NOTAR is a helicopter system which avoids the use of a tail rotor. It was developed by McDonnell Douglas Helicopter Systems (through their acquisition of Hughes Helicopters). The system uses a fan inside the tailboom to build a high volume of low-pressure air, which exits through two slots and creates a boundary layer flow of air along the tailboom utilizing the Coandă effect.
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. Directional yaw control is gained through a vented, rotating drum at the end of the tailboom, called the direct jet thruster. Advocates of NOTAR believe the system offers quieter and safer operation.
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
The use of directed air to provide anti-torque control had been tested as early as 1945 in the British Cierva W.9. During 1957, a Spanish prototype designed and built by Aerotecnica flew using exhaust gases from the turbine instead of a tail rotor. This model was designated as Aerotecnica AC-14.
Development of the NOTAR system dates back to 1975, when engineers at Hughes Helicopters began concept development work. In December 1981, Hughes flew an OH-6A fitted with NOTAR for the first time. The OH-6A helicopter (serial number 65-12917) was supplied by the U.S. Army for Hughes to develop the NOTAR technology and was the second OH-6 built by Hughes for the U.S. Army. A more heavily modified version of the prototype demonstrator first flew in March 1986 (by which time McDonnell Douglas had acquired Hughes Helicopters).
The original prototype last flew in June 1986 and is now at the U.S. Army Aviation Museum in Fort Rucker, Alabama.
A production model NOTAR 520N (N520NT) was later produced and first flew on May 1, 1990. It crashed on September 27, 1994 when it collided with an AH-64D while flying as a chase aircraft for the Apache.
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. 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 causing the downwash from the main rotor to hug the tailboom, producing lift, and thus a measure of directional control. This is augmented by a direct jet thruster and vertical stabilisers.
1 Air intake 2 Variable pitch fan 3 Tail boom with Coandă Slots 4 Vertical stabilizers 5 Direct jet thruster 6 Downwash 7 Circulation control tailboom cross-section 8 Anti-torque lift
Diagram showing the movement of air through the NOTAR system
Denel South Africa also did some research on the NOTAR system in the late 80’s, The prototype Denel Cristel was seen at a few airshows the Christel was a converted Alouette III. The Cirstel principle splits the high pressure air from the engine for use in the Coanda slots, while the low pressure air is bled-off for the tail thruster and to mix and cool the engine exhaust. The thruster nozzle is a Denel-patented clamshell design, unlike McDonnell Douglas' `rotating can' concept. Cooling the exhaust air has the added benefit of minimising the aircrafts Infra-Red footprint.
Benefits of the NOTAR system include greatly reduced external noise (NOTAR-equipped helicopters are among the quietest certified helicopters). This is because up to 60% of the noise from conventional helicopters is produced by the interaction of the tip vortices of the main and tail rotor.
Helicopter accidents may be caused by the tail rotor striking tree branches, power lines, the ground or other obstructions. Eliminating the tail rotor removes this hazard and enables NOTAR helicopters to go where tail rotor layout helicopters cannot i.e. close to trees or buildings. They are also safer for ground crews to work near as there is no danger posed from a spinning tail rotor.
Since there is no interaction between tip vortices of the main and tail rotor, the operational vibration is reduced.
The thrust force of the coandă effect caters to the need of antitorque force. As the torque effect requires more antitorque, the Coandă effect provides more lift to provide that antitorque.
Mc Donnell Douglas MD-900 Explorer
The NOTAR system is not as efficient as the tail rotor, and NOTAR helicopters sacrifice some power as a result.
Although generally agile and stable, at high forward airspeeds the properties of the airflow over the tail boom change, and the Coandă effect fails. The 'H'-shaped tail characteristic of NOTAR helicopters is used to provide anti-torque at speed using conventional moving control surfaces. As a result, the helicopter can be difficult to turn when traveling at speed, and the large control surfaces of the tail inhibit maximum sideways velocity.
The translating tendency and the tail rotor roll forces continue to exist.