The S-52 was the first sikorsky to loop. It was in 1949, and the pilot was Harold "Tommy" thompson, he's still alive and has footage the loops. Funny thing, the FAA (then CAA) didn't like what he did and they took his license away, but only for a few days.
I have also read that a Piasecki tandem looped successfully sometime in the late 40s due to a mechanical malfuction.
As for gyros, I've seen old black and white footage of the old gyros with wings tail and fabric covered blades doing consecutive loops at low level.
If any of you has ever seen a good model helicopter pilot fly, then you know that a helicopter is capable of almost any maneuver you can think of if the aircraft is properly designed and rigged. Obviously none of the people out there designing and building full size helicopters seem to find it of much value or they would have built something by now.
It is not illegal to do aerobatics in all helicopters. You can do anything you want if you certify the aircraft in the experimental category.
It seems to me that people in the helicopter industry shy away from this aerobatic stuff. Just look at all of the responses on this forum. I'm not saying anyone should go do aerobatics, the aircraft just aren't designed for it. But what if this same subject was brought up concerning fixed wing? We'd be talking about Pitts specials, extras, eagles, sukois and all kinds of homebuilts. And all of the maneuvers we've done and techniques. The fact is fixed wing aerobatics aren't anymore useful or easier to make happen than rotory winged aerobatics. The only reason we rotorheads are not out flying upside down is that no one makes any helicopter equivilants to a pitts or an extra.
[This message has been edited by helisphere (edited 14 January 2001).]