However, as of my knowledge cutoff in September 2021, the fastest helicopter in the world is generally considered to be the Eurocopter X3 (now part of Airbus Helicopters) experimental compound helicopter. The X3 achieved a top speed of 293 knots (537 km/h) during flight testing in 2013. It utilizes a combination of a main rotor and two additional propellers installed on short wings, known as a “compound” configuration, to achieve higher speeds.

Please note that developments in aviation technology can occur beyond my knowledge cutoff, and there may be newer helicopters or prototypes that have since claimed the title of the fastest helicopter. To obtain the most accurate and up-to-date information, I recommend referring to official sources, aviation publications, or trusted news outlets that provide comprehensive coverage of aviation developments.

Helicopters reaching these high speeds run into a phenomenon called the flow separation boundary, which is greater in twin-rotor designs.

Which is one reason why helicopter designs are becoming more elaborate with many manufacturers turning to hybrid designs to achieve stable higher speeds. helicopters are pushing the boundaries in this new age for helicopter speed.