View Full Version : Altitude
Dude~
23rd May 2001, 01:34
I know the rough conversion is 1mb or hp per 30 feet. But is this exact and also, is it dependable on temperature, density etc?
The standard atmosphere is defined in metric units........the standard atmosphere is assumed to have a lapse rate of 0.0065 deg/m or 0.021325 deg/ft, and a gas constant of R=287.26 J/kg*deg Kelvin
In the tropopause (below 11000 m 36089 ft)its mainly a function of temperature..
p2/p1=(t2/t1)^5.256
rho2/rho1=(t2/t1)^4.256
rho2=P2/(R*T2)
where p=pressure, t= temperature, rho=density
at sea level
p1=101325 N/m^2
t1=288.15 deg Kelvin
rho1=1.225 kg/m^3
Above 11000m in the stratosphere the temperature is constant at -56.5 deg C (216.65 deg Kelvin), a different approach is used to calculate the properties.
I good web site to have a look at is
1976 Standard Atmosphere Calculator (http://www.digitaldutch.com/atmoscalc/index.htm). You can do all the calculations online, and it has the graphs in metric and english units.
:rolleyes:
[This message has been edited by Zeke (edited 22 May 2001).]
Yes, these 30ft are only exact on sea level. But as pilot you don't need to know any other value.
If you change your altimeter setting, you change only the reference pressure at sea level so the correction is made at sea level.
The same thing with te pressure altitude
380