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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




Airfoil Behavior
Although specific examples can be cited in which each of
the principles predict and contribute to the formation of lift,
lift is a complex subject. The production of lift is much more
complex than a simple differential pressure between upper
and lower airfoil surfaces. In fact, many lifting airfoils do
not have an upper surface longer than the bottom, as in the
case of symmetrical airfoils. These are seen in high-speed
aircraft having symmetrical wings, or on symmetrical rotor
blades for many helicopters whose upper and lower surfaces
are identical. In both examples, the relationship of the airfoil
with the oncoming airstream (angle) is all that is different. A
paper airplane, which is simply a flat plate, has a bottom and
top exactly the same shape and length. Yet these airfoils do
produce lift, and "flow turning" is partly (or fully) responsible
for creating lift.

As an airfoil moves through air, the airfoil is inclined
against the airflow, producing a different .ow caused by the
airfoil's relationship to the oncoming air. Think of a hand
being placed outside the car window at a high speed. If the
hand is inclined in one direction or another, the hand will
move upward or downward. This is caused by deflection,
which in turn causes the air to turn about the object within
the air stream. As a result of this change, the velocity about
the object changes in both magnitude and direction, in turn
resulting in a measurable velocity force and direction.

A Third Dimension

To this point the discussion has centered on the .ow across
the upper and lower surfaces of an airfoil. While most of the
lift is produced by these two dimensions, a third dimension,
the tip of the airfoil also has an aerodynamic effect. The high pressure area on the bottom of an airfoil pushes around the tip
to the low-pressure area on the top. [Figure 3-9] This action
creates a rotating flow called a tip vortex. The vortex flows
behind the airfoil creating a downwash that extends back to the
trailing edge of the airfoil. This downwash results in an overall
reduction in lift for the affected portion of the airfoil.

Tip vortex
Figure 3-9. Tip vortex.

Manufacturers have developed different methods to
counteract this action. Winglets can be added to the tip of
an airfoil to reduce this flow. The winglets act as a dam
preventing the vortex from forming. Winglets can be on the
top or bottom of the airfoil. Another method of countering
the flow is to taper the airfoil tip, reducing the pressure
differential and smoothing the airflow around the tip.

Chapter Summary

Modern general aviation aircraft have what may be considered
high performance characteristics. Therefore, it is increasingly
necessary that pilots appreciate and understand the principles
upon which the art of flying is based. For additional
information on the principles discussed in this chapter, visit
the National Aeronautics and Space Administration (NASA)
Beginner's Guide to Aerodynamics at http://www.grc.nasa.