In 2010 and 2011, I experimented with attaching an ArduPilot Mega to a ready to fly remote controlled airplane. The Arduino-based device navigated with a GPS, IMU, and other sensors.
The ArduPilot Mega 1.0 (no longer sold) provided a variety of built in autopilot functions, including following a GPS path, circling, returning to launch, and stabilized flight.
Beginning in 2013, I began to construct a customized airplane from my own planes. This included the development of a hot-wire foam cutter and a custom autopilot system.