NOTAR

The boundary layer changes the direction of airflow around the tailboom, creating thrust opposite the motion imparted to the fuselage by the torque effect of the main rotor.

A more heavily modified version of the prototype demonstrator first flew in March 1986 (by which time McDonnell Douglas had acquired Hughes Helicopters).

Although the concept took over three years to refine, the NOTAR system is simple in theory and works to provide some directional control using the Coandă effect.

[2][3] A variable pitch fan is enclosed in the aft fuselage section immediately forward of the tail boom and driven by the main rotor transmission.

This fan forces low pressure air through two slots on the right side of the tailboom,[notes 1] causing the downwash from the main rotor to hug the tail boom, producing lift, and thus a measure of directional control.

MD Explorer air ambulance
The Cierva W.9 showing the long tailboom from which the efflux from the engine-driven fan emerged from a directable vent on the left side at the tip of the tailboom
MD Helicopters 520N NOTAR