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Aircraft Control and Movement

An aircraft has six degrees of freedom but the niceity of control sometimes needed means that there can be eighteen or even more aspects of movement to control. To further complicate matters there is often a interaction between different movements - roll always causes yaw or banking can produce sideslip for example.

There have always been three primary actions for a pilot - pitch (movement of the nose up or down relative to axial direction), roll (axial rotation) and yaw (normal rotation). On commercial aircraft these are controlled using a handlebar or spectacle grip mounted on a control column. While on military aircraft, as on the earliest aircraft, a control stick or joystick is used. Following the introduction of fly-by-wire, where there is no mechanical connection from the control to the control surfaces, a sidestick was introduced - a small joystick designed for one handed use.

Conventionally pulling back causes a nose-up pitch action. Turning or moving the control to the right or left produces roll (and turn), turning the control effects the rate of roll rather than indicating the angle to which the aircraft will roll. Yaw is induced by foot pedals where pressure on the right or left pedal produces yaw in the indicated direction.

In micro-lights and gliders the pitch action is reversed - pulling back produces a nose-down pitch action.

Control Surfaces

Yaw is induced by a moveable rubber attached to a vertical fin at the rear of the aircraft, sometimes the entire fin is movable. Movement of the rudder cambers the vertical surface producing force. Since the force is created a distance behind the centre of gravity this sideways force causes a yawing motion. On a large aircraft there may be several independent rudders on the single fin for both safety and to control the inter-linked yaw and roll actions.

It should be realised that an aircraft cannot execute a level turn by yaw alone - there is no surface to use to create cornering forces. A precise combination of bank and lift must be generated to cause the required centripetal forces without producing a sideslip.

Pitch is caused by the rear part of the tailplane's horizontal stabiliser being hinged to create an elevator. By moving the elevator up a position of negative camber the tailplane is pulled down and the anle of attack on the wings increased so the nose is pitched up and lift is generally increased. There is however an initial period where lift is reduced, this is especially noticable in larger aircraft which can drop some way before the increased angle of attack on the wings takes effect.

The system of a fixed tail surface and moveable elevators is standard in subsonic aircraft. Craft capable of supersonic flight often have an slab (all-moving) tail surface. Pitch is changed here by moving the entire horizontal surface of the tail. This system is used becaouse in supersonic flight the change in camber has less effect on lift and a slab surface produces less drag.

Aircraft that need control at extreme angles of attack are sometimes fitted with a canard configuration where piching movement is created using a forward foreplane (roughly level with the cockpit). Such a system produces an immediate increase in lift and so a better response to pitch controls. This system is common in delta-wing aircraft, which use a slab-type canard foreplane.

A further design of tailplane is the vee-tail. So named because that instead of the standard inverted T there are two vertical fins angled away from each other in a V. To produce force like a rudder the two trailling edge control surfaces move in opposite directions. To act as a elevator both surfaces move together. The supposed advantage is the reduction in weight and drag from the reduction in the number of control surfaces from three to two.

Roll is controlled by movable sections on the trailing edge of the wings called ailerons. The ailerons move differentially - one ups and one downs. The difference in camber cause a difference in lift and thus a rolling movement. As well as ailerons there are also spoilers - small hinged plates on the upper surface of the wing, originally they were used to produce drag to slow the aircraft down. But on modern aircraft with the benefit of automation they can be used in combination with the ailerons to provide roll control.

The earliest powered aircraft did not have ailerons. The whole wing was warped using wires. Wing warping is efficient since there is no discontinuity in the wing geometry. But as speeds increased unintentional warping became a problem and so ailerons were developed.

The actual linkages within the aircraft are discussed under:

aircraft mechanical control systems aircraft powered control systems

See also Aerodynamics.