Request answer for this question on airspeed indicator
Thread Starter
Joined: Mar 2009
Posts: 3
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From: india
Request answer for this question on airspeed indicator
Q Compressiblity error
A Causes overreading of the ASI at all levels whenever TAS exceeds 300kt.
B Causes overreading of the ASI whenever CAS exceeds 300kt.
C is calibrated out of the ASI at MSL ISA conditions and only causes overeading when air density is decreased and CAS exceeds 300kt.
D is calibrated out of the ASI at MSL ISA conditions and only causes overeading when air density is decreased and TAS exceeds 300kt.
As per my understanding answer is one of A or D as compressibility error occurs at TAS more than 300kts, also airspeed indicator is calibrated at seal level for density error, but I am not sure if its A or D.
Please list the answer and the reason also. Thanks alot in advance for the help
A Causes overreading of the ASI at all levels whenever TAS exceeds 300kt.
B Causes overreading of the ASI whenever CAS exceeds 300kt.
C is calibrated out of the ASI at MSL ISA conditions and only causes overeading when air density is decreased and CAS exceeds 300kt.
D is calibrated out of the ASI at MSL ISA conditions and only causes overeading when air density is decreased and TAS exceeds 300kt.
As per my understanding answer is one of A or D as compressibility error occurs at TAS more than 300kts, also airspeed indicator is calibrated at seal level for density error, but I am not sure if its A or D.
Please list the answer and the reason also. Thanks alot in advance for the help
Joined: Sep 2007
Posts: 7
Likes: 0
From: UK
Hi,
This is my understanding of compressibility......
It has to be taken into consideration when Either;
1) TAS > 300kts
2) CAS > 200Kts
3) Mach > 0.4m
The ASI is calibrated for standard ISA condidtions (hence why at sea level in ISA conditions if you fly at SLOW speed IAS/CAS = EAS = TAS).
The ASI gives you a reading of IAS/CAS - therefore you would not want to 'calibrate' out compressibility - as if you did you are then showing EAS, which is not what the purpose of the ASI is.
Hope this helps!
G-A-G
This is my understanding of compressibility......
It has to be taken into consideration when Either;
1) TAS > 300kts
2) CAS > 200Kts
3) Mach > 0.4m
The ASI is calibrated for standard ISA condidtions (hence why at sea level in ISA conditions if you fly at SLOW speed IAS/CAS = EAS = TAS).
The ASI gives you a reading of IAS/CAS - therefore you would not want to 'calibrate' out compressibility - as if you did you are then showing EAS, which is not what the purpose of the ASI is.
Hope this helps!
G-A-G
Last edited by Get-a-Grip; 30th October 2009 at 09:05.
Joined: Aug 2009
Posts: 4
Likes: 0
From: india
Compressebility error
Compressebility error always causes air speed indicator to over read at a TAS of 300 knots or above. This is because at such high speed air is brought to rest at pitot tube and is compressed . Due to which total pressure which is measured by pitot tube increases and thus causing air speed indicator to over read.
Hope this clears your doubt
Hope this clears your doubt
Joined: Apr 2004
Posts: 734
Likes: 10
From: London, GB
...except that modern airspeed indicators are designed for compressible flow and the whole argument about 'compressibility corrections' is largely irrelevant here.* The solution derived from Euler's dynamic equations and the equation of state by Jean Claude Barré de Saint-Venant (used in modern FAA/EASA-approved ASIs) is valid below M 1.0, and reasonably accurate until about M 1.3, after which the Rayleigh 'supersonic' Pilot tube formula should be used.
* See Old Smokey's 2006-01-12 post on dynamic -v- impact pressure here: http://www.pprune.org/questions/2057...ml#post2318100
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See also NACA Technical Report 837 (Langley, 1946).
* See Old Smokey's 2006-01-12 post on dynamic -v- impact pressure here: http://www.pprune.org/questions/2057...ml#post2318100
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See also NACA Technical Report 837 (Langley, 1946).
Last edited by selfin; 31st October 2009 at 15:51. Reason: Note on NACA-TR-837.
