Bombardier's latest Canadair Regional Jet is more than just a stretch of its smaller sisters
To describe the Bombardier Canadair Regional Jet (CRJ) Series 700 as a simple stretch of its smaller CRJ-200 sister would do a disservice to the extensive engineering that has gone into the new 70-seater. The aircraft represents a major structural, system and interior redesign by the Canadian aerospace manufacturer.
The Canadair Regional Jet series has proved to be an enormous success for both Bombardier and the aircraft's operators since the original CRJ-100 entered revenue service with Lufthansa CityLine in November 1992. The 50-seat jet has rewritten the book on regional air transportation, offering a 460kt (850km/h) cruise speed, 3,200km (1,700nm)-plus range and improved passenger cabin comfort. For the US travelling public, growing increasingly weary of the cramped and noisy confines of a turboprop, the 50-seat jet has proved to be a tonic. There was also a nationwide reaction against propeller-driven airliners after the highly publicised crash of a turboprop near Chicago in 1994, which killed 68 people. Sales of the CRJ-100 and improved -200 have ballooned to over 550 aircraft, with US regional Comair alone accounting for 110 of these orders.
Bombardier Regional Aircraft president Robert Gillespie says: "The development of the market is not a surprise to us. It's coming a little bit faster than some people had foreseen, but the fact there is market for the aircraft is not a surprise. The size of aircraft has been moving up quite steadily for some time and that's why we decided some time ago to invest in a 70-seat jet."
Bombardier launched the CRJ-700 programme in January 1997 after several years of preliminary study. By 1997's end, it had clocked up 1,230h of windtunnel testing, conducted 38 preliminary design reviews and hosted six airline advisory panel meetings. A 12-month detailed definition phase followed, encompassing critical design reviews and the start of fabrication.
The stated design objectives were to "meet or exceed" the performance of the CRJ-100/200, with a nominal range of 3,150-3,750km at Mach 0.8, while maintaining maximum airframe, system and operational commonality with the 50-seat jet. The aircraft looks set to easily meet the first criteria, but the second goal has proven more of a challenge.
"Structurally, the fuselage is significantly different," concedes Douglas Erlandson, CRJ-700 product planning manager. Accommodating a 40% increase in seat capacity, without enlarging the CRJ-100/200's baseline 2.69m (106in)- diameter fuselage barrel, has entailed much more change beyond a 4.72m fuselage stretch.
Two fuselage plugs have been inserted, adding 3.96m to the forward fuselage and 0.76m aft of the wing. To create additional cabin space, the rear pressure bulkhead has been relocated 1.3m further aft, which in turn has required the jet's auxiliary power unit (APU) to be relocated into the tail. The longitudinal floor beams have been redesigned, lowering the cabin floor by 3cm and in doing so raising the overall ceiling clearance to 1.91m.
Designers have also tackled the CRJ's obscured low window line which, according to Erlandson, is "a carry-over from its Challenger business jet heritage". Moving the apertures higher and lowering the floor have raised the window line by 147mm and widened the field-of-view from 31í to 49í. The windows have been switched from a bolt-in to a new plug design for easier replacement.
The materials content of the Shorts-manufactured fuselage remains the same, but the structural design is modified. Frame depth has been reduced from 89mm to 76mm, providing an additional 26mm of shoulder room in the cabin. The frames have also been designed to take new cleat attachment "Z" section stringers in place of the earlier "T" stringers, with the exception of the fuselage crown.
An increase in passengers inevitably entails more baggage, which Bombardier has addressed by incorporating a new 3.1m3 (109ft3) under-floor baggage compartment forward of the wing. The 5.26m-long hold is accessed from the left-hand side of the aircraft via an inward opening plug door measuring 1.07 x 0.51m.
Although the hold is longer than the forward plug, it has not displaced anything from the adjacent avionics bay, claims CRJ-700 project planner Eric Harpin. He says: "A reduction of the avionics bay volume was achieved by reducing the size of several avionics line replaceable units and a more efficient use of the bay itself."
The compartment is only 560mm high, which though ideal for skis or golf clubs, is too small for the average North American baggage handler. Designers have addressed this problem with a proposed sled-type baggage retrieval system. In addition to the new 455kg (1,000lb) capacity hold, the CRJ-700 retains a larger baggage compartment aft of the cabin. It has a moveable bulkhead and can take up to a maximum load of 2,040kg.
Bombardier has extensively re-configured and enlarged the wing, while retaining the CRJ-100/200's basic metallic double spar design. The wing area has been enlarged by 32% to 68.7m2 (740ft2), with the largest add-on being a 910mm root plug. The front spar depth has been increased and its separation from the rear spar widened.
The CRJ-100/200 wing's leading-edge kink has been straightened. The new jet incorporates three new extendable leading-edge slats for better field performance. The aircraft's fast landing speed has been cut from 113kt to 106kt, while typical take-off field length at the CRJ-700's new 33t maximum take-off weight (MTOW) has been shortened from 1,765m to 1,565m.
