight control units(PFCU)); fly-by-wire FCS.

6.1.1. MECHANICAL FLIGHT CONTROL SYSTEMS

The mechanical FCS are the most basic designs. They were used in early aircraft and currently in small aeroplanes where the aerodynamic forces are not excessive. The FCS 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. Since an increase in control surface area in bigger airplane leads to an exponential increase in 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 An-24 or An-140.

Some mechanical FCS use servo tabs that provide aerodynamic assistance to reduce complexity.

6.1.2. HYDROMECHANICAL FLIGHT CONTROL SYSTEM

The complexity and weight of a mechanical FCS increases considerably with size and performance of the airplane. A hydraulic FCS has 2 parts: the mechanical circuit; the hydraulic circuit. The mechanical circuit links the cockpit controls with the hydraulic circuits. Like the mechanical FCS, 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 most jet transports and high performance aircraft. In the mechanical FCS, the aerodynamic forces on the control surfaces are transmitted through the mechanisms (artificial feel devices) and can be felt by the pilot. This gives a tactile feedback of airspeed and aids flight safety. The hydromechanical FCS lacks this "feel". The aerodynamic forces are only felt by the actuators. Artificial feel devices are fitted to the mechanical circuit of the hydromechanical FCS 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 FCS.

6.1.3. FLY-BY-WIRE FLIGHT CONTROL SYSTEMS

With the invention of the autopilot, it is possible to control an aircraft electrically. The pilot utilizes switches on the autopilot for control. Later autopilots can accept steering commands directly from the cockpit controls. The cockpit controls must be fitted with transducers.

As an autopilot's reliability improves, the next logical stage of FCS evolution was to totally remove the mechanical circuit, creating the fly-by-wire FCS.

Analog fly-by-wire FCS. The fly-by-wire FCS eliminates the complexity, fragility and weight of the mechanical circuit of the hydromechanical FCS and replaces it with an electrical circuit. The cockpit controls now operate signal transducers which generate the appropriate commands. The commands are processed by an electronic controller. The autopilot is now part of the electronic controller. The hydraulic circuits are similar except that mechanical servo valves are replaced with electrically controlled servo valves. The valves are operated by the electronic controller.

In this configuration, the FCS must simulate "feel". The electronic controller controls electrical feel devices that provide the appropriate "feel" forces on the manual controls. This is still used in the EMBRAER 170 and EMBRAER 190 and was used in the Concorde, the first fly-by-wire airliner.

On more sophisticated versions, analog computers replaced the electronic controller. Digital fly-by-wire FCS. A digital fly-by-wire FCS is similar to its analogue conterpart. However, the signal processing is done by digital computers. The pilot literally can "fly-via-computer". This increases flexibility as the digital computers can receive input from any aircraft sensor. It also increases stability, because the system is less dependent on the values of critical electrical components in an analog controller.

Power-by-wire FCS. Having eliminated the mechanical circuits in fly-by-wire FCS, the next step is to eliminate the bulky and heavy hydraulic circuits.The hydraulic circuit is replaced by an electrical power circuit. The power circuits power electrical or self-contained electrohydraulic actuators that are controlled by the digital flight control computers. All benefits of digital fly-by-wire are retained.

The biggest benefits are weight savings, the possibility of redundant power circuits and tighter integration between the aircraft FCS and its avionics systems. The absence of hydraulics greatly reduces maintenance costs.

Intelligent FCS. A newer flight control system, called Intelligent Flight Control System (IFCS), is an extension of modern digital fly-by-wire FCS. The aim of IFCS is to intelligently compensate for aircraft damage and failure during mid-flight, such as automatically using engine thrust and other avionics to compensate for severe failures such as loss of hydraulics, loss of rudder, loss of ailerons, loss of an engine, etc.

Fly-by-optics FCS. Fly-by-optics is sometimes used instead of fly-by-wire because it can transfer data at higher speeds, and it is immune to electromagnetic interference. In most cases, only the electronic interface to the wire changes. The data generated by the software and interpreted by the controller remain the same.

6.2. AN-140 FLIGHT CONTROL SYSTEM

The aircraft is equipped with an integrated system of electromechanical controls including: rudder control system (Fig. 6.1); elevator control system (Fig. 6.2) ailerons control system (Fig. 6.3); spoilers control system. The elevator, rudder, and ailerons can be controlled either by the pilots, or automatically, in response to the automatic flight control system input. The aileron trim tabs, rudder and elevator trim tabs have remote electrical control.

The manual control unit and the pedal unit are kinematically interconnected and ensure the elevator, rudder, and aileron control from the pilots` and co-pilots` stations.

Use is made of the combined push-pull/cable control linkage: the cable control linkage is used in the fuselage, and the control rods are used in the flight compartment, wing, and tail unit. The flaps have: electro-hydraulic control in the primary control mode; remote control using electric actuators in the standby mode.

The spoilers control system is electro-hydraulically controlled with automatic extension, and either automatic or manual retraction in response to pilot input with the aid of the return springs of the hydraulic actuator depending on the aircraft takeoff/landing configuration.

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Fig. 6.1. Rudder Control Diagram

Fig. 6.2. Elevator Control Diagram

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Fig. 6.3. Aileron Control Diagram