[Flightgear-devel] Phidot not zero.. And bank angle constant...

[Nicol C. <nic...@ho...>]

Hi guys.

First of all (before I start "complaining") let me start off by saying: 
thanks for sharing your work publicly! MUCH appreciated. If you don't mind, 
and have some time to spare me, I need to ask a question...

About myself and my project: I am qualified as an electronic engineer (did a 
masters in control of a model RC helicopter), and have about 6 years 
simulation experience in the Aerospace industry. I am trying to use 
FlightGear for autopilot/AHRS development work... I am looking at the data 
as received from the Native-FDM UDP packet (at 20-40Hz). Maybe I am missing 
something (not at all impossible)... but I think something might be wrong... 
I am using FlightGear Version 0.9.10.

My question:
If I put the aircraft (Cub or 172) into a constant bank and pitch angle turn 
("trimmed"), I expect phidot and thetadot to be near zero, and psidot 
non-zero... Yet, I see thetadot near zero, and phidot and psidot non-zero 
(as a matter of fact: phidot is almost as "large" as psidot). Surely this 
can't be right? According to the FlightGear (and MathsWorks / Matlab) 
documentation, this is not p,q,r but Euler/Gimbal angular rates in 
rads/sec... not body rates...

I know that the pitch and roll angles are constant because I can see it 
visually in the sim, and confirmed looking at the UDP data (roll, pitch, 
yaw)...

My goal: I want to derive p,q,r (simulate rate gyroscope sensors) and 
calculate the horizon... Building an AHRS. But if I use the current data, 
the angles are drifting even faster than using the real hardware!!

Am I missing something, or is there a bug??

Kind Regards,
Nicol.

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