ArduPlane SITL using JSON structure

I have been working on creating an SITL of ArduPlane in Unity using the JSON structure and wanted some guidance for making it.
So firstly I tried the ArduCopter and Rover examples that are given in the ArduPilot repository using MATLAB and Simulink and then transferred them to Unity by creating theirs DLLs and they have been working pretty fine, the only problem was that in the Rover example even when I run it using Simulink, the movement of the Rover is very unstable, it keeps on moving in a zig zag movement
But rest of it is fine, so after that I tried to create an SITL for ArduPlane and for that I used the physics model mentioned in this book, so firstly I created this same model in Simulink and then imported it into Unity but the problem that I am facing is that according to this model I need to give the Plane some initial velocity without which the model crashes due to some values getting changed to zero in the denominator, but then it keeps moving in the SITL without sending any commands, when try to run it using Mavproxy and Unity.

So I needed to know if anyone can provide any kind of suggestion to solve this problem, it will be very helpful. Thanks
Also do let me know if I need to provide any other information for making you understand it better