To: "Pinhole Listserv" <pinhole@exploratorium.edu>,
Date: Tue, 22 Dec 1998 12:27:13 -0700
From: "BenP" <bpitt@n2.com>
Subject: Re: Flying & wing shapes - Pinhole Digest #95 - 11/26/98
Re: Question #2 - Wings & flight & symmetry, etc.
One factor not discussed in the original transmission, or the response about wings, is the "angle of attack" of the wing. The angle of attack is the angle between a line which traces the airflow past the plane (the relative wind), and a line drawn from the leading edge to the trailing edge of the wing (the wing's Chordline). An aircraft's wings do not necessarily travel through the air in a level orientation. For example, if the leading edge of the wing is higher than the trailing edge, this results in an angle of attack greater than zero.
Even if a wing is symmetrical relative to the surface formed by the infinite number of chordlines along the length of the wing, it may not be symmetrical with respect to the relative wind. It may have a positive angle of attack, with its lower surface a bit more exposed to the oncoming air than is the top surface. As a result, a symmetrically shaped wing may not function as a symmetrical wing as experienced by the air molecules through which it is moving.
The leading edge of a wing is rounded and the trailing edge is sharp. In front of the leading edge there is a shock wave that precedes the wing. As this shock wave meets air in front of the wing, air molecules part to pass above and below the wing, even before the wing itself reaches the air molecules. Between these parting air molecules and the leading edge of the wing is a pocket of (relatively) stagnant air known as the stagnation point. Picture a volume of (relatively) still air in in a line along the front of the leading edge shaped like a very small wedge. Because this line of air is not moving over the surfaces of the wing, it ends up being the area of highest air pressure against the wing surface. If, as the wing moves through the air, the angle of attack is greater than zero, then the leading edge is slightly higher than the trailing edge. When this occurs, the stagnation point is slightly below the leading edge of the wing. As a result, in flight, the functional top
surface of a 'symmetrical' wing is greater in area than the bottom surface. Also, because of the shifted stagnation point, there is an area of relatively high pressure below the leading edge of the wing, adding to the lift.
As the angle of attack increases, the ratio of the upper surface area : lower surface area will increase. A craft with wings designed for inverted flight (as 'symmetrical' ones likely are) will have controls which allow a positive angle of attack even when the craft is upside down. (In normal flight, a pilot pulls back on the yoke to raise the nose and increase the angle of attack. During inverted flight, however, the pilot must actually push the yoke forward to raise the nose and keep the leading edge of the wing higher than the trailing edge.)
In sum, although a wing may be symmetrical in shape, it may not be symmetrical as 'seen' by the air molecules passing by it. Therefore I don't believe the theories of Bernoulli, et al. are brought into question by the existence of 'symmetrical' wings.
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