The world's fastest helicopter is the Westland Lynx, which could reach Mach 0.3, which is about 324km/h (200mph).
The Westland Lynx is a British helicopter designed by Westland and built at Westland's factory in Yeovil, first flying on 21 March 1971 as the Westland WG.13. Originally intended as a utility craft for both civil and naval usage, military interest led to the development of both battlefield and naval variants, which went into operational usage in 1977 and were later adopted by the armed forces of over a dozen nations. The helicopter is now produced and marketed by AgustaWestland.