CUTAWAY: CSeries strikes balance between innovation and proven technology

Washington DC
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Nearly five years have passed since Bombardier formally launched the CSeries at the Farnborough air show. While the narrowbody market has completely reshaped since then, the CSeries has remained almost the same.

A high-density version unveiled in March lengthens the CS300 slightly, raising the baseline seat count from 130 to 135. The first CS100 flight test vehicle is on track to fly within two weeks in exactly the configuration that Bombardier envisioned in 2007.

Some aspects of the CSeries programme are clear. A supply chain now exists with several national clusters playing key roles. Americans have the power and control systems, Italians have the control surfaces, the Germans have the landing gear while the Chinese and Canadians are splitting the fuselage work for the programme.

bombardier cseries bombardier

 Bombardier

The system appears to be working. After a six-month delay that Bombardier attributed to supplier problems, the imminent first flight milestone is a sign the programme is recovering internally. The next four flight test vehicles are in various stages of final assembly.

If the CSeries is defined by its fuel-saving potential, most of the credit belongs to the engine. Indeed, Bombardier estimates that the installation of the Pratt & Whitney PurePower PW1500G alone is responsible for driving 50-60% of the fuel burn improvement claimed.

There are two ways to produce thrust from a jet engine - either accelerate a small amount of air really fast or a large volume of air relatively slowly. The former is a technique that works very well for gas-guzzling fighter ­aircraft. ­Airlines, however, prefer the fuel-sipping technique of the latter, which is already widely used by modern commercial aircraft.

The PurePower engine is different because it takes that idea to a new level. By using a larger fan diameter and a reduction gear, this geared fan is able to push twice the amount of air backward through the engine at one-third of the speed of a conventional engine like the International AeroEngines V2500.

bombardier cseries bombardier

Bombardier

Bombardier considered a largely composite fuselage but rejected it because of maintenance concerns

For the CSeries, Bombardier faced an interesting design challenge involving the installation of the 1.85m (73in)-wide fan of the PW1500G. While Bombardier is familiar with wing-mounted turboprop engines, the CSeries is the first to use wing-mounted turbofan engines. Working with supplier Spirit AeroSystems, Bombardier began working in 2007 to optimise the wing integration with the pylon to the wing. Bombardier was responsible for the overall design while Spirit AeroSystems completed the design of the strut-to-wing hardware and aft fairing package.

The CS100 and CS300 share a common wing that is the second major innovation in Bombardier's configuration for the CSeries. The efficiency of the 35.1m-wide aerofoil is partly aided by a signature Bombardier canted winglet at the tip, but its real performance is derived from the composite material.

The CSeries programme marks Bombardier's first foray into designing an all-­composite wing for a commercial aircraft. ­Researching and ­developing a process to manufacture the 112m² (1,210ft²) of composite wing area for the CSeries represented the largest investment ever poured into the Bombardier ­Aerospace-Belfast (formerly Short Brothers) facility in Northern Ireland, including commitment by the UK ­government to the tune of £520 million ($814 million).

MATERIAL WOES

The Belfast facility developed a unique process combining resin transfer moulding and autoclave curing techniques into a new method called resin transfer infusion. It is a method that fabricates several large wing skin panels as a single piece, with stringers and stiffeners integrated in the process rather than bolted on later.

Bombardier mates the trailing edge of the wing to carbon fibre reinforced plastic (CFRP) control surfaces. Brindisi, Italy-based Salver uses a resin transfer moulding and infusion process to make the inboard and outboard flaps, and builds the spoilers using a process based on honeycomb stiffened pre-preg. Meanwhile, Sonaca manufactures the wing fixed leading edges, slats and de-icing system.

The decision to use the composite material on the wings was relatively easy for Bombardier because of the weight savings offered by a single-piece design. No such convenient answer presented itself for the fuselage pressure vessel. As with the Global-series business jets, Bombardier decided to use carbonfibre in the aft section and for the Alenia-supplied horizontal and vertical stabilisers. Forward of the aft bulkhead, however, Bombardier decided to use a new and more reliable generation of aluminium-lithium metal.

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Bombardier

The Pro Line Fusion flightdeck represents the new industry standard for graphic interface

Bombardier studied using carbon fibre for the forward and centre fuselage barrels, but ultimately rejected the idea. Unlike the single-piece wing panel with integral stiffeners and stringers, no such consolidation of parts and assemblies could be achieved within the complexity of the fuselage barrel. Potential airline customers voiced another concern about the carbon fibre fuselage. They feared the front-line maintenance requirements for a narrowbody aircraft with a high-cycle mission that is exposed to collisions with ground vehicles more frequently than a widebody vehicle.

Bombardier still needed to drive real weight savings out of the fuselage barrel. Since 2005, the company had been working with Alcoa and Alcan (now Constellium) to develop a new generation of aluminium-lithium alloys that were still lighter than aluminium metal, but were not as susceptible to corrosion and fatigue stress as earlier alloys.

Bombardier eventually selected Constellium to deliver the Airware-branded aluminium-lithium panels, which are billed as 25% lighter than aluminium and a 12-year maintenance cycle guarantee. Finally, Bombardier could expand the maintenance interval from base aluminium, but without accepting the weight and front-line maintenance concerns posed by composite materials.

