now would like to introduce a new mental image of a wing. One is used
to thinking of a wing as a thin blade that slices though the air and
develops lift somewhat by magic. For this we would like to adopt a
visualization aid of looking at the wing as a virtual scoop that
intercepts a certain amount of air and diverts it to the angle of the
downwash. This is not intended to imply that there is a real, physical
scoop with clearly defined boundaries, and uniform flow. But this
visualization aid does allow for a clear understanding of how the
amount diverted air is affected by speed and density. The concept of
the virtual scoop does have a real physical basis but beyond the scope
of this work.
The virtual scoop diverts a certain amount of air from the horizontal to roughly the angle of attack, as depicted in Figure 13. For wings of typical airplanes it is a good approximation to say that the area of the virtual scoop is proportional to the area of the wing. The shape of the virtual scoop is approximately elliptical for all wings, as shown in the figure. Since the lift of the wing is proportional to the amount of air diverted, the lift is also proportional to the wing’s area.
As stated before, the lift of a wing is proportional to the amount of air diverted down times the vertical velocity of that air. As a plane increases speed, the virtual scoop diverts more air. Since the load on the wing does not increase, the vertical velocity of the diverted air must be decreased proportionately. Thus, the angle of attack is reduced to maintain a constant lift. When the plane goes higher, the air becomes less dense so the virtual scoop diverts less air at a given speed. Thus, to compensate the angle of attack must be increased. The concepts of this section will be used to understand lift in a way not possible with the popular explanation.
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