Loops and aerobatics
Lo Richard,
A quick answer .. yes certain helicopters CAN do a loop like a BK 117 (with certain mod's like clipping part of the vertical stabilizer)
The big criteria is the Rotor HEAD must be of the Fixed - type and not the type that you find on a Allouette / Gazelle / Puma or Mi-24 for that matter .
They all have dragging hinges and use hydraulics to lead and lag ... eischhh this is a long subject - where do I start ..
To answer you question - yes they can and a very well known video clip is of a massivve CH53 "jolly Green Giant" doing a slow roll ... sorry ran out of power om Laptop - power out !
Gunsss