Using JSBSim as an external FDM for DCS

[seanmcleod70]

@Zaretto has a repo - https://github.com/Zaretto/acEFM that allows you to make use of JSBSim FDMs within DCS.

Currently he maintains a personal JSBSim fork with some custom changes as part of acEFM.

A couple of PRs were submitted recently aiming to do away with the need for maintaining a modified JSBSim fork, and to potentially include changes that would be useful for other cases where JSBSim needs to be incorporated into another simulation environment.

• Add FGAuxiliary setters for host-owned propagation
• Add a per-model execution-enable property

There was some discussion in one of the PRs that wasn't entirely focussed on the specific PR and which includes some information others may find useful if they're interested in using JSBSim as an external FDM with DCS or other similar simulation environments, so this separate discussion item has been created to continue the discussion.

I've taken a new look after your tidy up, in particular glancing through these 2 files.

https://github.com/Zaretto/acEFM/blob/master/flyt-EFM-dcsJSBSim/JSBSim_interface.cpp
https://github.com/Zaretto/acEFM/blob/master/flyt-EFM-dcsJSBSim/DCS_interface.cpp

Given your comment about simply needing to disable the integrators in JSBSim, i.e.

    // DCS owns the state, so set all four Propagate integrators to eNone which
    // disables integration and permits injection of the state injected in
    // set_current_state_body_axis
    fgSetDouble("/fdm/jsbsim/simulation/integrator/rate/rotational", 0);
    fgSetDouble("/fdm/jsbsim/simulation/integrator/rate/translational", 0);
    fgSetDouble("/fdm/jsbsim/simulation/integrator/position/rotational", 0);
    fgSetDouble("/fdm/jsbsim/simulation/integrator/position/translational", 0);

I guess my initial thoughts were that the breakdown between DCS and JSBSim was along the lines in the following diagram, with JSBSim's responsibilities in the grey block, and with DCS taking care of the blocks I've circled in pale blue.

However, looking at the code, it isn't the case that DCS simply takes $\dot{X}$ from JSBSim, integrates it and returns $X$.

Rather, you supply DCS with the total force and total moment (excluding gear) from JSBSim and then DCS uses that, with mass information etc. to run the 6-DOF dynamics, and integrates the result that you then feed back into JSBSim. In other words, the blocks I've highlighted in green are also DCS blocks, including the atmosphere.

void ed_fm_add_local_force(double &x, double &y, double &z, double &pos_x, double &pos_y, double &pos_z)
{
    FGJSBsim *model = get_model();
    x = model->fgGetDouble("/fdm/jsbsim/forces/fbx-total-lbs") * LBS_TO_N;
    y = -model->fgGetDouble("/fdm/jsbsim/forces/fbz-total-lbs") * LBS_TO_N;
    z = model->fgGetDouble("/fdm/jsbsim/forces/fby-total-lbs") * LBS_TO_N;

    pos_x = center_of_mass.x;
    pos_y = center_of_mass.y;
    pos_z = center_of_mass.z;
}

void ed_fm_add_local_moment(double &x, double &y, double &z)
{
    FGJSBsim *model = get_model();

    // jsb
    // x + back
    // y + right
    // z + up

    // dcs
    // x + forward
    // y + up
    // z + right

    x = model->fgGetDouble("/fdm/jsbsim/moments/l-total-lbsft") * LBSFT_TO_NM;
    y = -model->fgGetDouble("/fdm/jsbsim/moments/n-total-lbsft") * LBSFT_TO_NM;
    z = model->fgGetDouble("/fdm/jsbsim/moments/m-total-lbsft") * LBSFT_TO_NM;
}

You then feed $X$ back into JSBSim via these and a couple of other functions. Although DCS's alpha and beta are ignored.

void FGJSBsim::set_current_state_body_axis(
    double ax, double ay, double az,                      // linear acceleration component in body coordinate system
    double vx, double vy, double vz,                      // linear velocity component in body coordinate system
    double wind_vx, double wind_vy, double wind_vz,       // wind linear velocity component in body coordinate system
    double omegadotx, double omegadoty, double omegadotz, // angular accelearation components in body coordinate system
    double omegax, double omegay, double omegaz,          // angular velocity components in body coordinate system
    double yaw, double pitch, double roll,                // radians
    double alpha_rads,                                    // AoA radians
    double beta_rads,                                      //AoS radians
    double dT,
    double ro_kgm3
)

void FGJSBsim::set_altitude(double h_ft)
{
    Propagate->SetAltitudeASL(h_ft);
}