PIONEER INNOVATIONS

The combination of aluminium-lithium alloy in the fuselage and carbon fibre material in the wings presents its own design problems. Corrosion is always a concern in the joints between a composite and metallic material, even when it involves an alloy that only includes aluminium. Bombardier, however, is confident the problem can be overcome by how the joints are designed.

Such galvanic corrosion is also a concern in the aft fuselage, which mixes carbon fibre panels and aluminium frames. Unlike the composite wing or geared turbofan, this is one area where Bombardier can claim operational experience. The Global-series business jets employed the same material combination on the aft sections. Bombardier's solution to preventing corrosion is inserting a thin film at the joint, protecting the metallic alloy from developing rust.

Both the CS100 and CS300 will share identical forward and aft fuselage sections, which are both supplied by Bombardier internally. Shenyang Aircraft, which assembles the centre fuselage, has the only section that differs between the variants, with two plugs fore and aft to accommodate the added length of the CS300.

Bombardier's list of innovations pioneered by the CSeries continues - and by no means ends - inside the Liebherr-supplied landing gear system. For Liebherr, the size of the CSeries offered a strategic opportunity for the traditional supplier of smaller regional and business jets. Goodrich and Messier-Dowty had a duopoly on supplying landing gear on all jets larger than 120 seats until Liebherr won the contract for the CSeries. Subsequently, Airbus selected Liebherr to supply the nose landing gear for the significantly larger A350 XWB.

However, the real innovation for the CSeries programme is found inside the gear system, with the Meggitt E-brake system. Electrically-actuated and even powered brakes in aircraft were nothing new when Bombardier signed Meggitt to a supply deal for the CSeries. However, the CSeries was the first to use electric brakes without a hydraulic system back-up.

Through avoiding the use of hydraulics to actuate the brake-by-wire control system, Bombardier seeks to reduce weight and maintenance costs, as hydraulic leaks posed a fire risk for the aircraft. However, the electrically-driven breaks would prove more of a departure onboard the CSeries than was the rule.

Relaunched in 2008, the CSeries was in full-scale development when the first hints of Boeing's struggles with the more-electric ­architecture of the 787 were coming to light. Perhaps consequently or even coincidentally, Bombardier decided to take a less aggressive approach to using electricity for actuation and power distribution.

For example, Boeing had selected Thales in 2005 to provide the power conversion system that included the GS Yuasa lithium-ion batteries for the 787. Like all modern aircraft, the 787 uses batteries to start the auxiliary power unit (APU) and to briefly supply power to the cockpit systems if all other onboard power generators fail. Lithium-ion provides more power in a lighter package than nickel cadmium batteries, so Boeing decided it was worth the risk. Indeed, the Seattle-based aircraft maker still stands behind the technology despite two over-heating incidents last January that caused a worldwide grounding.

CHOOSING WISELY

Bombardier faced a similar decision as Boeing in 2008, but determined with largely the same data that the technology was not yet ready to be installed on the CSeries. Instead, the main and APU batteries on the CSeries, installed on the right-hand mid-body fairing, are proven and reliable nickel-cadmium batteries.

That same philosophy guided further decisions on the actuation systems. Bombardier says it has studied replacing bleed air-driven pneumatic systems to pressurise the cabin with an electric system, but rejected the idea for the same reasons as the lithium-ion battery and the composite fuselage barrels: it was not ready for operational service.

Aircraft systems are supplied by three hydraulics systems driven by two power generators on the engines, with one main hydraulics line on each side and a back-up line located in the aft fuselage. In addition to the engine-mounted generators, Bombardier selected the Honeywell 131-9 APU for the CSeries.

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Electrical power is managed throughout the aircraft using the UTC Aerospace Systems ­(formerly Hamilton Sundstrand) electric power generation and distribution system. The system includes the two engine-mounted, variable frequency generators to feed power to the avionics and other loads. Emergency power is supplied by an air-driven electric generator.

The power is distributed in three main electrical equipment bays - the forward bay just aft of the cockpit, the mid-equipment bay just aft of the wing and a small bay in the aft ­section of the fuselage.

Perhaps the most critical electrical load on board the CSeries is the Rockwell Collins Pro Line Fusion integrated avionics system.

The flightdeck reflects the new industry standard for a graphical interface with the crew: five 15.1in LCD displays, including two outboard primary flight displays, two inboard multifunction displays and a multifunction display on the centre console.

CONTROLLED AUTHORITY

The sidestick controllers represent a more dramatic departure from the norm, at least within Bombardier. They are also a harbinger of even more important changes in the unseen compartments of the aircraft. The CSeries is the first Bombardier aircraft to use a three-axis fly-by-wire system, replacing a system of pulleys driven by hydro-mechanical systems. Instead, the pilots inputs are interpreted by the Rockwell Collins primary flight control computer and distributed to a Parker fly-by-wire system.

Fly-by-wire allows aircraft makers to decide how much authority is given to the pilot. Bombardier describes itself as somewhere in the middle between Airbus' policy of full envelope protection and Boeing's philosophy of leaving the pilot in complete control. The CSeries will be operated by the pilot using only two flight control modes: normal and direct. Airbus uses six different flight control modes, which Bombardier fears can sow confusion.

The CSeries also bears elements of Boeing's direct control philosophy. An example is the Woodward throttle quadrant assemblies. Although the engine throttles are automatic, Bombardier designed them with back-driven levers to provide tactile cues to the flightcrew.