Page 33 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 20

20 CHAPTER TWO

Eleanor Roosevelt shocked This is fairly straightforward. When you sit in a chair, you
military officers when she told put a force on the chair and the chair puts an equal and
them she wanted to take a ride opposite force on you. Another example is seen in the case of a
with an African-American pilot bending flow of air over a wing. The bending of the air requires
from Tuskegee. a force from Newton’s first law. By Newton’s third law, the air
must be putting an equal and opposite force on whatever is
bending it, in this case the wing.
Newton’s second law is a little more difficult but also more useful
in understanding many phenomena associated with flight. The most
common form of the second law is

F ma, or force equals mass times acceleration

The law in this form gives the force necessary to accelerate an object
of a certain mass. For this work we use an alternative form of Newton’s
second law that can be applied to a jet engine, a rocket, or the lift on a
wing. The alternate form of Newton’s second law can be stated:

The force (or thrust) of a rocket is equal to the amount of gas
expelled per time, times the velocity of that gas.

The amount of gas per time might be in units such as pound mass
per second (lbm/s) or kilograms per second (kg/s). The velocity of
that gas might be in units such as feet per second (ft/s) or meters per
second (m/s). This form of Newton’s second law says that if one
knows the rate at which gas is expelled from a rocket motor, and the
speed of that gas, the thrust of the rocket motor can be easily
calculated. To double the thrust, one can double the amount of gas
expelled per second, double the velocity of the gas, or a combination
of the two.
Let us now look at the airflow around a wing with Newton’s laws in
mind. In Figure 2.2 we see the airflow around the wing as many of us
have been shown at one time or another. Notice that the air approaches
the wing, splits, and reforms behind the wing going in the initial
direction. This wing has no lift. There is no net action on the air and
thus there is no lift, the reaction on the wing. If the wing has no net
effect on the air, the air cannot have any net effect on the wing. Now,