As
an airplane approaches the speed of sound, conditions around it begin to change.
The
air ahead of it starts to compress. Shock waves form on its wings and drag
increases dramatically.
Shock wave
formation in front of the Bell X-1, the first airplane to fly faster than
sound; the Concorde, a supersonic airliner; and the Space Shuttle during reentry into the atmosphere.
Credit:
National Air and Space Museum, Smithsonian Institution
A
moving airplane causes a disturbance in the air—a wave of pressure—similar to a
sound wave. Just like sound waves, any object in motion, such as an airplane,
causes a chain reaction of colliding air molecules to spread outward in all
directions at the speed of sound. Keep in mind that it is the wave that
travels; the air simply moves back and forth. This wave of molecular collisions
is called a pressure wave.
Sound
consists of waves transmitted through the air (or another substance) by
molecules bumping into each other. When these sound waves reach your ear, they
cause your ear drums to vibrate. Your brain “decodes” the vibrations into
voices, music, and noises.
The
shock waves created by an airplane flying faster than sound are nearly cone
shaped and extend outward until they dissipate. However, if the airplane is
flying low enough so the shock wave reaches the ground, anyone in the shock
wave's path will experience a sonic boom. The sound is caused by a sudden,
momentary change in air pressure that the ear registers as a
loud bang.
We
use Mach numbers to describe an airplane’s speed in terms of the speed of sound.
A
Mach number is derived by comparing the speed of an airplane with the speed of
sound in the air it’s moving through. An airplane moving at Mach 1 is traveling
at the speed of sound. The Mach number is named in honor of
Ernst Mach, a late 19th century physicist who studied gas dynamics.
● Subsonic: Usually
less than Mach 0.8. Air is flowing slower than sound over every part of the
airplane.
● Transonic: About Mach
0.8 to Mach 1.2. Air is flowing faster than sound over some parts of the
airplane.
● Supersonic: Greater
than about Mach 1.2. Air is flowing faster than sound over the entire airplane.
● Hypersonic: Greater
than about Mach 5. Heat becomes a critical factor.