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Pilot's Handbook of Aeronautical Knowledge
Principles of Flight
Theories in the Production of Lift

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Air circulation around an airfoil
Figure 3-4. Air circulation around an airfoil occurs when the front stagnation point is below the leading edge and the aft stagnation point is beyond the trailing edge.

This low-pressure area produces an upward force known as
the Magnus Effect, the physical phenomenon whereby an
object's rotation affects its path through a fluid, to include
air. Two early aerodynamicists, Martin Kutta and Nicolai
Joukowski, eventually measured and calculated the forces for
the lift equation on a rotating cylinder (the Kutta-Joukowski

To summarize the Magnus effect, an airfoil with a positive
angle of attack (AOA) develops air circulation about the upper
surface of the wing. Its sharp trailing edge forces the rear
stagnation point to be aft of the trailing edge, while the front
stagnation point falls below the leading edge. [Figure 3-4]

Bernoulli's Principle of Differential Pressure

A half-century after Newton formulated his laws, Daniel
Bernoulli, a Swiss mathematician, explained how the pressure
of a moving fluid (liquid or gas) varies with its speed of
motion. Bernoulli's Principle states that as the velocity of a
moving fluid (liquid or gas) increases, the pressure within
the fluid decreases. This principle explains what happens to
air passing over the curved top of the airplane wing.

A practical application of Bernoulli's Principle is the venturi
tube. The venturi tube has an air inlet that narrows to a throat
(constricted point) and an outlet section that increases in
diameter toward the rear. The diameter of the outlet is the
same as that of the inlet. At the throat, the airflow speeds up
and the pressure decreases; at the outlet, the airflow slows
and the pressure increases. [Figure 3-5]

Since air is recognized as a body and it is accepted that it must
follow the above laws, one can begin to see how and why an
airplane wing develops lift. As the wing moves through the
air, the .ow of air across the curved top surface increases in
velocity creating a low-pressure area.

Although Newton, Magnus, Bernoulli, and hundreds of other
early scientists who studied the physical laws of the universe
did not have the sophisticated laboratories available today,
they provided great insight to the contemporary viewpoint
of how lift is created.

Air pressure decreases in a venturi tube.
Figure 3-5. Air pressure decreases in a venturi tube.