Transparency Master
The physics of powered aircraft flight is far from obvious. In fact there is some debate as to whether the lift generated in powered flight is best explained by invoking Bernoulli's Principle or by appealing to Newton's 3rd Law.
An aircraft wing is shown (in cross-section) in the diagram below. The red lines indicate how the air flow diverges as it passes over the wing.
Lift generated by Bernoulli's Principle arises from the shape of the wing
An aircraft gets its lift from the shape of its wing(s). As air is forced over the upper surface of the wing its velocity increases, causing the pressure against the upper surface of the wing to decrease. The underside of the wing experiences a pressure greater than the upper surface. This pressure difference causes the phenomena we call lift .
There is obviously considerable merit in this explanation since a close examination of all airplane wings will show that they all have this characteristic cross-sectional shape.
However this cannot be the whole story, otherwise how would it be possible for airplane to fly upside down?
Lift generated from Newton's 3rd Law arises from the angle of attack of the wing
The Newtonian theory of lift rests on the fact that all aircraft wings moving through the air at a slight angle of attack1 must push the air downwards. The relative motion of the air molecules under the wing would appear as shown in the red lines of the diagram below.
The reaction (recall Newton's 3rd Law of Motion) to the downward deflection of the air below the wing is the upwards thrust of the air molecules on the under side of the wing, thus generating lift.
 Transparency Master |
As anyone who has ever held the palm of their hand against the onrushing wind while they were seated in a moving automobile will know, the impact of the air
on a flat open palm exerts considerable force. The amount of force depends upon the speed of the automobile (the speed of the onrushing air) and the angle of attack.
The same is true for airplane wings. The diagram to the left shows a typical wing in cross-section at various angles of attack. For any given air speed, the larger the angle of attack, the larger the force exerted on the lower surface of the wing.
Similarly, for any given angle of attack, the upward force increases as speed of the wing through the air increases. |
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