Is the plane in which the X2's tail rotor rotates parallel with the airframe or perpendicular to the airframe? From the image it appears to be perpendicular to the airframe, a bit different than an ordinary helicopter.
Perpendicular.
For the non mechanical engineer types of APS, any non coaxial rotor helicopter needs something to counter the conservation of angular momentum caused by the exerting torque on the rotor blades by the engine. Usually a tail rotor, but sometimes something novel like NOTAR (using Coanda effect to counteract the torque of the main rotor by routing fan exhaust through slots in the tail boom).
Another point of the coaxial design is to drastically reduce dissymmetry of lift, which causes one of the most fundimental speed limits on helicopters. Your advancing blade is moving into the wind, causing faster than normal airflow (thus greater lift). Your retreating blade has the opposite problem, it's moving with the wind, so less lift. Once you get enough of an imbalance, you get "retreating blade stall". In layman's terms, when you want to go faster than the warning labels tell you, the bird wants to flip itself onto the 'weaker' side and pancake the bird. That's bad.
BUT, when you have two blades going in opposite directions, they cancel out the dissymmetry. Well, they're supposed to. When Mr Rotor Tip takes a drive to Supersonic Aerodynamic Lane, unstability can occur. Tis why we're tossing a nifty FBW package. So that's why we think we can make this thing go 287.5 mph.
(Not to nitpick, but it technically doesn't have a tail rotor. It's a six-bladed pusher-type propeller, classed as "auxiliary propulsion")
