Adding a new SIM Aircraft to SITL

I am attempting to add a new style of aircraft into the SITL environment that can run on FlightGear as well. I am basing it on the X-Frame in the quadcopter set ups.

So far I have found that to add the sim, I will need to edit the following sections:
As well as add a new Flightgear aircraft model.

The AP_Motors library is easy enough to understand and edit.

I have now been working through the libraries/SITL folder and I am a bit confused by the Motors::calculate_forces function. They define an arm_scale and a yaw_scale. My understanding is the arm_scale should be the rotor arm length from the centre and I am not sure about the yaw_torque. The values then make sense

Please help and answer, What are these values, Where do these values come from and in general am I taking the right approach to adding a new model into the SITL environment?

Thanks very much in advance.