I have built a custom helicopter autopilot system. it isn't anything incredible, but it took a lot of effort for me to put it together. It is divided into two parts, first is the altitude hold. Use a small keypad, set your desired altitude, and flip a switch or toggle a button to activate, and the helicopter should go up to your chosen altitude.
Second is the Autopilot. Map input A to the target X and Input B to target Y. Once you set the target in the keyboard, turn your autopilot on and it will point in the right direction and pitch forward until you get there. After you wire everything you can tweak it to your particular craft.
If you click on the MC, you will see it has properties to edit for the P/D values for both the AP and the altitude hold so you don't have to go into the MC editor every time you tweak something, for your convenience. It also has other values to tweak such as desired pitch angle (i.e. if your altitude can't hold with AP, decrease this). Feel free to use in your builds, just please give me credit, I worked my tail off :) (I'm not an engineer or a programmer, so I had to learn Lua just for this project, it may be sloppy but gosh darn it, it works!)
It's basically plug and play, so it should work fine. Keep in mind, GPS sensor has to be able to see the sky, it can't shoot through blocks! I designed it for a normal main rotor and tail rotor helicopter, so i have no clue if or how well it works with tandem or coaxial rotors. that said, if you notice any issues please let me know :)
