gros,
I had a simular problem on mine but it happened after a little hard landing & what I found is that the main shaft on the big gear was actually spinning inside the "hub" in the center of the big gear and only friction & a small portion of the retaining pin (inside the hub) was turning the blades. When I set her down a little hard everything upstairs tried to go down stairs through the big gear if you know what I mean. There was only a slight crack visable in the hub but I replaced it & the problem was solved.

Dont know if this is your problem or not but might be worth a look see