IAS versus TAS

[MajorMagee]

I was trying to find the location of the parameter that controls the max airspeed damage this evening and fell into a bit of a trap.

I looked in the air file, cfg, and xpd, and each has some likely looking values to set. No matter what I changed the damage continued to occur at the same speed.

Oh, I forgot to delete the bdp file. Nope still no apparent.

Finally I switched from flying with my default Indicated Air Speed instruments, to displaying True Airspeed, and suddenly the damage was happening when the airplane reached the value set in the xpd. That lead me to testing at a variety of altitudes and the damage was happening at different IAS values and constant TAS.

To my way of thinking this is backwards from the way it should be. Damage does not occur from hitting a particular true airspeed, but from the pressure forces on the structure, and Indicated Air Speed is a better indication of this.

Am I thinking about this correctly, and the programmers got it backwards, or do I misunderstand how this should work in real life?

 

[gecko]

Yes, you are thinking correctly, but I am confused by your results. I think in the past I have adjusted this using a maximum mach value in the air file, and thus not tied directly to IAS or TAS.

 

[MajorMagee]

I had changed the Mach setting in the Air File as well, so that may well have been what was controlling it, but then it should have been at a fixed value using IAS, right?

Is there a Mach Meter Gauge for one of the aircraft?

 

[MajorMagee]

Okay, I see there is one in the F-86. I'll do some experimenting with that.

 

[MajorMagee]

Well, it turns out the Mach Meter in the F-86 does vary with altitude as it should (see quote below), and the overspeed damage limit is set in the Air File as a Mach Number, and not the True Airspeed as I thought it was doing in the xpd.

For example, if the M[SUB]MO[/SUB] is Mach 0.83, then at 30,000 feet (9,144 m) where the speed of sound under standard conditions is 590 knots (1,093 km/h; 679 mph), the true airspeed at M[SUB]MO[/SUB] is 489 knots (906 km/h; 563 mph). The speed of sound increases with air temperature, so at Mach 0.83 at 10,000 feet (3,048 m) where the air is much warmer than at 30,000 feet (9,144 m), the true airspeed at M[SUB]MO[/SUB] would be 530 knots (982 km/h; 610 mph).

Checking in game for Mach 0.83 I get:

Altitude_____TAS_____IAS
Sea_Level___ 645____700
10,000______625_____580
20,000______595_____475
30,000______570_____385

It appears that CFS3 IAS is calibrated to match TAS at about 5,000 ft.

 

[MajorMagee]

It sure would be nice to have a Mach Meter as part of the on-screen HUD Gauges (F5) when you nose over into a steep dive from high altitude.

I don't suppose there's any way to add one?

 

[gecko]

That all sounds about right Andy. As far as I can tell, CFS3 models a standard day atmosphere at all times. While I wish we had more control over those variables, it does make calculations easier. https://en.wikipedia.org/wiki/Standard_day