Page 60 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 47

How Airplanes Fly 47

Fig. 2.19. Illustration of the lift distribution on the wings of an F-14A. (Photo
courtesy of NASA.)

temperature, causing condensation in the form of a kind of fog. This fog
displays the variation in load along the wing. One can see that the load
is greatest near the root of the wing and it tapers to zero at the wingtip.
The large fuselage causes the lift to decrease along the centerline of the
fighter. The lift must go to zero at the wingtip because the relatively
high pressure under the wing communicates around the wingtip with
the lowered pressure above the wing.
The greater the load on a section of wing, the greater the amount of
air that section diverts and the farther above and behind the wing the
effect of the wing is felt. Let us look at the downwash sheet behind the
wing. At the trailing edge of a wing the downwash is roughly constant
in velocity. But as one goes farther back from the wing, the sections
with greater load will have a higher vertical velocity, as shown in
Figure 2.20. So, after the wing passes, the load along the wing
expresses itself in the change in velocity of the downwash, with the
higher load having the highest velocity.
In the discussion of the Coanda effect, we saw that a difference in
velocity in adjacent layers of air caused the air to bend toward the
more slowly moving air. Likewise the difference in velocity of the
downwash sheet causes the air to bend, wrapping from higher load to