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at a cylinder rotating in an airstream. The local
velocity near the cylinder is composed of the airstream
velocity and the cylinder’s rotational velocity, which
decreases with distance from the cylinder. On a cylinder,
which is rotating in such a way that the top surface
area is rotating in the same direction as the airflow, the
local velocity at the surface is high on top and low on
the bottom.
As shown in Figure 3-4, at point “A,” a stagnation
point exists where the airstream line meets on the surface
and then splits; some air goes over and some
under. Another stagnation point exists at “B,” where
the two air streams rejoin and resume at identical
velocities. We now have upwash ahead of the rotating
cylinder and downwash at the rear.
Figure 3-2. The forces that act on a glider in flight.
Figure 3-3. The lift equation is mathematically
expressed by the above formula.
3-3
The difference in surface velocity accounts for a difference
in pressure, with the pressure being lower on
the top than the bottom. This low pressure area produces
an upward force known as the “Magnus Effect.”
This mechanically induced circulation illustrates the
relationship between circulation and lift.
BERNOULLI’S PRINCIPLE
An airfoil with a positive angle of attack develops air
circulation as its sharp trailing edge forces the rear
stagnation point to be aft of the trailing edge, while the
front stagnation point is below the leading edge.
[Figure 3-5]
Air flowing over the top surface accelerates. The airfoil
is now subjected to Bernoulli’s Principle, or the
“venturi effect.” As air velocity increases through the
constricted portion of a venturi tube, the pressure
decreases. Compare the upper surface of an airfoil
with the constriction in a venturi tube that is narrower
in the middle than at the ends. [Figure 3-6]
The upper half of the venturi tube can be replaced by
layers of undisturbed air. Thus, as air flows over the
upper surface of an airfoil, the camber of the airfoil
causes an increase in the speed of the airflow. The
increased speed of airflow results in a decrease in pressure
on the upper surface of the airfoil. At the same
time, air flows along the lower surface of the airfoil,
building up pressure. The combination of decreased
pressure on the upper surface and increased pressure
on the lower surface results in an upward force.
[Figure 3-7]
As angle of attack is increased, the production of lift is
increased. More upwash is created ahead of the airfoil
as the leading edge stagnation point moves under the
leading edge, and more downwash is created aft of the
trailing edge. Total lift now being produced is perpendicular
to relative wind. In summary, the production of
lift is based upon the airfoil creating circulation in the
airstream (Magnus Effect) and creating differential
pressure on the airfoil (Bernoulli’s Principle).
NEWTON’S THIRD LAW OF MOTION
According to Newton’s Third Law of Motion, “for
every action there is an equal and opposite reaction.”
Thus, the air that is deflected downward also produces
an upward (lifting) reaction. The wing’s construction is
designed to take advantage of certain physical laws that
generate two actions from the airmass. One is a positive
pressure lifting action from the airmass below the
Figure 3-4. Magnus Effect.
Figure 3-5. Stagnation points on an airfoil.
Figure 3-6. The upper surface of an airfoil is similar
to the constriction in a venturi tube.
3-4
wing, and the other is a negative pressure lifting action
from the lowered pressure above the wing.
As the airstream strikes the relatively flat lower surface
of the wing 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, airstream striking the upper curve section of the
leading edge of the wing is deflected upward, over the
top of the wing. The speed up of air on the top of the
wing produces a sharp drop in pressure. Associated
with the lowered pressure is downwash, a downwardbackward
flow. In other words, a wing shaped to cause
an action on the air, and forcing it downward, will provide
an equal reaction from the air, forcing the wing
upward. If a wing is constructed in such form that it
will cause a lift force greater than the weight of the
glider, the glider will fly.
If all the required lift was obtained from the deflection of
air by the lower surface of the wing, a glider would need
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Glider Flying Handbook(24)
