True Altitude?

[Soldier_Fortune]

Hi FrankB.

Yes: I use ISA for all my above calculations, because the E6-B Flight Computer is based on that standard and, as far as I can see, also the atmosphere model in IL2 is based on ISA.

The equation to calculate manually TAS is:

TAS = IAS* SQRT(AD@MSL / AD_ALT) [1]

Where:

AD@MSL: Air Density @ MSL = 1.225 kg/m3
AD_ALT: Air Density at a given altitude

Surely already you've found the AD_ALT equation. But if not, that is:

AD_ALT = AD@MSL* [(Tmap - ALT*0.0065)/Tmap]^4.25 [2]

Where Tmap is the outside air temperature @MSL of the chosen map.

If you combine both equations, you can calculate TAS directly as an E6-B does it:

TAS = IAS*[Tmap / (Tmap - ALT*0.0065)]^2.125 [3]

WARNING:All the above temperatures are expressed in Kelvin (ºK = ºC + 273), and altitudes in meters. If you prefer to use feet for altitude, then you must change 0.0065 for 0.002 in all the above equations.

The eq. [3] clearly shows the link among TAS, IAS, actual altitude, and OAT at such altitude.
Being (Tmap - ALT*0.0065) the OAT at a given altitude, the eq. [3] allows you to calculate TAS directly with your altitude, speed and OAT gauges readings. And the best is that you can use any speed units: km/h, m/sec, MPH or knots, without a previous conversion.

I.e: The "Betty's" altimeter returns the altitude in meters but the IAS is read in knots (the G4M1-11 has an OAT gauge but it doesn't work; therefore you must calculate OAT by yourself... but you know how to do it.).
No problem: use altitude and IAS into eq. [3] as you read them..!

I hope this help you. Anyway, don't hesitate to ask me.