I remember Tim C telling us about this in his series of helo PofF lessons, aka "101 ways to kill yourself in a helicopter". He showed us some interesting pics of the tip vortices off a Gazelle gathering together and inpinging quite neatly right where the Fenestron was, once you had yawed about 30 - 40 degrees. The rotation direction of this vortex was the same as that of the Fenestron, thus giving rise to the theory that the Fenestron might lose effectiveness. There was also the eye-watering footage of the French tests, but I seem to remember him saying that the French might have changed the rotation direction of the Fenestron on their Gazelles. Any truth in this?