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Pilot's Handbook of Aeronautical Knowledge
Principles of Flight
Airfoil Design

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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




Applying Bernoulli's Principle of Pressure, the increase in
the speed of the air across the top of an airfoil produces a
drop in pressure. This lowered pressure is a component of
total lift. The pressure difference between the upper and
lower surface of a wing alone does not account for the total
lift force produced.

The downward backward flow from the top surface of an
airfoil creates a downwash. This downwash meets the flow
from the bottom of the airfoil at the trailing edge. Applying
Newton's third law, the reaction of this downward backward
flow results in an upward forward force on the airfoil.

High Pressure Below
A certain amount of lift is generated by pressure conditions
underneath the airfoil. Because of the manner in which air
flows underneath the airfoil, a positive pressure results,
particularly at higher angles of attack. But there is another
aspect to this airflow that must be considered. At a point
close to the leading edge, the airflow is virtually stopped
(stagnation point) and then gradually increases speed. At
some point near the trailing edge, it again reaches a velocity
equal to that on the upper surface. In conformance with
Bernoulli's principle, where the airflow was slowed beneath
the airfoil, a positive upward pressure was created i.e., as the
fluid speed decreases, the pressure must increase. Since the
pressure differential between the upper and lower surface
of the airfoil increases, total lift increases. Both Bernoulli's
Principle and Newton's Laws are in operation whenever lift
is being generated by an airfoil.

Pressure Distribution
From experiments conducted on wind tunnel models and on
full size airplanes, it has been determined that as air flows
along the surface of a wing at different angles of attack, there
are regions along the surface where the pressure is negative,
or less than atmospheric, and regions where the pressure is
positive, or greater than atmospheric. This negative pressure
on the upper surface creates a relatively larger force on the
wing than is caused by the positive pressure resulting from
the air striking the lower wing surface. Figure 3-8 shows the
pressure distribution along an airfoil at three different angles
of attack. The average of the pressure variation for any given
angle of attack is referred to as the center of pressure (CP).
Aerodynamic force acts through this CP. At high angles of
attack, the CP moves forward, while at low angles of attack
the CP moves aft. In the design of wing structures, this CP travel is very important, since it affects the position of the air loads imposed on the wing structure in both low and high AOA conditions. An airplane's aerodynamic balance and controllability are governed by changes in the CP.

Pressure distribution
Figure 3-8. Pressure distribution on an airfoil and CP changes
with AOA.