I've scratch built an RC plane and ground station (hardware, software, and frame all scratch built - not using Ardupilot nor a pixhawk) and want to develop an autopilot for it.
I'm familiar with control systems and I'm considering using a software PID controller for a simple pitch and roll holding autopilot (nothing too fancy). The only problem is that I'm not sure what range of gains will keep my plane stable. If the initial gains are too high, it'll go uncontrollable and crash. If the initial gains are too low, it won't be able to keep altitude or correct for any disturbances.
In order to get a good idea as to what sort of gains might be acceptable, I would like to find a transfer function based on the physical properties of the plane (if such a generic transfer function even exists). Does anyone know of such? Maybe there is a better way to pick initial gains?