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One
might ask what the downwash from a wing looks like. The downwash comes
off the wing as a sheet and is related to the details on the load
distribution on the wing. Figure 18 shows, through condensation, the
distribution of lift on an airplane during a high-g maneuver. From the
figure one can see that the distribution of load changes from the root
of the wing to the tip. Thus, the amount of air in the downwash must
also change along the wing. The wing near the root is "virtual
scooping" up much more air than the tip. Since the wing near the root
is diverting so much air the net effect is that the downwash sheet will
begin to curl outward around itself, just as the air bends around the
top of the wing because of the change in the velocity of the air. This
is the wing vortex. The tightness of the curling of the wing vortex is
proportional to the rate of change in lift along the wing. At the wing
tip the lift must rapidly become zero causing the tightest curl. This
is the wing tip vortex and is just a small (though often most visible)
part of the wing vortex. Returning to Figure 7 one can clearly see th
development of the wing vortices in the downwash as well as the wing
tip vortices.
 Fig 18. Condensation showing the distribution of lift along a wing
Winglets
(those small vertical extensions on the tips of some wings) are used to
improve the efficiency of the wing by increasing the effective length,
and thus area, of the wing. The lift of a normal wing must go to zero
at the tip because the bottom and the top communicate around the end.
The winglet blocks this communication so the lift can extend farther
out on the wing. Since the efficiency of a wing increases with area,
this gives increased efficiency. One caveat is that winglet design is
tricky and winglets can actually be detrimental if not properly
designed.
MORE (Ground Effect)
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