Hi Iman
I advise to use an airframe that you are able to control manually. While tuning your airframe for autonomous flight, for landing in difficult terrain and whenever anything goes wrong with the autopilot you need to be able to fly your airframe manually.
If you have troubles controlling a very unstable airframe manually, you can stabilize it using standard RC gyros. It also helps to use some expo on your RC transmitter. And RC training is essential, on the field and if you like on a PC simulator. With these measures you can control any fixed wing airframe manually.
Regards
Reto