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.