McDonnell Douglas MD 520N

And thereby hangs a tail

Last September 17, the MD 520N became the first new helicopter in almost a decade to receive FAA type certification. It also became the first single- main-rotor helicopter without a tail rotor for antitorque and directional control to enter production. In the past, efforts to counteract the torque generated in turning a helicopter's rotor — which results in the fuselage attempting to rotate in the opposite direction — have followed two main paths. The most common solution has been to mount small tail rotor at the end of a tailboom. The drawbacks to the design are the obvious safety hazard of location the tiltrotor where it can easily come into contact with people and objects on the ground, its susceptibility to foreign-object damage, and the noise it generates. Another solution was to use two or more main rotors — mounted at opposite ends of the fuselage, at the tips of wings or outriggers, side by side, or coaxially. The downside here is weight and complexity — which help drive up cost of manufacture and operation. When Hughes Helicopters (which later became the McDonnell Douglas Helicopter Company) began developing its Notar system (the acronym, which stands for "no tail rotor," is a trademark of MDHC) in 1976, engineers sought a more elegant solution. They found it in the theories of Henri Marie Coanda (1885-1972), a Romanian-born aeronautical engineer and inventor who postulated that air flowing over a curved surface tends to follow the curve of that surface. We see Coanda Effect at work when air flows over the surface of an airfoil. Using Coanda Effect, the aerodynamics of the tailboom itself could be tapped to provide the bulk of the antitorque force required.

(Coanda, by the way, designed the first jet-powered airplane to fly, in 1910, using a remarkably advanced ducted-fan concept. It crashed on its first flight, due more to Coanda's shortcomings as a pilot than to any design deficiency. He later designed a solar-powered desalinization system. He was a little ahead of his time.)

The first flight of a Notar concept-demonstrator was made in December 1981. The commitment to full-scale development and production came in 1987. The first production prototype flew in December 1989, and the first production unit followed last June.

Looking at the MD 520N, you first note the seemingly massive tailboom, tipped by what is alternatively called a "direct jet thruster" or " variable- area thruster" (pilots call it the "can"). Studying the machine in greater detail, you find that appearances are deceiving.

First, the tailboom is extraordinarily light, its size being related to its aerodynamic role. Made of Kevlar and graphite, the boom, even with its fiberglass stabilizer unit, weighs less than 100 pounds, only a few pounds more than a conventional tail. It is attached to the fuselage by four bolts. Two cannon plugs provide power connections for the anticollision lights and a servo actuator for the right vertical stabilizer. A quick-disconnect plug links the control system. Two people can remove the tailboom in five minutes. The unit is so simple the company hopes to offer a 5,000-hour TBO on it. (Ironically, according to MDHC, the boom's composite construction was of greater concern to the FAA in certification than the Notar system itself.)

Second, the appearance of the can implies that vectored thrust is used for antitorque and directional control. This is only partially true, as 60 to 70 percent of the antitorque force in hover is provided by Coanda Effect. Directional control in forward flight above about 20 knots comes largely from the left vertical stabilizer, which is linked to the pilot's pedals.

In airplanes, they're generally called rudder pedals because they move that control surface; in conventional helicopters, they're called antitorque pedals because they control the pitch of the tail rotor blades. In Notar, the pedals in essence do both (and more, as we'll see below) because they also control the can, which rotates through 210 degrees. Rotating the can governs the amount of thrust by regulating the width of a slot through which air is blown and provides directional control by regulating the direction in which the air is blown. There's tremendous reserve thrust available: The helicopter can perform a pedal turn at the literally breathtaking rate of 120 degrees per second; in a hover, the slot is only open about a three-finger width.

The effectiveness of this system is such that the 52ON is approved for hovering in 30-knot crosswinds and has been flown sideways at speeds significantly higher. Pilots will find the 52ON requires only about one fifth the pedal displacement of a comparable conventional helicopter. Why? To take one example, the blades of a conventional tail rotor undergo constantly changing angles of attack due to changing airflow, particularly in gusty wind conditions, requiring the pilot to compensate with pedal inputs. With Notar, the issue is largely moot.

