Page 56 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 43

How Airplanes Fly 43

may wonder how the design of the wing affects the induced power
requirements. In other words, what is the wing’s efficiency for lift?
Efficiency for lift has to do with the amount of induced power it
takes to produce a certain lift. The lower the induced power needed
the greater the efficiency. The most obvious way to improve
the efficiency of a wing is to increase the amount of air
Approximately 600 million people
diverted by the wing. If more air is diverted, the vertical
fly on domestic commercial
velocity of the air is reduced for the same lift and so is the
routes per year. That’s over 1.6
induced power. This is accomplished by increasing the size of
million a day!
the wing.
Another important contribution to the induced power requirements
of a wing is the additional loading due to the upwash. In brief, the
upwash puts an additional load on the wing on the order of 20 percent
of the lift for a general-aviation airplane. This additional load
decreases with increasing aspect ratio. This is discussed in the section
on ground effect.
In consideration of the total efficiency of a wing, the parasitic power
(or parasitic drag) must also be considered. The parasitic power of a
wing is proportional to its area. So for cruise speeds where parasitic
power dominates there is a limit to how much the area of the wing can
be increased to reduce the induced power. There are additional
problems with increasing the wing’s area, particularly by making it
longer. The first is that large wings are heavy and increase the weight
of the airplane. The second is that long wings are not as structurally
strong as shorter wings. If two wings have the same area, the longer
one will be the most efficient due to reduced upwash loading, though
it will be weaker. Gliders operate at speeds where induced power
dominates. Thus the high-performance gliders have long
wings. Figure 2.17 shows a glider with a 60:1 glide ratio. This
Sea birds, which must fly long
means that in still air, the glider travels 60 ft horizontally for
distances without landing, also
every foot it descends to provide power. It is interesting to note
have high aspect ratio wings for
that sea birds, which must fly long distances without landing,
optimum efficiency.
also have high aspect ratio wings for optimum efficiency.
Most fast airplanes have shorter wings. An exception is the U-2 spy
plane, which flew at 460 mi/h (740 km/h) above 55,000 ft (16,700 m).
The U-2 had long wings because of the extreme altitudes at which it