MobiFlight Community Support

Hi !
I want to calculate density altitude and using this formula ; Density altitude = Pressure altitude + [120 x (OAT – ISA Temp)].
In MF transform ; $*3+120*(#-?), where
$ = Pressure altitude,
#= Placeholder-OAT,
?= Placeholder-ISA temp where I use pressure altitude as reference, interpolation;500=15,1000=14, 1500=13 aso,
120 is a constant and
$*3 means I have pressure altitude in feet.
The interpolated ISA-temp-placeholder doesen't work. If I replaces "?" with "15" it works. What's wrong ?

Cheers, Peter

Hi

Thats a known Bug! ( Or better its a feature on a other spot that ocure in trouble on that point)

A Interpolation result internal not in a Integer..... It is like a String !
You can see this in the "Output Collumn" of your Config, where you calculate the ISA Temp.
For example for 1000 feet your Config not show as expected "14" .... It show instead "14 (1000)"
(Means it remain as "text" the original Value within brackets)

If we use THIS Value within a Formula by set it to a placeholder, the formula crash with a syntax error.....
Cause technical your formula now is .....

$*3+120*(#- 14(1000) ) And thats mathematical nonsense!

****************

Solution:

You can wait.... Maybe we solve this "bug" in one of the next releases.
OR
Much more profitable.... Change the Config and caluclate the ISA without a Interpolation. Use a tranform instead !
If i understand right.... ISA is 15 -(Press Alt/1000*2)
For example current Alt = 8000ft .... 15-(8000/1000*2) --> 15-(8*2) --->15-16 = -1

Or maybe much more easy.... 15-Alt/500
15-8000/500 ---> 15-16=-1

Right ?

Then you have a "real Value" instead of the interpolation String !

Thanks for the tip/workaround
My ISA-temp ("?") transformed ; 16-($*3/500).
Works perfect.

Cheers, Peter

Nice....

Just some details.....

Stephan and myself checkt this out for personal interest.

The Factor ( source Wikipedia) is NOT 16 ..... it´s 15 !
So in your case the Temp is wrong by 1 degree ( so Altitude is wrong by 120 ft at all time)

Additional the $*3 is not correct. Altitude Meters in feet is 3,28084

So ... Your Calcualtion is working.... But not accurate !

***************
By the way.... Do you use FS2020 ?
If Yes.... there you can read the Values in the needed Units directly.... AND as i know there is a Variable for the ISA without need of own calculations !

You are right. Replaced those values and replaced even 120 to 118.6. Hopefully reading more precise now. I am on MSFS 2020 but did not find an ISA var or similar. Searched the net and of course on MF hubhop.

Cheers, Peter

Try out the follow AVars...

STANDARD ATM TEMPERATURE Outside temperature on the standard ATM scale

Maybe this is also interesting....
AMBIENT DENSITY Ambient density
SEA LEVEL PRESSURE Barometric pressure at sea level

And always remember... You can easy change the unit within the code....

e.g.

(A:BAROMETER PRESSURE,Millibars)
Can be chaged to ....
(A:BAROMETER PRESSURE,Pascal)
(A:BAROMETER PRESSURE,inHg)
(A:BAROMETER PRESSURE,bar)
and so on.... No recalculation by yourself is needed anymore !



https://docs.flightsimulator.com/html/Programming_Tools/SimVars/Simulation_Variable_Units.htm

New knowledge to me. Nice to know and shall use/test.
Thank you, Peter