Re: True Air Speed?

Date:         22 Feb 99 03:30:25 
From:         David Lednicer <>
Organization: Analytical Methods, Inc.
References:   1 2
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> On 1/17/99 2:37 AM, Scott ( said:
> Can anyone tell me how to work out TAS for IAS?

According to the Pratt & Whitney handbook, to get from IAS, you first
correct for airspeed position error - this gives Calibrated Air Speed
(CAS).  Then, you apply compressibility corrections to get Equivalent
Air Speed (EAS).  Finally, you apply a density correction to get True
Air Speed (TAS).

The CAS to EAS to TAS correction is necessary due to how airspeed
indicators work.  They measure the difference between total pressure and
static pressure, to get dynamic pressure.  They then assume density to
be a constant to produce airspeed.  However, density changes with Mach
number (hence EAS) and altitude (hence TAS).  To make the corrections,
you need to know local sonic velocity, altitude and temperature.

David Lednicer             | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc.   |   email:
2133 152nd Ave NE          |   tel:     (206) 643-9090
Redmond, WA  98052  USA    |   fax:     (206) 746-1299