Reply To: Force VS Airpseed Profile ( Flaw)

[Diego Bürgin]

Hello folks

Diego here.

This problem has been on my issue list for a longer time and I’ve been asked about it from multiple people.
I’ve had a try at it multiple times unsuccessfully, so I’m reaching out to the community now.

What is the problem exactly?

CLS2Sim can read true and indicated airspeed from X-Plane.
(In fact, it can read any value that is in this List.)
Airspeed is used for force calculations, which affect the “Force Scalefactor”, a scaling value for the deflection force.

Indicated Airspeed:
+ It will scale according to air pressure, so dynamic loading using indicated airspeed would be correct.
– When failing the pitot tube, or any indicated airspeed related system in X-Plane, the indicated airspeed will fail also and controls will loose forces.

True Airspeed:
– It will not scale according to air pressure, so the further up you go, the more it will diverge from indicated airspeed.
+ it will continue to give reliable data, regardless of system failure in X-Plane.

So we have two factions of people. The faction wanting realistic dynamic forces, and the faction wanting to be able to simulate failures.
The easy solution would be to let people choose between TAS and IAS, we can have that in the next update, no problem.
But I’m sure we all agree that it would be nicer to have the correct behavior, even if it has to be somewhat simplified,
Or maybe a choice between IAS, TAS, and self calculated IAS?

Now I’ve tried to calculate indicated airspeed myself, but I just don’t arrive at the same or even similar values as I see in X-Plane.
So my question is to you people from the community:

Can you give me the concrete formula to get to indicated airspeed using the available data?
A simplified version that holds up to standard?
I am willing to experiment. I can upload different versions to try out.

The constraint is, that all variables in that formula would have to be available from Datarefs:
http://www.xsquawkbox.net/xpsdk/docs/DataRefs.txt

For example I have the barometric pressure at the point the current flight is at:
sim/weather/barometer_current_inhg

I have the true airspeed in m/s:
sim/flightmodel/position/true_airspeed

I can read the density of the air in kg/cubic meters:
sim/weather/rho

Here’s one of the formulas I tried to implement:
https://aviation.stackexchange.com/questions/25801/how-do-you-convert-true-airspeed-to-indicated-airspeed
(In the first link got stuck on the correction variable “Ki” as I didn’t find it in the Datarefs.
As a last measure, we could give you an expert option, where you enter that variable yourself. But meh. Not happy with that.)

And here’s another one:
https://aviation.stackexchange.com/questions/42793/why-does-indicated-airspeed-change-with-altitude

Want to see the datarefs live in X-Plane, their actual values during flight?
Use this plugin: http://www.xsquawkbox.net/xpsdk/mediawiki/File:DataRefEditor_12-11.zip

So here are my cards. What do you think?

EDIT:
Proposals to test:

Gradual change from IAS to TAS on failure using LP filter.

Regards
Diego