SIM_Plane.cpp uses torque as rotation acceleration when updating simulation dynamics

[magate]

Bug report

Issue details

I'm pretty sure there is a bug in the simulation in SIM_Plane.cpp , rot_accel = getTorque(aileron, rudder, thrust, force); returns a torque not an angular acceleration. https://github.com/ArduPilot/ardupilot/blob/e2767f899ffae99b5778745bac5c9cfbbbcdfc88/libraries/SITL/SIM_Plane.cpp#L343C5-L343C15

If we do a quick unit conversion just on the first term in la = qbar*b*(c_l_0 assuming c_l_0 is unit less means (qbar = 1.0/2.0*rho*pow(effective_airspeed,2)*s which would be [kg/m^3*m^2/s^2*m^2] ==> [kg*m/s^2] b = [m] ==> kg*m^2/s^2 which is correct for torque torque = Fxd N*m ==> kg*m/s^2*m ==> kg*m^2/s^2.

ardupilot/libraries/SITL/SIM_Plane.cpp

Line 202 in e2767f8

la = qbar*b*(c_l_0 + c_l_b*beta + c_l_p*b*p/(2*effective_airspeed) + c_l_r*b*r/(2*effective_airspeed) + c_l_deltaa*inputAileron + c_l_deltar*inputRudder);

This is a long way to say that getTorque is returning a torque. This is also clear form adding the alignment where it is force*distance.

ardupilot/libraries/SITL/SIM_Plane.cpp

Line 210 in e2767f8

la += CGOffset.y * force.z - CGOffset.z * force.y;

However, rot_accel which is actually a torque is used to directly update the simulation dynamics https://github.com/ArduPilot/ardupilot/blob/2a218221f06b7ec5b6bb78ed82fdcf35c4db1e32/libraries/SITL/SIM_Plane.cpp#L397C5-L397C20.

This is OK if we assume moment of inertia is Vector3f moment_of_inertia = {1.0, 1.0, 1.0};.

Where this becomes an issue is in QuadPlane if a user wanted to get the moments of inertia used in the copter simulation.

https://github.com/ArduPilot/ardupilot/blob/2a218221f06b7ec5b6bb78ed82fdcf35c4db1e32/libraries/SITL/SIM_Frame.cpp#L572C1-L575C56

This has been corrected there where the variable reflects that it is a torque. #20529

Note

Unfortunately, dividing the plane rotational acceleration by the copter moments of inertia makes the quadplane sim unflyable.

Version
Went through the code on master and it looks like there are no plane moment of inertias. It's possible I'm missing it. It looks like the other sims do this.

Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ x ] Plane
[ ] Rover
[ ] Submarine

Airframe type
QuadPlane

Hardware type
SITL

Logs
Not applicable.