Page 39 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 26

26 CHAPTER TWO

Some claim that Gustave wing the uniform flow is slower than the free-stream velocity,
Whitehead, of Germany, flew on because the pressure is slightly increased.
Aug. 14, 1901, beating the Wright Look again at Figure 2.6, while paying attention to the black
brothers by over 2 years. arrows representing the forces on the wing. There are two points
to notice. The first is that most of the lift is on the forward part
of the wing. In fact, half of the total lift on a wing is typically produced
in the first one-fourth of the chord length. The second thing to notice is
that the arrows on the leading part of the wing are tilted forward. Thus
the force of lift is pulling the wing along as well as lifting it. This would
be nice if it were the entire story. Unfortunately, the horizontal forces on
the trailing part of the wing compensate the horizontal forces on the
leading part of the wing. Thus the net result is that the forces produce lift
only on the wing. We are ignoring skin friction for the moment.
This is a good time to look back at the popular description of lift. It
attributes lift to the acceleration of the air over the wing, causing a
reduction in pressure. As stated, the bending of the air causes the
reduction in pressure above the wing. This reduced pressure causes
the acceleration of the air via the Bernoulli effect. The
The acceleration of air over the
acceleration of air over the top of a wing is the result of the
top of a wing is the result of
lowered pressure and not the cause of the lowered pressure.
the lowered pressure and not the
The pressure difference across the wing is the cause of lift.
cause of the lowered pressure.
But the lowering of the pressure above the wing is the result
of the production of the downwash. As we will see, it is the
adjustment of this downwash in direction and magnitude which
allows the wing to adjust for varying loads and speeds.

Downwash

From Newton’s second law one can state the relationship between the
lift on a wing and its downwash.

The lift of a wing is proportional to the amount of air diverted per
time, times the vertical velocity of that air.

Similar to the rocket, one can increase the lift of a wing by increasing
the amount of air diverted, the vertical velocity of that air, or a
combination of the two. The concept of the vertical velocity of the