WITH 95 AIRCRAFT delivered in the first year, production running at 10-12 a month and the orderbook full for the next year, Bell Helicopter Textron has a success on its hands, the scale of which the helicopter industry has not seen for some years. By offering a combination of improved performance and aggressive pricing, Bell has successfully tapped the long-sought market to replace its wildly successful Model 206 series.
Costing at its introduction just $115,000 more than a 206L-4 LongRanger, the Model 407 has generated an enthusiasm among operators which Bell has not witnessed for some time, says product manager, commercial development, Dale Cato. For a marginal increase in acquisition price and operating cost, Bell has produced a commercial helicopter with a greater payload, faster cruise, longer range and higher hover ceiling than that of the 206L-4.
When the 407 programme was begun in late 1993, engineers were set a tough design-to-cost goal which stated that the aircraft price could be increased by no more than $100,000 relative to the 206L-4. Incorporating a more expensive rotor system meant that savings had to be found elsewhere. Cato says that Bell "came close" to its goal and adds that efforts to take cost out of the rotor and airframe continue.
NEW DYNAMICS
At the heart of the 407 is a new dynamic system - engine, transmission and rotors. The 206 uses Bell's venerable two-blade "teetering" main rotor, but the new design demanded something more advanced. To reduce development costs, the company chose the four-blade rotor from the OH-58D Kiowa Warrior, a military derivative of the 206. The tail rotor and transmission also come from the OH-58D.
The main rotor is 10.7m in diameter, with four 255mm-chord blades. Bell says that the rotor provides increased agility without significantly increasing gust-sensitivity; vibration is reduced and noise is less intrusive than that produced by a two-blade rotor. Cato says that Bell is developing the capability to cruise at reduced rotor-RPM to reduce fly-over noise further.
The composite "soft-in-plane" hub is hingeless; the blades are attached to a flexible glassfibre-reinforced plastic (GFRP) yoke, which accommodates flapping motion, via elastomeric dampers and bearings, which accommodate lead-lag and pitch-change motion, respectively. Atop the hub is a Frahm damper, which attenuates rotor vibration.
The blades, also, are composite. Each has a GFRP skin wrapped around a GFRP spar and Nomex-honeycomb core. Bell says that the hub and blades have unlimited fatigue life. Compared with the OH-58D rotor, the 407 hub has an improved composite material to increase fatigue life. Longer-lasting stainless-steel leading-edge erosion strips replace the estane rubber used to protect OH-58D blades.
Compared with that of the 206, the two-blade tail-rotor provides increased thrust capability, enabling the 407 to be operated in winds up to 35kt (65km/h) from any direction, at altitudes up to 10,000ft (3,050m). Two Kevlar/ Nomex blades are attached to a stainless-steel yoke which has two elastomeric flapping bearings and four Teflon-coated, bronze-lined "monoball-type" pitch-change bearings.
The tail rotor is driven from the engine via a shaft running along the top of the tailboom to a 90degree reduction gearbox at the tip. Attached to this shaft, just aft of the engine, is the impeller for the oil-cooler blower.
LONGER LIFE
Bell has uprated the OH-58D transmission for the 407, with larger bearings and redesigned gears to increase fatigue life. The engine drives the main transmission via a free-wheeling unit and Kaflex flexible driveshaft. Manufactured by Kaman subsidiary Kamatics, the Kaflex driveshaft accommodates movement of the transmission via truss-like flexible plates which provide fail-safe dual load-paths.
The main transmission has two reduction stages; engine speed is 6,317RPM and rotor speed 413RPM. In the first, a spiral-bevel input gear drives a collector gear to which is attached a sun gear, which drives the planetary assembly providing the second stage of reduction. The latter consists of the sun gear, five pinion gears and a fixed ring gear. The rotor mast is connected to the pinion gears.
While the 206 uses a "nodal-beam" transmission-mounting system to reduce the transfer of rotor vibration into the fuselage, the 407 uses a lighter and cheaper solution which is known as SAVITAD - system for attenuating vibration independent of tuning and damping. This consists of two beams to which the transmission is attached via elastomeric mounts. Each beam is bolted to the fuselage roof and restrained by a spring which has a natural frequency lower than the rotor excitation-frequency.
DIGITAL CONTROL
Bell elected to stay with the 206's proven Allison, now Rolls-Royce Allison, Model 250 turboshaft in the 407, but to upgrade to the latest, digitally controlled, "Series IV" engine. The 250-C47B used produces 505kW (675shp) take-off power, and a maximum-continuous 470kW, and is equipped with a Chandler Evans single-channel, full-authority digital engine-control (FADEC) system.
