thumb|alt=Moving the control stick to the left moves the aileron in the left wing up, that in the right wing down, making the plane lower the left wing. Pulling on the stick moves the elevators up, making the plane raise the nose. Pressing the right rudder pedal moves the rudder to the right, making the plane turn the nose to the right.|upright=1.2|Primary aircraft control surfaces and motion: <br> A. [[Aileron B. Control stick C. Elevator D. Rudder]]

Flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude. The primary function of these is to control the aircraft's movement along the three axes of rotation. Flight control surfaces are generally operated by dedicated aircraft flight control systems.

Development of an effective set of flight control surfaces was a critical advance in the history of development of aircraft. Early efforts at fixed-wing aircraft design succeeded in generating sufficient lift to get the aircraft off the ground, however with limited control. The development of effective flight controls allowed stable flight.

A conventional fixed-wing aircraft uses three primary flight control surfaces&ndash; aileron, rudder and elevator to control the roll, yaw, and pitch respectively. Secondary flight control surfaces might include spoiler, flaps, and slats on the wings. The main control surfaces of a fixed-wing aircraft are attached to the airframe in such a way that they can perform the intended range of motion. These usually work by deflecting the air stream passing over them, to create the intended effect.

Certain fixed-wing aircraft configurations may use different control surfaces however the basic principles remain. For other airborne vehicles, these vary depending on the controls required. For rotary wing aircraft such as a helicopter, the stick and the rudder is used to accomplish the same motions about the three principal axes and the rotating flight controls such as main rotor and tail rotor disks. Certain elements are considered as a generalized fluid control surface, such as the rudders, which are shared between aircraft and watercraft.

Development

The Wright brothers are credited with developing the first practical control surfaces, registered as a part of their patent on flying. While elevator and rudder were used to control the pitch and the yaw, early aircraft had trouble controlling the roll of the flight. The Wright brothers used wing warping, a technique where the outer trailing edges of the wing were manipulated to control the roll. However, this technique put additional pressure on the wings, making it more prone to structural failure. In an attempt to circumvent the Wrights' patent, Glenn Curtiss developed hinged movable surfaces called ailerons, attached to the wings, which helped controlling the roll of the aircraft. These hinged control surfaces have the advantage of not causing stresses that are a problem of wing warping and are easier to build into structures, and have since become a standard in fixed wing aircraft.

Axes of motion

thumb|Rotation around the three axes|alt="pitch" is a nose-up/nose-down rotation, "roll" is a rotation lifting one wing and lowering the other, "yaw" rotation moves the nose in a left/right direction

An aircraft is free to rotate around three axes that are perpendicular to each other and intersect at its center of gravity. To control position and direction, a pilot is required to be able to control rotation about each of them. These axes move with the aircraft and change relative to the earth as the aircraft moves. For example, for an aircraft whose left wing is pointing straight down, its "vertical" axis is parallel with the ground, while its "transverse" axis is perpendicular to the ground.

  • Transverse axis or lateral axis passes through an aircraft from wingtip to wingtip, and rotation about this axis is called pitch. Pitch changes the vertical direction that the aircraft's nose is pointing. Elevators are the primary flight control surfaces used to control the pitch.
  • Longitudinal axis passes through the aircraft from nose to tail, and the rotation about this axis is termed as roll. The bank angle can be changed by applying differential lift to each of the wings. The ailerons are the primary control surfaces to effect this, while spoilerons on the upper wing surface aids rolling in larger aircraft. The rudder also has a secondary effect on bank through moving one wing forward and the other backwards, affecting the lift they produce. They are used during low speed, high angle of attack flight including take-off and descent for landing. Some aircraft are equipped with "flaperons", which are more commonly called "inboard ailerons". These devices function primarily as ailerons, but on some aircraft, will "droop" when the flaps are deployed, thus acting as both a flap and a roll-control inboard aileron.

