Kind of speed in FMB

[greybeard_52]

2024-02-29 17_10_34-Greenshot.png

[ATAG_Lenny]

Indicated Air Speed. So for same True Air Speed at higher altitudes the IAS will start to decrease, and vis-a-versa. For those who have not dealt with this before, as aircraft climb the air gets thinner and so less air molecules flowing through the pitot tube resulting in the mechanical observation that the aircraft is flying slower. It's a little more complicated than that but you get the gist. So to maintain the same true airspeed at a higher altitude will result in a slower indicated air speed. It's like 1.5% slower per 1000' (~300 metres).

Attached is a little spreadsheet that allows one to calculate IAS and TAS at different altitudes. Just change the variables in the yellow highlighted cells. It is in metric and imperial. There is also a worksheet with some performance data for aircraft in the sim.

Lenny

Indicated Airspeed (IAS) to True Airspeed (TAS) Conversion.zip

[greybeard_52]

Indicated Air Speed.

Thank you very much! I asked because often I see AI keeping a cruising speed significantly lower than that set in FMB (I check in flight during simulation on my airspeed indicator, while following my AI leader). I got the doubt that that velocity set in FMB could be TAS.

[GANIX]

Hello,

Before we do any more calculations, please recall that the impact pressure is the only directly measurable quantity in a WWII single-seat fighter that relates to the aircraft's speed with respect to the air. So, with considerable accuracy to annual average conditions at sea level, WWII airspeed tables usually were corrected for certain air pressure conditions in the battle area they were used.
A very convenient way was the vertical displacement of the standard atmosphere from its standard position required to obtain the observed pressure at the height above mean sea level considered. In other words, this vertical displacement of the standard atmosphere is necessary to obtain the same weight of air above the height considered in both the actual and standard atmosphere. The approach was not considered the theoretically best method for providing an immediate display of speed based on dynamic or impact pressure measurement at that time. However, it was considered the best possible compromise solution, coming quite close to the ideal solution.

Back to Greybeard's screenshot and the question: 'TAS or IAS?'

Doubtless, the quantity 347 represents airspeed for Standard Mean Sea Level where TAS = CAS = IAS = EAS. The following observations confirm this.

When the mission starts, the aircraft in question will be created at 1832m geometric altitude above MSL with 347 km/h TAS (airspawn). As long as the AI is the pilot in command, the aircraft accelerates to approximately 380 km/h TAS at 1832 pressure altitude and tries to hold the speed. Approx. 347 kmh airspeed produce the same compressible dynamic pressure qc(h, TAS) at Standard MSL as 380.0 km/h true airspeed at 1832 meters pressure altitude.
Please note: the ratio 380kmh : 347kmh @1832 is valid exclusively for a standardised atmosphere model based on 15° Celsius (t(ref) = 273.15+15) at mean sea level, where h(ref) = 0m.

Watch out, the Team Fusion airspeed indicator (Blitz! 5.041) shows the ratio 380kmh : 341kmh (TAS:IAS) at 1832 pressure altitude. 341 km/h airspeed produce the same incompressible dynamic pressure q(h, TAS) at Standard MSL as 380 km/h true airspeed at 2185 metres pressure altitude. In other words, the ASI 'assumes' that the plane is flying at 2185m above standard mean sea level.

Please note: the ratio 380kmh : 341kmh @2185m is valid exclusively for an atmosphere model based on 25° Celsius (t(ref) = 273.15+25) at mean sea level, where h(ref) = -354m.

The screenshot shows waypoint 5/8. The AI as pilot in command accelerates to approx. 380km/h at 1832m AMSL and will hold the speed as long as no other event forces a change.




greybeard_52_2024-02-29.png

[ATAG_Lenny]

So does that mean it is really TAS? Everything I have read, including the manual said it was IAS.
Lenny

[GANIX]

Hello Lenny,

IAS is essential for human pilots. However, the AI-Pilots do not know anything about IAS. So, there are a couple of good reasons to assume, that 1C:Maddox implemented this value as 'Calibrated Airspeed'. It 'really was' CAS in iL2COD_v1.11.20362 and I have reason to believe that a couple of still hidden equations 'behind the curtain' work with it.
If you like, I can give a more detailed answer. Please tell me what manual you refer to?

Postscript: In the meantime, there are loads of posts dealing with airspeed, whatever it may be. It would be great if we could gather and reference them here in a subforum, like 'All About Airspeed' for example.

So does that mean it is really TAS? Everything I have read, including the manual said it was IAS.
Lenny

[GANIX]

Thank you very much! I asked because often I see AI keeping a cruising speed significantly lower than that set in FMB (I check in flight during simulation on my airspeed indicator, while following my AI leader)....

@greybeard_52, your observation is nearly correct!

The quantity “Speed 347 km/h” represents the Calibrated Airspeed (CAS) at TF-Blitz mean sea level, where TAS, CAS and EAS/IAS are equal.

When the mission starts, your aircraft will spawn at a geometric altitude of 1,832 m above mean sea level (MSL) with a true airspeed (TAS) of 347 km/h. Engage the autopilot and observe how the AI pilot in command responds.
The underlying equations “behind the curtain” treat 347 km/h as a function of compressible pressure qc(h,TAS). Specifically, 347 km/h CAS produces the same compressible pressure qc(h,TAS) at standard mean sea level as 380.0 km/h TAS at a pressure altitude of 1,832 m. Therefore, as long as the AI remains in command, the aircraft will accelerate from an initial 347 km/h TAS to approximately 380 km/h TAS at a pressure altitude of 1,832 m and will attempt to maintain that speed.

Watch out, the German Team Fusion airspeed indicator (Blitz! 5.046) shows the ratio 380kmh : 341kmh (TAS:IAS) at 1,832 pressure altitude. The airspeed indicator should indicate approximately 347 km/h, but it doesn’t.
The German airspeed Indicator urgently needs repair or recalibration.

[Artist]

Short version:

A. The speed set in the Full Mission Builder is Ground Speed (GS)

B. What you see on your speedometer in the cockpit is Indicated Airspeed (IAS)

Details:

A. The speed set in the FMB is Ground Speed (GS)

Only this makes sense, because in the FMB you plan the flights ("start here at this time, be there at that time"), so you must use GS.
It is your responsibilty here to not demand a Tiger Moth to achieve 200 km/h GS against a headwind of 50 km/h while trying to ascend from 1000m to 5000m.
(BTW the FMB activly prevents you from entering 500 km/h GS for a Tiger Moth, replacing it with the model's maximum speed - based on sea level with no wind GS=TAS=CAS=IAS)

IAS/TAS/CAS would make no sense here, because if you change altitude, temperature, wind (direction and/or strength) you'd need to adjust speed in all waypoints.


B. What you see on your speedometer in the cockpit is Indicated Airspeed (IAS)

It takes into account: height, temperature and wind (both strength and direction)

[FTC_Rostic]

Short version:

A. The speed set in the Full Mission Builder is Ground Speed (GS)

According to simple time measurements it seems that it is not GS at all. At least in running mission.

[Artist]

According to simple time measurements it seems that it is not GS at all. At least in running mission.

I am not talking about AI (yet). I am talking about FMB value and what the player sees in the cockpit.

When you make an airspawn you see that the GS of the FMB translates correctly into the IAS that you initially see on your speedometer.