MAXIMUM ALLOWABLE AIRSPEED
aircraft speed. The Mach number is the ratio of
INDICATOR. --Figure 6-4, view B, shows the
the speed of a moving body to the speed of sound
face of a maximum allowable airspeed indicator.
in the surrounding medium. For example, if an
The dial face measurements are in knots from 50
aircraft is flying at a speed equal to one-half the
to 450 with an expanded scale below 200 knots.
local speed of sound, it is flying at Mach 0.5. If
The dial has an indicating pointer and a maximum
it moves at twice the local speed of sound, its
safe airspeed pointer. The maximum safe airspeed
speed is Mach 2. (The term Mach number comes
from the name of an Austrian physicist, Ernest
pointer moves as the maximum safe airspeed
changes because of static pressure changes at
Mach, pioneer in the field of aerodynamics.)
Figure 6-5 shows the front view of a typical
different altitudes.
No matter where the pitot-static tube is
airspeed and Mach number indicator. The
located, it is impossible to keep it free from all
instrument consists of altitude and airspeed
air disturbances set up by the aircraft structure.
mechanisms incorporated in a single housing. This
You must make allowances for this installation
instrument gives the pilot a simplified presentation
error when reading the indicator. Temperature is
of both indicated airspeed and Mach number.
another cause of error. Also, imperfect scaling of
Both indications are read from the same pointer.
the indicator dial with respect to the air-speed-
The pointer shows airspeed at low speeds, and
differential pressure relationship will cause
both indicated airspeed and Mach number at high
an error in reading. You can make simple
speeds. Pitot pressure on a diaphragm moves the
adjustments to the instrument mechanism to
pointer, and an aneroid diaphragm controls the
correct the tendency to read fast or slow.
Mach number dial. The aneroid diaphragm reacts
to static pressure changes because of altitude
MACH NUMBER INDICATORS. --In some
changes. Figure 6-6 is a mechanical schematic of
cases, the term Mach number is used to express
an airspeed and Mach indicator.
Figure 6-6.-Airspeed/Mach number indicator mechanical schematic.
6-5
