While considering an aircraft's speed may seem straightforward, there are several nuances that must be considered. In fact, there are six methods to determine airspeed, each of which is used to represent slightly different events. Since all six methods are used routinely to help flight and ground crews quantify the aircraft's velocity, it is beneficial for anyone interested in aviation to appreciate the differences between them.
The most straightforward method of airspeed calculation is the indicated airspeed. This is the value read from the airspeed indicator in the cockpit and is also used to determine stalling speeds and other performance metrics. Indicated airspeed is determined by the pitot tube, which is a device that contains two holes that measure air pressure as it enters the system. Containing two perpendicular holes, the pitot tube can produce two pressure readings, allowing the difference to be plugged into Bernoulli's equation to calculate velocity.
Calibrated airspeed is derived from indicated airspeed, but with corrections for several common errors. These errors are generally caused by an incorrect pressure reading from the pitot tube based on its location on the aircraft. Throughout the plane's exterior, there are several velocity gradients that exist due to the many lift-generating components. The equation for calculating calibrated airspeed varies based on the geometry of the aircraft and is typically published in the plane's operating manual.
True airspeed is calculated using the calibrated airspeed after correcting for altitude and temperature. As such, this is the most accurate representation of the aircraft's speed in a given environment, and is therefore used in navigation systems and flight planning. While indicated airspeed may be listed in manuals and performance reports, many experts prefer using true airspeed to accurately represent this information. At low speeds or when the pressure is measured at sea level conditions, the indicated airspeed is congruent with true airspeed, but after the first few minutes of flight, these values begin to diverge rapidly. While the formula for calculating true airspeed is eloquent and quickly performed by onboard computers, it may be roughly estimated by adding 2% to the indicated airspeed for every 1000ft above mean sea level.
Groundspeed is another value of aircraft velocity that may be calculated, but its use in routine operation is limited. It is defined as the speed of the aircraft relative to the Earth's surface on a horizontal plane. Generally calculated from the true airspeed corrected for wind vectors, the groundspeed is commonly displayed in real-time on the entertainment system for passengers. Another measurement less commonly used is equivalent airspeed, which is calculated from calibrated airspeed considering the compressible nature of air. Its use is generally limited to testing environments and in education.
Finally, the Mach number is a value used to describe velocity compared to the local speed of sound which varies based on the temperature of the medium. On most supersonic aircraft, the meter reads from zero to two, with faster, experimental aircraft requiring a wider range of readings. Aircraft capable of traveling at or above the speed of sound may be labeled transonic, supersonic, or hypersonic. Transonic aircraft average a top speed of 1.0 Mach, which is a typical speed for most modern aircraft. If the plane can reach speeds over 1.0 Mach, it is considered to be supersonic, as is the case with many military fighter jets. Finally, experimental aircraft like the NASA X-plane are considered hypersonic and are capable of reaching Mach 5.0 and above.
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