Flying Mechanism of an Airplane
An airplane flies because air flowing over its wings produces
lift as a result of lower pressure above the wing than below.
This effect, known as Bernoulli's principle, varies
according to the nature of both the airstream and the wing over
which it flows. Bernoulli's principle states that any fluid,
including air, that speeds up must lose pressure. When air passes
over the specially curved upper surface of a wing, it must travel
a relatively greater distance than the same air mass that moves
across the bottom of a wing, and therefore must speed up. As the
air speeds up, it loses pressure. A difference in pressure is
created between the high-speed, lower-pressure air above the wing,
and the lower-speed, high-pressure air below. This pressure
difference produces a force known as lift, which is the essence
Lift is one of the four primary forces acting upon an airplane. The others are weight, thrust, and drag. Weight is the force that offsets lift, because it acts in the opposite direction. The weight of the airplane must be overcome by the lift produced by the wings. If an airplane weighs 4.5 metric tons, then the lift produced by its wings must be greater than 4.5 metric tons in order for the airplane to leave the ground. Designing a wing that is powerful enough to lift an airplane off the ground, and yet efficient enough to fly at high speeds over extremely long distances, is one of the marvels of aircraft technology.
Thrust is the force that propels an airplane forward through the air. It is provided by the airplane's propulsion system; either a propeller or jet engine or combination of the two.
A fourth force acting on all airplanes is drag. Drag is created because any object moving through a fluid, such as an airplane through air, produces friction as it interacts with that fluid and because it must move the fluid out of its way to do its work. A high-lift wing surface, for example, may create a great deal of lift for an airplane, but because of its large size, it is also creating a significant amount of drag. That is why high-speed fighters and missiles have such thin wings-they need to minimize drag created by lift. Conversely, a crop duster, which flies at relatively slow speeds, may have a big, thick wing because high lift is more important than the amount of drag associated with it. Drag is also minimized by designing sleek, aerodynamic airplanes, with shapes that slip easily through the air.
Managing the balance between these four forces is the challenge of flight. When thrust is greater than drag, an airplane will accelerate. When lift is greater than weight, it will climb. Using various control surfaces and propulsion systems, a pilot can manipulate the balance of the four forces to change the direction or speed. A pilot can reduce thrust in order to slow down or descend. The pilot can lower the landing gear into the airstream and deploy the landing flaps on the wings to increase drag, which has the same effect as reducing thrust. The pilot can add thrust either to speed up or climb. Or, by retracting the landing gear and flaps, and thereby reducing drag, the pilot can accelerate or climb.
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