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From: mlf@druxn.UUCP (FontenotML)
Newsgroups: net.aviation
Subject: Re: Pressure vs Altitude
Message-ID: <1164@druxn.UUCP>
Date: Sat, 23-Mar-85 11:13:53 EST
Article-I.D.: druxn.1164
Posted: Sat Mar 23 11:13:53 1985
Date-Received: Sun, 24-Mar-85 04:05:16 EST
Organization: AT&T Information Systems Laboratories, Denver
Lines: 25

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The dependence of pressure on altitude can be derived from the
hydrostatic equation:
   (dp/dz) = - rho g
and the ideal gas law:  p = R rho T, together with the assumption
that temperature decreases 2 degrees C per 1000 feet of altitude increase
(the "standard atmosphere" assumption). The result is that 

     (p/pzero) = ((288-2z)/288)**5.2072,

where p is the pressure at altitude z (with z expressed in thousands
of feet) and pzero is the sealevel pressure.  The constant 288 in the
equation is the standard sealevel temperature (15 degrees C), expressed
in degrees kelvin.  The exponent 5.2072 is g/(2R), converted to units
of degrees per thousand feet.  Once this equation is available, TAS
can be easily obtained from CAS by dividing CAS by
   
     ((p/pzero)(288/T))**(1/2), 

where T is the temperature at altitude z, expressed in degrees kelvin.
(and degrees kelvin = 273 + degrees C).

                                           Mike Fontenot
                                           Denver, Colo
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