Aircraft flight mechanics
From Wikipedia, the free encyclopedia
An Aeroplane (Airplane in US usage), is defined as: a power-driven heavier than air Aircraft, deriving its lift chiefly from aerodynamic reactions on surface which remain fixed under given conditions of flight. (ICAO Doc 9110)
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[edit] Straight and level flight of aircraft
In steady, level flight, an aircraft can be considered as being acted on by four forces in equilibrium: lift, weight, thrust, and drag. Thrust is the force generated by the engine and acts along the engine's thrust vector. Lift acts perpendicular to the motion of the aircraft. Drag acts parallel to the motion of the aircraft. Weight acts, through the aircraft's centre of gravity, towards the centre of the Earth. Very roughly, in straight and level flight, lift can be assumed equal to weight and thrust equal to drag. By altering the balance of these basic forces, an aircraft can be maneuvered in three dimensions.
[edit] Aircraft control and movement
There are three primary ways for an aircraft to change direction. Pitch (movement of the nose up or down), roll (rotation around the longitudinal axis, that is, the axis which runs along the length of the aircraft) and yaw (swinging turn (change of heading direction) includes both roll and yaw of the aircraft).
In micro-lights and hang gliders the pitch action is reversed - pulling back produces a nose-down pitch action.
[edit] Aircraft control surfaces
Yaw is induced by a moveable rudder, attached to a vertical fin usually at the rear of the aircraft. Sometimes the entire fin is movable. Movement of the rudder changes the size and orientation of the force the vertical surface produces. 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 realized that using yaw alone is not a very efficient way of executing a level turn in an aircraft and will result in some sideslip. A precise combination of bank and lift must be generated to cause the required centripetal forces without producing a sideslip.
Pitch is controlled by the rear part of the tailplane's horizontal stabilizer being hinged to create an elevator. By moving the elevator up (a position of negative camber) the tailplane is pulled down and the angle 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 noticeable 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 a stabilator, an all-moving tail surface. Pitch is changed in this case by moving the entire horizontal surface of the tail. This seemingly simple innovation was one of the key technologies that made supersonic flight possible. In early attempts, as pilots exceeded Mach 0.9, a strange phenomena made their control surfaces useless, and their aircraft uncontrollable. It was determined that as an aircraft approaches the speed of sound, the air approaching the aircraft is compressed and shock waves are produced in a conical shape as the aircraft meets and exceeds the sound barrier. These shock waves made the elevator control surfaces freeze and so the problem was solved by moving the entire horizontal surface of the tail. Also, in supersonic flight the change in camber has less effect on lift and a stabilator produces less drag.
Aircraft that need control at extreme angles of attack are sometimes fitted with a canard configuration, in which pitching movement is created using a forward foreplane (roughly level with the cockpit). Such a system produces an immediate increase in lift and therefore a better response to pitch controls. This system is common in delta-wing aircraft (deltaplane), which use a stabilator-type canard foreplane. A disadvantage to a canard configuration compared to an aft tail is that the wing cannot use as much extension of flaps to increase wing lift at slow speeds due to stall performance. A combination tri-surface aircraft uses both a canard and an aft tail (in addition to the main wing) to achieve advantages of both configurations.
A further design of tailplane is the V-tail, so named because that instead of the standard inverted T or T-tail, there are two vertical fins angled away from each other in a V (if they're arranged like a V, at least one of them isn't vertical). To produce yaw like a rudder, the two trailing edge control surfaces move in the same direction. To produce pitch like an elevator, the surfaces move in opposite directions.
Roll is controlled by movable sections on the trailing edge of the wings called ailerons. The ailerons move differentially - one goes up as the other goes down. The difference in camber of the wing cause a difference in lift and thus a rolling movement. As well as ailerons, there are sometimes also spoilers - small hinged plates on the upper surface of the wing, originally used to produce drag to slow the aircraft down and to reduce lift when descending. On modern aircraft, which have the benefit of automation, they can be used in combination with the ailerons to provide roll control.
The earliest powered aircraft built by the Wright brothers 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.
[edit] Aerodynamics
A false explanation for lift has been put forward in mainstream books, and even in scientific exhibitions. Known as the equal transit-time fallacy, it states that the parcels of air which are divided by an airfoil must rejoin again; because of the greater curvature (and hence longer path) of the upper surface of an aerofoil, the air going over the top must go faster in order to "catch up" with the air flowing around the bottom. Therefore, because of its higher speed the pressure of the air above the airfoil must be lower. Despite the fact that this "explanation" is probably the most common of all, it is false in that there is no requirement that divided parcels of air rejoin again, and in fact they do not do so.
The actual linkages within the aircraft are discussed in aircraft flight control systems.
