Page 67 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 54

54 CHAPTER TWO

The hummingbird has a wing Until now we have not considered the additional loading
2
2
area of 2 in (12 cm ). The caused by upwash in the description of flight. The total load
albatross has a wing area of on the wing is equal to the lift of the wing plus the upwash
2
2
6200 in (960 cm ). loading. This additional load due to upwash is equal to
(2/AR) lift, where AR is the wing’s aspect ratio (span/mean
*
chord). Most small airplanes have aspect ratios of 7 to 8. An airplane
with an aspect ratio of 7 will experience approximately a 20 percent
reduction of wing loading in ground effect. Since induced power is
proportional to the load squared, this corresponds to almost a 40
percent reduction in induced power. The glider in Figure 2.17 gets a
substantial reduction in wing loading due to upwash from its very
long wings.
It is reasonable to ask if this additional loading due to upwash changes
the physical description of flight. The answer is no. The increased loading
is proportional to the lift. Let us look at an airplane going through a 2g
turn at a constant speed. Before the turn the wing had a certain load due
to the weight of the airplane and to the upwash. In a 2g turn the apparent
weight of the airplane is doubled, and the load due to upwash has also
doubled. Thus the load on the wing has doubled. Since the angle
of attack must also be doubled to maintain altitude, nothing has
Charles Lindbergh flew 50
changed in our description. All the descriptions above remain the
combat missions during WWII,
same. It is just that the load on the wing is higher than the weight
even though he was a civilian.
of the airplane, but still proportional to it.

Wrapping It Up

The key thing to remember about lift is that it is a reaction force
caused by the diversion of air down. The lift of a wing is proportional
to the amount of air diverted times the vertical velocity of that air. For
a given wing the amount of air diverted is proportional to the speed of
the wing and the density of the air. The vertical velocity of the down-
wash is proportional to the angle of attack and the speed of the wing.
The induced power is proportional to 1/speed and to load squared.
The parasitic power of an airplane is proportional to speed cubed.
With these basic concepts the phenomena of flight can be easily
understood.