History and development


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Tail rotors have been a necessary evil on conventional helicopters with single, main rotors. They help counter the torque, or twisting force, applied to the airframe by the engine drive as it proceeds through the transmission to the main rotor blades. Without a counter force, especially when hovering, the airframe, or body, would tend to spin in the opposite direction to the drive rotation. The tail rotor on a conventional single rotor craft also provides directional control.

Essentially, a tail rotor is a variable-pitch propeller mounted sideways at the rear of a helicopter. It operates exposed and close to the ground. This makes it an unsavoury piece of machinery to anyone re anything that gets in its way. This is especially true when traditional helicopter flight patterns are compared to conventional fixed-wing patterns. Helicopters take off, land, and hover in confined areas; the change direction abruptly. In combat and law enforcement, they often "hide" in foliage or other cover. A tail rotor can present problems, and it is highly vulnerable.

According to U.S. Army safety reports, 20 percent of all peacetime helicopter accidents are caused by tail rotors. User surveys have shown that operators want high tail rotors to increase clearance and add safety margins. Another user concern is that tail rotors are noisy. This is a distinct disadvantage, not only to the military and law enforcement, but to commercial users who must operate in populated areas.

Answers to these problems seem to involve replacing or eliminating the tail rotor. In some large-capacity units, like the tandem-rotor Chinook and the twin-rotor Kaman Husky, the problems are solved by two counter rotating main rotors. They balance, or cancel out, the torque effect. However, a high percentage of operating helicopters is single-rotor, small capacity units. Replacing the tail rotors on these requires new technology, a technology that retains the helicopter's handling characteristics. After all, that's why people fly them in the first place. Now, McDonnell Douglas Helicopter Company has developed an antitorque, no-tail-rotor-system - NOTAR.


Helicopter designers have been working on replacing the tail rotor since the 1940s. Stephan a. Hanvey of McDonnell Douglas Helicopter cites the XV-9 and the XH-17 as examples of single-rotor helicopters that have flown without a tail rotor. Need for an antitorque force was eliminated by applying the force developed through a hot or cold cycle to the blade tips of the main rotor. This approach, however, only addressed the counter force function of the tail rotor. With the XH-17, a tail rotor (albeit smaller) was still needed for differential directional control. "No tail rotor concepts that have failed," says Hanvey, "have tried to use pressure air through jet thrusters only, turning vanes, and other devices. The generally had low control power and low efficiency".

MDH (formerly Hughes Helicopters Inc.) began its research on the no-tail-rotor system in 1975. Preliminary flight tests were conducted on a standard OH-6 model. Based on the results, the company developed its first version of the NOTAR system. From a flying test bed, the first fully integrated NOTAR helicopter emerged in 1981. It was flown successfully during the early 1980s. However, in-flight characteristics were not at the level the engineers wanted.

The concept verified the expected control responsiveness, which, according to Hanvey, "was essentially identical to a conventional tail rotor with improved handling qualities and vibration levels." The practical application, however, did not live up to the concept. Air flow around the tailboom was not as predicted. The adjustment (6" high fences mounted on the tailboom) worked well when the craft was hovering, but not in forward flight. Further simulator evaluation increased the engineers' understanding of the problem. Emphasis was placed on understanding the flow patterns around the tailboom.

Using a scale model place in water, which replicated the results of a full-scale model, MD made a breakthrough in 1985. Shimmering fish scales in the water enhanced the flow patterns, and fluorescent dye laser light. With this method, system changes that once took months to complete could be made in days. Ultimately, an effective air pattern was developed. It used two air slots and no fences. By 1987, it was possible to send a modified OH-6A with a NOTAR system on a tour of the country. This gave pilots access to the new design and gave the company valuable pilot input.

The demonstration model had a 7 degree-twist fan that was only 50% efficient or 33% less than was needed to equal the efficiency of the original tail rotor. While the army predicted that no more than 65% could be achieved, James R. Van Horn, chief of NOTAR technology, reports that final design refinements developed a 16 degree-twist fan (more turbine fan than propeller) that achieves an efficiency of 85% of better. The last corner had been turned. In 1987, the American Helicopter society presented its Howard R. Hughes Trophy to Van Horn; Andrew H. Logan, director of technology, and Evan P. Sampatacos, chief LHX helicopter design engineer, for their work on NOTAR concept demonstrator.

On 29 December 1989, MD successfully flew the first NOTAR-equipped production model, the MD 530N. Is was a 15 minute flight and took place at the company's facilities at Mesa, Arizona. On 4 February 1990, the MD 530N, with 5.6 hours of logged flight time, made its first public flight at the Heli Expo '90 in Dallas.


