The speed of sound depends on the outside air temperature at any altitude, i.e.,, so for a given true airspeed the flight Mach number increases with increasing altitude in the atmosphere. Of all the ways to categorize aerodynamic flows about a wing or flight vehicle, the free-stream Mach number is one of the most useful. Recall that the Mach number is the ratio of airspeed to the speed of sound, so the flight Mach number at any altitude will be given by where will be the true airspeed and is the ambient speed of sound at that altitude. ” The achievable airspeed (or airspeed range) of a flight vehicle is best classified in terms of its corresponding flight Mach number, as shown in the figure below. This means the speed through the air or airspeed and specifically the true airspeed of the aircraft, or what is referred to in engineering terms as the quantity “. When the performance of a flight vehicle is addressed, then its “speed” capabilities are obviously important. Be able to calculate the true airspeed of an aircraft.Know the difference between indicated, equivalent, calibrated, and true airspeeds.Understand the aerodynamic principles associated with airspeed measurement.Therefore, to avoid confusion and potentially misinterpretations of what is called “airspeed,” then the proper basis of airspeed measurement and the various definitions of airspeed must be understood, and how such measurements are actually used in both engineering and aviation practice. True airspeed and ground speed (including the effects of the winds) are also needed for navigation purposes. However, engineers are more often concerned with the true airspeed of the aircraft through the air, which will, in general, not be the same as the indicated airspeed or the equivalent airspeed. This particular airspeed is significant because it is a measure of the actual dynamic pressure acting on the aircraft. If the indicated airspeed can be corrected for errors then it is called the equivalent airspeed. Pilots are usually concerned more about the indicated airspeed, which is what is read off the airspeed indicator in the cockpit. However, determining an aircraft’s airspeed must be done with great care and the process requires a knowledge of aerodynamics and other engineering principles. The performance characteristics of an aircraft are generally presented in terms of it airspeed, as well as a function of its flight altitude and in-flight weight. Knowing how fast an aircraft travels through the air is critical to both its piloting and its engineering.
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