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Just got home and thought I would do a quick ROC TTC test..

In this test I set the Altimeter_set to 1025mB..

And note in the TTC you can see the I_Altitude changes (red) as I change mB.

Anyway, not a formal test, so don't draw any conclusions from these results.

Quote:

Originally Posted by ACE-OF-ACES (Post 446648)

OK, couple of things.

Rethinking what I said about:

if ( 15 - Pressure_Alt_ft*0.0019812 < -56.5) <--- determines the Tropopause altitude
{
TropAlt = PressureAlt - 56.5/0.0019812 <--- calculates altitude element above Tropopause
TempStd = 288.15 - 56.5;
Density_Alt_ft = Pressure_Alt_ft + (TempStd/0.0019812 + TropAlt) * (1 - Math.Pow((TempStd / I_AmbientAirTemperature),0.2349690));
else
TempLapse= Pressure_Alt_ft*0.0019812;
TempStd = 288.15 - Templapse;
Density_Alt_ft = Pressure_Alt_ft + (TempStd/0.0019812) * (1 - Math.Pow((TempStd / I_AmbientAirTemperature),0.2349690));
}
TempStd = 288.15 - TempLapse;
Density_Alt_ft = Pressure_Alt_ft + (TempStd/0.0019812) * (1 - Math.Pow((TempStd / I_AmbientAirTemperature),0.2349690));

I'm not sure about the part:
Density_Alt_ft = Pressure_Alt_ft + (TempStd/0.0019812 + TropAlt) * (1 - Math.Pow((TempStd / I_AmbientAirTemperature),0.2349690));

TempStd/0.0019812 is the correction of Pressure_Alt_ft within the Troposphere, beyond which there is no/negligible further Temperature Lapse so no correction seems to be required for that so I think the original formula applies, just
Density_Alt_ft = Pressure_Alt_ft + (TempStd/0.0019812) * (1 - Math.Pow((TempStd / I_AmbientAirTemperature),0.2349690));

EDIT 2: OK, regarding I_AmbientAirTemp, as you say it is only present where the gauge is fitted and it isn't in the Spitfire or Hurricane and returns zero.

so, how about this to get I_AmbientAirTemperature ?
if (I_Altitude / Z_AltitudeMSL_m > 2.5) //is I_Altitude in feet?
{
Z_TempLapse = (Z_AmbientAirTemp - 288.15) / Z_AltitudeMSL_ft;
else
Z_TempLapse = (Z_AmbientAirTemp - 288.15) / Z_AltitudeMSL_m;
}
I_AmbientAirTemperature = I_Altitude * Z_TempLapse;

Also, what are you using to create your charts? I tried plotting the Y axis as Altitude (as is often done for these tests) but Excel threw a wobbly when the Level Speeds or ROC reversed with altitude.

PS Further apologies to Felipe for turning his Spitfire thread into a general "Aircraft Performance Measurement" thread but I think it is just about On Topic :)

Hey klem

Im at work right now.. But Ill take a look at your Java code when I get home tonight.. I don't want to rush it and make a silly mistake like I did with T_slr vs TLR

As for the units, The variables that start with 'I_' as in indicated as in the cockpit guages will change betwen IMPERIAL and SI units depending on the country of origin. For example, when test the Spit the I_Altitude will display 'feet' but when testing the 109 it will display 'meters'. Where as the 'Z_' values are allways in SI units.

As for what I use to graph, I use matlab. As for excel, you can flip the axis, simply right-mouse-click on the graph and select 'Source Data' at which point it will display the data for the X and Y axis.. You just have to flip the X and Y sources.. You can type it in, which can be messy, or cut-in-paste swap, or simply click on the button and drag-n-drop the columns of data.. Which can be messy if you have alot of data. See attached for excel examples

Hey klem..

Have not had a chance to work on it yet, got home friday night and the girl friend had 'other ideas' as to how I was going to spend my time this weekend! I am hoping to get some time tonight to work on it! S!

Hey klem

Sorry this reply took so long.. I have been busy with some of my own stuff..

On that note I still have not writen the C# code for you yet, but wanted to share with you my matlab code, i.e.

