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). The air molecules moving over the upper surface would be
forced to move faster than would the molecules moving along the bottom
of the airfoil, since the upper molecules must travel a greater
distance due to the curvature of the upper surface. This increased
velocity reduces the pressure above the airfoil.
Bernoulli's
principle of pressure by itself does not explain the distribution of
pressure over the upper surface of the airfoil. A discussion of the
influence of momentum of the air as it flows in various curved paths
near the airfoil will be presented. Momentum is the resistance a moving
body offers to have its direction or amount of motion changed. When a
body is forced to move in a circular path, it offers resistance in the
direction away from the center of the curved path. This is "centrifugal
force." While the particles of air move in the curved path AB,
centrifugal force tends to throw them in the direction of the arrows
between A and B and hence, causes the air to exert more than normal
pressure on the leading edge of the airfoil. But after the air particles
pass B (the point of reversal of the curvature of the path) the
centrifugal force tends to throw them in the direction of the arrows
between B and C (causing reduced pressure on the airfoil). This effect
is held until the particles reach C, the second point of reversal of
curvature of the airflow. Again the centrifugal force is reversed and
the particles may even tend to give slightly more than normal pressure
on the trailing edge of the airfoil, as indicated by the short arrows
between C and D.
Therefore,
the air pressure on the upper surface of the airfoil is distributed so
that the pressure is much greater on the leading edge than the
surrounding atmospheric pressure. This causing strong resistance to
forward motion; but the air pressure is less than surrounding
atmospheric pressure over a large portion of the top surface (B to C).
As
seen in the application of Bernoulli's theorem to a venturi, the
speed-up of air on the top of an airfoil produces a drop in pressure.
This lowered pressure is a component of total lift. It is a mistake,
however, to assume that the pressure difference between the upper and
lower surface of a wing alone accounts for the total lift force
produced.
One
must also bear in mind that associated with the lowered pressure is
downwash; a downward backward flow from the top surface of the wing. As
already seen from previous discussions relative to the dynamic action
of the air as it strikes the lower surface of the wing, the reaction of
this downward backward flow results in an upward forward force on the
wing. This same reaction applies to the flow of air over the top of the
airfoil as well as to the bottom, and Newton's third law is again in
the picture.
HIGH PRESSURE BELOW
In
the section dealing with Newton's laws as they apply to lift, it has
already been discussed how a certain amount of lift is generated by
pressure conditions underneath the wing. Because of the manner in which
air flows underneath the wing, 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 has
again reached a velocity equal to that on the upper surface. In
conformance with Bernoulli's principles, where the airflow was slowed
beneath the wing, a positive upward pressure was created against the
wing; i.e., as the fluid speed decreases, the pressure must increase.
In essence, this simply "accentuates the positive" since it increases
the pressure differential between the upper and lower surface of the
airfoil, and therefore increases total lift over that which would have
resulted had there been no increase of pressure at the lower surface.
Both Bernoulli's principle and Newton's laws are in operation whenever
lift is being generated by an airfoil.
Fluid
flow or airflow then, is the basis for flight in airplanes, and is a
product of the velocity of the airplane. The velocity of the airplane
is very important to the pilot since it affects the lift and drag
forces of the airplane. The pilot uses the velocity (airspeed) to fly
at a minimum glide angle, at maximum endurance, and for a number of
other flight maneuvers. Airspeed is the velocity of the airplane
relative to the air mass through which it is flying.
