It’s been a while—about 22 years—since certification was awarded for a helicopter design that then and now gets a second look from many who see it. Aviation enthusiasts stare at it and say, “Oh, I get it,” though they often don’t. And non-aviation types just know that something is missing, even if it takes them a little while to figure out what.
The technology is called NOTAR, an acronym for “no tail rotor.” And the concept was pretty simple: Get rid of that pesky, noisy, dangerous tail rotor, and replace it with something safer and more effective.
Hughes Aircraft—the developer and original patent holder of the NOTAR, as well as the builders of the model 500 helicopter it was first made available on—would change owners and names several times before being acquired by its current owner, billionaire investor Lynn Tilton, founder of parent company Patriarch Partners. Tilton retained the most recently used name, MD Helicopters, and continues to offer three aircraft lines with the NOTAR system: the single-engine MD520N, its stretched brother the MD600, and the twin-engine MD902 series dubbed the Explorer. (The company still sells several variations of its tail rotor-equipped MD500, as well.)
Oddly enough, the science behind NOTAR was not new in 1990 when the first MD520N was delivered. In fact, a portion of it predates the first fully functional helicopters ever produced.
To paraphrase Sir Isaac Newton’s Third Law of Motion, for every action there is an equal and opposite reaction. For a helicopter, it means that when the main rotors are spinning in one direction, the rest of the aircraft wants to spin in the opposite direction, unless acted upon by some other force. In early helicopters, the “other force” was generated by a smaller rotor flipped vertically, so it would force the tail—and by extension the rest of the airframe—to resist that unwanted torque. Other designers would use a second rotor system located next to, in tandem with, or atop the first, and turn that second set in the opposite direction. But the most popular system has remained the conventional tail rotor or “penny farthing” layout, a term that originally referred to old bicycles that had a huge wheel in the front and a much smaller one in the back.
Hughes engineers, as well as most everyone in the aviation world, were aware of the biggest hazard associated with the penny farthing layout: People were either getting chopped to bits by inadvertently walking into the tail rotor, or pilots were generating all kinds of damage and injuries if they accidentally struck something with the tail of their aircraft.
At first glance, the NOTAR systems seems to fix that by replacing the tail rotor with a nozzle that directs thrust from the engine’s exhaust to the left or right as needed to counteract main rotor torque. But there are actually three things going on that make the NOTAR system work, and harnessing engine exhaust is not one of them.
In 1924, Romanian-born engineer Henri Coanda (1886-1972) discovered that a stream of air will hug and conform to a surface if it passes closely enough to it, and can apply a force while doing so. He called it the Coanda Effect, and in the 1930s received a French patent for it. The first of the three aspects used by the NOTAR system employs that effect, in that it uses the main rotor’s downwash over the tail boom to help keep the aircraft from spinning in the opposite direction.
To make that happen, they installed a fan—dubbed the NOTAR fan—near the root of the tail boom to draw air through an inlet located behind the main rotor mast. The fan forces that air down the inside of the boom and then overboard through long, horizontal slots at the 3 o’clock and 5 o’clock positions. And while those slots are barely noticeable from the outside, they vent enough air to help lower the pressure of the main rotor’s downwash as it hugs the curvature of the right side of the tail. The higher pressure remaining on the left side of the boom creates lift that acts horizontally to the right, thus resisting a substantial amount of the unwanted, clockwise hull rotation induced by the main rotor.
The second aspect of the NOTAR system also uses the airflow generated inside of the tail boom by the fan, but does so by allowing it to escape through the vents on the direct-jet thruster, the signature bobbed tail assembly at the very end of the boom. Quite simply, the air that reaches the thruster passes through a fixed cone that has several stationary, vertical vanes on the left and right. That assembly is enclosed by a rotating cone that has an opening equal to about one quarter of its circumference. A hidden system of pulleys and roller allows it to rotate around the fixed cone, thus metering air delivered by the NOTAR fan out either or both sides, depending on which way the helicopter is to be yawed and at what rate.
The third and final part of the NOTAR system is the vertical stabilizers. The one on the left works just like a rudder on an airplane: deflect it to the right, it yaws the nose of the aircraft to the right; deflect it to the left, and the nose yaws left. However, the two stabilizers do not work in unison. Unlike the left stabilizer, which is controlled by the pilot’s pedal inputs, the right unit is moved by a yaw stability augmentation system (YSAS). The YSAS consists of a small electro-mechanical actuator that moves the right stab based on information received from a yaw rate gyro and lateral accelerometer installed in the cockpit. With the switch on, the system counteracts any Dutch roll the aircraft might experience much quicker than the pilot could.
From a pilot’s perspective, the main rotor downwash, jet thruster, and vertical stabs that make up the NOTAR system do not create anything terribly foreign to even the newest helicopter aviator. From an engineering standpoint, the three facets of the system work seamlessly.
When the pilot begins pulling in power to lift the ship off the ground, the counterclockwise rotation of the main rotor blades still requires a left pedal input to keep the nose straight. Upon doing so, a mixing unit makes three simultaneous adjustments. First, it changes the pitch of the blades on the NOTAR fan, which increases the volume of air it pumps down the tail boom to the slots and jet thruster. Second, it rotates the thruster cone to pass more air out of the left side of the assembly than the right. And third, the mixer turns the trailing edge of the left stabilizer to the right. The amount of left pedal applied matches the amount of movement applied to each member of the NOTAR system. But this does not mean that the Coanda Effect, the thruster and the vertical fins are playing an equal role.
At any given phase of flight, counteracting main rotor torque requires different amounts of help from the three aspects of NOTAR. In a hover, 60 percent of the anti-torque function is gained from the downwash of the main rotor through the Coanda Effect, while the remaining 40 percent is handled by the thruster. With no airflow passing around them, the vertical stabs, while still following commands from the mixing unit, aren’t helping at all. As the aircraft passes through effective translational lift—roughly between 16 and 24 knots—the downwash associated with the Coanda Effect becomes less helpful, forward airflow brings the stabilizers into play, and the thruster continues to provide a significant amount of yaw control. At approximately 60 knots and above, things will change again, as the stabilizers take on most of the anti-torque duties, the thruster helps out, and the Coanda Effect provides only minimal assistance.
In the event of an autorotation, the pilot will work the pedals the way he or she would in any other flight profile, because the NOTAR fan will still be driven by the main rotor transmission, which will be driven by the autorotating main rotor. The thruster and stabilizers will also continue to function because they are mechanically attached to the pedals by cables and control rods. Even the YSAS is helping out, as long as electrical power is still present.
Toward the end of the NOTAR’s development, engineers were already happy to find that a NOTAR-equipped helicopter cruising at 110 knots and 500 feet. AGL created a cone of noise that extended only half the distance of other aircraft in its class. This, they say, is due to the elimination of the turbulent air generated by a tail rotor, and makes it arguably the quietest helicopter in the world.
Best of all, NOTAR is a very safe anti-torque system. Even when running at full power, there isn’t thing that will hurt ground personnel. Stick the tail in the trees, and you’ll only scratch the paint. Come near the thruster, and your hat will be blown off.