Introduction
This chapter examines the fundamental physical laws
governing the forces acting on an aircraft in flight, and
what effect these natural laws and forces have on the
performance characteristics of aircraft. To control an aircraft,
be it an airplane, helicopter, glider, or balloon, the pilot
must understand the principles involved and learn to use or
counteract these natural forces.

Structure of the Atmosphere
The atmosphere is an envelope of air that surrounds the
Earth and rests upon its surface. It is as much a part of the
Earth as the seas or the land, but air differs from land and
water as it is a mixture of gases. It has mass, weight, and
indefinite shape.

The atmosphere is composed of 78 percent nitrogen, 21
percent oxygen, and 1 percent other gases, such as argon
or helium. Some of these elements are heavier than others.
The heavier elements, such as oxygen, settle to the surface
of the Earth, while the lighter elements are lifted up to the
region of higher altitude. Most of the atmosphere's oxygen
is contained below 35,000 feet altitude.

Air, like fluid, is able to flow and change shape when
subjected to even minute pressures because it lacks strong
molecular cohesion. For example, gas completely fills any
container into which it is placed, expanding or contracting
to adjust its shape to the limits of the container.