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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 Design

An airfoil is a structure designed to obtain reaction upon its
surface from the air through which it moves or that moves
past such a structure. Air acts in various ways when submitted
to different pressures and velocities; but this discussion
is confined to the parts of an aircraft that a pilot is most
concerned with in flight—namely, the airfoils designed to
produce lift. By looking at a typical airfoil profile, such as
the cross section of a wing, one can see several obvious
characteristics of design. [Figure 3-6] Notice that there is
a difference in the curvatures (called cambers) of the upper
and lower surfaces of the airfoil. The camber of the upper
surface is more pronounced than that of the lower surface,
which is usually somewhat flat.

NOTE: The two extremities of the airfoil profile also differ in
appearance. The end, which faces forward in flight, is called the leading edge, and is rounded; the other end, the trailing
edge, is quite narrow and tapered.

Typical airfoil section.
Figure 3-6. Typical airfoil section.

A reference line often used in discussing the airfoil is the chord
line, a straight line drawn through the profile connecting the
extremities of the leading and trailing edges. The distance
from this chord line to the upper and lower surfaces of the
wing denotes the magnitude of the upper and lower camber at
any point. Another reference line, drawn from the leading edge
to the trailing edge, is the mean camber line. This mean line is
equidistant at all points from the upper and lower surfaces.

An airfoil is constructed in such a way that its shape takes
advantage of the air's response to certain physical laws.
This develops two actions from the air mass: a positive
pressure lifting action from the air mass below the wing,
and a negative pressure lifting action from lowered pressure
above the wing.

As the air stream strikes the relatively .at lower surface of
a wing or rotor blade when inclined at a small angle to its
direction of motion, the air is forced to rebound downward,
causing an upward reaction in positive lift. At the same time,
the air stream striking the upper curved section of the leading
edge is deflected upward. An airfoil is shaped to cause an
action on the air, and forces air downward, which provides
an equal reaction from the air, forcing the airfoil upward. If
a wing is constructed in such form that it causes a lift force
greater than the weight of the aircraft, the aircraft will fly.

If all the lift required were obtained merely from the
deflection of air by the lower surface of the wing, an aircraft
would only need a .at wing like a kite. However, the balance
of the lift needed to support the aircraft comes from the flow
of air above the wing. Herein lies the key to flight.

It is neither accurate nor useful to assign specific values to
the percentage of lift generated by the upper surface of an
airfoil versus that generated by the lower surface. These are
not constant values and vary, not only with flight conditions,
but also with different wing designs.

Different airfoils have different flight characteristics. Many
thousands of airfoils have been tested in wind tunnels and in
actual flight, but no one airfoil has been found that satisfies
every flight requirement. The weight, speed, and purpose
of each aircraft dictate the shape of its airfoil. The most
efficient airfoil for producing the greatest lift is one that has
a concave, or "scooped out" lower surface. As a fixed design,
this type of airfoil sacrifices too much speed while producing
lift and is not suitable for high-speed flight. Advancements
in engineering have made it possible for today's high-speed
jets to take advantage of the concave airfoil's high lift
characteristics. Leading edge (Kreuger) flaps and trailing
edge (Fowler) flaps, when extended from the basic wing
structure, literally change the airfoil shape into the classic
concave form, thereby generating much greater lift during
slow flight conditions.

On the other hand, an airfoil that is perfectly streamlined
and offers little wind resistance sometimes does not have
enough lifting power to take the airplane off the ground.
Thus, modern airplanes have airfoils that strike a medium
between extremes in design. The shape varies according to
the needs of the airplane for which it is designed. Figure 3-7
shows some of the more common airfoil sections.

Airfoil designs.
Figure 3-7. Airfoil designs.

Low Pressure Above
In a wind tunnel or in flight, an airfoil is simply a streamlined
object inserted into a moving stream of air. If the airfoil profile
were in the shape of a teardrop, the speed and the pressure
changes of the air passing over the top and bottom would be
the same on both sides. But if the teardrop shaped airfoil were
cut in half lengthwise, a form resembling the basic airfoil (wing) section would result. If the airfoil were then inclined so
the airflow strikes it at an angle (angle of attack (AOA), the
air moving over the upper surface would be forced to move
faster than the air moving along the bottom of the airfoil. This
increased velocity reduces the pressure above the airfoil.