Page 35 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 22

22 CHAPTER TWO

On May 9, 1926, Commander R.E. means that the volume of a mass of air remains constant and
Byrd made the first flight over that flows of air do not separate from each other to form voids
the North Pole. On Nov. 28–29, (gaps). For the moment let us consider the Coanda effect with
1929, Byrd made the first flight water. This effect can be demonstrated in a simple way. Run a
over the South Pole. small stream of water from a faucet and bring a horizontal
water glass over to it until it just touches the water, as in Fig-
ure 2.4. As in the figure, the water will wrap partway around the glass.
From Newton’s first law we know that for the flow of water to bend
there must be a force on it. The force is in the direction of the bend.
From Newton’s third law we know that there must be an equal and
opposite force acting on the glass. The same phenomenon causes
forces between the airflow around a wing and the wing. So why do
fluids tend to bend around a solid object?
The answer is viscosity, that characteristic that makes a fluid thick
and makes it stick to a surface. When a moving fluid comes into
contact with a solid object, some of it sticks to the surface. A small
distance from the surface the fluid has a small velocity with respect
to the object. As in Figure 2.5, the farther one looks from the surface,
the faster the fluid is flowing, until it eventually comes to the speed
of the uniform flow some distance away from the object. The
transition layer between the surface and the fluid at the uniform flow
is called the boundary layer, which is discussed in the next chapter.

Force on glass

Force on water

Fig. 2.4. The Coanda effect.