Hi Remi,
Very Interesting! Your suggestion that a normally aspirated engine
will develop more power at 8000' than at 1000' is quite curious.
The fraction of oxygen is stable in the troposphere at 20.9%
The standard lapse rate of 2C per 1000' is included in the
rule of thumb that the pressure drop is an inch of Hg per 1000'
Therefore the roughly 7" drop in pressure include the temperature
change of roughly 14C.
Look at this page from Wikipedia which has the formulae for pressure
at altitude (with or without temperature lapse):
http://en.wikipedia.org/wiki/Atmospheric_pressure
NB a 7" drop in MP is roughly a 25% drop in power between 1 and 8
k'. The change in temperature is (following the Gay-Lussac's law)
P1/T1 = P2/T2 where temperature is in Kelvin. So P1/P2 = T1/T2
or P1/P2 = (273 + 15)/(273 + 1) = 288/274 which is only a 5% change
in density or partial pressure, so even if temperature was not
accounted for in the pressure drop, the temperature effect is
completely swamped by the pressure change.
Remi, you might try the calculator for engine power at
http://wahiduddin.net/calc/calc_hp_dp.htm
or look here:
http://www.2-stroke-porting.com/altiden.htm
or here:
http://www.nappf.com/nappf_density_altitude.htm
or better yet read this delightful old book on Aircraft Engines
(I reference it here for you because it is free on the web):
http://books.google.com/books?id=qiBFAAAAMAAJ
and a good book on aerodynamics and gas laws. I like Aerodynamics
for Naval Aviators. See in particular the standard Atmosphere table on
page 5 and pages 100-104, and page 135-149 on Recip engines.
Cheers,
--------
Ira N224XS
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http://forums.matronics.com/viewtopic.php?p=221271#221271
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