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IAP Ratio

WarpD

WarpD

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us-ohio
Ok.. having problems calculating IAP ratio... sortof... I know that IAP ratio is P_0 / p where P_0 is 29.92126 and p is pressure at altitude. In a spreadsheet the formula I'm using to calculate pressure at altitude is
p = P_0 * POWER((1-0.0000068755856 * altitude),5.2558797).

So, in the sim I slew to 35000 feet and the ambient pressure is 7.0599220670618816. My spreadsheet calculates it at 7.040620526 which is off a tad, and I'm really uncertain as to why that would be.

Any thoughts? Suggestions?
 
I suspect it is because the calculation has to be applied to geopotential altitude instead. At least it gives a much better fit
gh = alt - (alt*alt)/(alt+6356766) for alt in meters (6356766 is earth mean radius)
At high altitudes it gives concordant values with what FS reports (within 0.00001 on standard pressure ratio and IAP)

Hope it will help

Hervé
 
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