Add FGAuxiliary setters for host-owned propagation

[Zaretto]

Together with the per-model enable property (#1481), these five FGAuxiliary setters are the main JSBSim-core changes that permit running the engine inside an externally hosted simulation such as DCS World.

Adds to FGAuxiliary:

• Setadot, Setbdot (alpha/beta rates)
• SetNx, SetNy, SetNz (body-axis load factors)

When a host owns the state integration it injects the derived quantities Auxiliary would otherwise compute from the propagation it no longer runs. The host-owned domain is propagation together with ground reactions, tightly coupled through the 6DoF integration and externalised as one unit. These rates and load factors are among the few quantities Auxiliary cannot recompute once that integration is external, so the host supplies them.

Follows the state-propagation review @agodemar raised in #1390 and #1391.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 25.10%. Comparing base (0b99c70) to head (21c95aa).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1482   +/-   ##
=======================================
  Coverage   25.10%   25.10%           
=======================================
  Files         171      171           
  Lines       18843    18843           
=======================================
  Hits         4731     4731           
  Misses      14112    14112           

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[agodemar]

Overall, it looks good to me, thanks @Zaretto for this solution.
@bcoconni and @seanmcleod70 any comments on this?

[seanmcleod70]

Will take a look on the weekend at this PR and #1481 to refresh my memory of the details.

[seanmcleod70]

@Zaretto how come there are so few new FGAuxiliary functions?

Looking at my comment - #1390 (comment) in the previous discussion I noted 12 new FGAuxiliary functions that you had added to your fork at the time but with this PR you've only added 5.

[bcoconni]

There again I agree with @seanmcleod70 and in addition to that, this looks to me like a quick and dirty implementation to solve the particular issues that the DCS interface is facing.

The problem is that in most cases these methods will be no-op's because the content passed to the setters will be overwritten by the method FGAuxiliary::Run, unless the FGAuxiliary model is disabled using the feature submitted by PR #1481. That does not look like a great API design to me.

My point here is not to reject the PR upfront but to tell that:

1. This PR and PR Add a per-model execution-enable property #1481 are strongly interconnected. And they should not be designed and reviewed separately.
2. We need to put much thoughts into these 2 PRs before proceeding. This implies suggesting and reviewing alternative designs to select the most reasonable one based on a discussion about their relative merits.

Also why are the setters not used to allow modifying their related properties ?

jsbsim/src/models/FGAuxiliary.cpp

Lines 438 to 440 in f46a071

	PropertyManager->Tie("accelerations/Nx", this, &FGAuxiliary::GetNx);
	PropertyManager->Tie("accelerations/Ny", this, &FGAuxiliary::GetNy);
	PropertyManager->Tie("accelerations/Nz", this, &FGAuxiliary::GetNz);

jsbsim/src/models/FGAuxiliary.cpp

Lines 454 to 455 in f46a071

	PropertyManager->Tie("aero/alphadot-deg_sec", this, inDegrees, &FGAuxiliary::Getadot);
	PropertyManager->Tie("aero/betadot-deg_sec", this, inDegrees, &FGAuxiliary::Getbdot);

[Zaretto]

@seanmcleod70 With only propagate and ground reactions disabled, Auxiliary still runs, so the only things that
cannot be calculated are the five variables derived from the propagation the host owns.

@bcoconni I think making these five properties that are already bound read/write is a much better solution than adding methods, and one that will permit this all to be done from the XML.

This is not designed as a Q&D way to integrate DCS as I've already done that, as discussed on #1390. This is a real attempt to bring extra functionality into JSBSim that will have more widespread use and increase the flexibility of what JSBSim can be used for. I've already seen one real world case where JSBSim couldn't be used without patches

Merging #1481 into this is my preferred approach but I will take guidance.

[seanmcleod70]

With only propagate and ground reactions disabled, Auxiliary still runs, so the only things that
cannot be calculated are the five variables derived from the propagation the host owns.

Yep, I relooked at the code in that context, i.e. FGAuxiliary::Run() still being called, and the 5 variables this PR enables to be set externally, but I'm a bit confused given that FGAuxiliary::Run() calculates adot and bdot.

jsbsim/src/models/FGAuxiliary.cpp

Lines 176 to 177 in d371f38

	adot = (vAeroUVW(eU)*in.vUVWdot(eW) - vAeroUVW(eW)*in.vUVWdot(eU))/mUW;
	bdot = (in.vUVWdot(eV)*Vt - vAeroUVW(eV)*Vtdot)/(Vt*sqrt(mUW));

I must be missing something?

[bcoconni]

Yes I agree with @seanmcleod70, if FGPropagate is disabled then the PR needs to manually update the properties from FGPropagate that are no longer updated but are still fed to the other models? The code of FGAuxiliary needs not be modified? 🤔

I think making these five properties that are already bound read/write is a much better solution than adding methods, and one that will permit this all to be done from the XML.

Yes, I like this idea with one caveat: the properties are currently read-only (they only have a getter) so you will need to re-wire them on the fly to make them read/write as soon as the model is disabled.

[Zaretto]

Since I first had the idea of using JSBSim inside DCS it's been quite a journey, and the last few months have achieved real progress because now I've got acEFM working with zero changes to JSBSim. Credit for this goes to @seanmcleod70 and @bcoconni for the thorough review of my pull requests that has consistently guided what I've been doing into the right places in the codebase.

In the end all that was needed was simply to set the integrators in FGPropagate to eNone and ensuring the gear stays retracted; which makes #1481 not important.

[seanmcleod70]

@Zaretto great to hear it's as simple as disabling the integrators in JSBSim.

Just trying to understand the comment about in.vUVWdot being stale?

    // Alpha/beta rates - Auxiliary::Run() zeros adot/bdot then recomputes
    // from in.vUVWdot (which is stale in DCS mode). We compute from
    // frame-to-frame differencing and apply AFTER Run() in update().
    if (init_body) {
        double factor = 1.0 / dT;
        pending_adot = (alpha_rads - last_alpha_rads) * factor;
        pending_bdot = (beta_rads - last_beta_rads) * factor;
    }
....
....
    // DCS-provided Vt for debug logging (Auxiliary will compute its own from vAeroUVW)
    double Vt_ms = Magnitude(vx - wind_vx, vy - wind_vy, vz - wind_vz);