Page 32 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 19

How Airplanes Fly 19

pressure that is formed above the wing accelerates the air Wings develop lift by diverting
down, with almost all of the lift of a well-designed wing air down.
coming from the diversion of air from above the wing.
One should be careful not to form the image in their mind of the air
striking the bottom of the wing and being deflected down. This is a
fairly common misconception, and was also made by Sir Isaac
Newton himself. Since Newton was not familiar with the details of
airflow over a wing, he thought that the air was diverted down by its
impact with the bottom of birds’ wings. It is true that there is some lift
due to the diversion of air by the bottom of the wing, but the majority
of the lift is due to the top of the wing.
The physical description of lift is very powerful. It allows one to
have an intuitive understanding of such diverse phenomena as the
dependence of lift on the angle of attack of the wing, inverted flight,
ground effect, high-speed stalls, and much more. As stated before, this
description of lift is that it is useful to the pilot in flight. This cannot
be said for the popular description of lift.

Newton’s Three Laws

The most powerful tools for understanding flight are Newton’s three
laws of motion. They are simple to understand and universal in appli-
cation. They apply to the flight of the lowly mosquito and the motion
of the galaxies. Let us look at these laws, not quite in order.
A statement of Newton’s first law is:

A body at rest will remain at rest, and a body in motion will con-
tinue in straight-line motion unless subjected to an external applied
force.

In the context of flight this means that if an object (such as a mass
of air), initially motionless, starts to move, there has been force acting
on it. Likewise, if a flow of air bends (such as over a wing), there also
must be a force acting on it.
Newton’s third law can be stated:

For every action there is an equal and opposite reaction.