Page 37 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 24

24 CHAPTER TWO

NASA had plans to fly an Lift on a Wing
autonomous airplane on Mars
We now have the tools to understand why a wing has lift. In
to coincide with the 100th
brief, the air bends around the wing because of the Coanda
anniversary of the Wright
effect. Newton’s first law says that the bending of the air
brothers’ first flight.
requires a force on the air, and Newton’s third law says that
there is an equal and opposite force on the wing. All this is
true. But there is a little more to it than that. First, let us look at the air
bending around the wing in Figure 2.6. To bend the air requires a
force. As indicated by the gray arrows, the direction of the force on the
air is perpendicular to the bend in the air. The magnitude of the force
is proportional to the “tightness” of the bend. The tighter the air bends
the greater the force on it. The forces on the wing, as shown by the
black arrows in the figure, have the same magnitude as the forces on
the air but in the opposite direction. When the air bends around the
surface of the wing it tries to separate from the airflow above it. But
since it has a strong reluctance to form voids, the attempt to separate
lowers the pressure and bends the adjacent streamlines above. The
lowering of the pressure propagates out at the speed of sound, caus-
ing a great deal of air to bend around the wing. This is the source of
the lowered pressure above the wing and the production of the down-
wash behind the wing. Figure 2.7 is a good example of the effect of
downwash behind an airplane. In the picture the jet has flown above

Force on wing Force on air
Fig. 2.6. Forces on the air and the corresponding reaction forces on the airfoil.