(57) Kaman Helicopters History Montage - YouTube

I met a guy who said he flew these syncrocopters in Viet Nam, fun chopper he said. The early versions would get into RBS in a heartbeat, not only that but you had to be real careful with a tail wind because you could also get inot VRS. And sometimes you could get into both at the same time, you know like any 206A. Gotta hand it to ol Charlie, he stuck with that design when everyone said it was bogus.

I have an old friend who flew them with the Air Force in Viet Nam and Thailand.  There call sign was PEDRO,  They used them for search and rescue and runway fire suppression.  He says it was a good helicopter.  When they got N model Hueys (212) he didn't think it was quite as good as he thought. Before the Kmax, operators  here in the Northwest used them for logging and had a number of crashes and killed some people more to helicopter abuse than fault of the machine IMHO. 

CK was ahead of his time

The Marines had one that did pilotless external load lifts in Arganistan.

K-Max, not the original USAF Pedro type.