How to obtain the course angle from the simulation

[Dobid]

Hello, me again ! I'm still trying to implement a PID controller for the x8 UAV.

In the book I'm following, they introduce the notion of successive loop closure. As showed in the book, I'm trying to implement an inner loop controlling the roll angle by commanding the ailerons, and an outer loop controlling the course angle by commanding the roll angle.

My problem is that I can't find in JSBSim, a property that returns the "course angle" denoted $\chi$ as described in these figures from the book :

I've tried attitude/psi-rad, attitude/heading-true-rad and flight-path/psi-gt-rad. When plotted, all of them seem to be the same and representing the yaw angle staying at zero (which I can't control, I only have elevons).

Is there a way of retrieving this course angle $\chi$ ?

Thanks a lot !

[seanmcleod]

Isn't $\chi$ simply based on the angle with regards to the North and East velocities?

  PropertyManager->Tie("velocities/v-north-fps", this, eNorth, (PMF)&FGPropagate::GetVel);
  PropertyManager->Tie("velocities/v-east-fps", this, eEast, (PMF)&FGPropagate::GetVel);

[Dobid]

You're right, it is.

Now, what are the differences between the 3 properties I mentionned in my question ? They all seem to be the same.

[seanmcleod]

Now, what are the differences between the 3 properties I mentionned in my question ?

Think about the wind.

PropertyManager->Tie("flight-path/psi-gt-rad", this, &FGAuxiliary::GetGroundTrack);

double GetGroundTrack(void)        const { return psigt;         }

psigt = atan2(in.vVel(eEast), in.vVel(eNorth));
if (psigt < 0.0) psigt += 2*M_PI;

PropertyManager->Tie("attitude/psi-rad", this, (int)ePsi, (PMF)&FGPropagate::GetEuler);

  /** Retrieves a vehicle Euler angle component.
      Retrieves an Euler angle (Phi, Theta, or Psi) from the quaternion that
      stores the vehicle orientation relative to the Local frame. The order of
      rotations used is Yaw-Pitch-Roll. The Euler angle with subscript (1) is
      Phi. Various convenience enumerators are defined in FGJSBBase. The
      relevant enumerators for the Euler angle returned by this call are,
      ePhi=1, eTht=2, ePsi=3 (e.g. GetEuler(eTht) returns Theta).
      units radians
      @return An Euler angle.
  */
  double GetEuler(int axis) const { return VState.qAttitudeLocal.GetEuler(axis); }

PropertyManager->Tie("attitude/heading-true-rad", this, (int)ePsi, (PMF)&FGPropagate::GetEuler);

[Dobid]

Not sure about what you mean about the wind. However thanks to the pointers to the code you showed I'm able to understand better what those properties return :

jsbsim/src/FGFDMExec.cpp

Line 481 in b2172a4

Auxiliary->in.vVel = Propagate->GetVel();

jsbsim/src/models/FGPropagate.h

Lines 285 to 296 in b2172a4

	/** Retrieves a Local frame velocity component.
	Retrieves a Local frame velocity component. The velocity returned is
	extracted from the vVel vector (an FGColumnVector). The vector for the
	velocity in Local frame is organized (Vnorth, Veast, Vdown). The vector
	is 1-based. In other words, GetVel(1) returns Vnorth. Various convenience
	enumerators are defined in FGJSBBase. The relevant enumerators for the
	velocity returned by this call are, eNorth=1, eEast=2, eDown=3.
	units ft/sec
	@param idx the index of the velocity component desired (1-based).
	@return The body frame velocity component.
	*/
	double GetVel(int idx) const { return vVel(idx); }

So psi-gt-rad indeed corresponds to $\chi$ right ? If the wind blows, the 'drift' induced is taken into account since the frame of reference is the inertial NED frame.

And since they both call to the same GetEuler method, psi-rad and heading-true-rad are both the same euler angle ?

[seanmcleod]

So psi-gt-rad indeed corresponds to $\chi$ right ?

Yep.

And as you can see in the code snippets attitude/psi-rad and attitude/heading-true-rad are always identical.

Now if there is no wind then flight-path/psi-gt-rad will also be the same. But if there is some cross-wind component then flight-path/psi-gt-rad will differ compared to the other two.

[Dobid]

Alright I got it, very clear now. Thanks !