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.
Principles of
Flight
Chapter 4
Leading edge of wing under 1,500x magnification
Figure 4-1. Microscopic surface of a wing.
Air is a Fluid
When most people hear the word “fluid,” they usually think
of liquid. However, gasses, like air, are also fluids. Fluids
take on the shape of their containers. Fluids generally do not
resist deformation when even the smallest stress is applied,
or they resist it only slightly. We call this slight resistance
viscosity. Fluids also have the ability to flow. Just as a liquid
flows and fills a container, air will expand to fill the available
volume of its container. Both liquids and gasses display these
unique fluid properties, even though they differ greatly in
density. Understanding the fluid properties of air is essential
to understanding the principles of flight.
Viscosity
Viscosity is the property of a fluid that causes it to resist
flowing. The way individual molecules of the fluid tend to
adhere, or stick, to each other determines how much a fluid
resists flow. High-viscosity fluids are “thick” and resist flow;
low-viscosity fluids are “thin” and flow easily. Air has a low
viscosity and flows easily.
Using two liquids as an example, similar amounts of oil and
water poured down two identical ramps will flow at different
rates due to their different viscosity. The water seems to flow
freely while the oil flows much more slowly.
As another example, different types of similar liquids will
display different behaviors because of different viscosities.
Grease is very viscous. Given time, grease will flow, even
though the flow rate will be slow. Motor oil is less viscous
than grease and flows much more easily, but it is more viscous
and flows more slowly than gasoline.
All fluids are viscous and have a resistance to flow, whether
or not we observe this resistance. We cannot easily observe
the viscosity of air. However, since air is a fluid and has
viscosity properties, it resists flow around any object to
some extent.
Friction
Another factor at work when a fluid flows over or around
an object is called friction. Friction is the resistance that one
surface or object encounters when moving over another.
Friction exists between any two materials that contact each
other.
The effects of friction can be demonstrated using a similar
example as before. If identical fluids are poured down two
identical ramps, they flow in the same manner and at the
same speed. If the surface of one ramp is rough, and the other
smooth, the flow down the two ramps differs significantly.
The rough surface ramp impedes the flow of the fluid due
to resistance from the surface (friction). It is important to
remember that all surfaces, no matter how smooth they
appear, are not smooth on a microscopic level and impede
the flow of a fluid.
The surface of a wing, like any other surface, has a certain
roughness at the microscopic level. The surface roughness
causes resistance and slows the velocity of the air flowing
over the wing. [Figure 4-1]
Molecules of air pass over the surface of the wing and actually
adhere (stick, or cling) to the surface because of friction. Air
molecules near the surface of the wing resist motion and have
a relative velocity near zero. The roughness of the surface
impedes their motion. The layer of molecules that adhere to
the wing surface is referred to as the boundary layer.