Calibrated Airspeed - Calculation From Impact Pressure

Calculation From Impact Pressure

Since the airspeed indicator capsule responds to impact pressure, CAS is defined as a function of impact pressure alone. Static pressure and temperature appear as fixed coefficients defined by convention as standard sea level values. It so happens that the speed of sound is a direct function of temperature, so instead of a standard temperature, we can define a standard speed of sound.

For subsonic speeds, CAS is calculated as:

where:

• = impact pressure
• = standard pressure at sea level
• is the standard speed of sound at 15 °C

For supersonic airspeeds, where a normal shock forms in front of the pitot probe, the Rayleigh formula applies:

The supersonic formula must be solved iteratively, by assuming an initial value for equal to .

These formulae work in any units provided the appropriate values for and are selected. For example = 1013.25 hPa, = 661.48 knots. The ratio of specific heats for air is assumed to be 1.4.

These formulae can then be used to calibrate an airspeed indicator when impact pressure is measured using a water manometer or accurate pressure gauge. If using a water manometer to measure millimeters of water the reference pressure may be entered as 10333 mm .

At higher altitudes CAS can be corrected for compressibility error to give equivalent airspeed (EAS). In practice compressibility error is negligible below about 10,000 feet and 200 knots.