Why do engines NEED to be in the "Engines" directory in FlightGear?

[Octal450]

Hi,
It seems I MUST place engines in the "Engines" directory in FlightGear JSBSim FDE. Anywhere else, and JSBSim "can't find" the file even if I specify the path like I do for every other thing.

Why is this?

Kind Regards,
Josh

[seanmcleod]

So here is the logic in JSBSim for looking for an engine file.

jsbsim/src/models/FGPropulsion.cpp

Lines 440 to 461 in 57361d0

	SGPath FGPropulsion::FindFullPathName(const SGPath& path) const
	{
	SGPath name = FGModel::FindFullPathName(path);
	if (!ReadingEngine && !name.isNull()) return name;
	#ifdef _WIN32
	// Singular and plural are allowed for the folder names for consistency with
	// the default engine folder name "engine" and for backward compatibility
	// regarding the folder name "Engines".
	const array<string, 2> dir_names = {"Engines", "engine"};
	#else
	// Allow alternative capitalization for case sensitive OSes.
	const array<string, 4> dir_names = {"Engines", "engines", "Engine", "engine"};
	#endif
	for(const string& dir_name: dir_names) {
	name = CheckPathName(FDMExec->GetFullAircraftPath()/dir_name, path);
	if (!name.isNull()) return name;
	}
	return CheckPathName(FDMExec->GetEnginePath(), path);
	}

Basically it first looks for the engine file, e.g. CFM56 in a Engine or Engines sub-directory of the aircraft, see lines 455-458.

If that fails, then via line 460 it looks for it in the specified EnginePath.

Now EnginePath in JSBSim stand-alone:

jsbsim/src/JSBSim.cpp

Lines 359 to 363 in 57361d0

	FDMExec = new JSBSim::FGFDMExec();
	FDMExec->SetRootDir(RootDir);
	FDMExec->SetAircraftPath(SGPath("aircraft"));
	FDMExec->SetEnginePath(SGPath("engine"));
	FDMExec->SetSystemsPath(SGPath("systems"));

Where the root path can be specified via the command line, otherwise it defaults to the current directory.

--root=<path> specifies the JSBSim root directory (where aircraft/, engine/, etc. reside)

Now, when it comes to FlightGear they may setup EnginePath differently. Although even if they do, you should still be able to bundle your engine file in a engine sub-directory of the aircraft.

For example here a snippet from FlightGear.cxx that lives in the JSBSim repo, although note that this was last updated 7 years ago, so you would need to double-check FlightGear's current source code.

jsbsim/examples/FlightGear.cxx

Lines 224 to 230 in 57361d0

	SGPath aircraft_path( fgGetString("/sim/aircraft-dir") );
	SGPath engine_path( fgGetString("/sim/fg-root") );
	engine_path.append( "Aircraft/Generic/JSBSim/Engines" );
	SGPath systems_path( fgGetString("/sim/fg-root") );
	systems_path.append( "Aircraft/Generic/JSBSim/Systems" );