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| [[Image:Machmeter.SVG|thumb|right|Illustration showing the face of a Machmeter reading a [[Mach number]] of 0.83]]
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| A '''Machmeter''' is an [[aircraft]] [[pitot-static system]] [[flight instrument]] that
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| shows the [[ratio]] of the [[true airspeed]] to the [[speed of sound]],
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| a [[dimensionless quantity]] called [[Mach number]]. This is shown on a Machmeter as a [[decimal fraction]].
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| An aircraft flying at the speed of sound is flying
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| at a Mach number of one, expressed as ''Mach 1''.
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| ==Use==
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| As an aircraft in [[transonic]] flight approaches the speed of sound,
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| it first reaches its [[critical mach]] number, where air flowing
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| over low-pressure areas of its surface locally reaches the
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| speed of sound, forming [[shock wave]]s. The [[indicated airspeed]]
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| for this condition changes with ambient temperature,
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| which in turn changes with [[altitude]].
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| Therefore, indicated airspeed is not entirely adequate to
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| warn the pilot of the impending problems. Mach number is
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| more useful, and most [[High-speed flight|high-speed aircraft]] are limited to a maximum operating Mach number, also known as '''M<sub>MO</sub>'''.
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| For example, if the '''M<sub>MO</sub>''' 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 M<sub>MO</sub> 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 M<sub>MO</sub> would be {{convert|530|kn|0}}.
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| ==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.
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| ==Calibration==
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| In subsonic flow the Mach meter can be calibrated according to:
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| :<math>
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| {M}=\sqrt{5\left[\left(\frac{p_t}{p}+1\right)^\frac{2}{7}-1\right]}\,
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| </math>
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| where:
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| :<math>\ M\,</math> is Mach number
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| :<math>\ p_t\,</math> is [[stagnation pressure|total pressure]] and
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| :<math>\ p</math> is [[static pressure]]
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| :and assuming the [[heat capacity ratio|ratio of specific heats]] is 1.4
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| When a shock wave forms across the pitot tube the required formula is derived from the Rayleigh Supersonic Pitot equation, and is solved iteratively:
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| :<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>
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| where:
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| :<math>\ p_t</math> is now impact pressure measured behind a normal shock.
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| Note that the inputs required are impact pressure (or [[stagnation pressure|total pressure]]) and static pressure. Air temperature input is not required.
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| ==See also==
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| * [[Airspeed indicator]]
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| * [[Mach number]]
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| * [[Pitot-static system]]
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| ==References==
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| *{{cite book
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| | title = Instrument Flying Handbook
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| | date = 2005-11-25
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| | publisher = U.S. Federal Aviation Administration
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| | location = U.S. Government Printing Office, Washington D.C.
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| | id = FAA-H-8083-15
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| | page = 3–8
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| <!-- | url = http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/ -->
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| }}
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| *{{cite book
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| | title = Instrument Flying Handbook
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| | date = 2007
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| | publisher = U.S. Federal Aviation Administration
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| | location = U.S. Government Printing Office, Washington D.C.
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| | id = FAA-H-8083-15A
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| | page = 3–10
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| | url = http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/
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| }}
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| {{USGovernment|sourceURL=[http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/ Instrument Flying Handbook]}}
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| {{Flight instruments}}
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| [[Category:Aircraft instruments]]
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| [[Category:Measuring instruments]]
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| [[Category:Speed sensors]]
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| {{aviation-stub}}
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Also visit my blog post free online tarot card readings (www.indosfriends.com)