It worked this time 'cos it has stiffness and dampness
As the tail prop runs off the same drive as the main rotors, it too will slow its spin as the X2 speeds up, but it delivers more thrust by increasing the angle at which its blades cut the air. Thus as an X2-type craft accelerates towards maximum speed, all its blades will actually be slowing right down.
A somewhat similar effort to produce a high-speed stacked rotor chopper was attempted decades ago in the form of the XH-59A experimental craft. But the XH-59A suffered from severe, damaging vibration at high speeds and the idea never caught on. Today's X2 boasts modern superstiff rotors and active vibro-damping gear, and Sikorsky says that it can go at 250 knots while shuddering no more than a regular copter at top speed. Another big score for the X2 over the XH-59A is that all three of its eggbeaters run off a single turbine, where the earlier effort required no less than four engines (two for the rotors and two for thrust). Engine hours are one of the main factors in helicopter maintenance and running costs, so craft developed from the X2 should be considerably cheaper to buy and run.
Not many military aircraft are single-engined, of course, but there are a few: for instance the US Army's veteran Kiowa Warriors, used in roles collectively described as "armed scout". The Kiowa can spot targets for more heavily-armed Apache attack choppers or other aircraft, or attack them itself using Hellfire missiles, Hydra rockets or a variety of guns (it can also carry Stinger missiles for defending itself against enemy aircraft). Unlike an Apache it can instead carry a handful of passengers or other small payloads about.
The US Army really ought to replace its Kiowas soon, and the X2 "Raider" would seem an ideal candidate: it too could carry light armament or small numbers of personnel. Not that many helicopter roles genuinely call for doubling of speed, but presumably military scouting is one area where the X2's legs would be really useful. The only other VTOL aircraft with high speed right now is the famous V-22 Osprey tiltrotor, but this is twin-engined and has various disadvantages including severe downblast and a propensity to melt its landing pad. (Though the Osprey is somewhat faster at 270 knots top end.)
So Sikorsky ought to be onto a winner here with its Raider. But the US Army, like others around the world, is looking down the barrel of spending cuts in the near future: and helicopter "armed scouting" is scarcely the most treasured of its many missions and types of kit. Certainly the service has been happy enough to let Sikorsky make all the running so far, coughing up nothing to help with the development of the X2 to date.
Whirlybird watchers may be waiting a good bit more than four years for the Raider's first flight. ®
*The famous G-LYNX, a specially pimped Westland Lynx, still holds the ordinary-helicopter speed record at just over 216 knots (249 mph/401 km/h).