Spitfire Mk IIa performance tests - Patch 1.07.18301

[=AN=Felipe]

Sure Klem, ill use the real tests too in comparasion. So the altimeter was setted to 985mb 0ft at manston airfield, then i start to run the tests, so its about -400ft if we do by your technics, no winds and other things, if you are talking about that i do this way

thx klem

[klem]

Originally Posted by =AN=Felipe

Sure Klem, ill use the real tests too in comparasion. So the altimeter was setted to 985mb 0ft at manston airfield, then i start to run the tests, so its about -400ft if we do by your technics, no winds and other things, if you are talking about that i do this way

thx klem

Hi Felipe,

Manston elevation is around 144 ft and Tangmere ~40 feet. I checked the Altimeter error two ways. I deliberately ditched in the sea and checked the output file for ditched altitude reading I_Altitude (when it hit the water) and also the 'True Height' or PosHeight from this:
Point3d pos = me.Place().Pos();
PosLongitudinal = pos.x;
PosLateral = pos.y;
PosHeight_m = pos.z
PosHeight_ft = PosHeight_m * 3.2808399
That indicated that the altimeter should be set to 992Mb.

Of course for low flying its the Sea level you want which in theory can change with the tides but I don't think that's modelled.

I also use HudLog to display some parameters on screen all the time including Altimeter Altitude and True Height so I can easily set the Altimeter. The frustrating thing is that I just loaded the mission again and found that it has changed to 995mb and I don't know why! Anyway its only something like 100 ft difference so no big deal with the FMs being as far off as they are. Funny thing is that if I ignore that and set it to 992 I seem to be about right over the sea low level.

[bongodriver]

Why dont you guys set altimeter to 1013? this is what will give you standard pressure altitude which is what most charted data is based on, theres not much need to actualy test at sea level, simply start the testing at 1000ft and the data can be projected for the last 1000ft to sea level.

Manstons surveyed elevation was 150ft during wartime and tides are never factored, an elevation is based on mean sea level i.e. the average of all tide heights.

are you guys using the density altitude conversion for the tests?

[TomcatViP]

Originally Posted by bongodriver

.... an elevation is based on mean sea level i.e. the average of all tide heights.

?????!!!!???

Don't pull tht hairs too much... You'll need it when getting older !

[bongodriver]

Originally Posted by TomcatViP

?????!!!!???

Don't pull tht hairs too much... You'll need it when getting older !

Problem?

[klem]

Originally Posted by bongodriver

Why dont you guys set altimeter to 1013? this is what will give you standard pressure altitude which is what most charted data is based on, theres not much need to actualy test at sea level, simply start the testing at 1000ft and the data can be projected for the last 1000ft to sea level.

Manstons surveyed elevation was 150ft during wartime and tides are never factored, an elevation is based on mean sea level i.e. the average of all tide heights.

are you guys using the density altitude conversion for the tests?

Because the CoD environment doesn't seem to be set to 1013. In any case the True Airspeeds are calculated in the script using IAS, Altitude and the CoD environment data (apparent pressures and temperatures) including Density Altitude:

//calculate TrueAirspeeds and MACH
//Pressure Altitude P_alt=Ind_Alt+145442.2*(1- (alt_set/1013.25)^0.190261)
Pressure_Alt = I_Altitude+145442.2*(1- Math.Pow((C_AltimeterPinion/1013.25),0.190261));
//Density altitude (D_Alt) = P_Alt +(StdTemp0/.0019812)*(1-(StdTemp0/OAT)^0.2349690) ; = P_Alt +(273.15/.0019812)*(1-(273.15/OAT)^0.2349690)
Density_Alt = Pressure_Alt + 137870.9872804361 * (1 - Math.Pow((273.15 / Z_AmbientAirTemperature),0.2349690));
//TrueAirspeed_mph = CS/(1-6.8755856*10^-6 * ''DA'')^2.12794 (NB assume IAS = CS)
TrueAirspeed_mph = I_VelocityIAS / Math.Pow((1 - 6.8755856 * Math.Pow(10, -6) * Density_Alt), 2.12794);
TrueAirspeed_kph = TrueAirspeed_mph / 0.621371192;
TrueAirspeed_kt = TrueAirspeed_mph / 0.868976242;
//Mach Number (M) = TAS/CS; CS = sound speed= 38.967854*sqrt(T+273.15) where T is the OAT in celsius; TAS is true airspeed in knots.
MACH = TrueAirspeed_kt / (38.967854 * Math.Sqrt(Z_AmbientAirTemperature + 273.15));

