Aircraft flight control systems
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A flight control system consists of the flight control surfaces, the respective cockpit controls, connecting linkage, and necessary operating mechanisms to control aircraft in flight
The fundamentals of aircraft controls have been explained in aeronautics. Discussion here centers on the underlying mechanisms of the flight controls. Generally the cockpit controls are arranged like this:
- control 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.
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Classification
Flight control systems (FCS) are classified as follows:
- mechanical FCS
- hydromechanical FCS (powered flight control units(PFCU))
- fly-by-wire FCS
Mechanical FCS
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. The Cessna Skyhawk is a typical example.
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 Fokker 50.
Some mechanical FCS 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 can even be found in early jet transports such as the all mechanical Boeing 707.
Hydromechanical FCS (powered flight control units(PFCU))
The complexity and weight of a mechanical FCS increases considerably with size and performance of the airplane. Hydraulic power overcomes these limitations. With hydraulic FCS aircraft size and performance are limited by economics rather than a pilot's strength.
A hydraulic FCS has 2 parts:
- the mechnical 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. These include the Antonov An-225, the Lockheed SR-71 and most aircraft in-between.
Artificial feel devices
In the mechanical FCS, the aerodynamic forces on the control surfaces are transmitted through the mechanisms 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 mechnical 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.
Fly-by-wire FCS
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. The autopilot is still equipment furnished by the buyer of an aicraft.
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. This is the simplest and earliest configuration, an analog fly-by-wire FCS, first fitted to the Avro Vulcan in the 1940s.
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. The cancelled supersonic Canadian fighter, the Avro CF-105 Arrow, was built this way in the 1950s. Analog computers also allowed some customization of flight control chracteristics, inluding relaxed stability. This was exploited by the early versions of F-16, giving it impressive maneuverability.
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.
F-8C_FBW.jpg
The computers read positions and forces from the pilot's controls and aircraft sensors. They calculate differential equations that move the flight controls to carry out the intentions of the pilot.
The program in the digital computers let aircraft designers tailor an aircraft's handling characteristics precisely. For example the software can prevent the aircraft from being handled dangerously by preventing pilots from exceeding preset limits (the aircraft's envelope).
Sidesticks or conventional control yokes can be used to fly such an aircraft. Some authorities say that the lack of visual feedback from the side stick is a problem, which is why Boeing utilized conventional yokes in the 777 and the upcoming 787.
As the computers continuously fly the aircraft, pilot workload is reduced. It is now possible to fly aircraft with relaxed stability. The primary benefit for military aircraft is more responsive flight performance. Digital FCS enabled inherently unstable aircraft such as Lockheed Martin F-117 Nighthawk to fly. A modified NASA F-8C Crusader was the first digital fly-by-wire aircraft, in 1972. In 1984, the Airbus A320 was the first airliner with digital fly-by-wire controls. In 2005, the Dassault Falcon 7X was the first business jet with fly-by-wire controls.
On military aircraft, FBW's light weight permits true dual-redundant controls. A wire can be severed, and the backup wire can still convey the control information to the actuators.
For airliners, digital FCS reduces weight, both by eliminating bulky mechanical items and by reducing the flight control surfaces. The reduced drag of the smaller controls also lowers operating costs.
Boeing and Airbus differ in their FBW philosophies. In Airbus aircraft, the computer always retains ultimate control and will not permit the pilot to fly outside the airplane's normal flight envelope. In a Boeing 777, the pilot can override the system, allowing the plane to be flown outside this envelope in emergencies. The pattern started by Airbus A320 has been continued with the Airbus family and every new Boeing design since the Boeing 777.
Aircraft-engine integration
The advent of FADEC (full authority digital engine control) engines permits operation of the FCS and engines to be fully integrated. On modern military aircraft other systems such as autostabilisation, navigation, radar and weapons system are all integrated with the FCS.
FADEC allows maximum performance to be extracted from the aircraft without fear of engine misoperation, airplane damage or high pilot workloads. Movable exhaust ducts jointly controlled by the FCS and FADEC delivers maximum agility through thrust vectoring.
In the civil field, the integration increases flight safety and economy. The Airbus A320 and its fly-by-wire bretheren are protected from low-speed stall. In such conditions, the FCS commands the engines to increase thrust without pilot intervention. In economy cruise modes, the FCS adjusts the throttles and fuel tank selections more precisely than all but the most skillful pilots. FADEC reduces rudder drag needed to compensate for sideways flight from unbalanced engine thrust. The fuel management controls keep the aircraft's attitude accurately trimmed with fuel weight, rather than draggy aerodynamic trims in the elevators.
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. This system is used in the Lockheed Martin F-35.
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. Several demonstrations were made on a flight simulator where a Cessna-trained small-aircraft pilot successfully landed a heavily-damaged full-size jet operated by IFCS, without prior experience with large-body jet aircraft. This development is being spearheaded by NASA Dryden Flight Research Center[1] (http://www.dfrc.nasa.gov/Newsroom/FactSheets/FS-076-DFRC.html). It is reported that IFCS is mostly a software upgrade to an existing fully computerized digital fly-by-wire FCS.
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.
See Also
- MIL-STD-1553 A standard data bus for fly-by-wire.