Page 59 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 46

46 CHAPTER TWO

Fig. 2.18. The V-22 Osprey. (Used with the permission of the Boeing Management
Company.)

Wing Vortices

The downwash behind the wing is sometimes called the downwash
sheet. This downwash sheet has a curl in it, producing the wing vor-
tex. Near the tip of the wing the wing vortex curls very tightly, creat-
ing the wingtip vortex. The wingtip vortex initially contains a small
amount of the energy of the wing vortex, but because it is often so vis-
ible, it is the only part of the wing vortex with which most people are
familiar. Eventually, the entire wing vortex curls into a single trailing
vortex on each side. To understand why the wing vortex curls, we
must first consider the lift distribution of the wing.
In our discussion of the scoop, we illustrated in Figure 2.12 that the
amount of air diverted by a wing is a maximum near the root and
decreases to zero at the wingtip. The height of the scoop at any point
along the wing represents the load and the momentum transferred at
that point. The load on a wing is nicely illustrated in Figure 2.19, which
shows the condensation on top of the wing of a fighter aircraft during a
high-g maneuver. The lowered pressure above the wing reduces the air