Density Altitude Calculation?

[richblake74]

A strange question but I know someone will know this.
I am trying to come up with a Formula for Density Altitude..

DA = density altitude in feet
P0 = atmospheric (static) pressure
PSL = standard sea level atmospheric pressure (101.325 kPa)
T = true (static) air temperature in kelvins (K) [add 273.15 to the Celsius (°C)] figure
TSL = standard sea level air temperature (288.15 K)
b = 0.235

Is there anyone aware of a formula, that can be used on excel for this problem?

I cannot seem to post the formula.

Just a stab in the Dark here.
Merry Xmas to you all...

 

[Sal Paradise]

=145426*((1-exp((p0/psl)/(t/tsl),2))

 

[richblake74]

Thank you, so where would I put the linked cell in this formula?
It just does not seem tot work for me right now. Excel is questioning the 2...

A1 is the altitude, and in B1 I would like to get the density altitude based on the temp in cell A2, and the altitude in A1.
A1 lets say equals to 1,500ft
A2 equals to 15 Degrees C
I would like the Density altitude result in cell B1
Can this be done.


Cheers

 

[Sal Paradise]

You could really help out by providing the actual math you want done rather than forcing us to rely on wikipedia to get the thing you want calculated.

So no, I cannot tell you how to do it because I don't know what P0 would be at 1500 feet since you didn't give the calculation.

If you want the answer, then you have to provide more than what you have.

 

[JB_Scotland]

Density Altitude = Pressure Altitude + (Standard Temperature / Standard Lapse Rate) * (1-(Standard Temperature / Actual Temperature)^0.2349690)

Standard Temperature and Actual Temperature are in degrees Kelvin and the Standard Lapse Rate is .0019812°C/ft below the tropopause.

Therefore if cell A1 is Pressure Altitude and cell A2 is Actual Temperature in degrees Centigrade the formula for cell B1 (Density Altitude) would be:

=A1+((288.15-0.0019812*A1)/0.0019812)*(1-((288.15-0.0019812*A1)/(273.15+A2))^0.234969)

http://williams.best.vwh.net/avform.htm refers.

 

[JB_Scotland]

In your example 15C at 1500ft would give a DA of 1850ft.

 

[richblake74]

thank you JB_Scotland, worked great

 

[RadioSaigon]

Greetings and apologies for breathing new life into what is patently an old thread... but, in trying to crack a similar problem I found this site, thread and formula. I've been playing with it over the last couple of days, and think maybe there is a discreet bug somewhere in here.


If for example, across a range of pressure altitudes, you manipulate the temperature in °C to ISA+10, the resultant Density Altitude is always Pressure Altitude + 1161 feet. Now, I'm no mathematician -just a pilot trying to create a solution. It strikes me however, that the resultant DA should probably not be quite so remarkably linear. I expected that as PA increases, the difference between PA and DA would likewise increase, despite a constant ISA+ difference.


Am I being too singular in my thoughts? Any assistance would be appreciated!


regards


RS

 

[JB_Scotland]

RS,

In a "Standard Atmosphere" Density Altitude = Pressure Altitude.

Ignoring the effects of humidity, in a non-standard atmosphere at a particular pressure altitude the non-standard thing is the temperature. As the temperature varies from standard the density will vary from standard: warmer less dense, cooler more dense. For dry air it is a linear relationship.

For a one degree temperature change away from standard there is a 116 feet difference in Density Altitude hence for your ISA + 10 the formula will give the 1161 feet.

Google "Density Altitude", there are many explanations out there. The aviation formulary Aviation Formulary V1.46 has an approximation formula of: D_Alt=P_Alt+118.6*(T-T_s) Here they are using the constant of 118.6 feet per degree but again the approximation is a linear relationship.

JB

 

[RadioSaigon]

Many thanks for he clarification JB -I must have slept thru that part of my Meteorology "education". In my defence perhaps, I have never been accused of being a world-class student... and it was quite som time ago!