Code:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% ISA Standard

Alt_Tropopause_m    = 11000.0;      % ISA Altitude of Tropopause in meters

Tmp_Tropopause_K    = 216.65;       % ISA Temperature of Tropopause in Kelvin

To_K                = 288.15;       % ISA Temperature @ SL in Kelvin [288.15(°K) = 15(°C) = 59(°F)]

Po_mB               = 1013.25;      % ISA Pressure @ SL in millibar [1013.250(mB) = 29.92126("Hg) = 101325.0(Pa) = 2116.2166(lbs/ft^2) = 760.0(mmHg) = 14.69595(psi) = 1.0(atm)]

Po_Hg               = 29.92126;     % ISA Pressure @ SL in inch of mercury [1013.250(mB) = 29.92126("Hg) = 101325.0(Pa) = 2116.2166(lbs/ft^2) = 760.0(mmHg) = 14.69595(psi) = 1.0(atm)]

TLR_Cpm             = 0.0019812;    % ISA Temperature Lapse Rate in Celsius/meter [6.5(°C)/1,000(m) = 1.9812(°C)/1,000(ft)]

TLR_Kpm             = 0.0019812;    % ISA Temperature Lapse Rate in Celsius/meter [6.5(°K)/1,000(m) = 1.9812(°K)/1,000(ft)]

PLR_ftpHg           = 1000;         % ISA Pressure Lapse Rate in feet/inch of mercury [1,000ft/"Hg]



% Where

% 29.92000 "Hg = 1013.207489 millibar

% 29.92126 "Hg = 1013.250158 millibar



if (units_si == 1)

    To = To_K;

    Po = Po_mB;

    PLR = 1;

    TLR = TLR_Cpm;

else

    Po = Po_Hg;

    To = To_K;

    PLR = PLR_ftpHg;

    TLR = TLR_Cpm;

end



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% STEP 1) Find the PRESSURE DIFFERENCE

%

% Pdif = Po - Altimeter_Set

Pdif = Po - Altimeter_Set;



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% STEP 2) Find the ALTITUDE CORRECTION due to the PRESSURE DIFFERENCE @ SL

%

% Alt_pcorr = (To/TLR) × [1 - (Altimeter_Set/Po)^0.190261]

%

% Or you can use the following simple approximation

%

% Alt_pcorr = Pdif × PLR

%

% Where:

% Altimeter_Set = Is the altimeter set/offset (CoD: Altimeter Pinion)

Alt_pcorr = (To./TLR) .* (1 - power((Altimeter_Set./Po),0.190261));



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% STEP 3) Find the PRESSURE ALTITUDE due to the ALTITUDE CORRECTION

%

% Alt_pressure = Alt_ind + Alt_pcorr

%

% Where:

% Alt_ind       = The Indicated Altitude

% To/TLR        = 44330.77(m) = 145442.15(ft)

Alt_pressure = Alt_indicated + Alt_pcorr;



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% STEP 4) Find STANDARD TEMPERATURE due to the TEMPERATURE DIFFERENCE @ SL

%

% Tstd = To - TLR × Alt_pressure        

%

% Tstd = 15(°C) - 0.0019812(°C/ft) × 8,000(ft)

% Tstd = 15(°C) - 15.8496(°C)

% Tstd = -0.8496(°C) = 272.30(°K)

%

% or in Kelvin

%

% Tstd = 288.15(°K) - 0.0019812(°K/ft) × 8,000(ft)

% Tstd = 288.15(°K) - 15.8496(°C)

% Tstd = 272.30(°K)

Tstd = To - TLR .* Alt_pressure;



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% STEP 5) Find DENSITY ALTITUDE

%

% NOTE: TLR, Tstd, and OAT must be in units of Kelvin for this formula to work!!

%

% Alt_denisty = Alt_pressure + (Tstd/TLR) × [1 - (Tstd/OAT)^0.2349690]

Alt_density = Alt_pressure + (Tstd. / TLR) .* (1 - power((Tstd ./ Temp_indicated),0.2349690));

I you have not used matlab, know that it does some things under the hood. The big thing being arrays, if you have an array, and you want to do stuff with it, you don't have to do the indexing for example, say we have an array called 'dog' that I want to apply an offset to, and create a new array called 'cat'. In matlab all you have to do is the following:

Code:

cat = dog + 100;

Where as in most other languages you would have to do the indexing yourself, i.e.

Code:

for (ii=0; ii<dog.Lenght-1;ii++)

{

    cat(ii) = dog(ii) + 100;

}

Same goes for mul or div

Code:

cat = dog .* 100;

cat = dog ./ 100;

The period before the * and / means apply this to each value (each index), instead of the sum of the array

Feel free to contact me if you have any questions

PS still debating on wether we can use the ingame temp, or not, ill keep you posted

You'll understand if I don't take up matlab, C# is enough of a challenge!

Those formulas are generally what I am using.

btw I assume our TLR_Cpm is for altitude in feet not metres (TLR_Cpf?).

The problem we have for calculating Density Altitude is that there is no OAT indicator. Then there is the question of whether Z_AmbientTemp relates to the cockpit altitude.

I did calculate a lapse rate from Z_Temp and Z_AltMSL and assuming 288.15 at SL but that gave daft results.

So I have assumed Z_AmbientTemp as OAT. I crashed at SL and it returned 290.1K or ~17C. At 15000ft it gave 260.2, a lapse rate of 0.0019933 for the CoD environment so I guess its OK for our purposes.