THEORY OF FLIGHT AERODYNAMIC FORCES
There are four aerodynamic forces which affect an airplane; lift, gravity, thrust, and drag. These forces operate within Newton's 3rd law of Motion which states "For every action, there is an equal and opposite reaction".
• “Lift” is generated by the design of the wing which creates a differential air pressure between the top and bottom of the wing. The design of a wing provides a curved surface over its top which forces the air to travel a further distance over the top of the wing when compared to the bottom of the wing. The faster air travels over a surface, the less pressure it exerts. Therefore, as the aircraft moves through the air, the air passing over the wings (called relative wind) creates a force of “lift” as the pressure on top of the wing is less than on the bottom of the wing.
• “Gravity” is the counter force to lift. It is the weight of the aircraft. When the force of “lift” overcomes the force of “gravity,” the aircraft flies! During straight and level flight, when the aircraft is not climbing, descending, or turning, the force of lift and gravity are equal.
• “Thrust” is the forward acting force produced by the power of an aircraft’s engine which is converted to thrust by a propeller which pulls an aircraft through the air; or the power of a jet engine which pushes the aircraft through the air.
• “Drag” is the counter force of thrust. Anything which disturbs airflow creates drag. Just as when you place your hand out your car window when traveling down the highway. That force you feel against your hand is drag. When you rotate your hand so a greater surface is facing the direction of the relative wind (the air passing over the car), the greater the force. The faster the car’s speed, the greater the force. The same is true of an airplane. There are two types of drag: “parasitic,” that generated by an object in the airflow; and “induced,” That generated as a component of lift. The air passing over the wings actually creates some drag.
Airfoil The shape of the aircraft wing makes flight possible and is called an airfoil.
The upper surface of the airfoil is shaped such that the air speeds up relative to the underside. When air speeds up pressure reduces on the upper surface of the wing relative to the lower part. High pressure wants to escape to low pressure but the wing is in the way, thus lift is created. For an aircraft to fly it must produce more lift than its own weight.
Aviation Basics 2
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