Page 99 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 86

86 CHAPTER THREE

Smooth Ball

Dimpled Golf Ball

Fig. 3.24. The dimples on a golf ball decrease form drag.

Since a wing at an angle of attack generates a trailing vortex sheet, so
do vortex generators. So, for vortex generators to work they only need
to be as high as the boundary layer is thick. They are usually added a
little aft of the region of highest air velocity.
Unfortunately, there is a penalty for adding vortex generators. Since
vortex generators put a force on the air, they do work, yet without
adding to the lift of the wing. Therefore, additional power is needed
when vortex generators are used. The reality of vortex
In 1911, Orville Wright made a generators is that they are used most often to correct an existing
9-minute flight in a glider. It problem. Because they introduce a power penalty, there is no
remained a world record for incentive for putting them in the original wing design. A well-
10 years. designed wing should not need vortex generators.
Nevertheless, there are several cases where it is desirable to
utilize vortex generators. The first is in situations where the vortex
generators are used only for takeoff and landing but are stowed for
cruise. A particular situation where this can be seen is on the leading
edge of some wing flaps. The use of flaps is discussed in the next section.