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What is mach number?

Asked by: 4912 views Aerodynamics, Flight Instructor, General Aviation

Hi there Mach number is the ratio of the airspeed of given object over the local speed of sound under the same conditions. Then what is: Free stream Mach number (Mfs)? Critical Mach number (Mcrit)? Compressibility Mach Number ?   Thank you!  

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1 Answers



  1. Alex Weeks on Mar 21, 2015

    I’ll preface my answer by saying I’m simply a pilot, not an engineer, so consider the source.

    Your questions appear to deal with the concept of transonic flight, so I’ll first briefly define transonic flight..

    As an airplane passes through air, some parcels of air travel at relatively higher speeds than other parcels. This happens in a number of places on the airframe, but the easiest to understand is the wing. All pilots learn Bernoulli’s principle which, in air foil terms, states that due to the shape of the wing, air travels faster over the top of the wing, creating a relative low pressure area and thus creating lift. So the air going over the top of the wing is going faster than the air going under the wing.

    When we start talking about flight near the speed of sound, This effect means that in a certain speed range, while the airplane itself is subsonic, the speed of the air over the wing is supersonic (the speed of the air is faster and the local air pressure is lower). We refer to this speed range as “transonic.” Some parts of the airplane are supersonic while others are subsonic.

    Free Stream Mach Number is the theoretical speed at which sound travels through undisturbed air. Air that is not being pushed, compressed, diverted, etc by the airplane.

    Critical Mach Number is the lowest speed at which a part of the airframe goes supersonic. It’s the lower limit of transonic flight.

    I’m not familiar with the term “Compressibility Mach Number” but I imagine it has to do with the fact that in air, the speed of sound is higher for higher temperatures and higher densities. So as air gets compressed in front of the wing, the air is slowed and the local speed of sound is higher creating a subsonic area in front of the leading edge even at supersonic speeds.

    There are other places besides the wing where we see interesting things related to the speed of air. The space between the engines and the fuselage is a classic venturi and in some aircraft like the Lear 35 the space between the engine nacelle and the top of the wing can be a venturi at high speed. As air passes through these venturi-like spaces it accelerates, goes supersonic and creates significant drag even though the airframe itself is well below the speed of sound. Air can go supersonic around other parts of the airframe too.

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