Density Altitude Equation on HP 35s
07-06-2014, 02:01 PM
Post: #1
 ptros Junior Member Posts: 3 Joined: Jul 2014
Density Altitude Equation on HP 35s
For fun I hammered out a density altitude app in Java and decided to change things up so it works on the HP 35s. I used the NOAA calculators and associated formulas from here: http://www.srh.noaa.gov/epz/?n=wxcalc

Here is the EQN formula for the HP 35s...

D = 145442.16 * (1 - ((17.326 * B * ((288 - 0.0065 * (A / 3.2808)) / 288) ^ 5.2561 / ((9 / 5 * ((5 / 9 * (T - 32)) + 273.15 / (1 - 0.379 * ((6.11 * 10 ^ ((7.5 * (5 / 9 * (W - 32))) / (237 + (5 / 9 * (W - 32))))) / (33.8638816 * B))) - 273.15) + 32) + 459.69)) ^ 0.235))

D = Density Altitude
B = Barometer(inHg)
A = Station Altitude
W = DewPoint(F)
T = Temp(F)

Example…
Solve for D (Density Altitude)...
B = 30.11
A = 5130
W = 39
T = 89
Result for D = 8040.64

In the formula I’m calculating Vapor Pressure, Virtual Temperature, and Station Pressure to get the Density Altitude.

I seem to get a very slight variance on the 35s vs the NOAA online Density Alt calculator but still its very accurate.
Cheers!
07-09-2014, 07:36 PM
Post: #2 Eddie W. Shore Senior Member Posts: 1,131 Joined: Dec 2013
RE: Density Altitude Equation on HP 35s
Very cool! Putting this equation on my 35S is now on the list of things to do.
07-09-2014, 07:56 PM
Post: #3 Geoff Quickfall Senior Member Posts: 740 Joined: Dec 2013
RE: Density Altitude Equation on HP 35s
Yep, must add that to my aviation suite. Strictly for interest as the takeoff and landing data on the b777 is computed on board.

But leaving Bogata, altitude of airport is 8000 feet on a hot day (40'C) with a low pressure 29.80 it would be interesting to see what the density altitude is. Also comparing these answers to my circular JEPP cr3 which has the log curve built in to the display.

Thanks, but what are those brackets things (((()))))?

:-)
07-10-2014, 12:28 AM
Post: #4 CosmicTruth Member Posts: 164 Joined: May 2014
RE: Density Altitude Equation on HP 35s
I dropped this into the 50G equation solver and D=8040.64949872?

Thanks
~~~~8< Art >8~~~~

PS: Please post more 50G stuff :)
07-10-2014, 08:40 AM
Post: #5
 r. pienne Member Posts: 53 Joined: Dec 2013
RE: Density Altitude Equation on HP 35s
There's a much simpler version on Wikipedia: https://en.wikipedia.org/wiki/Density_altitude (though I haven't tested it as I know nothing about the subject)

It's also generally much simpler for people to stick to standard international units instead of laboriously converting to and from their local units that the rest of the world knows nothing about.

R.
07-13-2014, 01:56 PM
Post: #6
 ptros Junior Member Posts: 3 Joined: Jul 2014
RE: Density Altitude Equation on HP 35s
The calculation on Wikipedia is okay but it assumes dry air therefore your Density Altitude will indicate lower than actual. We are calculating humidity (water vapor) indirectly by using the dew point. Humidity displaces air which decreases lift which results in a higher density altitude. That said formulas which assume dry air are not specifically dangerous to use given temperature is the primary driver. If using a dry air Density Altitude formula results in an aircraft's performance to be at threshold then one might assume calculating humidity will be more definitive in finding that threshold, and it will. But on those hot humid days its better to leave the plane in the hanger and do something else until temps cool down.
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