What does the new Bell 407 offer over its predecessors?
Graham Warwick/MONTREAL
IF BELL IS renowned for two things in the commercial-helicopter field, they are the Model 206 and the two-blade rotor. These traditional Bell strengths have threatened to become weaknesses, however, as the prolonged recession in the commercial-helicopter market has encouraged operators to hold on to their 206s; and as environmental pressure on helicopter noise has increased awareness of the inherent disadvantages of two blades.
Bell has now tackled the weaknesses without sacrificing the strengths - and its reward has been the unlocking of the elusive 206-replacement market. Six months after launching the Model 407 light helicopter, Bell has firm orders for more than 110 aircraft, with company demonstrators and other commitments taking delivery positions accounted for beyond 140.
Planned production at Bell Helicopter Textron Canada's Mirabel Quebec plant, has been increased to more than seven a month, a rate which will be achieved by June 1996 and which is almost double that of the current 206L-4 LongRanger IV, which will remain in production alongside the 407, "for as long as the market demands", says Bell.
The key, undoubtedly, has been Bell's aggressive design-to-cost approach to upgrading the 206. The 407 costs just $100,000 more than the 206L-4, but offers substantially improved capacity, payload and performance. Its direct operating costs are only marginally higher, than those for the 206L-4.
In upgrading its most popular helicopter, Bell concentrated its efforts on the rotor and cabin. The 407's four blade main rotor offers noise, vibration, performance and maintainability improvements, while its bulged fuselage side panels, make available greater cabin comfort and interior flexibility.
Costs have been contained by retaining much of the 206L-4 airframe and by using a proven dynamic system - the main rotor and transmission are adapted from those of Bell's OH-58D Kiowa Warrior armed-scout helicopter.
Other changes include uprated, digitally controlled engines, increased use of composite materials, and solid-state cockpit instruments - all designed to improve productivity and reduce operating costs.
Bell initially looked at a four-bladed light helicopter in the mid-1970s. The concept perished in the severe recession, which then enveloped the commercial-helicopter industry, but was revived in late 1993. A concept demonstrator was built and flown in just 107 days, says programme manager Dale Cato, by modifying a 206L-4 with the OH-58D rotor and side fairings simulating the wider fuselage.
Parent company Textron, gave the go ahead in July 1994, launching an aggressive programme, which calls for delivery of the first 407s to customers, at the Helicopter Association International (HAI) show, in Bell's home town of Fort Worth Texas, in February 1996, just one year after the 407 was announced, at the 1995 HAI show in Las Vegas, Nevada.
This schedule means that Bell's Canada plant is developing two new four-bladed helicopters almost simultaneously. The single turbine 407 and the Model 430 intermediate twin launched in 1994, are scheduled to be certificated within days of each other, in December 1995.
The 407 programme is on schedule and on budget, says Cato. The first certification aircraft was flown at Mirabel on 29 June, followed by the second certification aircraft on 22 July. The first production aircraft will be flown early in September and used in the US/Canadian certification programme before being delivered to Bell's Fort Worth training school.
The certification aircraft are powered by the definitive Allison 250-C47 turbo-shaft. This 590kW (790shp) engine, flat-rated to 500kW in the 407, is a derivative of the -C40 engine used in the 430 and includes a Chandler Evans full-authority digital engine-control (FADEC) system. The FADEC improves engine reliability and extends overhaul intervals, says chief of 407 project engineering, Rodney Taylor.
DYNAMIC UPGRADE
In upgrading the 206, Bell started with the heart of the helicopter - its dynamic system. While the 206 has a two blade "teetering" main rotor, the 407 has a four-blade, "soft-in-plane" rotor. Blades are attached to the rotor hub via a glassfibre-reinforced-plastic yoke, which flexes to allow horizontal and vertical blade movement, while elastomeric bearings allow blade pitch to be varied. Bell is aiming for on-condition maintenance of the blades, which are composite with steel leading-edge abrasion strips.
More than 240,000 flight hours have been accumulated on the OH-58D rotor. Changes introduced for the 407 include an increased- fatigue-life resin matrix for the yoke; the addition of elastomeric lead-lag dampers; a reduced-maintenance hub-mast joint; and vertical pitch-links. The inclined pitch-links used on the OH-58D result in vibration which Bell wanted to eliminate on the 407, says Taylor.
