Hearing is believing!

to hear more. One of the highlights of the recent Paris Airshow was Bombardier's new C Series CS300 airliner making its debut flying display. Well, the CS300 flew every day (apparently), yet I managed to miss its maiden flight because it was so quiet, I didn't even know it was up there!

Claimed to be the industry's first all-new designed, single-aisle aircraft for over 25 years, Bombardier says the C Series aircraft exceeds fuel burn, payload range and other performance targets. Its airfield performance also exceeds the original targets and it is set to meet noise performance targets, making it the quietest commercial jet in production. The company also adds it has increased the maximum range by 350Nm more than originally targeted.

Much of the credit must go to Bombardier's selection of Pratt & Whitney's PurePower PW1500G Geared Turbofan jet engines. The engine manufacturer's Fan Drive Gear System offers advanced combustion technology, the highest by-pass ratio of any certified turbofan engine in the world and fewer stages and airfoils: a combination which delivers reduced emissions, reduced noise and reduced fuel burn.

A fresh start

The C Series family benefits from a ‘clean-sheet' design which includes leading-edge technology and systems integration, advanced materials and latest-generation aerodynamics. So, from an engine propulsion perspective, did this present Pratt & Whitney with an exciting opportunity to break the mould of aero engine design?

“We've built our Geared Turbofan engine using a very large diameter fan, which is enabled by our fan drive gear system,” begins Pratt & Whitney's vice-president of PW1000G programmes, Graham Webb. “Acting as a speed reduction gearbox, it makes the fan turn slower and also amplifies torque, which allows us to employ smaller diameter turbomachinery to drive this large diameter fan. We're trying to drive the largest diameter fan possible, because this provides improvements in what we call ‘propulsive efficiency', which is basically increasing the bypass ratio of the engine and reducing the fan pressure ratio.

“Having a ‘clean sheet' design meant we were able to incorporate the largest diameter fan possible into the aircraft as in the early phases we are dealing with a ‘rubber' aircraft. This enabled Pratt to optimise the engine propulsive efficiency with the highest by-pass ratio engine possible to reduce aircraft fuel burn. We're now seeing the clear benefits of the Bombardier-Pratt & Whitney collaborative design efforts in terms of the recent announcements Bombardier recently made during the Paris Airshow, where they stated that they are exceeding their previous fuel burn and range specification values.

It's very rare for an aircraft to come into the market ‘out of the box', having performance higher than the original specifications. This achievement is a clear testimony to the fact that Bombardier has done a great job in the way that they have integrated a number of new innovative aircraft technologies and brought them to fruition.”

I'm interested to know how Pratt & Whitney has approached Bombardier's desire for an aircraft that satisfies its demands for both ‘hot and high' and city-centre airport operations?

“The air can become so thin that the engines get hotter when flying around high altitude airports. This provides a challenge for the turbomachinery in terms of temperature and also other areas of the engine too. The Geared Turbofan engine's larger bypass ratio enables us to provide support for these types of high elevation operations because it acts more like a propeller-driven engine than a typical turbofan. Our engine is focused on having more of its thrust being generated from the fan as opposed to the turbomachinery, which enables us to keep the temperatures down relative to what we would have to do if we didn't have the fan drive gear system and the larger diameter fan.

“We've kept our turbomachinery margins relative to temperature down to a level that has enabled us to recently obtain a higher thrust capability from the existing engine design. We've come out with a 25,000lb thrust rating for the C Series aircraft. This has become the PW1525G and it is approximately a 5% increase in thrust relative to the prior highest rating. It's another opportunity to provide value to customers, and these high elevation airports where they can make use of this thrust to extend the range of their aircraft, which becomes limited as a result of the high elevation and especially high ambient temperatures. Airline operators can also use it for higher payload and actually optimise what they want to perform based upon the operational characteristics of the engine and the aircraft.”

Hot and high flying

Webb says that in order to meet these hot and high operating requirements, Pratt & Whitney employed the latest heat resistant super alloys, composite materials and thermal barrier coatings in the manufacture of its engine components to cope the hotter engine temperatures encountered.