The 250's layout is unchanged: air flows into the single-stage centrifugal compressor, then through a compressor-discharge tube to the combustor at the back of the engine, to flow forward through the two-stage gas-producer and power turbines and exhaust at the middle of the engine, via a collector to a curved duct projecting above the fuselage.
Bell says that the FADEC improves performance and safety, reduces pilot workload, increases time between overhauls (TBO), simplifies maintenance and reduces life-cycle costs. The 407 is the first single-turbine helicopter with the FADEC, and its introduction has presented challenges. In the first year of operation, there were two crashes caused by engine shutdowns resulting from a FADEC wiring fault. The fleet was grounded temporarily as a result.
In normal, automatic mode, the FADEC electronic-control unit controls fuel-metering by a hydromechanical unit. In manual, back-up mode, throttle input mechanically controls fuel flow. Bell says that the FADEC offers faster, cooler engine starts and provides torque, temperature and speed limiting. Data recorded for engine-condition monitoring can be downloaded to a lap-top computer via a data-port in the cockpit.
SPACESHIP
Airframe changes from the 206L-4 are small, but significant. The forward-fuselage structure - nose, cockpit and cabin - is aluminium-honeycomb, with a new honeycomb-core alloy used in the nose and lower fuselage to increase corrosion resistance. Bulkheads behind the crew and rear-passenger seats provide roll-over protection. Bell did not redesign the airframe to meet the latest crashworthiness requirements (the 407 is approved on an amended 206 type certificate), but is "comfortable" with the survivability rate in 206 accidents, says Cato.
Bulged composite side panels represent the major airframe change. These increase rear-seat width by 480mm and provide stronger, stiffer frames for the enlarged windows and composite doors. The panels are carbonfibre-reinforced plastic (CFRP), with GFRP skins incorporating a copper-mesh layer for lightning-strike protection.
Doors are pre-hung and are interchangeable between 407s - a major advance over the 206, Bell admits. Improved fit and increased rigidity reduce airflow noise and eliminate water leakage. All the doors have recessed, automotive-style handles inside and out and are held open by gas struts. The left passenger door is hinged on a narrower, litter door which can be opened to allow longer loads, such as a stretcher, to be lifted into the cabin.
The 407 is a seven-place helicopter, with the pilot and one passenger in the cockpit, and five seats in the cabin: two aft-facing and three forward-facing. The bulged side panels increase rear-seat shoulder room by 10%, while the 35% larger windows improve cabin natural lighting and outside view. Detail improvements include new methods of attaching the windshields, which are screwed (rather than riveted) in place, and windows, which are attached with automotive-style adhesive tape and screws. Polycarbonate mouldings cover exposed structure in the cockpit, to improve appearance.
Aft of the cabin is the intermediate airframe section, which includes the engine deck and baggage compartment, and to which the tailboom is attached. This section has an aluminium internal structure and one-piece CFRP skins either side, saving weight and cost. The engine deck has a heat-resistant titanium pan and fore and aft firewalls. The 0.45m3 (16ft3) baggage compartment has a side-opening door.
The tailboom is aluminium and carries the aluminium-honeycomb stabiliser and vertical fin. The stabiliser is fixed, but has leading-edge slats and auxiliary "end-plate" fins to increase its effectiveness. Competing the airframe structure are the GFRP and CFRP cowlings and fairings that enclose the transmission and engine.
HAND FLOWN
While the OH-58D has a stability- and command-augmentation system to reduce pilot workload, Bell could not afford such a luxury of the 407. Instead, the flight controls were redesigned to reduce the rotor forces. The swashplate which transfers control inputs from the fixed airframe controls to the rotating rotor controls was rephased and vertical pitch links installed, connecting the swashplate with pitch horns on the blade roots. The OH-58D has inclined pitch links, which result in vibration.
These changes have enabled the 407 to be certificated for single-pilot visual-flight-rules (VFR) operation with simple, hydraulically boosted flight controls, as used in the 206. Mechanical linkages from the cyclic stick, collective lever and rudder pedals run under the pilot's seat and up through a vertical control-tunnel between and behind the crew (also called the "broom closet") to the hydraulic power-boost support fitting on the roof. Bell says that the control tunnel, which is part of the airframe primary structure, plays a key role in protecting the occupants in a crash.
