Page 22 - Understanding Flight

CH01_Anderson PFS2 7/25/01 8:55 AM Page 9

Basic Concepts 9

ceases to separate before the wing arrives and instead collides ener-
getically with the wing.
The speed of sound is not a constant in air. In particular The Soviet TU-144 supersonic
it changes with air temperature and thus with altitude. As transport crashed at the Paris
the air temperature decreases with altitude, so does the Airshow in 1973. It was later
speed of sound, though not as quickly. At sea level the value found that the pilot was trying to
of Mach 1 is about 760 mi/h (1220 km/h). The speed of avoid a French Mirage fighter that
sound decreases with altitude to about 35,000 ft (about was trying to take unauthorized
11,000 m), where the value is about 660 mi/h (1060 km/h). pictures of the TU-144 in flight.
The speed of sound then remains essentially constant to an The TU-144 was planned to
altitude of 80,000 ft (24,000 m). No airplanes fly above this compete with the Concorde.
altitude, with the exception of the Space Shuttle on its way
back from space.

Kinetic Energy

Kinetic energy is the energy of an object because it is moving. The
energy difference between a bullet sitting on a table and one flying
through the air is the kinetic energy. To be technical, if the bullet had a
mass m (say in grams, for example) and were moving at a velocity v,
2
1
its kinetic energy would be ⁄2mv . (There, we have reached the high-
est level of math complexity that one needs in order to understand
flight.)
Because we are going to be discussing the movement of air and the
production of propulsion by the acceleration of air or exhaust, it is
important that when we say kinetic energy it be understood that we
mean the energy due to motion. It is that simple.

Air Pressures

Before we go into understanding flight and airplanes, we should
spend just a little time discussing air pressure to remove some com-
mon misunderstandings. The following discussion is only strictly true
for air moving at speeds below about Mach 0.3 (three-tenths the speed
of sound) where the air can be considered incompressible. This will
be discussed in greater detail in the next chapter.