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.
Read more about this topic: Calibrated Airspeed
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