How do you double the speed of a helicopter? Maybe it shouldn't be as obvious as just stapling a couple extra engines onto it, but it seems to have worked, as the Eurocopter X3 is now the fastest helicopter ever built.
Most helicopters have a top speed of 130 or 140 knots, which works out to about 160 mph. With the seemingly simple addition of two turboprops mounted on little wings, the Eurocopter X3 prototype hybrid helicopter has managed to make it to 267 mph in stable, level flight.
It only takes one extra engine to drive the additional propellers, and no fancy autopilots or stabilizing systems are needed to keep the X3 flying just like a normal helicopter. You probably won't get the same gas mileage, but you will get "impressive climb and descent rates, as well as excellent maneuverability" along with "outstanding capabilities for acceleration and deceleration."
While Eurocopter is willing to entertain the notion that the X3 be made available for disgustingly rich people to smugly cruise around in, you'll probably see it first in search and rescue, medivac, and military operations, where it can act as a little brother to this fearsome beast.