Editing Aircraft flight control system (section)

==Flight control systems==

===Mechanical===
[[File:Tiger cables.JPG|thumb|right|[[de Havilland Tiger Moth]] elevator and rudder cables]] 
Mechanical or manually operated flight control systems are the most basic method of controlling an aircraft. They were used in early aircraft and are currently used in small aircraft where the aerodynamic forces are not excessive. Very early aircraft, such as the [[Wright Flyer|Wright Flyer I]], [[Blériot XI]] and [[Fokker Eindecker]] used a system of [[wing warping]] where no conventionally hinged control surfaces were used on the wing, and sometimes not even for pitch control as on the Wright Flyer I and original versions of the 1909 [[Etrich Taube]], which only had a hinged/pivoting rudder in addition to the warping-operated pitch and roll controls.<ref>Taylor, 1990. p. 116.</ref> A manual flight control system uses a collection of mechanical parts such as pushrods, tension cables, pulleys, counterweights, and sometimes chains to transmit the forces applied to the cockpit controls directly to the control surfaces. [[Turnbuckle]]s are often used to adjust control cable tension. The [[Cessna Skyhawk]] is a typical example of an aircraft that uses this type of system. [[Gust lock]]s are often used on parked aircraft with mechanical systems to protect the control surfaces and linkages from damage from wind. Some aircraft have gust locks fitted as part of the control system.<ref>Thom,1988. p. 153.</ref>
 
Increases in the control surface area, and the higher airspeeds required by faster aircraft resulted in higher aerodynamic loads on the flight control systems. As a result, the forces required to move them also become significantly larger. Consequently, complicated mechanical [[gear]]ing arrangements were developed to extract maximum [[mechanical advantage]] in order to reduce the forces required from the pilots.<ref name="Taylor118">Taylor, 1990. p. 118.</ref> This arrangement can be found on bigger or higher performance [[Propeller (aircraft)|propeller]] aircraft such as the [[Fokker 50]].

Some  mechanical flight control systems use [[servo tab]]s that provide aerodynamic assistance. Servo tabs are small surfaces hinged to the control surfaces. The flight control mechanisms move these tabs, aerodynamic forces in turn move, or assist the movement of 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.<ref>Thom,1988. p. 86.</ref> The Boeing 737 incorporates a system, whereby in the unlikely event of total hydraulic system failure, it automatically and seamlessly reverts to being controlled via servo-tab.

===Hydro-mechanical===
[[File:Hydromechanical flight control system.png|thumb|right|Hydromechanical designs, consisting of a mechanical circuit and a hydraulic circuit, were used to reduce the complexity, weight, and limitations of mechanical flight controls systems.<ref>{{cite book |title=Pilot's Handbook of Aeronautical Knowledge |date=2016-08-24 |publisher=[[Federal Aviation Administration]] |page=6-2}}</ref>]]
The complexity and weight of mechanical flight control systems increase considerably with the size and performance of the aircraft. [[hydraulic machinery|Hydraulically powered control surface]]s help to overcome these limitations. With hydraulic flight control systems, the aircraft's size and performance are limited by economics rather than a pilot's muscular strength. At first, only-partially boosted systems were used in which the pilot could still feel some of the aerodynamic loads on the control surfaces (feedback).<ref name="Taylor118" />

A hydro-mechanical flight control system has two parts:
*The ''mechanical circuit'', which links the cockpit controls with the hydraulic circuits. Like the mechanical flight control system, it consists of rods, cables, pulleys, and sometimes chains.
*The ''hydraulic circuit'', which has hydraulic pumps, reservoirs, filters, 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 [[electro-hydraulic servo valve]]s control the movement of the actuators.

The pilot's movement of a control causes the mechanical circuit to open the matching servo valve in the hydraulic circuit. The hydraulic circuit powers the actuators which then move the control surfaces. As the actuator moves, the [[electro-hydraulic servo valve|servo valve]] is closed by a mechanical [[feedback]] linkage - one that stops movement of the control surface at the desired position.

This arrangement was found in the older-designed jet transports and in some high-performance aircraft. Examples include the [[Antonov An-225]] and the [[Lockheed SR-71]].

