I wanted to bring attention to the work that Tridge and the rest of the CUAV team have done to demonstrate the possibility of flying a Helicopter with a standard FBL controller handling the rate control duties. They have been testing gas powered helicopter for next round of the Outback Challenge which will require a long-range VTOL aircraft. Helicopters are a natural target for this mission of course.
Several of the CUAV team are experienced RC helicopter pilots, but not as familiar with installing a Pixhawk on a helicopter which can be difficult, especially in the case of gas engine helis. As such, they were more comfortable having a normal Flybarless Controller handling rate control. In this case, the Skookum Robotics 540. Tridge has made changes to the code which allow for a pure control pass-through from the RC Rx, through the Pixhawk, and straight to the FBL controller. This pass-through occurs in Acro mode, whereby the Pixhawk running ArduCopter has no effect on the flight control, so even if the Pixhawk should have a major AHRS/EKF failure, the helicopter is still controllable.
Of course, the Pixhawk is capable of controlling a Helicopter without any FBL system, and this is the most common arrangement. But the possibility of running an FBL controller in series with the Pixhawk helps lower the barrier to entry for many existing RC Helicopter pilots. And also offers helicopters similar failsafe-function to airplanes, where they can survive the loss of the autopilot.