Measuring Different Types of Airspeed

An Airspeed indicator is the measuring instrument of dynamic pressure, which is caused by the airplane’s movement through the atmosphere. Moreover, it also measures static air, taking advantage of the fact that atmospheric pressure decreases as the vehicle rises upward. Ram air, a combination of static and dynamic pressure, is a critical value which airspeed indicators measure through the use of pitot static components like the pitot tube and static port. While situated on the ground, ram pressure includes just the static component due to a lack of forward motion, but both static and dynamic pressure are measured while in flight.

Simply put, an airspeed indicator measures the difference between static pressure and ram pressure collected from the pitot tube, resulting in calculated dynamic pressure. To do this, the pitot tube fills with static air when a plane is in motion, while an additional component called the pressure diaphragm simultaneously fills with ram air. As the diaphragm is filled with more ram pressure, it expands, and airspeed also rises.

Indicated Airspeed (IAS)

Indicated Airspeed (IAS) is the measurement that an airspeed indicator itself produces without any calculations done to account for variations in atmospheric density, installation error, or instrument error. Manufacturers often use airspeed as the grounds for determining aircraft performance.

Calibrated Airspeed (CAS)

Another type of measured airspeed is calibrated airspeed (CAS), which is an IAS measurement that has been corrected for both installation and instrument errors. There is no way for manufacturers to completely avoid airspeed errors, which may total several knots at certain airspeeds and with specific flap settings. At low airspeeds, the error is most significant. However, while an aircraft is cruising at higher altitudes and speeds, this causes the IAS and CAS to function quite similarly. To measure for calibrated airspeed, one must look in the calibration chart of the respective aircraft manual.

True Airspeed (TAS)

True airspeed is the CAS that has been corrected for altitude and unusual temperatures. Seeing as air density decreases as altitude increases, this means that aircraft need to be flown at faster speeds the more they increase in elevation to allow for the same pressure difference between pitot impact pressure and static pressure. As such, for any given CAS, the TAS increases at higher altitudes, while for any given TAS, the CAS decreases when altitude decreases.

True airspeed can be measured by using a flight computer or by approximating it with the following calculation: Add 2 percent to the CAS for every 1000 feet of altitude. This means that at 10,000 feet, you would be flying about 20% faster than your indicated airspeed. Typically, the TAS is used during flight planning stages.

Ground Speed (GS)

The groundspeed (GS) represents the speed of an aircraft as it flies over the earth, and it is calculated as the true airspeed adjusted for wind. As the plane faces a tailwind, the GS increases; however, it decreases when facing a headwind. This measure is important for navigation and it impacts the amount of time it takes to get to an intended destination.

Equivalent Airspeed (EAS) And Mach Number (M)

Tipically used for structural calculations and testing, equivalent airspeed represents the calibrated airspeed that has been corrected for the compressibility of air at a non-trivial Mach number, as well as the airspeed at sea level.      

The Mach number is the ratio of the aircraft’s true airspeed (TAS) to the Local Speed of Sound (LSS) as displayed on the Machmeter. This measurement is impacted by atmospheric conditions, air temperature, and density, as well as is significant in jet aircraft that fly at high speeds.

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