A prototype I made for a quadcopter controller with autopilot way back in Unity 5.5 (Ported to Unity 2018.2). It uses PIDs for stabilizing the whole drone so that greater control is achieved. Forward movement is done by adding more force to appropriate propellers like in real life.
It also has basic pathfinding and obstacle avoidance so it can go to a specified waypoint.
Use the WASD keys to move the drone and Shift/Space to move it up and down. A waypoint can be chosen by clicking the left mouse button on a surface. To cancel the auto pilot just click the right mouse button.
The drone model is a free one I found on the interwebs. I don't remember where exactly it might have been https://free3d.com/, but I think it just might be ripped from COD MW3
You can try the web build here https://simeonradivoev.itch.io/physical-quadcopter-controller