novice
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

RAS/EAS Errors

Sat Sep 29, 2012 3:21 pm

If rectified airspeed is indicated airspeed corrected for pressure error and equivalent airspeed is rectified airspeed corrected for compressibility error, what is the differences in these two corrections?
 
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larshjort
Posts: 1434
Joined: Wed Dec 19, 2007 6:54 pm

RE: RAS/EAS Errors

Sat Sep 29, 2012 6:51 pm

Quoting Novice (Thread starter):
rectified airspeed is indicated airspeed corrected for pressure error

RAS=CAS Calibrated Airspeed
As far as I remember calibrated airspeed is corrected for the pressure drop that comes with increasing altitude. EAS then takes that number and corrects it for the compressability that happens when you reach near supersonic speeds.

At low speeds CAS=EAS

/Lars
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Mir
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Joined: Mon Jan 05, 2004 3:55 am

RE: RAS/EAS Errors

Sat Sep 29, 2012 7:40 pm

Quoting larshjort (Reply 1):
As far as I remember calibrated airspeed is corrected for the pressure drop that comes with increasing altitude.

Calibrated airspeed is indicated airspeed corrected for instrument error. When you correct for pressure dropping with altitude, you end up with true airspeed (correction for non-standard temperature is a part of that as well).

-Mir
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Pihero
Posts: 4248
Joined: Mon Jan 31, 2005 5:11 am

RE: RAS/EAS Errors

Sat Sep 29, 2012 8:47 pm

A simpler way to remember all the relationships between the airpeeds :
IAS + ki --> CAS x kc -->EAS x 1/sqrt d --> TAS
where
- ki is the instrument and position (of the pitot tube) error
- kc is the compreessibility error (to be ignored below 200 kt and 5000 ft )
- d is the air density

EAS is only ueed for aircraft performance, never in everyday piloting.
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