====Artificial feel devices====
With purely mechanical flight control systems, the aerodynamic forces on the control surfaces are transmitted through the mechanisms and are felt directly by the pilot, allowing tactile feedback of airspeed. With hydromechanical flight control systems, the load on the surfaces cannot be felt and there is a risk of overstressing the aircraft through excessive control surface movement. To overcome this problem, artificial feel systems can be used. For example, for the controls of the [[RAF]]'s [[Avro Vulcan]] jet [[bomber]] and the [[RCAF]]'s [[Avro Canada CF-105 Arrow]] supersonic interceptor (both 1950s-era designs), the required force feedback was achieved by a spring device.<ref>The Arrowheads, pages 57-58, 83-85 (for CF-105 Arrow only).</ref>  The [[Fulcrum (mechanics)|fulcrum]] of this device was moved in proportion to the square of the air speed (for the elevators) to give increased resistance at higher speeds. For the controls of the American [[Vought]] [[F-8 Crusader]] and the LTV [[A-7 Corsair II]] warplanes, a 'bob-weight' was used in the pitch axis of the control stick, giving force feedback that was proportional to the airplane's normal acceleration.{{Citation needed|date=July 2010}}

====Stick shaker====
{{Main|Stick shaker}}
A [[stick shaker]] is a device that is attached to the control column in some hydraulic aircraft. It shakes the control column when the aircraft is approaching [[Stall (fluid dynamics)|stall]] conditions. Some aircraft such as the [[McDonnell Douglas DC-10]] are equipped with a back-up electrical power supply that can be activated to enable the stick shaker in case of hydraulic failure.<ref>{{Cite journal|last=Daniels|first=Taumi|title=Regarding Pilot Usage of Display Technologies for Improving Awareness of Aircraft System States|url=https://ntrs.nasa.gov/api/citations/20200002619/downloads/20200002619.pdf|journal=NASA Langley Research Center}}</ref>

====Power-by-wire====
In most current systems the power is provided to the control actuators by high-pressure hydraulic systems. In fly-by-wire systems the valves, which control these systems, are activated by electrical signals. In power-by-wire systems, electrical actuators are used in favour of hydraulic pistons. The power is carried to the actuators by electrical cables. These are lighter than hydraulic pipes, easier to install and maintain, and more reliable. Elements of the [[F-35]] flight control system are power-by-wire.<ref>{{Cite web |url=https://www.aviationtoday.com/2001/05/01/power-by-wire/ |title=Power-By-Wire - Avionics<!-- Bot generated title --> |access-date=2018-08-09 |archive-url=https://web.archive.org/web/20170627215836/http://www.aviationtoday.com/2001/05/01/power-by-wire/ |archive-date=2017-06-27 |url-status=live |date=May 2001 }}</ref><ref>{{cite journal|title=Review on signal-by-wire and power-by-wire actuation for more electric aircraft|journal=Chinese Journal of Aeronautics|volume=30|issue=3|pages=857–870|doi=10.1016/j.cja.2017.03.013|year=2017|last1=Maré|first1=Jean-Charles|last2=Fu|first2=Jian|doi-access=free|bibcode=2017ChJAn..30..857M }}</ref><ref>{{Cite journal |title=C-141 and C-130 power-by-wire flight control systems - IEEE Conference Publication<!-- Bot generated title --> |pages=535–539 vol.2 |doi=10.1109/NAECON.1991.165802 |date=May 1991 |s2cid=109026952 }}</ref> The actuators in such an electro-hydrostatic actuation (EHA) system are self-contained hydraulic devices, small closed-circuit hydraulic systems. The overall aim is towards more- or all-electric aircraft and an early example of the approach was the [[Avro Vulcan]]. Serious consideration was given to using the approach on the Airbus A380.<ref>{{Cite web |url=https://www.aviationtoday.com/2001/10/01/a380-more-electric-aircraft/ |title=A380: 'More Electric' Aircraft - Avionics<!-- Bot generated title --> |access-date=2018-08-12 |archive-url=https://web.archive.org/web/20180812212940/https://www.aviationtoday.com/2001/10/01/a380-more-electric-aircraft/ |archive-date=2018-08-12 |url-status=live |date=October 2001 }}</ref>

===Fly-by-wire control systems===
{{Main|Fly-by-wire}}
A fly-by-wire (FBW) system replaces manual flight control of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires (hence the term ''fly-by-wire''), and flight control computers determine how to move the [[actuators]] at each control surface to provide the expected response. Commands from the computers are also input without the pilot's knowledge to stabilize the aircraft and perform other tasks. Electronics for aircraft flight control systems are part of the field known as [[avionics]].

Fly-by-optics, also known as ''fly-by-light'', is a further development using [[fiber-optic cable]]s.