In article ,
Roy Smith wrote:

One of the coolest things I've seen is a helicopter do a roll (at Farnboro
airshow). That was amazing. I had always though helicopters were not
capable of doing anything like that.

Rigid rotar system.
From aging memory...
Back in the 1960's, Lockheed built the Cheyenne prototypes, predecessor
of today's Apache. It had a rigid rotor system and could fly loops and
rolls. The program was cancelled in favor of the cheaper Huey Cobra.
The first commercially built helicopter with the rigid rotor system
approved for aerobatics is the BO-105, followed by the BK-117, now part
of Eurocopter.
Those helos have been performing at the big Eurpean airshows since the
1980's.