ArduPilot now supports collective pitch quadcopters, which we are calling "HeliQuads". The above video is the first flight test on a WLToys V383. The support for this type of airframe is in this pull request:
The control system for this quad uses independently controlled collective pitch on each of 4 rotors, with a single electric motor that powers all 4 rotors at the same RPM via belts and a torque tube. It is highly aerobatic, with extremely rapid control available.
As part of the support for this frame a new inverted-flight CH7_OPT was also added, which is available for all helicopter frame types. This allows you to fly any flight mode upside down, including AUTO missions. Here is the first test flight of inverted flight in ALT_HOLD
One thing to watch out for in this model is vibration. In the videos above vibration was running at between 6G and 8G, which is really far too much. The EKF was coping, but only just. I will be adding more mass to the flight controller and changing the mounting system for the next flight tests.
Here is a photo of the current mounting system:
Many thanks to members of CanberraUAV who helped out with the build and testing, especially Greg for his work on the build, and Justin for piloting the flight tests.
UPDATE: this has now been merged into ArduPilot master