The McDonnell Douglas Explorer was designed with the customer more than just in mind
Guy Norris/MESA, ARIZONA
As the latest commercial machine from the manufacturer of the AH-64 Apache attack helicopter, it is reasonable to expect the McDonnell Douglas Helicopter Systems (MDHS) Explorer to incorporate the latest in rotary-wing technology.
What is less obvious, but more important, is the way in which customers were involved from the skids-up in how, what and where these advances were used. As a result, the true hallmark of the Explorer is not so much the technology itself, but the way in which MDHS' approach has produced a tailor-made helicopter which has been designed virtually by its own marketplace.
The genesis of the Explorer dates to 1986, when company engineers hit upon the idea of using the latest technology, such as an all-composite main rotor and MDHS' own no-tail-rotor (NOTAR) anti-torque system, as a basis for a new eight-seater design which would give excellent performance at affordable cost. Eight years later, MDHS has begun delivery of the Explorer with a target direct-operating cost of $389/h and a base price of $3.16 million at 1995 exchange rates.
In 1987, MDHS began canvassing more than 2,700 turbine helicopter operators around the world to define the preferred characteristics of a 2 to 4t-class utility helicopter. What emerged was a twin-engined machine capable of carrying a 900kg internal payload, or a 1,360kg external-sling load, and which would be able to be flown for 685km (370nm) at a sea-level cruise speed of 150kt (280km/h) in still air.
MDHS decided to go ahead with design work in January 1989. despite a stagnating marketplace. In a major departure from its normal design practice, MDHS invited 15 operators to send representatives to its Mesa, Arizona, headquarters to form a "Blue Team." This customer-advisory group kept the MDHS "Red" design team focused on the market-driven principle and introduced several changes during the evolution of the helicopter, which was christened the MDX.
"The Blue Team represented all segments of the operator community by looking over our shoulder every step of the way through design, development and testing of the Explorer. When we got off track, they nudged us," says company senior vice-president and general manager, Dean Borgman. "Sometimes the nudging was more like a shove and, maybe once or twice, they had to swat us with a rotor blade just to get our attention. But we listened, and are very glad we did."
MDHS also listened to a senior advisory council formed from risk-sharing partners in the $200 million programme. These included AIM Aviation, Canadian Marconi, Hawker de Havilland, Hughes-Treitler Manufacturing, Israel Aircraft Industries, Kawasaki Heavy Industries, Lucas Aerospace, Pratt & Whitney Canada and Turbomeca.
Designed in cyberspace
The Explorer is the first helicopter to be created using computer-aided design (CAD) techniques, MDHC says. The company says that its use of CAD, based around the Unigraphics II system, pre-empted even Boeing. "We were well into computer-aided design before our friends at Boeing began using it on the 777," says Borgman.
Three-dimensional (3D) models of parts were defined by the CAD system and used as the "master" plan, instead of the normal 2D drawings. The models supported the development of an electronic development-fixture which eliminated the need for a physical mock-up. As well as allowing the use of a centralised design database, which was used to check that everything fitted together, it also allowed the Explorer to be designed differently to suit the needs of the assembly line.
"For the first time, we were able to take a unique approach and do the drawings by the way the ship is built, rather than by subsystem," says MDHS commercial engineering director, Evan Sampatacos. The complex rotor-support build-up model, for example, includes parts which would have otherwise been presented in separate rotor, airframe, control and drive system installations.
"A major challenge was in integration. Helicopters are beasts in which you can't change anything without changing something else. The CAD system helped there because it tends to make a much larger percentage of parts fit together first time," says Sampatacos.
Tough performance goals and a vicious attack on weight demanded the widespread use of composite materials. As a result, the Explorer is the first helicopter to have a major portion of its primary structure constructed from composites. This is most evident in the fuselage, which is manufactured by Hawker de Havilland in Australia. Skins, floors, tub/keel beam and aft-fuselage assemblies are made from a pre-impregnated carbon-fibre composite with a toughened epoxy resin system produced by Hexel. Hawker de Havilland refined manufacturing techniques after the first three fuselage units and standardised on a final design which is around 10% lighter than the development fuselages, weighing in at just 260kg.
