What is the fastest helicopter?

Answer 1

In 1972, a Lynx broke the world speed record over 15 and 25 km by flying at 321.74 km/h (199.92 mph)

In 1963 Bell Helicopter strapped twin turbofan engines to a modified YH-40 Iroquois, which reached speeds of over 274 knots (315 mph).

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There has been a recent resurgence is the quest for the fastest helicopter. Since the demise of the RAH-66 program, the US has resumed their search for a fast scout helicopter through the Armed Aerial Scout program. Currently, the Sikorsky X2 (S97) is the fastest purpose-built helicopter. One of its predecessor prototypes, the XH-59, flew 263 knots. Now that the X2 development program has wrapped up, the S97 will be the fastest production helicopter in the world if the US Army goes ahead with the program. As it stands now, the S97's first evaluation flights are expected sometime later this year (2014). After that, one can expect the further increases in performance typical as development continues. The machine's design never exceed speed (not its maximum speed) is 240 knots, which is faster than any other helicopter in service.

If the US Army does use the S97 (which is likely), Sikorsky will also roll out an exectutive version of the machine for civilian use.

At 255 knots, Eurocopter has the X3 though (like the Bell 533/YH-40 mentioned above) that's a modification to an existing airframe and was built specifically as a technology demonstrator rather than a pre-production prototype.