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Wings 87

Fig. 3.25. Vortex generators on the slat and wing of a Navy A-4 Sky Hawk. (Air
Classics Museum,Auora. IL.)

The second case where vortex generators are desirable is when an
existing airplane is modified to meet new performance parameters. A
typical change in performance requirements is to make an existing
airplane use less runway for takeoff and landing. Ideally, a new wing
would be designed. However, designing a new wing for an existing
airplane is usually too costly. Vortex generators are thus added to
increase takeoff and landing performance. There are many companies
that modify existing airplanes, converting them to STOL (short takeoff
and landing) airplanes, which is the case in Figure 3.25.

High-Lift Devices

Devices or modifications to the wing that increase the stall angle of
attack are called high-lift devices. Airplanes employ various high-lift
devices to improve takeoff and landing performance. The basic prin-
ciple behind these devices is that they allow the wing to divert more
air down without stalling. Vortex generators discussed above are a
type of high-lift device. During takeoff and landing the airplane needs
to fly at the lowest speed possible, since high takeoff and landing
speeds mean longer runways. Therefore, a major goal in designing a
wing is to reduce the stall speed as much as possible. The easiest way
to do this would be to design the wing with a great deal of wing area
and camber. Such a wing would be able to fly at a high angle of attack
without stalling. But as mentioned before, a high-cambered wing
would have a high drag at cruise speeds. The solution is to design a