I tried both the changes to xxxxjet-set and the afcs.nas file already has "if (getprop("/sim/systems/autopilot/path") == "Aircraft/Generic/generic-autopilot.xml") { setprop("/sim/systems/autopilot/path", "Aircraft/xxxxJet/zkv1000/Systems/autopilot.xml");" set.
The autopilot path is set correctly when looking at the property tree (in flight) but the angle won't move from 20 degrees.
I'd be surprised if that would work, because according to my understanding FDM/AP/property rules are set up at FG init time and Nasal is initialized somewhat later - so by the time you set the path property the value is no longer read.
I'm not sure what exactly you're trying to do - there's dozens of planes which come with their own APs - why can't you simply start with one of them and study how it's done?
Also, I'm not sure what exactly you're looking for, but if it's for a thesis, a JSBSim plane with the AP also implemented on the JSBSim level might give you much more flexibility in its ability to run it in standalone mode - if you're using the FG-side AP/property rules, you always have to deal with the (at times weird) interaction between two separate modules.
I can point you at APs I've implemented in JSBSim (or with high level Nasal decisionmaking code) - a simple example is the Alouette-III helicopter which has both a 'true' AP for level flight and heading bug and a more sophisticated AP simulating the co-pilot being able to take off and hover the helicopter.
Possibly the most complex example of an AP in FG is the Space Shuttle, it can basically do everything the real thing can (launch, entry, launch abort trajectories, low energy entries, contingency Nz holding entries, orbital attitude management, orbital correction burns, atmospheric flight,...) This is an example of a high-level decisionmaking layer written in Nasal combined with a low-level JSBSim system to hold attitude targets and transit between the various control modes.
Should I be avoiding the xxxxjet if there are legal issues? I am using flightgear as a simulator for a thesis.
The simple answer is - probably yes. Honda claimed that using the model is a copyright infringement, that wasn't contested by Curt and the model was removed instead, if you now dig up and use the plane and even publish something with it, chances are you might get a nice letter from Honda lawyers and find yourself arguing how far the freedom of research gets you (or you might not) - basically any other plane you can choose has no such history.