Page 106 - Planning and Design of Airports

76 Airp o r t Pl anning

the stall speed, the speed at which there is not enough airflow over
the wings to sustain lift, which is dependent on speed and air density.
At high altitudes an aircraft will stall at a higher speed than it does at
sea level. At higher altitudes, however, the airspeed indicator is indi-
cating speeds lower than true speeds; consequently this is on the safe
side and no corrections are necessary. Thus, an aircraft with a stalling
speed of 90 kn will stall at the same indicated airspeed regardless of
altitude. This is why aircraft manufacturers always report stalling
speeds in terms of indicated airspeed rather than true airspeed. With
the introduction of jet transports and high speed military aircraft, the
reference datum for speed is often the speed of sound. The speed of
sound is defined as Mach 1 (after Ernst Mach, Austrian scientist).
Thus Mach 3 means three times the speed of sound. Most of our cur-
rent jet transports are subsonic (slower than the speed of sound) and
cruise at a speed in the neighborhood of 0.8 to 0.9 Mach. Many mili-
tary aircraft are supersonic (faster than the speed of sound). Again the
reader is reminded that when the maximum speed of an aircraft is
quoted as 0.9 Mach, this is in terms of true airspeed and not ground-
speed. Such an aircraft can conceivably be traveling at a groundspeed
higher than the speed of sound, depending on the magnitude of the
tailwind.
The speed of sound is not a fixed speed; it depends on temperature
and not on atmospheric pressure. As the temperature decreases, so
does the speed of sound. The speed of sound at 32°F (0°C) is 742 mi/h
(1090 ft/s), at −13°F (−25°C) it is 707 mi/h, and at 86°F (30°C) it is
785 mi/h. In fact, the speed of sound varies 2 ft/s for every change in
temperature of 1°C above or below the speed at 0°C. The speed of
sound at the altitudes at which jets normally fly is less than 700 mi/h,
but at altitudes at which small aircraft normally fly (20,000 ft or less) it
is greater than 700 mi/h.
The speed of sound may be computed from the formula

V = 33.4T 0.5 (2-4)
sm
V = 49.04T 0.5 (2-5)
sf
where V = speed of sound in miles per hour at some temperature
sm
V = speed of sound in feet per second at some temperature
sf
T = temperature in degrees Rankine
For convenience in navigation, aircraft distances and speeds are
measured in nautical miles and knots, just like measurement on the
high seas. One nautical mile (6080 ft) is practically equal to 1 min of
arc of the earth’s circumference. One knot is defined as 1 nmi/h. One
nautical mile is approximately 1.15 land miles.
The performances of aircraft are, in part, defined by the various
speeds at which they can safely liftoff, cruise, maneuver, and approach