Looping a helicopter is a bit of a pants activity. Pointless, rather silly when we think about the mechanics and aerodynamics and, no tactical application whatsoever. In fact, they aren’t even a loop as such. Dive for speed, pitch and a back flip.
I’ve only done one in a helicopter, under tuition from a manufacturer’s test pilot. Was it fun? Kind of. Was it worthwhile? Not really.
You can “loop” any flying machine - once! The ability to maintain energy, SA, and bring weapons to target whilst manouevring said chopper is far more impressive than a “loop” in a helicopter.
As for a 135, Lynx, CH53, Blackhawk, Seahawk, Puma 407 or such doing a loop - why? They are LiftTard platforms. Why deliver the only weapon you have (The soldiers (or paying passengers)) covered in vomit? Time for some to appreciate hovering or go fly a fixed wing designed to loop. Too many frustrated/failed wanna be Knuckleheads?