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What Is the Mach number?

Article · November 2019

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Aymen Labidi
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Journal of Aviation/Aerospace Education & Research (JAAER) © 2018 IVAO,

2018, Vol. 1 , Nov 11 .
th
International Virtual Aviation Organization , French Division

What Is the Mach number?

LABIDI Aymen
Captain-Instructor, Commercial Virtual Airline Pilot on IVAO & on Transavia Virtual Airlines
& INSAT University Student.

As an aircraft moves through the air, the air molecules near the aircraft are disturbed and move around the
aircraft. If the aircraft passes at a low speed, typically less than 250 mph, the density of the air remains
constant. Nevertheless, for higher speeds, some of the energy of the aircraft goes into compressing the air and
locally changing the density of the air. This compressibility effect alters the amount of resulting force on the
o
f

aircraft. The effect becomes more important as speed increases. Near and beyond the speed of sound, about 330
m/s or 760 mph, small disturbances in the flow are transmitted to other locations isentropically or with constant
entropy. However, a sharp disturbance generates a shock wave that affects both the lift and drag of an aircraft.

Educational Impact and Implications Statement

The authors examined the importance of Mach number as it gives us an important information about the
type of flow. In general, the flow of a fluid is divided into the four types depending upon the Mach
number.

Keywords: MACH, Subsonic Flow, Transonic Flow, Supersonic Flow, Hypersonic Flow, Hyper Velocity Flow.
This article is Written By L..Aymen

In fluid dynamics, the Mach number (M or Ma) is a dimensionless where:

quantity representing the ratio of flow velocity past a boundary to the local
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sound. The Mach number is named after Austrian physicist and

philosopher Ernst Mach, and is a designation proposed by aeronautical qc is impact pressure (dynamic pressure) and
engineer Jakob Ackeret. As the Mach number is a dimensionless quantity p is static pressure
rather than a unit of measure, with Mach, the number comes after the unit;
the second Mach number is Mach 2 instead of 2 Mach (or Machs). In the  is the ratio of specific heat of a gas at a constant pressure to
decade preceding faster-than-sound human flight, aeronautical engineers heat at a constant volume (1.4 for air).
referred to the speed of sound as Mach's number, never Mach 1.
The formula to compute Mach number in a supersonic compressible flow
The Mach number at which an aircraft is flying can be calculated by is derived from the Rayleigh supersonic pitot equation:

where:
Classification of Mach Regimes
M is the Mach number
u is velocity of the moving aircraft and While the terms subsonic and supersonic, in the purest sense, refer to
c is the speed of sound at the given altitude speeds below and above the local speed of sound respectively,
aerodynamicists often use the same terms to talk about particular ranges of
Note that the dynamic pressure can be found as: Mach values. This occurs because of the presence of a transonic regime
around M = 1 where approximations of the Navier-Stokes equations used
for subsonic design no longer apply; the simplest explanation is that the
flow locally begins to exceed M = 1 even though the freestream Mach
number is below this value.

Meanwhile, the supersonic regime is usually used to talk about the set of
Assuming air to be an ideal gas, the formula to compute Mach number in a
Mach numbers for which linearised theory may be used, where for
subsonic compressible flow is derived from Bernoulli's equation for M <
example the (air) flow is not chemically reacting, and where heat-transfer
1:[6]
between air and vehicle may be reasonably neglected in calculations.

In the following table, the regimes or ranges of Mach values are referred
to, and not the pure meanings of the words subsonic and supersonic.

Generally, NASA defines high hypersonic as any Mach number from 10

to 25, and re-entry speeds as anything greater than Mach 25. Aircraft
operating in this regime include the Space Shuttle and various space
planes in development.
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Flight Can be roughly classified into six categories :

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