Bf109 - Measurement of Air Speed (Fahrt)

[Varrattu]

On 4th of May 2023, Team Fusion introduced Update - Patch v5.036.

Along with Patch v5.036, the Team Fusion ambient standard temperatures should have been updated to more realistic atmospheric values. Temperature should vary according to the time of day. For me, this idea is one of the most important steps since the first release of Team Fusion's standard atmosphere. Unfortunately, Team Fusion had to roll back the update to pre-5.036 versions.

Nevertheless, there was enough time for a few tests before the rollback. The tests under v5.036 atmospheric conditions give me reason to urgently request a thorough examination of the Team Fusion atmospheric model and a comparison of airspeed relationships against the former original Team Maddox model. I'm trying to keep my findings, conclusions, suggestions as brief as possible...

I'm going to use the Bf109F as an example because there is a reliable open-source document available about the performance and airspeeds of the real warbird:

http://kurfurst.org/Performance_test...1F2_DB601N.PDF

To compare the authentic values against 'TeamFusion-Bf109F' following values are required:

double indicatedAirspeed = bf109f.getParameter(part.ParameterTypes.I_Velocity IAS, -1);
double trueAirspeed = bf109f.getParameter(part.ParameterTypes.Z_Velocity TAS, -1);
double ambientAirTemperature = bf109f.getParameter(part.ParameterTypes.Z_AmbientA irTemperature, -1);
double geometricAltitude = bf109f.getParameter(part.ParameterTypes.Z_Altitude MSL, -1);

The Patch v5.036 autumn map tests with the Bf109F at approximately 5000m above the Channel revealed the following data.

[1]
Pressure Altitude: 4999.1 metres ~ 5000 metres AMSL
Ambient Air Temperature: 245 Kelvin (-28° Celsius)
True Airspeed: 586.4 km/h ~ 586 km/h
Indicated Airspeed: 459.6 km/h ~ 460 km/h

For comparison, the Pressure Altitude needs to be converted to standard atmospheric conditions [2]. In a sense, it's the altitude at which the Bf109F "feels" it is flying. I'm only going to mention the results here. The methods and formulas are fully described on the internet.

[2]
Density Altitude: ~4610 meters
True Airspeed: ~586 km/h,
Air Temperature at MSL t0 = 288.15°Kelvin = 15°Celsius.
Static Pressure at MSL p0 = 1013.25 hPa at MSL.
Air Density at MSL r0 = 1.225 kg/³ at MSL.

Concerning True Airspeed:
586 km/h max. at 5000 metres ISA Pressure Altitude [1] are possible.

Concerning Indicated Airspeed:
As far as I know, all real German warbirds playable in 'iL2CoD' were equipped with the Bruhn Fl.22231 (60-750kmh) airspeed indicator. The airspeed indicator depends on the Pitot tube Fl.22261 for its operation. The pressure generated by the Fl.22261 for airspeed and altitude is calibrated based on the relationship:

dynamic pressure q = 0.5 * r * V² = 0.5 * r * TAS²

In which q is usually known as the incompressible dynamic pressure,
r is the air density at current flight level,
and V is the True Airspeed at flight level, that would produce the same incompressible dynamic pressure that is produced at ISA sea level conditions

dynamic pressure q = 0,5 * 1,225 * V² = 0,5 * 1,225 * EAS²

Nearly all airspeed indicators that have been used on German warbirds during WWII function this way. It is this definition that makes IAS a useful airspeed measurement without the need to correct for altitude or temperature. Please note: Only under standardized conditions the Indicated Airspeed (IAS) is equal to Equivalent Airspeed (EAS).

So, 460 km/h True Airspeed [1] at Mean Sea Level produces the same incompressible dynamic pressure q(h,TAS) as 586 km/h True Airspeed [1] at 4770 metres. In fact, based on the relationship <q=0.5*r*V²>, the indicated speed depends on a Density Altitude of 4610 metres [2] plus 160 metres = 4770 metres...

From the document 'Flugleistungen, Blatt No.6" (see above mentioned link) we learn that, computed for standard atmospheric conditions the Team Fusion Bf109F should perform as follows.

Pressure Altitude: 5000 Meter above MSL
True Airspeed: ~592 km/h
Indicated Airspeed: ~450km/h

Where 450 km/h Indicated Airspeed at Mean Sea Level produce the same incompressible dynamic pressure q(h,TAS) as ~592 km/h True Airspeed at 5354 (5000+354) metres.

And at
Pressure Altitude: 4000 Meter above MSL
True Airspeed: ~573 km/h
Indicated Airspeed: ~459km/h

Where ~459 km/h Indicated Airspeed at Mean Sea Level produce the same incompressible dynamic pressure q(h,TAS) as ~573 km/h True Airspeed at 4354 (4000+354) metres.

So, 'Bruhn-Werke GmbH' calibrated the Speed Indicator Fl.22231 to Density Altitude +~ 354 metres. I suspect that this indicator had been adjusted for areas of operation in North Africa.Flying through Patch v5.036 autumn map atmosphere with that indicator at approximately 5000m above the Channel should reveal the following data:

[3]
Pressure Altitude: ~5000 metres AMSL
Ambient Air Temperature: ~245 Kelvin (-28° Celsius)
True Airspeed: ~586 km/h
Indicated Airspeed: ~455 km/h

586 km/h True Airspeed at 4964 meters ( Density Altitude [2] + 354m ) altitude produce the same incompressible dynamic pressure q(h,TAS) as 455 km/h True Airspeed at Mean Sea Level. Under standard atmosphere condition, at Mean Sea Level Indicated Airspeed is equal True Airspeed. Please note that the 4964m is not the altitude at which the Bf109F "feels" it is flying.

Conclusion:
When considering the described calibration/correction of the Bf109 air speed indicator accidently as pressure altitude, one might assume that the air density corresponds to ~0,7361 kg³ @5000m or ~0.7390 kg³ @4964m! However, that would be a misinterpretation. The air density actually corresponds to the calculated "density altitude" of ~0.7677 kg/m³ @4610m under standard atmosperic condition where density ~1.225 kg/m³ @0m. The air density is thus actually higher than one would initially assume. It's the atmospheric condition at which the Bf109F "feels" it is flying.

The higher the altitude or speed, the clearer it becomes how ingenious the 'Flugleistungen, Blatt No.6' (see above mentioned link) fits to the real world atmosphere ...
... under North African conditions. For pilots operating above the English Channel, the airspeed indicator correction according to 'Flugleistungen, Blatt No.6' is not suitable.


Kind regards

Varrattu

References:
Kennblatt für das Flugzeugmuster Bf109, Baureihe F1 und F2 mit DB601N, Berlin 1941
Fluglehre, 5.Auflage, Mises (1936)
Fahrtmessung, Prof.Dr.KOPPE (1940)
Normalatmosphäre nach DIN 5450 (1937), Artillerie und Ballistik, Springer Verlag 1939
Beschreibung für FUESS Hoehenmesser 7a,b,c
NACA Report No.110 - The Altitude Effect On Air Speed Indicators, Part I
NACA Report No.156 - The Altitude Effect On Air Speed Indicators, Part II
NACA Report No.420 - Aircraft Speed Instruments
NACA Technical Note No.99 - Notes On The Standard Atmosphere
NACA Technical Note No.616 - Measurement Of Air Speed
NACA Reference Publication 1046 - Measurement Of Speed And Altitude


Edit 2024-09-09:
Fl.22231 pressure q = 0.5 * D(h,m+360) * V²
corrected for
Fl.22231 pressure q = 0.5 * D(h,m+354) * V²