The OH-58D is difficult to fly without a stability- and command-augmentation system (SCAS), but this was deemed too expensive for the 407, so the rotor controls had to be redesigned to eliminate the need for a SCAS. This process was speeded up, Taylor says, by using stereolithography and rapid casting to produce flight-control components (Flight International, 24-30 May, P36). Cato says, that the two certification aircraft are "flying well, and handling qualities are acceptable".
The OH-58D's main transmission has been uprated for the 407, with larger bearings and redesigned gears to increase fatigue life. The 206's complex "nodal-beam" transmission mounting has been replaced by a simpler "Savitad" pylon-mounting system, which uses soft elastomeric mounts to absorb vibration. The result is less weight, cost and maintenance.
A "Frahm" damper mounted above the rotor head further reduces vibration. The 206's skid landing gear has been modified to match the new rotor's ground-vibration characteristics and the aft cross-tube now pivots in the middle, between outboard damping pads. These simple modifications raised the resonant frequency of the gear without resort to hydraulics or springs, Taylor says.
The four-blade rotor system is expensive, Bell admits, and savings have had to be found elsewhere to achieve the tough "no-increase" cost limit set by company president Webb Joiner. Most of these have been found in the way that the airframe is manufactured.
Use of composites has been increased, both to eliminate corrosion and to reduce cost. The aft fuselage is now made of carbonfibre composite, as are the new side panels. These panels are contoured to provide an additional 178mm of cabin width at the aft seats. The 407 will seat three abreast comfortably in the rear seat row, and a 35% larger window area provides better visibility for the passengers. "You can see almost straight down," says Taylor.
The new side panels have another advantage. Fitting the 206's doors has always been a problem, he admits. On the 407, the doors are pre-hung off the aircraft - dramatically easing assembly. Standard automotive door handles and locks are used, making the doors easier to open and more secure when closed, he says. The door windows and cabin skylights are attached using double-sided tape - another automotive technique, resulting in a smooth exterior line.
Cockpit improvements include the use of Litton liquid-crystal-display (LCD) instruments with built-in event/exceedance recording. These gauges are more readable, reliable and more accurate than conventional instruments, Bell says. Computer ports allow data recorded by the LCD gauges and FADEC to be downloaded and the entire instrument binnacle disconnects easily for maintenance, Taylor says.
The single-channel FADEC provides automatic starting, fault monitoring and hot-start protection and contributes to the engine's extended time between overhauls (TBO). Compressor and gearbox maintenance will be on-condition and the turbine TBO will start at 1,750h, compared with 1,500h for the non-FADEC Allison 250-C30, increasing to 2,650h is stages. Target transmission TBO is 4,500h.
System improvements include a larger overhead circuit-breaker panel, which hinges down at its forward edge for improved maintenance access. The fuel system is simplified, with elimination of the 206's venturi-type fuel-transfer system in favour of dual forward and aft boost pumps. An optional 75litre auxiliary tank, which can be installed or removed from the aft-fuselage baggage bay in 10min, Taylor says, extends range by 20%.
PRICE APPEAL
Price is the single most important factor in the 407's appeal. Bell president Joiner initially specified that there be no price increase over the 206L-4. That limit was later relaxed to reflect the 407's increased productivity, but the final price - $1.2 million - is only $100,000 more than that of the 206L-4. Direct operating-cost has risen only $4 an hour over that of the 206L-4, to $264/h, an increase described by Taylor as "insignificant".
The cost figures are remarkable when the 407's performance is taken into consideration. Payload is increased by 120kg, cruise speed by 18kt (33km/h), range by 15%, and hover ceiling by 1,400ft (430m), according to Bell figures. Maximum gross-weight has increased by 250kg, to 2,270kg - with a 2,270kg limit when carrying an almost 1,360kg external load. The higher gross-weight will be required when Bell develops the planned 407T twin-turbine version in 1996, Taylor says.
Cato says that Bell plans to have five 407s at the HAI show in Fort Worth next February, three customer aircraft and two company demonstrators. The first aircraft will be handed over to launch customers Niagara Helicopters and Greenlandair. The first 430s will also be handed over at the 1996 HAI - an event, which will mark the 50th anniversary of the first civil-helicopter certification, the Bell 47.
Source: Flight International
