Page 41 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 28

28 CHAPTER TWO

Fig. 2.8. A helicopter pushes air down. (Photo courtesy of the U.S.Air Force.)

DOES THE EARTH SUPPORT THE AIRPLANE?
Some insist that since the airplane exerts a force on the earth,
in straight-and-level flight, the earth is somehow holding the
airplane up.This is definitely not the case.The lift on the wing
has nothing to do with the presence of the surface of the
earth. Examining two simple examples can show this.
The first example is to consider the thrust of a propeller,
which is just a rotating wing. It certainly does not develop its
thrust because of the presence of the surface of the earth.
Neither could the presence of the earth provide the
horizontal component of lift in a steep bank.

The second example to consider is the flight of the Concorde.
It cruises at Mach 2, 55,000 ft (16,000 m) above the earth.The
pressure information of the jet cannot be communicated to
the earth and back faster than the speed of sound. By the time
the earth knows the Concorde is there, it is long gone.