Metallic parts consist of the titanium roof, which provides protection from fire in the engine area, the main frames, fittings and forward-cockpit structure. Two aluminium plough beams form the primary structural support for the nose and provide enhanced crash-protection. In the event of a forward impact with the ground, the beams are designed to keep the nose of the helicopter from tipping down.
"This and the [Kaman] K-MAX are the first new helicopters to be certificated with the new crashworthy requirements," explains Sampatacos. "There are three stages of energy absorbtion: the undercarriage, belly crushing and the stroking of the crash-resistant seats." Other safety features include rollover protection, self-sealing and puncture-resistant fuel tanks and break-away fuel couplings and valves.
In the passenger configuration, the Explorer's 1.44m-wide cabin provides enough space for two rows of three 480mm seats , with a seventh passenger seated in the co-pilot's position. Without seats, the helicopter has a completely flat floor which is accessible via a rear-access door and large sliding doors on either side of the cabin. The Blue team wanted a long flat floor to take objects up to around 4m long. It also wanted a door up to 1.3m wide so it could take 1.2m plywood sheets, according to Sampatacos. The two keel beams supporting the floor also provide walls for a 602litre underfloor fuel tank.
CAPACITY STATIC MAST
Mounted above the cabin is the rotor-support structure which, in MDHS tradition, uses the company's tried and tested static-mast concept used on the MD 500 and AH-64 Apache. Although torque is still passed from the rotor through the transmission, all lift and other rotor loads are transferred down to the keel beams via forward and aft "A" frames. At their base, the aluminium-alloy frames support the landing-gear skid mountings.
"By using the static mast, all loads are taken out before they reach the transmission. By tuning the stiffness of the mast, it means we don't need to add any parasitic vibration-absorbers," says Sampatacos. MDHS believes that the system is inherently safer because of this ability to reduce vibration through dynamic tuning, as well as the concept's in-built structural redundancy. The oil-lubricated transmission system, relieved of its lift loads, can therefore be made simpler and lighter with an expected mean time between removal of 5,000h. It is also easier to maintain because the static-mast and truss design allows the transmission to be removed without the need to remove the rotor head and blades. This also obviates the need for re-rigging, providing another major time saving.
The tailboom and empennage are all-composite primary structures made by MDHS using the same carbon composite and toughened resin as the fuselage. Like the fuselage, the early tailboom design was altered slightly for the final-production configuration to give a 25% weight saving. As the tailboom is hollow to accommodate the NOTAR system, it has aerodynamic surfaces on the inside, as well as the outside. Slots run the length of the right-hand side of the boom to allow air to escape and create the Coanda effect at the heart of the NOTAR principle.
At the end of the tailboom, the empennage is mounted on an elastomeric isolator. This flexible joint was not in the original design, but was introduced to allow the tail unit to ride the turbulent wake which is created by the relatively square-shaped fuselage. A two-spar composite tailplane supports all-moving rudders. The rudders themselves are split into upper and lower segments and are mounted on metal hinge posts.
Although it is not obvious at first glance, the vertical fins are set at slightly different angles, with the left fin canted in by a few more degrees. This, again, was not part of the original design intent, but was done after the first series of flight tests. "We had to move them around slightly until we got just the right loading on all of them," says Sampatacos.
Further optimisation was achieved during test flights by adding small Gurney flaps which project from the trailing edges of the vertical and horizontal fins. "They increase the lift curve of the tail and add stability. It's difficult to predict the hinge moments on something like this, and we used Gurneys on the vertical to tune the hinge moments," he adds.
NOTAR and rotor
At the end of the boom is the most obvious part of the NOTAR system. Instead of the conventional tail rotor, there is a "jet thruster" which swivels to provide yaw control. Air is directed out of the end of the tailboom through fixed louvres in the NOTAR nozzle. The amount and direction of thrust is controlled by swivelling the directional jet-thruster fairing which fits over the nozzle and which is linked to rudder pedals in the cockpit. The Explorer thruster is based on the same concept used on the MD 520N, but is some 30% larger.
