Airfoil Nomenclature
What is an Airfoil?
An airfoil is the cross-sectional shape of a
wing, blade, or similar surface designed to
produce an aerodynamic force, such as lift,
when it moves through a uid like air or
water. Airfoils work by creating di erent
air pressures on their upper and lower surfaces, which, according to Bernoulli's Principle, causes air to accelerate over the
curved top surface and decelerate underneath, resulting in a net upward force of
lift. Examples of airfoils include airplane
wings, helicopter rotor blades, and propeller blades.
The rst patented airfoil shapes were developed by Horatio F. Phillips in 1884.
Phillips was an Englishman who carried
out the rst serious wind-tunnel experiments on airfoils. In 1902, the Wright
brothers conducted their own airfoil tests
in a wind tunnel, developing relatively e cient shapes which contributed to their
successful rst ight on December 17, 1903
Clearly, in the early days of powered ight,
airfoil design was basically customized and
personalized. However, in the early 1930s,
NACA—the forerunner of NASA—embarked on a series of de nitive airfoil ex-
periments using airfoil shapes that were
constructed rationally and systematically.
Many of these NACA airfoils are in common use today. Therefore, in this chapter
we follow the nomenclature established by
the NACA; such nomenclature is now a
well-known standard.
Parts of an Airfoil
Consider the airfoil sketched in Figure 4.8.
The mean camber line is the locus of points
halfway between the upper and lower surfaces as measured perpendicular to the
mean camber line itself. The most forwards
and rearwards points of this line are called
the leading and trailing edges respectively.
The straight line connecting the leading
and trailing edges is the chord line of the
airfoil, and the precise distance from the
leading to the trailing edge measured along
the chord line is simply designated the
chord c of the airfoil.
The camber is the maximum distance between the mean camber line and the
chord line, measured perpendicular to the
chord line.
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fl
ff
fl
fi
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Fig 1 Airfoil Nomenclature
The thickness is the distance between the
upper and lower surfaces, also measured
perpendicular to the chord line. The shape
of the airfoil at the leading edge is usually
circular, with a leading-edge radius of approximately 0.02c. The shapes of all standard
The net e ect of the p and τ distributions
integrated over the complete body surface
is a resultant aerodynamic force R and
moment M on the body, as sketched
in Figure 3. In turn, the resultant R can be
split into components, two sets of which
NACA airfoils are generated by specifying
the shape of the mean camber line and
then wrapping a speci ed symmetrical
thickness distribution around the mean
camber line.
Aerodynamic Forces and
Moments on an Airfoil
No matter how complex the body shape
may be, the aerodynamic forces and moments on the body are due entirely to the
above two basic sources. The only mechanisms nature has for communicating a
force to a body moving through a uid are
pressure and shear stress distributions on
the body surface. Both pressure p and
shear stress τ have dimensions of force per
unit area (pounds per square foot or newtons per square meter)
Fig 3 Resultant aerodynamic force and
moment on the body
are shown in Figure 4.
In Figure 4, V∞ is the relative wind, de ned
as the ow velocity far ahead of the body.
The ow far away from the body is
called the freestream, and hence V∞ is also
called the freestream velocity.
As sketched in Figure 2, p acts normal to
the surface, and τ acts tangential to the
surface. Shear stress is due to the “tugging
action” on the surface, which is caused by
friction between the body and the air.
Fig. 4 Resultant aerodynamic force and the
components into which it splits
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fi
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fl
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Fig. 2 Illustration of pressure and Shear on
an Airfoil
In Figure 4, by de nition,
L ≡ lift ≡ component of R perpendicular to
V∞
D ≡ drag ≡ component of R parallel to V∞
N ≡ normal force ≡ component of R perpendicular to c
A ≡ axial force ≡ component of R parallel
to c
The angle of attack α is de ned as the angle between c and V∞. Hence, α is
also the angle between L and N and between D and A. The geometrical relation
between these two sets of components is,
from Figure 4.
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L= N cos α− A sin α
D= N sin α + A cos α