A prototype I made for a quadcopter controller with autopilot way back in Unity 5 days. 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.

How to use

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 autopilot just click the right mouse button. In the windows build controllers also work by using the analog sticks.

The code is open source. Check it out at https://github.com/simeonradivoev/Quadcopter-Controller