There is no CoD C# parameter for Pressure to set the Altimeter by so I have to measure it and/or set it by reference to True Height. I am assuming 'True Height' is from Mean Sea Level. Can't think what else it would be. I set the Altimeter to read as near as possible to True Height when on the runway so I won't hit the sea on low level tests and I'll have near-correct Altitude reading on he Altimeter. Having said that, at a setting of 992mb its about 4 feet out at sea level but its about 2,000 feet out at 20,000 ft. something wrong somewhere. I don't thionk it matters at the moment as he performance is so far off but I'll include some mB vs Altitude error checks after the next patch.

EDIT: Did some tests.
Max Alt setting available is 1045mB. At True Height 20,000 ft, Alt reading ~19,900. Closest I could get.
Alt setting 1045mB at TH 15,000ft, Alt reading ~15,500. Alt setting for correct TH reading 1027mB
Alt setting 1027mB at TH 10,000ft, Alt reading ~10,500. Alt setting for correct TH reading 1007mB
Alt setting 1007mB at TH 5,000ft, Alt reading ~5,800. Alt setting for correct TH reading 998mB

Am I missing something?

[Crumpp]

At 992mb at 19C, the density altitude at sea level is 1194 feet.

If the atmosphere is correct, then standing at sea level is the same as being at 1200 feet altitude on a standard day to the airplane.

Your altimeter is just a barometer and measures pressure. It will only show some 554 feet!!!

All the altimeter sees is pressure while the aircraft sees density. Test flying is done at pressure altitude and performance calculated according to density altitude.

Set the ALTIMETER to 29.92inHg or 1013Mb. That is your altitude for starting your performance calculations.

[klem]

Originally Posted by Crumpp

At 992mb at 19C, the density altitude at sea level is 1194 feet.

If the atmosphere is correct, then standing at sea level is the same as being at 1200 feet altitude on a standard day to the airplane.

Your altimeter is just a barometer and measures pressure. It will only show some 554 feet!!!

All the altimeter sees is pressure while the aircraft sees density. Test flying is done at pressure altitude and performance calculated according to density altitude.

Set the ALTIMETER to 29.92inHg or 1013Mb. That is your altitude for starting your performance calculations.

Well that would skip the first calculation of Pressure Altitude from indicated altitude and altimeter setting and I might try that but I don't see how that would change the resulting TAS.

I'm not a test pilot, I'm just doing the Maths and it took me a while to get my head around how to do these tests. I did it that way so I could fly at/very near sea level on the altimeter although as bongodriver says I could just project the 0ft value.

I'm not sure what your trying to say other than to collect the readings in a different way that skips one calculation step.

Do you think my results will be incorrect for some reason? See above fornulae drawn from
http://williams.best.vwh.net/avform.htm#Mach
and my Hurricane results
http://forum.1cpublishing.eu/showthread.php?t=33135

[ACE-OF-ACES]

Originally Posted by klem

Am I missing something?

I think so.. In that I found and error in your Density Altitude calculation

See the following REVIEW of your Java code.

Before we do that, I took the liberty of re-wording the formulas you used from Williams site, i.e.

http://williams.best.vwh.net/avform.htm#Altimetry

I also added more detailed definitions all in all with the goal of making them easier to read and follow

ISA Standard Temperature Lapse Rate

TLR = 0.0065°C/m = 0.0019812°C/ft

Relationship between pressure and indicated altitude

Alt_corr = (T_o/TLR) × [1 - (Altimeter_set/P_o)^0.190261]
Alt_corr = 145442.16 × [1 - (Altimeter_set/P_o)^0.190261] <-- in feet
Alt_corr =  44330.77 × [1 - (Altimeter_set/P_o)^0.190261] <-- in meters

Alt_pressure = Alt_ind + Alt_corr

Where:

Alt_ind       = Altimeter Indicated Altitude with corresponding Altimeter Setting
Altimeter_set = Altimeter Setting
P_o           = ISA Standard Pressure @ SL
T_o           = ISA Standard Temperature @ SL
TLR           = ISA Standard Temperature Lapse Rate

Relationship between pressure and density altitude

Alt_density = Alt_pressure + (T_std/TLR) × [1 - (T_std/OAT)^0.2349690]

Where:

T_std = T_o - Alt_ind × TLR = Standard Temperature in Kelvin
T_o   = ISA Standard Temperature @ SL
TLR   = ISA Standard Temperature Lapse Rate
OAT   = Outside Temperature in Kelvin

Now lets review your Java implementation of Williams formulas

Pressure Altitude

//Pressure Alt = P_alt = Ind_Alt + 145442.2*(1 - (alt_set/1013.25)^0.190261)
Pressure_Alt = I_Altitude + 145442.2 * (1 - Math.Pow((C_AltimeterPinion/1013.25),0.190261));

Everything looks fine here, but keep in mind this is only valid when I_Altitude is in feet, which will be the case when testing RAF planes, but it will switch to meters when testing LW planes.

