Page 31 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 18

18 CHAPTER TWO

discuss why the air goes more slowly below the wing and faster over
the top. While useful for calculations of lift, circulation is not something
a pilot needs to consider while flying an airplane. The concept of
circulation is useful, however, for the understanding of upwash and of
ground effect, and is discussed in greater detail later in this chapter.
Like the popular description of lift, the classical theories in the
mathematical description of lift do not illustrate the fact that lift
requires the action of a great deal of air, much more than you might
imagine. We will not spend much time on the mathematical
description of lift, though it will come up from time to time.
The mathematical description of lift is a general term for the
analysis tools of classical aerodynamics and computational
The Wright brothers knew
aerodynamics. If the objective is to accurately compute the
nothing of Bernoulli’s principle or
aerodynamics of a wing, these are the tools to use, though the
any mathematical aerodynamic
description is mathematical and not physical. This is a point
theories. They just watched how
lost on many of its proponents. Fortunately, the physical
birds fly.
description of lift, presented here, does not require
complicated mathematics.

The Physical Description of Lift

The physical description of lift is based primarily on Newton’s three
laws and a phenomenon called the Coanda effect. This description is
uniquely useful for understanding the phenomena associated with
flight. It is useful for an accurate understanding of the relationships in
flight, such as how power increases with load or how the stall speed
increases with altitude. It is also a useful tool for making rough esti-
mates (“back-of-the-envelope calculations”) of lift. The physical
description of lift is also of great use to a pilot who needs an intuitive
understanding of how to fly the airplane.
In this description, lift is recognized as a reaction force, that is,
wings develop lift by diverting air down. One knows that a propeller
produces thrust by blowing air back and that a helicopter develops lift
by blowing air down. Propellers and helicopter rotors are simply
rotating wings. Thus the concept of a wing diverting air down to
produce lift should not be difficult to accept. As we will see, the low