Although outwardly the most obvious "new" feature of the Explorer, the NOTAR system has been in operational service since October 1991 on the MD 520N. Apart from handling advantages, the absence of the fast-spinning tail rotor makes NOTAR-equipped helicopters exceptionally quiet. MDHS says that the Explorer generates 81.9 EPNdB (environmentally perceived noise decibels) during a flyover at 500ft (150m), compared with its nearest rival, the Eurocopter AS.355N, which produces around 86.9 EPNdB under identical conditions.
The NOTAR is also seen as a major contribution to improved helicopter safety. As one executive from Petroleum Helicopters, the first Explorer customer, puts it, "...you wouldn't believe the energy we expend on getting people to keep away from tail rotors".
Air is pumped into the tailboom by a variable pitch fan which forms the heart of the NOTAR system. The fan is the same diameter as that used in the MD 520N, but is designed with a more advanced three-dimensional aerofoil. "The blade is the same size [as on the MD 520N], but the aerofoil and camber are different. They've been adjusted for a higher mass-flow with better root and tip sealing," says Sampatacos. The injection-moulded, glass-fibre-reinforced-plastic-filled poly-propylene blades are relatively cheap to make and have demonstrated good erosion resistance, he adds.
The fan is mounted between bulkheads and inside the frames of a support structure located forward of the root of the tailboom and directly above the access door to the rear-baggage compartment. The fan's composite case cover is easily removable for inspection and on-site repair of the fan assembly from within the compartment entrance itself.
The Explorer is the first commercial helicopter with an all-composite, bearingless main rotor using a flexbeam and blade system. "The rotor system has been relatively trouble-free: the main issue has been getting the composite parts built. Otherwise, apart from adjustments to the stiffness of the rotor dampeners, it has been straightforward," says Sampatacos.
Much of the testing has concentrated on the flexbeam, which connects the blade to the hub. Developed originally from the Helicopter Advanced Rotor Programme (HARP), and later fine-tuned in a joint NASA/McDonnell Douglas technology effort, the Explorer's flexbeam is made from a glass fibre-reinforced-plastic composite material like the blade, and is sheathed in a carbon-fibre-composite pitchcase. Toughened resins are used in all the composite elements, while the hub and damper caps are made from aluminium. The leading edges of the blades, which have parabolically swept tips, are protected with a titanium sheath.
"One big advantage of the CAD system was that we could design the cockpit from the pilot's eye," says Sampatacos, who adds that adjustable seats "...keep the design eye in about the right position for different heights". A compact integrated instrument-display system (IIDS) is designed to keep the cockpit uncluttered and give an exceptionally large forward view.
The AlliedSignal-designed IIDS is composed of twin liquid-crystal displays. These show a wide range of data, ranging from health-monitoring and maintenance advisories to cargo-hook displays and rotor-track and balance solutions. The left-hand display shows a pictogram of the engines and fuel status. "There was so much available with the IIDS that we persuaded the Federal Aviation Administration that we didn't need a master caution and warning panel, which is a bit of a bonus," Sampatacos adds.
Power management of the two P&WC 206A (or Turbomeca Arrius 2C turboshafts as an option from aircraft No.129) is achieved with simple cockpit controls and a full-authority digital engine-control (FADEC) system. Each engine is controlled by a switch with four positions: off, idle, fly and train. Idle runs at 60-65% RPM, fly at 100% and train (for training) at 90-92%.
Following Blue Team recommendations, a back-up mechanical throttle was located on the collective in case of a FADEC failure. By selecting the centre detent on the twist grip, the pilot takes over control of the engine from the FADEC by controlling fuel flow. A red button mounted on the lever can be pressed to re-activate the FADEC.
Crew training has begun in earnest for the operators of the first 20 Explorers expected to be delivered in 1995. Some 36 are due to be handed over in 1996 and 48 in 1997, after which the same rate will be maintained.
Source: Flight International