Other flight control changes include 90mm longer-span inboard flaps and split multi-function spoilers, replacing the CRJ-100/200's earlier spoilerons and flight spoilers. The four new hydraulically powered spoilers feature several modes, including asymmetrically deployed roll assist, symmetrically deployed proportional lift dumping and ground lift dumping. The winglets have been redesigned and enlarged. The horizontal stabiliser has been scaled up in size by 70% and the vertical stabiliser reinforced for increased loads. Flap actuation has changed from a flexible shaft-driven system to torque tubes and ball-screw actuators, while the new two-position slats have driven by rack-and-pinion drives and rotary actuators. Both flaps and slats are activated by new dual power drive units, which are in turn controlled by two dual-channel flap track electronic control units.
The evolution of the Challenger series business jet and later the CRJ series regional jets has been paralleled by the incremental development of the General Electric CF34 turbofan. The engine, in its initial 9,000lb-thrust (40kN) CF34-1A commercial guise, first powered the 601-1A in 1983, followed by the improved -3A powered 601-3A in 1987, the CRJ-100 in 1991 and the -3B/3B1-powered CRJ-200 in 1998.
Bombardier's decision to launch the CRJ-700 has resulted in the most radical redesign of the engine to date, to generate the required 50% jump in thrust over the CF34-3B1. The new 13,790lb-thrust CF34-8C1 has full authority digital engine control, an enlarged 1.12m-diameter fan, new wide chord titanium fan blades, a reduced 10-stage high pressure compressor (HPC), the first three of which are comprised of integral blade/discs (blisks) - and fewer variable guide vane stages.
The machined-ring combustor has been designed for a longer burn life and reduced emissions. The high-pressure turbine module features directionally solidified second stage blades, three-dimensional aerodynamic refinements and extended-life boltless blade retainers, replicated in the four-stage low pressure turbine. The net result is a 10% improved thrust-to-weight ratio and a 30% reduction in parts.
A larger engine has necessitated a new pylon mounting and nacelle housing from Shorts. To accommodate the 37% heavier engine, the pylons have been enlarged and built to a thicker gauge, incorporating more titanium in place of aluminium. The mounting has also been designed to facilitate faster engine change. There is also a new leading edge fairing.
Mounted on the pylon are wide-opening clamshell cowlings to the CF34's fan, reverser and core, with a counterbalance operation for one-man operations. The nacelle has been designed for right-left commonality to reduce parts count and spares provisions. "The thrust reverser has a hydraulic actuator system and no blocker doors instead of the pneumatic system on the -200," says Harpin.
Changes have been made to the aircraft's electrical power system, including fitting three more powerful 40kVA generators and new AC power controllers. A new emergency 8,000rpm air driven generator has been installed. The CRJ-700 has fitted new external taxi and navigation lighting, while the passenger cabin has been equipped with self-illuminating floor lighting.
An increased demand for power has meant replacing the present AlliedSignal GTCP36-150 APU with the AlliedSignal RE-220. The new APU is housed in a 25% larger tailcone produced by GKN Westland Aerospace. The APU is mounted in its own cradle and can be accessed from below via clamshell doors for removal using fishpole hoists.
The new APU is large enough to drive a 40kVA generator and can be operated throughout the CRJ-700's flight envelope up to a maximum altitude of 41,000ft. It can be started at 37,000ft and can assist a main engine start at 25,000ft. "What drove the selection of the APU was the requirement of American Airlines to pull down the cabin temperature within 30min to 27¼C on a 40¼C day," says Erlandson.
A new Liebherr environmental control system (ECS) automatically switches between the APU and bleed air from the CF34-8C's LPC sixth stage and HPC 10th stage. Two air conditioning control systems with dual redundant channels regulate the air conditioning system from the aft avionics bay. The bleed air also feeds the wing leading edge and cowling anti-icing system and the cabin pressure system.
"The CRJ-200 uses 100% fresh air, while the CRJ-700 uses a mix of two-thirds fresh and one-third recycled. This is a more efficient use of the ECS and doesn't require quite the power air packs do," explains Erlandson. The recirculation fans can be switched off from the cockpit in the event of smoke being detected in the cabin.
A 42% increase in MTOW has required the CRJ-700 be fitted with a new undercarriage from Menasco. In place of the CRJ-200's trailing arm configuration is a cantilever main gear with folding side brace and new shock absorber strut. The nose gear is also new with a geometrically locked drag brace. Other changes include new tyres and a digital anti-skid control system.
Bombardier has resisted making major changes to the cockpit to ensure common crew qualification with the CRJ-100/200.
The CRJ-700 retains the same Rockwell Collins Pro Line 4 avionics suite with six-tube electronic instrumentation system. Any real differences are confined to a new Ratier-Figeac throttle quadrant, flap/slat lever, ECS display panel, additional overhead lighting and a three-speed windshield wiper.
Flight testing began just hours before the 28 May roll-out of the first aircraft SN 10001. Its first flight was the day before. The test schedule for the lead aircraft will include flutter, stability and control testing, stall and certification handling and performance testing. Aircraft 10002, due to be completed in July, will focus on system performance and certification testing.
The completion of aircraft 10003-05 will follow at monthly intervals from September. The third aircraft will used for avionics testing. The final two jets will be fitted with customer-specific interiors for certification. Flight testing is to end by August 2000, with Canadian certification aimed for October. Initial deliveries will begin in early 2001, with the first aircraft going to launch customer Brit Air. Orders total 96, American Eagle 25, Atlantic Southeast Airlines 12, Comair 20, Horizon Air 25 and Lufthansa 10. There are options for 138 aircraft.
Source: Flight International