| Aircraft Flight Mechanics |
Article Index for Aircraft |
Website Links For Aircraft |
Information AboutAircraft Flight Mechanics |
| CATEGORIES ABOUT AIRCRAFT FLIGHT MECHANICS | |
| aerodynamics | |
| aeronautics | |
|
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) STRAIGHT AND LEVEL FLIGHT OF AIRPLANE In steady, level flight, an airplane 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 airplane. Drag acts antiparallel to the motion of the airplane. Weight acts 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. AIRPLANE CONTROL AND MOVEMENT There are three primary ways for an airplane 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 airplane) and ''yaw'' (swinging the nose left or right relative to the airplane vertical axis). On a commercial airplane these are controlled using a handlebar or spectacle grip mounted on a control column in front of the pilot. Following the introduction of Fly-by-wire , some commercial airplanes now use a small controller mounted on the side of the flight deck. With ''Fly by wire'' there is no mechanical connection from the flight deck to the control surfaces, electrical signalling is used instead. On a Military Airplane , as on the earliest airplane, a control stick or joystick is used. Conventionally, pulling back causes a nose-up pitch action; pushing forward causes a nose-down pitch action. Turning or moving the control to the right or left produces roll; turning the control affects 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. A coordinated turn (change of heading direction) includes both roll and yaw of the airplane. In micro-lights and Hang Glider s the pitch action is reversed - pulling back produces a nose-down pitch action. AIRPLANE CONTROL SURFACES ''Yaw'' is induced by a moveable Rudder , attached to a vertical fin usually at the rear of the airplane. 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 airplane 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 using yaw alone is not a very efficient way of executing a level turn in an airplane 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 stabiliser 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 Wing s 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 airplane 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 airplane. 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 airplane uncontrollable. It was determined that as an airplane approaches the speed of sound, the air approaching the airplane is compressed and shock waves are produced in a conical shape as the airplane 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. Airplanes 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 airplane (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 airplane 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 Aileron s. 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 Spoiler s - small hinged plates on the upper surface of the wing, originally used to produce drag to slow the airplane down and to reduce lift when descending. On modern airplanes, which have the benefit of automation, they can be used in combination with the ailerons to provide roll control. The earliest powered airplane 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. AERODYNAMICS The Dutch ( 1700 - 1782 ) Mathematician Daniel Bernoulli has contributed greatly to the science of Aerodynamics and therefore to airplane flight mechanics. Without his theories no airplane would be able to fly in the manner they do so now. Especially Bernoulli's Principle , - which states that in fluid flow, an increase in velocity occurs simultaneously with decrease in pressure- is important. When air flows over the curved top of a wing it has to travel a longer path than the air flowing along the relatively flat underside of the wing, therefore the speed of air on the top of the wing has to increase so that discontinuancies in the flow of air do not occur. According to Bernoulli the pressure on the top of the wing decreases and this results in a lift force. For a mathematical formulation, see Bernoulli's Equation . This theory is considered to be essential for airplane flight mechanics. The actual linkages within the airplane are discussed under: Aircraft Flight Control Systems SEE ALSO |
|
|