often provide some directional stability, as many helicopters are equipped with a vertical stabilizer. The pilot would then be forced to autorotate and make an emergency landing with significant forward airspeed, which is known as a running landing or roll-on landing. The tail rotor itself is a hazard to ground crews working near a running helicopter. For this reason, tail rotors are painted with stripes of alternating colors to increase their visibility to ground crews while the tail rotor is spinning. Alternative technologies There have been three major alternative designs which attempt to solve the shortcomings of the tail rotor system. The first is to use an enclosured ducted fan rather than openly exposed rotor blades. This design is referred to as a fantail or "Fenestron", a trademark of Eurocopter (now Airbus Helicopters) for its Dauphin-series utility helicopters. The enclosure around the fan reduces tip vortex losses, shields the blades from foreign object damage, protects ground crews from potential hazard of an openly spinning rotor, and produces a much quieter and less turbulent noise profile than a conventional tail rotor. The ducted fan uses more numerous shorter blades, but otherwise works in very similar thrust principles to a conventional tail rotor. McDonnell Douglas developed the NOTAR (NO TAil Rotor) system, which eliminates having any rotating parts out in the open. The NOTAR system uses a variable pitch ducted fan driven by the helicopter's powerplant, but the ducted fan is mounted inside the fuselage ahead of the tail boom, and the exhaust passes through the tail boom to the end, where it is expelled out one side. This creates a boundary layer which causes the downwash from the main rotor to hug the tail boom according to the Coandă effect. This creates a force which cancels out the main rotor torque and provides directional control. The advantages of the system are similar to the Fenestron system discussed above. There are at least four ways to eliminate the necessity of a tail rotor altogether : Tandem/transverse rotors: to use two non-overlapping main rotors which rotate in opposite directions, so that the torque created by one rotor cancels out the torque created by the other. Such configurations are commonly seen on heavy-lift helicopters like the tandem-rotored CH-47 Chinook. The tiltrotor design, as seen on the V-22 Osprey, is a variation of the transverse rotor design, where the rotors are installed in tiltable nacelles