Blind equalization

2011-09-29T09:15:19Z

134.91.225.10: Undid revision 451920227 by 129.115.2.39 (talk)

[[Image:Machmeter.SVG|thumb|right|Illustration showing the face of a Machmeter reading a [[Mach number]] of 0.83]]

A '''Machmeter''' is an [[aircraft]] [[pitot-static system]] [[flight instrument]] that
shows the [[ratio]] of the [[true airspeed]] to the [[speed of sound]],
a [[dimensionless quantity]] called [[Mach number]]. This is shown on a Machmeter as a [[decimal fraction]].
An aircraft flying at the speed of sound is flying
at a Mach number of one, expressed as ''Mach 1''.

==Use==
As an aircraft in [[transonic]] flight approaches the speed of sound,
it first reaches its [[critical mach]] number, where air flowing
over low-pressure areas of its surface locally reaches the
speed of sound, forming [[shock wave]]s. The [[indicated airspeed]]
for this condition changes with ambient temperature,
which in turn changes with [[altitude]].
Therefore, indicated airspeed is not entirely adequate to
warn the pilot of the impending problems. Mach number is
more useful, and most [[High-speed flight|high-speed aircraft]] are limited to a maximum operating Mach number, also known as '''MMO'''.

For example, if the '''MMO''' is Mach 0.83, then at {{convert|30000|ft|m|0}} where the speed of sound under [[standard atmosphere|standard conditions]] is {{convert|590|kn|0}}, the [[true airspeed]] at MMO is {{convert|489|kn|0}}. The speed of sound increases with air temperature, so at Mach 0.83 at {{convert|10000|ft|m|0}} where the air is much warmer than at {{convert|30000|ft|m|0}}, the true airspeed at MMO would be {{convert|530|kn|0}}.

==Operation==
Some older mechanical Machmeters use an altitude [[Barometer#Aneroid_barometers|aneroid]] and an [[airspeed indicator|airspeed]] capsule which together convert pitot-static pressure into Mach number. Modern electronic Machmeters use information from an air data computer system.

==Calibration==
In subsonic flow the Mach meter can be calibrated according to:

:<math>
{M}=\sqrt{5\left[\left(\frac{p_t}{p}+1\right)^\frac{2}{7}-1\right]}\,
</math>

where:
:<math>\ M\,</math> is Mach number
:<math>\ p_t\,</math> is [[stagnation pressure|total pressure]] and
:<math>\ p</math> is [[static pressure]]
:and assuming the [[heat capacity ratio|ratio of specific heats]] is 1.4

When a shock wave forms across the pitot tube the required formula is derived from the Rayleigh Supersonic Pitot equation, and is solved iteratively:

:<math>{M}=0.88128485\sqrt{\left[\left(\frac{p_t}{p}+1\right)\left(1-\frac{1}{[7M^2]}\right)^\frac{5}{2}\right]}</math>
where:
:<math>\ p_t</math> is now impact pressure measured behind a normal shock.

Note that the inputs required are impact pressure (or [[stagnation pressure|total pressure]]) and static pressure. Air temperature input is not required.

==See also==
* [[Airspeed indicator]]
* [[Mach number]]
* [[Pitot-static system]]

==References==
*{{cite book
| title = Instrument Flying Handbook
| date = 2005-11-25
| publisher = U.S. Federal Aviation Administration
| location = U.S. Government Printing Office, Washington D.C.
| id = FAA-H-8083-15
| page = 3–8

}}
*{{cite book
| title = Instrument Flying Handbook
| date = 2007
| publisher = U.S. Federal Aviation Administration
| location = U.S. Government Printing Office, Washington D.C.
| id = FAA-H-8083-15A
| page = 3–10
| url = http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/
}}

{{USGovernment|sourceURL=[http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/ Instrument Flying Handbook]}}
{{Flight instruments}}

[[Category:Aircraft instruments]]
[[Category:Measuring instruments]]
[[Category:Speed sensors]]

{{aviation-stub}}

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Blind equalization