The idea of using Coanda Effect is not new, Lockheed tried it without real success in the 1950s. Hughes/MDHC engineers discovered why earlier attempts had failed. Simply put, the early assumption was that, to make Coanda Effect work, air must be under great pressure. As the designers learned, the Effect relies on moving a great mass of air, rather than putting air under high pressure. The Notar design relies on low-pressure, high-mass airflow, hence the large volume of the 52ON's boom. (The pressure in the boom is less than 2 psi higher than ambient air pressure.)

From the tailboom attach point forward, a 520N is virtually identical to MDHC's conventional 500-series helicopters. But where the tail rotor drive shaft once exited the transmission, there is now a 19-inch stub shaft connected to the gearbox of a high-speed (5,800-rpm) fan. A 15-inch shaft connects the fan gearbox to the fan, housed just forward of the tailboom attach point.

Air is drawn into the fan (assisted by rotor downwash) through a duct aft of the rotor mast. The pitch of the fan blades is controlled by the pilot's pedals to keep the air pressure in the boom constant. In addition to the variable slot in the can, air escapes through two long fixed slots on the right side of the boom. Air exits the slots at about four times the velocity of the rotor downwash to help keep the downwash attached to the curvature of the boom, resulting in a horizontal component of lift, just like a vertical wing. Presto: Coanda Effect generates antitorque force.

The exhaust stack for the 520N's derated 450-shaft-horsepower Allison 250-C20R turboshaft engine is canted to the left, contributing its own thrust to the antitorque effort.

But aside from the gleam of high technology, what does an investment in Notar technology buy for the helicopter operator? To answer that, Pilot went flying with the MD 520N's launch customer, the Phoenix Police Department's Air Support Unit. The unit took delivery of the first of a seven-ship order last October 31. The 520Ns, which should all have been delivered by the time you read this, will replace five 500Ds acquired in 1979 and 1980.

The most obvious benefit of Notar is safety. In 1985, in a crashworthiness study related to its LHX helicopter program, the U.S. Army found that one in five of its helicopter accidents resulting in fatalities or destruction of the aircraft were directly related to striking the tail rotor, loss of tail rotor effectiveness, or mechanical failure of the tail rotor system.

Phoenix's Air Support Unit has been flying since 1972. In almost 150,000 flight hours, its only mission-related accident involved a 500D whose tail rotor tangled with a chain-link fence. No injuries resulted, and the helicopter was soon repaired, but the event is an understandably frustrating spot on an outstanding safety record.

The unit works closely with other police department units and with other public safety services, assisting the fire department, for example, in about 25 rescues each year. The safety advantage offered by Notar for both flying crews and personnel on the ground was a major factor in the 520N's selection, according to Lieutenant Michael T. Casey, the officer in charge of the unit.

Another is quietness. According to data gathered during certification flight tests, the 520N is the quietest helicopter in its weight class (10 EPNdB — effective perceived noise decibels — below the FAA's Stage 2 regulations). It makes just half the noise of a comparable conventional helicopter. During a Pilot photo session, a 520N lifted off and flew to the far side of the police hangar. Subjectively, the noise coming from behind the building sounded more like a large air conditioning unit than a helicopter.

A quiet helicopter is a boon to police work, says Lt. Casey, where the "bread and butter" mission is night patrol conducted just 500 feet over the city. When orbiting in support of a ground search, helicopters fly at 300 feet agl. Being a good neighbor to slumbering, law-abiding citizens is an important concern, but so is getting the drop on the "bad guys." A 520N is virtually inaudible over the ambient daytime noise level of a city street. Casey, who appears before citizens groups to tell the Air Support Unit's story, says he's already received dozens of compliments on the quietness of the new helicopters.

