Notar makes the grade Tail rotors are a necessary evil on single-rotor helicopters. Aerospatiale made them less dangerous with its fan-in-fin fenestron but now McDonnell Douglas plans to abolish the tail rotor entirely. raham Warwick reports. very action has an equal and opposite reaction. In single-rotor helicopters this basic truth makes the fuselage rotate in the opposite direction to the rotor unless prevented from doing so by the anti-torque tail rotor. A tail rotor may be necessary, but it is a liability. Hovering in confined spaces carries the risk of the tail rotor striking something, while its scything blades are a menace to those on the ground and, in combat, a target for smallarms fire. Now McDonnell Douglas Helicopters has decided to seek commercial certifica tion for a no-tail-rotor (Notar) MD500, to pave the way for its tail-rotorless MDX light helicopter in the early 1990s. Th e decision is based on successful Notar flight tests on a modified OH-6, which first flew in 1981. W hile th e testbed tours the USA demonstrating the benefits of tail-rotorless flight, MDH is working towards commercial certification of the Notar concept by the end of 1989. The current-production MD500E and hot-and-high 530F were both intro duced by Hughes in 1982, the 500E powered by a 420 s.h.p. Allison 250-C20B and the 530F by the 650 s.h.p. C30 variant. Two new higher-performance versions are planned by MDH, a 520L to be certifi cated in December this year, and a hot- and-high 530K to be certificated in June 1989. The two variants will both feature increased-diameter main and tail rotors which increase lifting capability and oper ating altitude, and decrease noise. Based on the 500E, the 520L is powered by the new 450 s.h.p. Allison 250-C20R, while the 530K retains the C30 powerplant of the existing 530F. The tail-rotorless MD500, designated the 520N, will be based on the new 520L, with its C20R engine and increased- diameter main rotor. MDH plans to build two Notar-equipped prototypes, the first of which will fly in May 1989, with certifi cation set for late 1989. The tail-rotorless helicopters will be prototypes because MDH is not yet committed to producing the 520N, but the differences between prototype and pro duction aircraft will be small, it says. Whether the company proceeds into 520N production will depend on market response, however. MDH delivered 69 MD500s in 1987, down from 76 in 1986, but ended the year with its largest order backlog—28 aircraft—in recent years. This level of sales would not be enough to launch the 520N, however. The company is looking for a slightly better market, and believes it will materialise in the next 18 months. Already there are signs. So MDH is being cautious about intro ducing Notar, although it believes the concept to be the most exciting tech nology in recent years. The Notar tailboom is, in effect, a circular-section circulation-control aero foil. As main-rotor downwash flows over the boom it meets higher-velocity air exiting from slots running the length of the pressurised boom's starboard side. The mixing of these flows causes the downwash to adhere to the boom on that side while it separates on the other. This has the same effect as camber on an aero foil, generating a sideforce which opposes main-rotor torque. Th e Notar tailboom is pressurised to about 0-51b/in 2 by a fan driven from the engine gearbox. Fan pitch is varied to keep boom pressure, and therefore slot exit velocity, fairly constant at about four times the downwash velocity. Circulation control provides about two-thirds of the anti-torque mom ent 26 FLIGHT INTERNATIO NAL 27 February 1988