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Aerodynamic Testing 223

Fig. 8.22. Airspeed calibration probe on D-558-II. (Photo courtesy of NASA.)

THE STANDARD DAY

The pressure variation with altitude is not exactly the same
every day, so a “standard day” has been defined and used
internationally. A standard day is 59°F and 29.92 inches of
mercury (15°C and 1013 millibars) at sea level.The pilot adjusts
the altimeter for the true barometric pressure so that correct
altitude is indicated. When calling in for permission to land at
an airport, among the first things given to the pilot are the
barometric pressure and the wind’s speed and direction. Below
18,000 feet all altimeters are adjusted to local atmospheric
conditions so that two airplanes in the same airspace will both
read correctly. Above 18,000 feet, all altimeters are set for the
barometric pressure of a standard day.

As discussed in the first chapter, the airspeed indicator works The German aeronautical pioneer
off the difference between the static pressure and the dynamic Otto Lilienthal died after his
pressure measured by the Pitot tube. The static pressure is glider stalled and crashed. As he
usually taken from the same static port used by the altimeter. lay dying, he was quoted as
Unlike the static port, the value measured by the Pitot tube does saying “sacrifices must be made.”
not change with position unless the aircraft is flying supersonic.
But since the static port is position-sensitive, position error also
affects the accuracy of the airspeed indicator.