Safety and quiet address two of the public's biggest worries regarding helicopters, but the 520N has other qualities to recommend it. The helicopter boasts increased payload compared to its conventional forebears. It can lift a ton on its cargo hook, which the police use for rescue work. Speed — the ability to move from one side of the 460-square-mile sprawl of Phoenix to another in 10 minutes is a quality the Air Support Unit exploited in its 500Ds. The unit's 520Ns aren't quite as fast as the helicopters they're replacing because they carry several hundred pounds more mission equipment — some of it externally mounted. Standard in the new ships are a forward- looking infrared (FLIR) system, an improved spotlight, and a vastly more capable avionics suite that offers crews access to 5,000 communications frequencies, compared to 12 in the old ships.

The tactical advantage of the new equipment was demonstrated during a recent spate of uncharacteristically bad weather in Phoenix. Thick fog was used as cover for a series of automobile break-ins; the perpetrators were apprehended using FLIR from atop the solid cloud deck.

The handling improvements made possible by Notar also help the cops do their job. Six of the Air Support Unit's 16 pilots are specially qualified as part of a joint police/fire department Firebird Rescue Team. ("Firebird" is the Air Support Unit's call sign.) The day of Pilot's air-to-air photography was characterized by high winds and turbulence in the lee of the mountains; the pilots half-joked that it seemed mountain rescue calls always come on such days, as one did just before Christmas. A hiker on a steep trail near the top of the city's 2,600-foot Squaw Peak suffered what appeared to be a stroke. Rescuers feared the man would not survive being carried back down the trail. A 520N was dispatched and lifted the victim off the mountain in a litter steadied at the end of a 90-foot cable by a paramedic. The man was delivered to a waiting ambulance at the base of the mountain. (He is recovering.)

Rescue team pilots are trained for that kind of long-line work; for one-skid landings in the rough, mountainous desert terrain around the city, for white-water river rescues; and for high-rise building fire fighting and other efforts in which personnel rappel from a hovering helicopter.

Police crews typically fly two two-hour patrols each night. The unit responds to about 14,000 Priority One calls (generally involving felonies in progress or pursuits), assists in about 1,400 felony apprehensions, and amasses about 5,000 flight hours each year. At these performance levels, the improved handling and diminished work load that combine with Notar's greatly reduced vibration level to mitigate pilot and passenger fatigue are greatly appreciated. And the lower parts count — stemming mainly from the elimination of the tail rotor, drive shaft, and gearbox — should increase reliability, decrease maintenance, and lower operating costs.

Because the 520N's flying qualities of the tail rotor, drive shaft, and gear are very similar to the 500D's, Lt. Casey says, the unit was able to put greater emphasis on mission equipment familiarization than on aircraft handling during transition training. The similarity to the older ships is good news too for the unit's maintenance personnel. MDHC offers a five-hour pilot transition course and refresher training through factory approved training centers.

As of mid-January, MDHC held firm orders for 165 MD 520Ns and options on 25 more. The company had delivered two of the Phoenix police ships plus one of a two-ship order to a South Korean concern. Forty percent of the orders are from overseas. The production rate is building toward five per month; 60 units are scheduled for delivery in 1992. Orders taken today for the 520N and 530N (powered by a derated 650-shp Allison 250-C30) will be delivered in 1994. And MDHC is considering offering Notar as a retrofit/conversion on conventional 500-series helicopters.

The Air Support Unit's transition to new helicopters has not been without growing pains. An on-site MDHC technical representative helps solve the problems that come with breaking in new aircraft. But none of the glitches so far have involved the Notar system, which has been completely reliable. All in all, pilots, mechanics, and administrators are well-pleased with the capabilities of the 520N. "The taxpayers are getting a hell of a deal," says Lt. Casey. And McDonnell Douglas is getting a challenging proving ground for its latest — and most exciting — product.