Descripción De Vuelo

This material can be found in more detail in "Understanding Flight", by
David Anderson and Scott Eberhardt, McGraw-Hill, 2001, ISBN: 0-07136377-7
This article was written for "Fliegermagazin" (Flyer's Magazine) published in
Munich Germany

A Physical Description of Flight ©
David Anderson
Fermi National Accelerator Laboratory
Ret.
dfa180@aol.com

&
Scott Eberhardt
Dept. ofAeronautics and Astronautics
University of Washington
Seattle WA 91895-2400
scott@aa.washington.edu

Almost everyone today has flown in an airplane. Many ask the simple question "what makes an
airplane fly?" The answer one frequently gets is misleading and often just plain wrong. We hope
that the answers provided here will clarify many misconceptions about lift and that you will
adopt ourexplanation when explaining lift to others. We are going to show you that lift is easier
to understand if one starts with Newton’ laws rather than the Bernoulli principle. We will also
s
show you that the popular explanation that most of us were taught is misleading at best and that
lift is due to the wing diverting air down. Most of this diverted air is pulled down from above the
wing.
Let us startby defining three descriptions of lift commonly used in textbooks and training
manuals. The first we will call the Mathematical Aerodynamics Description of lift, which is used
by aeronautical engineers. This description uses complex mathematics and/or computer
simulations to calculate the lift of a wing. It often uses a mathematical concept called
"circulation" to calculate the acceleration ofthe air over the wing. Circulation is a measure of the
apparent rotation of the air around the wing. While useful for calculations of lift, this description
does not lend themselves to an intuitive understanding of flight.

The second description we will call the Popular Description, which is based on the Bernoulli
principle. The primary advantage of this description is that it is easy tounderstand and has been
taught for many years. Because of its simplicity, it is used to describe lift in most flight training
manuals. The major disadvantage is that it relies on the "principle of equal transit times", or at
least on the assumption that because the air must travel farther over the top of the wing it must
go faster. This description focuses on the shape of the wing and preventsone from understanding
such important phenomena as inverted flight, power, ground effect, and the dependence of lift on
the angle of attack of the wing.
The third description, which we are advocating here, we will call the Physical Description of lift.
This description of lift is based primarily on Newton's three laws and a phenomenon called the
Coanda effect. This description is uniquelyuseful for understanding the phenomena associated
with flight. It is useful for an accurate understanding 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 estimates ("back-of-the-envelope calculations") of lift. The Physical
Description of lift is also of great use to a pilot who needs anintuitive understanding of how to
fly the airplane.

The popular description of lift
Students of physics and aerodynamics are taught that an airplane flies as a result of the Bernoulli
principle, which says that if air speeds up the pressure is lowered. (In fact this is not always true.
The air flows fast over the airplane’ static port but the altimeter still reads the correct altitude.)
sThe argument goes that a wing has lift because the air goes faster over the top creating a region
of low pressure. This explanation usually satisfies the curious and few challenge the conclusions.
Some may wonder why the air goes faster over the top of the wing and this is where the popular
explanation of lift falls apart.
In order to explain why the air goes faster over the top of the wing,...