1.1.1 Types of Airspeed
The airspeed of an aircraft can be defined numerous ways depending on the measurements available and the application. Before advanced navigation systems such as GPS, where groundspeed can be found directly, pilots were required to calculate several intermediate airspeeds before true airspeed could be found. Relating true airspeed with information about wind velocity and direction would allow for the ground speed of the aircraft to be found. These intermediate airspeeds are:
Indicated Airspeed
The simplest airspeed to obtain is the indicated airspeed (IAS), this is found by measuring the total and static pressures using the pitot-static tube system and converting this first into dynamic pressure and finally into an airspeed. The conversion from dynamic pressure to airspeed is performed mechanically by the airspeed indicator unit, calibrated to the equation:
Where:
- , the standard atmosphere fluid density at sea level.
- is the dynamic pressure
- is the total pressure
- is the static pressure
Calibrated Airspeed
Calibrated airspeed is the indicated airspeed corrected for instrument and position error of the pitot-static tube and indicator units. The conversion for this is usually listed by the aircraft manufacturer and typically is affected by aircraft geometry such as flaps, vents and windows.
Figure 1, Cessna 172N Calibration Airspeeds Conversion Table
Calibrated airspeed is equal to true airspeed at atmospheric conditions; therefore, it can be found from:
Equivalent Airspeed
The equivalent airspeed (EAS) is the calibrated airspeed (CAS) corrected for any compressibility effects at a significant mach number. When the aircraft is travelling closer to the speed of sound, the air is compressed by the geometry of the aircraft and therefore no longer acts as an ideal gas; hence, this must be accounted for:
Where:
- P is the static pressure
- M is Mach Number
- The term can be neglected for speeds Ma < 0.85.
True Airspeed
True airspeed is the equivalent airspeed corrected for density affects due to the altitude of the aircraft and is required for aircraft cruising at altitudes where density is much lower. It should be noted that indicated airspeed is typically using in controlling the aircraft as it gives a better indication of margin above stall than true airspeed; but true airspeed is required for overall navigation of the craft before considering wind effects. True airspeed is a function of EAS and density:
Where;
- is the density of the air that the fluid is flying in.
As the density is a function of fluid temperature and pressure, which are in turn a function of altitude; this means that can be approximated using the altitude of the craft with the rule of thumb:
Groundspeed
Ground speed is the horizontal speed of an aircraft relative to the Earth’s surface, it can be determined by the vector sum of the aircraft’s true airspeed and the current wind speed and direction. Winds at other angles to the heading will have components of either headwind or tailwind as well as a crosswind component.