“Pratt & Whitney has long been a leader in materials and process technologies for aero engines. This Geared Turbofan engine family is no different. We've placed a lot of focus on advanced materials and processes in every module of the engine. In terms of the high pressure turbine, we've looked at advancements in our ‘single crystal' alloys; we're producing sulphur levels for improved oxidisation and we're using the latest generation of ½K thermo-barrier coatings, which are applied using unique processes in terms of both the bond coat and also the thermo-layers. We've applied the latest technology and advanced metallurgy high temperature disc alloys for the high pressure turbine, as well as for the back-end of the compressor. These new powder metallurgy alloys make the parts more difficult to machine and fabricate, so we've had to adapt our manufacturing processes in terms of types of tool cutters and the processes we use. In some cases we employ electrochemical and abrasive machining in order to produce very complex 3D aerofoils which are required for aerodynamic efficiency from these types of materials.

“For the hot section area of the engine, we're using very high toughness thermoplastic resins for the fabrication of our fan cases. These are the latest in terms of high toughness capabilities for impact resistance. The purpose is to make the lightest weight fan cases able to contain a fan blade, as well as satisfying the structural requirements. We also have hybrid-metallic fan blades, which are very lightweight, high aerodynamic efficiency constructions.

“They are a result of an evolving culmination of 3D machining which we use in order to obtain complicated shapes as well as very interesting fabrication assembly processes in order to build the entire composite up and meet all of the close tolerances required to meet the performance specification for that component. We employ aluminium and titanium materials, as well as a variety of coatings and composite fabrications for wear resistance. It's a very advanced next generation fan blade construction that offers both lightweight and high levels of aerodynamic efficiency.”

In terms of innovative manufacturing developments, Webb says the company has been using additive manufacturing processes for over 20 years, building its capabilities up from initial fabrication of prototypes and development tooling to the point where it can achieve production volumes.

“This is the starting point for a bright future for these capabilities, but there are obviously challenges that we have to work through in terms of addressing powder cleanliness and re-use – concepts that ensure that the quality of the components we manufacture consistently meet the requirements of the design.”

Working on the C Series programme since the initiation of the overall Geared Turbofan engine concept to deliver noise reduction, Webb is extremely proud of the C Series aircraft and Pratt & Whitney's engines.

“Analysing margin-to-noise requirements is one thing, but it's another when you actually see the aircraft flying and not be able to hear it. The C Series will revolutionise the way airports can operate because they are quiet aircraft that operate without disturbing anyone. This will enable extended flight operation and will lead to improved economics for airline operators.”

Ever-increasing circles

In terms of Pratt & Whitney's aero-engine designs of the future, Webb believes that the way forward means designing even larger diameter fans. However, he agrees this will require more focus on reducing weight to avoid increasing fuel burn.

“In the future we will see increased utilisation of composites, an increased integration of the engine with the nacelle and the pylons, the creation of more integrated and organic structures, and even more advances in gearbox technology. As we attempt to increase overall engine pressure ratio, we'll need to reduce the diameter of the cores, so we won't be able to put the low pressure turbine shaft through the centre. This will mean choosing alternative arrangements which will require advancements in gearbox technology, fan drive gear systems and other forms of advanced gearboxes.”

In terms of component manufacturing, Webb reckons we will see an increased use of composite materials, and gear manufacturing advancements in high temperature materials, ceramics, and refractory materials.

“From our perspective, we're very pleased to be the powerplant supplier for the C Series aircraft,” concludes Webb. “It's an advanced leading-edge aircraft which is going to be very successful. We're already starting to see the benefits of the technologies, not just the propulsion, but in other advancements of the C Series too. The Geared Turbofan engine family originated from this aircraft and it will continue to be a strong driver in terms of what we will set as the standard for propulsion in the future.”

www.purepowerengines.com

www.commercialaircraft.bombardier.com