Early airfoil
Later airfoil
Laminar flow airfoil
(Subsonic)
Circular arc airfoil
(Supersonic)
Double wedge airfoil
(Supersonic)
Clark 'Y' airfoil
(Subsonic)
Figure 4-6. Airfoil designs.
Camber of upper surface
Camber of lower surface
Trailing edge
Leading edge
Mean camber line
Chord line
Figure 4-5. Typical airfoil section.
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 flat 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 flat 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. They 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 4-6
shows some of the more common 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, 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.
Applying Bernoulli’s Principle of Pressure, the increase in
the speed of the air across the top of an airfoil produces a
Figure 2-8. Pressure distribution on an airfoil & CP changes
with an angle of attack.
High angle of attack
Normal angle of attack
Low angle of attack
attack
of
Angle
-8°
CP
attack
Angle of
+10°
CP
attack
of
Angle
+4°
CP
Figure 4-7. Pressure distribution on an airfoil and CP changes
with AOA.
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. However, 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
(AOA), 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 4-7 shows
the pressure distribution along an airfoil at three different
angles of attack. The average of the pressure variation for
any given AOA 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.
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