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Flight Principles

Flight requires lift, which occurs because wings move air downwards. Lift is created only when air strikes a wing at an angle (i.e., the angle of attack). When the leading edge of a wing is higher than the trailing edge, the bottom of the wing ‘pushes’ the air forward and creates an area of high pressure below and ahead of the wing. At the same time, air is deflected downward so, because of Newton’s Third Law of Motion (for every action there is an equal and opposite reaction), the wing is deflected upward. Both the upper and lower surfaces of the wing deflect the air. The upper surface deflects air down because the airflow “sticks” to the wing surface and follows the tilted wing (the “Coanda effect”).

Because of inertia, air moving over the top of the wing tends to keep moving in a straight line while, simultaneously, atmospheric pressure tends to force air against the top of the wing. The inertia, however, keeps the air moving over the wing from ‘pushing’ against the top of the wing with as much force as it would if the wing wasn’t moving. This creates an area of lower pressure above the wing. Because air tends to move from areas of high pressure to areas of low pressure, air tends to move from the high pressure area below and ahead of the wing to the lower pressure area above and behind the wing. This air moves, therefore, toward the trailing edge of the wing, or the same direction as the airflow created by the wing’s motion. As a result, air flows faster over the top of the wing. Because air under the wing is dragged slightly in the direction of travel, it moves slower than does the air moving over the top of the wing. Thus, air is flowing slower beneath the bottom of the wing. The faster-moving air going over the top of the wing exerts less pressure than the slower-moving air under the wing and, as a result, the wing is pushed upwards by the difference in pressure between the top and the bottom (the Bernoulli effect). So, both the development of low pressure above the wing (Bernoulli’s Principle) and the wing’s reaction to the deflected air underneath it (Newton’s third Law) contribute to the total lift force generated.

(Excerpt taken from this page.)