The primary structure of MD 520N/530N is made of lightweight, composite materials - a feature shared with only one other conventional production helicopter. An all-composite tailboom and empennage (tail assembly), weighing approximately 90 pounds (40,9 kilogram), represents an approximately 20% savings over the equivalent conventional aluminium tail assembly. The MD 520N/530N tail sections are fabricated primarily of graphite composites. Fibreglass is used in the vertical stabilator and Kevlar material, for some fairings and other secondary structures. Incorporating composites into the design increases the payload. It also reduces operating costs by extending component service life. The composite parts are designed to have operating lives of at least 5000 hours, with some parts needing replacement only when necessary.

"Our new design," explains Van Horn, "makes the MD 520N/530N more productive performers. The MD 520N weighs 50 pounds (22,7 kilogram) more than its predecessor, the MD 500E, but can carry 300 pounds (136,2 kilogram) more payload. The MD 520N weighs 48 pounds (21,8 kilogram) more than the MD 520F but has 202 pounds (91,7 kilogram) more payload." A unique feature of the new MD 530N is that is the first composite aircraft to be painted black. Normally, composite materials can be weakened by excessive heat. Previous commercial aircraft with composite structural materials have had to be pained white or heat-reflecting pastel colours. "Our composite tailboom" says Van Horn, "is designed to absorb heat without degrading the integrity of the aircraft structure."

Despite all the positives that can be said for the use of composites, there is a negative. It may lengthen the time it takes for the new helicopters to earn FAA certification. While the effectiveness of composites for primary structures has been proven in government-funded development programs, the methodology for testing these structures is new. The FAA will probably move slower in the certification program while its expands its data base for certifying composite aircraft.

Van Horn says that except for the introduction of turbine engines, "the NOTAR system represents the first significant configuration change to conventional helicopters since 1939 when Igor Sikorsky flew the first conventional rotocraft. "The new system uses the Coanda effect of air flowing over or around the surface of the tailboom to create lateral lift. This counteracts the torque of the main rotor. The NOTAR system eliminates drive shafts, gearboxes, and the rotor unit itself. This reduction in the parts count is a distinct advantage over conventional tail rotor craft.

In operation, the NOTAR system draws low-pressure air in through an air intake located at the top of the airframe to the rear of the main rotor shaft. A variable-pitch fan pressurises the tailboom to a relatively constant 0,5 psi. The air is fed to two starboard side slots and a direct jet thruster. The slots provide the necessary antitorque force. The rotating jet thruster provides direction control.

The two slots are located at 70 and 140 degrees. They allow ejected air to mix with the main rotor downwash to establish the Coanda effect. Hanvey explains: "the main rotor downwash energy (on the OH-6, downwash velocity was about 55 fps) is normally dissipated as essentially symmetric separation on both sides of the tailboom in a hover. The pressurised boom inject low-pressure air at 250 fps onto the Coanda surface (outer surface of the tailboom) which results in the deflection and produces about two-thirds of the required anti torque force. This force is predictable. It is controlled by the appropriate location of the slot and control of the air jet that exits from the slot."

In other words, the tailboom reacts like an airplane wing, only sideways. The increased air speed over the starboard side of the tailboom causes lateral lift, pushing against the torque forces trying to spin the helicopter clockwise. This, of course, is the same result that a tail rotor achieves when it "propels" the tail in a counter-clockwise motion.

"The main rotor downwash," Hanvey says, "skews as velocity is increased, and the circulation control slot is uncovered resulting in proportional loss of antitorque force. The vertical tail surface provides the directional stability with forward speed. In sideward flight, the effective angle of attack is changed as a function of the main rotor thrust and sideward velocity inflow effects." Again, what is happening is that, when the downwash is altered by motion other than hovering, the system reduces the Coanda effect, and the thruster picks up more of the load. This keeps the system forces balanced. The tail fin, which do not come into play when hovering, also become effective when flying forward.

The direct thruster provides the remaining one-third of the force needed to counter the torque of the main rotor. The thruster rotates, varying the opening either to the right or left. In this way, directional control is achieved.

An important by-product of the NOTAR system is the elimination of the medium - to high - frequency vibrations and the noise caused by a conventional tail rotor. Inside the cockpit, pilots report that the absence of vibrations is noticeable both in the pedals and the surrounding cockpit area.

This article was published in Compressed Air Magazine, November 1990 (IngerSoll Rand) and updated with information from Airnieuws, Scramble, Flight International, Air International and different sites of Internet.