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Introduction to Glider Flying > Aerodynamics of Flight > Forces of Flight > Lift > 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 hat 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 posi-tive pressure lifting
action from the airmass below thewing, 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 direc-tion of otion, 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 downward-backward flow. In other words, a wing shaped to cause
an action on the air, and forcing it downward, will pro-vide
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 only a flat wing like a kite. This, of course, is
not the case at all. The balance of the lift needed to support
the glider comes from the flow of air above the wing. Herein
lies the key to flight. The fact that the most lift is the result
of the airflow downwash from above the wing, forcing the wing
upward, must be thoroughly understood in order to continue further
in the study of flight.
It is neither accurate nor does it serve a
useful purpose, however, to assign specific values to the percentage
of lift generated by the upper surface of the airfoil versus
that generated by the lower surface. These are not con-stant
values, and will vary, not only with flight condi-tions, but
also with different wing designs.
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