Page 103 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 90

90 CHAPTER THREE

plus it is designed to take advantage of the gap
between the flap and the wing. The air in the
boundary layer, having passed over the top of
the wing, has lost most of its kinetic energy.
Thus when it reaches the extended flap it is
likely to separate from the flap and cause a stall.
However, the air passing under the wing does
not face the same problem. The slot between the
wing and the flap diverts some of the higher-
energy, lower-surface air to the top of the flap.
The air remains attached to the flap longer, thus
reducing drag and inhibiting stalls. A double-
Fig. 3.28. Fowler flap.
slotted flap (Figure 3.30) basically repeats this
step twice, using two separate flaps in tandem.
This provides the maximum lift from a flap
design. The disadvantage of this design is that
the operating mechanism is very complicated
and heavy. Multislotted flaps are seen on many
modern passenger jets, while large airplanes use
single-slotted flaps.
Until the 1990s airplane performance was the
key design criterion. Airplane companies were
proud of sophisticated triple-slotted flap
systems. During the 1990s a shift toward
reducing cost as a key design criterion has
pushed airplane companies to maximize the
performance of single-slotted flaps. One
Fig. 3.29. Slotted flap. technique that is used is to place vortex
generators on the leading edge of the single-
slotted flap. When the flap is retracted, the
vortex generators on the flap are hidden in the wing. Thus,
In 1991, the very first Boeing
the vortex generators do not penalize the airplane in cruise
727, the Spirit of Seattle, was
but are available for takeoff and landing.
retired after 64,492 flight hours,
The next time you fly a commercial airplane ask for a
which is equivalent to 7.4 years
window seat behind the wing. During the approach and
in the air.
landing phase of the flight, watch the wing unfold. It is truly