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Effect of Altitude...

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Subject: Effect of Altitude...
From: (Chris R. Burger)
Date: Thu, 26 Sep 1996 09:36:11 +200
As a rule of thumb, the ISA (International Standard Atmophere) looks 
like this:

Pressure:  29.92" Hg at sea level.  Lapses at roughly 1" Hg / 1000'.

           Round figures, at 5000':  (30 - 5)" Hg = 25" Hg, or
           83% of the sea level value.
Temperature:  15 deg C at sea level, lapsing 6.5 deg C per km, or 
           1.98 deg C / 1000'.
At a constant temperature, the density is proportional to pressure.  
Provided you're close to 15 deg C (59 deg F, if I'm correct), you can 
accept this proportionality with very little error.  A lower 
temperature leads to higher density, while a higher temperature leads 
to a lower density.  To quantify the effect, convert the temperature 
to kelvin:

   deg C + 273 = kelvin
Example:  15 deg C = 288 K

Then calculate the percentage change from this value.  For example, 
if the lowest expected ambient temperature (worst case) is -10 deg C, 
the change is 25 K, or almost 10%.  This means the dynamic pressure 
will be 10% more than expected on the basis of the pressure alone.

Dynamic pressure (as mentioned before) is 
    0.5 (density) (velocity) (velocity)
Make sure you check the units.  

I hope this helps.  Didn't have time to design a well reasoned 
treatise, but this should get you started.

Chris R. Burger ZS6EZ

Hobbyist pilot
South African Airline Transport Pilot License, Instructor's Rating
US FAA Commercial SEL/MEL, Instrument

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