Density Altitude

//Density Alt = P_Alt +(StdTemp0/.0019812)*(1-(StdTemp0/OAT)^0.2349690);
//Density Alt = P_Alt +( 273.15 /.0019812)*(1-( 273.15 /OAT)^0.2349690);
Density_Alt = Pressure_Alt + 137870.9872804361 * (1 - Math.Pow((273.15 / Z_AmbientAirTemperature),0.2349690));

Here you made a Standard Temperature calculation error..

It is not a constant!

It varries with Alt_ind (see above)

Which causes several problems..

1) You created a constant (137870.9872804361) when it should not be a constant!
2) You subtracted the wrong value from 1

Here is the Java code you want to use

Density_Alt = Pressure_Alt + (T_std/TLR) * (1 - Math.Pow((T_std / Z_AmbientAirTemperature),0.2349690));

Where:

T_std = 288.15 - Alt_ind × TLR
TLR   = 0.0065

If you plan on doing any testing above 36Kft than you will want to adjust T_std as follows

if ( height < tropopause ) then
   T_std = T_o - Alt_ind × TLR
else
   T_std = T_tropopause

Where:

tropopause starts at 11.0km (36089.24ft) and ends at 20km (65,616.78ft)
TLR = 6.49(°C/1,000m) = 1.98(°C/1,000ft) = 3.56(°F)/1,000ft
T_Topopause = -56.5(°C) = 216.65(°K)

Thus no need to check for height above 65kft in that I don't think the X-15 is in the game.. Yet! Or if the sequal includes the Bf-109K-4 with the uber FM you might have to add that check!

SUMMARY
I don't know if I would use Williams formulas.. I have some concerns with how he handles some of the units, but since you were using his formulas I thought it best to review those formulas. Also not sure if you can use Z_AmbientAirTemperature directly in that it is not realitve to I_Altitude.. And as you know small temp differences can have a big effect on Density Altitude calculations, so hopefully these are small differences. Might be better of using the ISA Standard Temp formulas adjusted for temp.

Hope this helps! S!

PS see attached where I did a quick ROC test and ploted the different altitudes

[klem]

Originally Posted by ACE-OF-ACES

I think so.. In that I found and error in your Density Altitude calculation

See the following REVIEW of your Java code.
...............................................
Where:
T_std = T_o - Alt_ind × TLR = Standard Temperature in Kelvin
T_o = ISA Standard Temperature @ SL
TLR = ISA Standard Temperature Lapse Rate
OAT = Outside Temperature in Kelvin[/code]

Now lets review your Java implementation of Williams formulas

Here is the Java code you want to use

Density_Alt = Pressure_Alt + (T_std/TLR) * (1 - Math.Pow((T_std / Z_AmbientAirTemperature),0.2349690));

OK I get the Constant versus T_std/TLR error, I made the mistake of taking it directly from his example

Where:
T_std = 288.15 - Alt_ind × TLR
TLR = 0.0065

OK

If you plan on doing any testing above 36Kft than you will want to adjust TLR as follows

if ( height < tropopause ) then
   TLR = 6.49(°C/1,000m) = 1.98(°C/1,000ft) = 3.56(°F)/1,000ft
else
   TLR = -56.5(°C) = 216.65(°K)

Where:
tropopause starts at 11.0km (36089.24ft) and ends at 20km (65,616.78ft)

I don't but I might build that into the code in case anyone else wants to use it.

Also not sure if you can use Z_AmbientAirTemperature directly in that it is not realitve to I_Altitude.. And as you know small temp differences can have a big effect on Density Altitude calculations, so hopefully these are small differences. Might be better of using the ISA Standard Temp formulas adjusted for temp.

The Z_AmbeintAirTemperature appears to be the OAT at the altitude, it decreases with alt at around 0.0021C/ft

Hope this helps! S!

It does, thanks. Also learning from Robo that the BCO doesn't work unless I wiggle the throttle.