Rich Lee:
To my knowledge, in over 25 years of aerobatic displays in the 500 series aircraft (I have never flown aerobatics in the MD600N as they have not been tested in that aircraft)...
Maybe not tested but certainly tried. Once. Unsuccessfully. Recall a certain Mr. Ruben Rivero, renown coke dealer/ENG pilot in Miami, Florida and his infamous, "Watch this!" to the impressionable R-44 pilot flying formation with him. Oops. Hey Ruben, ya sure don't get much Coanda Effect from that Notar with the tailboom severed, do ya!
It is interesting that amid all the hand-wringing and tongue-clucking and tsk-tsk'ing going on over the 407 loopyroll, some helicopters have had a careerful of aerobatics. The aforementioned 500-series being one of them, the other being the (drumroll, please) lowly
Enstrom F-28! that is dissected in another thread. Enstrom test-pilot Mott Stanchfield perfected the loop, which was later demonstrated routinely by routine demonstration pilot Mike Meager (for those of us old enough to remember the 1970's). How times have changed.