Page 93 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 80

80 CHAPTER THREE

efficiency it would be better to have the wing working harder.
The B-2 bomber would cost less
So, the canard sacrifices efficiency so that the horizontal
if it cost its weight in gold.
stabilizer always stalls first.
Choosing a canard configuration over a conventional
configuration has to be considered carefully. The total power required
for lift must be analyzed before the designer decides whether canard
or conventional configurations are best.

Boundary-Layer Energy

The concept of the boundary layer was introduced in Chapter 2. In
order to understand certain aspects of wing design, it is necessary to go
into more detail. In this section the effects of pressure and energy on the
boundary layer will be discussed as an introduction to high-lift devices.
The boundary layer on a wing section is shown in Figure 3.20. In this
figure the boundary-layer thickness is greatly exaggerated. In reality, the
boundary layer is quite thin. For example, at the trailing edge of a Boeing
747-400 (where the boundary layer is the thickest) the boundary layer is
approximately 1 inch (2.5 cm) thick. Figure 3.21 is a blowup of the
boundary layer showing how the speed relative to the wing changes

Boundary layer

Fig. 3.20. The boundary layer.

Edge of boundary layer Speed profile
through layer

Surface

Fig. 3.21. How the airspeed changes in the boundary layer.