The company made extensive use of a technique known as stereolithography (SL) on the 407 flight controls, to reduce the time needed to produce complex parts such as the power-boost support fitting. SL is a photochemical process for producing plastic models of parts direct from computer-design data. Using a rapid-casting method called QuickCast, developed by 3DSystems, Bell was able to use these models as patterns for investment casting of production parts. This avoided the expensive and lengthy process of machining the hard tooling required to produce the wax patterns used in conventional investment casting. The manufacturer estimates that it saved six months by using SL, including producing the complex power-boost fitting within 60 days.
Bell does not plan to certificate the 407 for instrument-flight-rules (IFR) operations, but is working with several autopilot suppliers interested in performing the work. "We are getting a lot of requests for IFR, but it is not economically feasible for Bell to do, so we've talked to the autopilot manufacturers and offered them technical support," Cato says.
SIMPLE SYSTEMS
Fuel is housed in two interconnected, rupture-resistant bladder tanks with a total capacity of 477litres, giving a range of 655km (340nm) at the 2,270kg maximum take-off weight. The forward cell is under and between the aft-facing cabin seats and the main cell is under and behind the forward-facing cabin seats. An optional 76litre auxiliary tank can be installed in the baggage compartment and extends range by 20%. The fuel system has been drop-tested 80% full from 50ft without rupture, Bell says.
System changes in the 407 include dual-element boost pumps which replace the 206's venturi-type fuel-transfer system. The electrical system is similar to that of the 206L-4, but with room in the nose for the optional 28Ah cold-weather battery.
Avionics changes centre on replacement of hydromechanical gauges with liquid-crystal-display (LCD) instruments produced by Litton Special Devices. There are eight of these LCDs on the instrument panel, three of which - torque, measured gas temperature and gas-producer speed - incorporate event and exceedance recording. Data can be downloaded to a lap-top computer for analysis.
The new gauges are more readable and reliable than conventional instruments, with dimmable backlight and a demonstrated 79,800h mean time between failures. Other maintainability features include the ability to disconnect and remove the entire instrument binnacle, and an overhead circuit-breaker panel which hinges down for improved access.
Another change is to the skid landing-gear, to avoid ground resonance from the four-blade rotor. The aft crosstube now pivots in the middle, between damping pads, a simple modification which raised the resonant frequency of the gear without resorting to hydraulics or springs.
GETTING BETTER
Bell is now working to improve the 407, beginning with increasing the never-exceed speed from the certificated 130kt to the planned 140kt. This was achieved with aircraft 75, Cato says, and strengthened parts are being retrofitted into earlier aircraft. Five parts did not meet the direct-operating-cost (DOC) goal and have been redesigned to extend life, he says. The DOC target was $264/h, just $4 more than that of the 206L-4, and is currently running at "around $270," says Cato.
FADEC software is being improved and the Quiet Cruise concept of reducing rotor RPM to reduce fly-over noise is being certificated. A cockpit switch will enable the 407 pilot to reduce rotor RPM to 90-93% in the cruise. Never-exceed speed will be reduced to 110kt in this mode, but Cato says that this is within the operating profile of customers interested in reducing fly-over noise, including sightseeing and law-enforcement operators flying over noise-sensitive areas.
Cato says the maximum take-off weight, with internal load, will be increased to 2,380kg. The airframe was designed to accommodate the higher weight for the once-planned 407T light twin, which has been abandoned and replaced by the all-new Model 427 (see box).
Bell is also flight-testing multiple height-velocity "avoid" curves, Cato says. These curves outline the safe combinations of height and speed at which the helicopter can suffer an engine failure and still be landed safely. The current flight manual has only one curve, for use at all weights, but at lighter weights the avoid areas are smaller, Cato explains. Multiple curves will give the 407 operator more flexibility, he says.
Certification efforts are now under way in 36 countries, Cato says, following Canadian and US certification in early 1996. "We are penetrating new markets because of the 407," he says. The fleet, meanwhile, has passed the 10,000h mark with some 800h having been accumulated on the high-time 407 - an aircraft which been in use at Bell's Fort Worth, Texas, training school for a year.
Bell is now considering increasing the price differential between the 206L-4 and the 407, to reflect the increased value offered by the new helicopter. While the price is likely to rise above $1.2 million, Bell will not be party to any "double-digit" increases, says senior vice-president Fred Hubbard, because the company has been vocal in blaming the high acquisition and operating costs of today's commercial helicopters for the industry's present depressed state.
With the 407, and the upcoming 427 light twin, Bell has moved set a new price standard for the industry. Sales of the 407 and orders for the 427 suggest that the company is succeeding in its bid to inject new life into the commercial-helicopter market - at the same time laying to rest the criticism that Bell helicopters represent old technology.
Source: Flight International