Slats

Slats, also known as leading edge devices, are extensions to the front of a wing for lift augmentation, and are intended to reduce the stalling speed by altering the airflow over the wing. Slats may be fixed or retractable - fixed slats (e.g. as on the Fieseler Fi 156 Storch) give excellent slow speed and STOL capabilities, but compromise higher speed performance. Retractable slats, as seen on most airliners, provide reduced stalling speed for take-off and landing, but are retracted for cruising.

Air brakes

thumb|right|Air brakes on the rear fuselage of a [[Eurowings BAe 146-300]]

Air brakes are used to increase drag. Spoilers might act as air brakes, but are not pure air brakes as they also function as lift-dumpers or in some cases as roll control surfaces. Air brakes are usually surfaces that deflect outwards from the fuselage (in most cases symmetrically on opposing sides) into the airstream in order to increase form-drag. As they are in most cases located elsewhere on the aircraft, they do not directly affect the lift generated by the wing. Their purpose is to slow down the aircraft. They are particularly useful when a high rate of descent is required. They are common on high performance military aircraft as well as civilian aircraft, especially those lacking reverse thrust capability.

Control trimming surfaces

Trimming controls allow a pilot to balance the lift and drag being produced by the wings and control surfaces over a wide range of load and airspeed. This reduces the effort required to adjust or maintain a desired flight attitude.

Elevator trim

Elevator trim balances the control force necessary to maintain the correct aerodynamic force on the tail to balance the aircraft. Whilst carrying out certain flight exercises, a lot of trim could be required to maintain the desired angle of attack. This mainly applies to slow flight, where a nose-up attitude is required, in turn requiring a lot of trim causing the tailplane to exert a strong downforce. Elevator trim is correlated with the speed of the airflow over the tail, thus airspeed changes to the aircraft require re-trimming. An important design parameter for aircraft is the stability of the aircraft when trimmed for level flight. Any disturbances such as gusts or turbulence will be damped over a short period of time and the aircraft will return to its level flight trimmed airspeed.

Trimming tail plane

Except for very light aircraft, trim tabs on the elevators are unable to provide the force and range of motion desired. To provide the appropriate trim force the entire horizontal tail plane is made adjustable in pitch. This allows the pilot to select exactly the right amount of positive or negative lift from the tail plane while reducing drag from the elevators.

Control horn

thumb|Mass balance protruding from an aileron used to suppress flutter

A control horn is a section of control surface which projects ahead of the pivot point. It generates a force which tends to increase the surface's deflection thus reducing the control pressure experienced by the pilot. Control horns may also incorporate a counterweight which helps to balance the control and prevent it from fluttering in the airstream. Some designs feature separate anti-flutter weights.

(In radio controlled model aircraft, the term "control horn" has a different meaning)

Spring trim

In the simplest arrangement, trimming is done by a mechanical spring (or bungee) which adds appropriate force to augment the pilot's control input. The spring is usually connected to an elevator trim lever to allow the pilot to set the spring force applied.

Rudder and aileron trim

Most fixed-wing aircraft have a trimming control surface on the elevator, but larger aircraft also have a trim control for the rudder, and another for the ailerons. The rudder trim is to counter any asymmetric thrust from the engines. Aileron trim is to counter the effects of the centre of gravity being displaced from the aircraft centerline. This can be caused by fuel or an item of payload being loaded more on one side of the aircraft compared to the other, such as when one fuel tank has more fuel than the other.

See also

  • Aircraft engine controls
  • Aircraft flight control systems
  • Aircraft flight mechanics
  • Flight with disabled controls
  • Ship motions
  • Six degrees of freedom
  • V-tail
  • Wing warping

Notes

References

  • <cite>Private Pilot Manual</cite>; Jeppesen Sanderson; (hardcover, 1999)
  • <cite>Airplane Flying Handbook</cite>; U.S. Department of Transportation, Federal Aviation Administration, FAA-8083-3A. (2004)
  • Clancy, L.J. (1975) Aerodynamics Pitman Publishing Limited, London
  • A clear explanation of model aircraft flight controls by BMFA
  • See How It Flies By John S. Denker. A new spin on the perceptions, procedures, and principles of flight.

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