Page 104 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 91

Wings 91

remarkable how the wing evolves into a high-lift wing from its normal
cruise configuration.

With the design of the Boeing 747, many felt that the ultimate
wing had been built.The wing boasted triple-slotted flaps and
was the pride of the engineers who designed it. Indeed it is a
marvel of performance. Unfortunately, building a triple-slotted
flap is costly. In the new paradigm where cost is one of the
leading design constraints, the goal is to design the simplest (in
other words cheapest) flap possible.Airbus has a single-slotted
flap on their Airbus A320.

Slots and Slats

Leading-edge devices, like flaps, are sometimes used to increase the
camber of the wing and increase the stall angle of attack. But the
details are somewhat different. Other times, the purpose of the leading-
edge devices is much like that of the slot in a slotted flap. These
devices allow the high-energy air from below the wing to flow to the
upper surface of the wing. This energizes the boundary layer. Thus, the
wing stalls at a higher angle of attack and the maximum lift is
increased.
The simplest leading edge device is the fixed slot shown in Figure
3.31. This is a permanent slot near the leading edge of the wing. The
high-pressure air below the wing is drawn up through the slot and
flows over the top of the wing. This energizes the boundary on top of
the wing. A permanent slot can increase the critical angle of attack
significantly. The disadvantage of the fixed slot is that it causes
increased power consumption and drag at cruise speeds.
A device similar to the slot is the fixed slat, shown in Figure 3.32. It
is added onto the wing, increasing the wing’s cord length as well as
energizing the boundary layer. Like the fixed slot, the fixed slat causes
increased drag at cruise speeds.
The solution to the drag caused by fixed slots and slats is to design
a slat that is deployed only at slow speeds and causes little or no drag
in cruise. The Handley-Page retractable slat, shown in Figure 3.33,
extends to large droop angles to give the wing large leading-edge