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RAS/EAS Errors  
User currently offlineNovice From United Kingdom, joined Aug 2012, 90 posts, RR: 0
Posted (1 year 11 months 3 weeks 2 days 14 hours ago) and read 2629 times:

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?

3 replies: All unread, jump to last
 
User currently offlinelarshjort From Denmark, joined Dec 2007, 1471 posts, RR: 0
Reply 1, posted (1 year 11 months 3 weeks 2 days 11 hours ago) and read 2584 times:

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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User currently offlineMir From United States of America, joined Jan 2004, 21626 posts, RR: 55
Reply 2, posted (1 year 11 months 3 weeks 2 days 10 hours ago) and read 2569 times:

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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User currently offlinePihero From France, joined Jan 2005, 4445 posts, RR: 76
Reply 3, posted (1 year 11 months 3 weeks 2 days 9 hours ago) and read 2554 times:
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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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