Page 40 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 27

How Airplanes Fly 27

downwash may seem a little foreign at first. We are all used to thinking
of the airflow across a wing as seen by the pilot, or as seen in a wind
tunnel. In this “rest frame” the wing is stationary and the air is moving.
But, what does the world look like in the rest frame where the air is
initially standing still and the wing is moving? Picture yourself on top of
a mountain. Now suppose that just as a passing airplane is opposite
you, you could take a picture of all the velocities of the air. What would
you see? You might be surprised.
Though we go into this in greater detail in a later section, the
There are currently over 13,000
first thing you would notice is that the air behind the wing is
airports in the United States.
going almost straight down when seen from the ground.
(Because of friction with the wing it in fact has a slight forward
direction.) The plausibility of this statement is fairly easy to
demonstrate. Turn on a small household fan and examine the tightness
of the column of air. If the air were coming off the trailing edges of the
fan blades (which are legitimate wings) other than perpendicular to the
direction of the blades’ motion, the air would form a cone rather than a
column. This can also be seen in the picture of a helicopter hovering
above the water (Figure 2.8). The pattern on the water is the same size
as the rotor blades. It is fortunate that nature works this way. If the air
behind the propeller of an airplane came off as a cone rather than a tight
column, propellers would be a much less efficient means of propulsion.
The air expanding out on one side of the airflow would just compensate
for the air expanding out on the other side and would not contribute to
the thrust.
The air behind the wing is going
The wing develops lift by transferring momentum to the air.
almost straight down when seen
Momentum is mass times velocity. In straight-and-level flight
from the ground.
the momentum is transferred toward the earth. This
momentum eventually strikes the earth. If an airplane were to fly over
a very large scale, the scale would weigh the airplane. This should not
be confused with the (wrong) concept that the earth somehow
supports the airplane. It does not. Lift on a wing is very much like
shooting a bullet at a tree. The lift is like the recoil that the shooter
feels, whether the bullet hits the tree or not. If the bullet hits the tree,
the tree experiences the event but has nothing to do with the recoil of
the gun.