T_Richard
The answer to your question is YES. Some helicopter types CAN loop - BK117, BO105 & Eurocopter Tiger (they are the three that I have seen).
Not all helicopters can loop due to the characteristics of their rotor systems. The beauty of the BO105 and BK117 is that their rotor systems are hingeless (or rigid for want of a better word). What this means is that they have no hinges to allow leading, lagging or flapping of the blades as they fly through the air. The blades themselves do these three functions. The beauty of this system is that they retain control power even down to -1g. To be precise, the BK117 and BO105 retains approx. 90% control power at -1g. What this means is that the rotor system has 90% ability to move into a new attitude at -1g. This is pretty good for a helicopter.
Down side - the ride can be a little bumpy but after the pilot becomes familiar with the flying characteristics - not too bad.
Enough of the Nerdy stuff.
