In a wind tunnel or in flight, an airfoil is simply a streamlined
object inserted into a moving stream of air. If the airfoil profile were
in the shape of a teardrop, the speed and the pressure changes of the air
passing over the top and bottom would be the same on both sides. But if
the teardrop shaped airfoil were cut in half lengthwise, a form resembling
the basic airfoil (wing) section would result. If the airfoil were then
inclined so the airflow strikes it at an angle (angle of attack), the air
molecules moving over the upper surface would be forced to move faster
than would the molecules moving along the bottom of the airfoil, since
the upper molecules must travel a greater distance due to the curvature
of the upper surface. This increased velocity reduces the pressure above
|Bernoulli's principle of pressure by itself does not explain the distribution of pressure over the upper surface of the airfoil. To understand more fully, let us investigate the influence of momentum of the air as it flows in various curved paths near the airfoil (Fig. 17-6). Momentum is the resistance a moving body offers to having its direction or amount of motion changed. When a body is forced to move in a circular path it offers resistance in the direction away from the center of the curved path.||
This is "centrifugal force." While the particles of air move in the curved path AB, centrifugal force tends to throw them in the direction of the arrows between A and B and hence, causes the air to exert more than normal pressure on the leading edge of the airfoil. But after the air particles pass B (the point of reversal of the curvature of the path) the centrifugal force tends to throw them in the direction of the arrows between B and C (causing reduced pressure on the airfoil). This effect is held until the particles reach C, the second point of reversal of curvature of the airflow. Again the centrifugal force is reversed and the particles may even tend to give slightly more than normal pressure on the trailing edge of the airfoil, as indicated by the short arrows between C and D.
Therefore, the air pressure on the upper surface of the airfoil is distributed so that the pressure is much greater on the leading edge than the surrounding atmospheric pressure, causing strong resistance to forward motion; but the air pressure is less than surrounding atmospheric pressure over a large portion of the top surface (B to C).
As we have seen in the application of Bernoulli's theorem to a venturi, the speedup of air on the top of an airfoil produces a drop in pressure. This lowered pressure is a component of total lift. It is a mistake, however, to assume that the pressure difference between the upper and lower surface of a wing alone accounts for the total lift force produced.
One must also bear in mind that associated with the lowered pressure is downwash; a downward backward flow from the top surface of the wing. As we have already seen from our previous discussion relative to the dynamic action of the air as it strikes the lower surface of the wing, the reaction of this downward backward flow results in an upward forward force on the wing. This same reaction applies to the flow of air over the top of the airfoil as well as to the bottom, and Newton's third law is again in the picture.