NOTAR

September 4, 2009 by Tim McAdams

A conventional tail rotor arrangement dates back to the first helicopters designed and produced. However, in addition to the added complexity of drive shafts, bearings, and gearboxes, tail rotors are noisy and susceptible to foreign object damage. In the 1970s engineers at Hughes Helicopters began working on concepts to eliminate the tail rotor for the US Army. They used the acronym NOTAR for NO Tail Rotor. In December 1981, Hughes flew an OH-6A fitted with a version of the NOTAR design for the first time.

The technology that made this possible is based on the Coanda Effect. Discovered in 1932 by Henri Coanda, it is the tendency of a fluid jet to adhere to a solid wall even when the wall curves away from the jet’s axis or direction. The NOTAR system takes advantage of this effect by using an enclosed fan driven by the transmission to force low-pressure air through two slots in the tail boom. This causes the rotor downwash to hug the contour of the tail boom creating a lateral lift vector that counteracts the majority of the rotor torque. The remaining rotor torque and directional control is managed by a direct jet thruster, which is controlled by pedal input.

The system works well and reduces the possibility of loss of directional control from tail rotor strikes in confined areas. Safety of offsite landings where personnel are walking around an operating helicopter is also enhanced. In addition, the system helps reduce maintenance costs and extends the life of structural components by reducing the vibrations levels that are caused by a high-speed tail rotor.

Three years after the NOTAR helicopter’s first flight Hughes sold its helicopters business to McDonnell Douglas. In May of 1990, McDonnell Douglas flew the first civil production NOTAR model, the 520N. In 1997 McDonnell Douglas merged with Boeing to become the Boeing Company. In 1999, Boeing sold the former MD commercial helicopter product lines to MD Helicopter Holdings Inc., but maintained the rights to the NOTAR system.

MD Helicopters Holdings Inc. was acquired in July 2005 by Patriarch Partners, LLC, an investment fund. The company was recapitalized as an independent company and is based in Mesa, Arizona. It currently manufactures several models with the NOTAR system. More information can be found at MD helicopters website.

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9 Responses to “NOTAR”

  1. Dale Long Says:

    Thank you, Sir. Loved the history listen.

  2. Paul Lisowski Says:

    There is a kit helicopter under development in Europe using a NOTAR antitorque system. Check out http://www.youngcopter.com/ for interesting details.

  3. Chris Says:

    Any disadvantages to a NOTAR? Is it as responsive as a conventional tail rotor, and how does it compare with respect to power demand?

  4. Kevin Says:

    Disadvantages of NOTAR? Mainly, it’s more expensive to order an MD ship with NOTAR, and some buyers are still opting for the conventional bladed tail in the 500 series (go to the MD helicopter website for more info). Power demand? A conventional tail rotor also feeds off the main engine, but the NOTAR loses some energy pushing all that air. I agree with a previous poster, thanks for the history lesson on the company.

  5. Fritz Cripe Says:

    The NOTAR system is not only more costly, it also eats away at the cruise speed by approx. 10 knots. According to the MD website, the 500E cruises at 135, the 530F at 134 and the 520N at 123 (lowering the gross weight by 350 lbs. will get you up to 128). This is one of the main reasons the military stayed with the conventional tail rotor system on the MH-6/AH-6 platforms. MD also offers the Quiet Knight tail rotor system, a 4 -bladed system turning at a slower RPM, which greatly reduces the noise footprint.

    Tim, keep up the great articles. It’s nice to see us rotor-heads represented so well.

  6. mick fincher Says:

    Around 1942, a german aircraft designer, Anton Flettner, perfected the FL282 “Kolibri” a helicopter with side-by-side, counter-rotating, intermeshing rotors. It had a rudder bot no tail rotor. Further, I suspect that, provided the rotors where advancing outboard of the fuselage, this arrangement would go a long way to eliminating retreating blade stall, one of the speed limiting factors faced by conventional rotorcraft. Comments?

  7. Ehud Gavron Says:

    Without a tail rotor, the two rotors will cancel the torque of the engine and transmission, and to a certain point will provide lift on opposite sides. After the blade-stall point an increase in airspeed means that each rotor’s dissymmetry of lift meant it was LESS effective on its retreating-blade cycle (and conveniently this was made up by the other blade’s advancing cycle) so in effect there ended up being ONE rotor blade at any given time ADVANCING and providing lift.

    So yes, this helps eliminate retreating blade stall, since the other rotor’s blade makes up for it, and also makes up for dissymmetry of lift across the entire “rotor zone” (when we include the two counter-rotating rotors).

    However, whereas before the blade stall on each 1/2 of 2 rotors there was sufficient lift and power, now there’s less… so if you have a super power-available aircraft where you can pile on the turbines and bring more power to bear, than yes, after such loss of lift (speed greater than blade stall per rotor) you can now function with “1 effective rotor” and continue to have 100% lift and 100% thrust (requiring somewhere >100% normal power). If you don’t have that power, you’re still limited.

    I look at it from an engineering standpoint. My mechanic and pilot friends just shake their heads and walk away slowly. Sometimes they call me for beers. I hope Tim explains it better ;)

    Ehud

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