08 January 2016, ©. Leeham Co: It’s the first Corner for the year and a look at 2015 as a year of technology advancements is due. 2015 will be remembered as the year when three clean sheet airliners passed important milestones. This will not happen for many years to come, so it will be worth to look at what they brought to world of aviation.
I’m thinking of Bombardier’s (BBD) CSeries getting certification for its first variant; the Mitsubishi MRJ doing its first flight’ and COMAC’s C919 being rolled out. Going forward, we will only have derivatives progressing through such milestones for years except for the roll-out of United Aircraft’s MS-21 single aisle airliner in 2016.
The Airbus A320neo was certified in 2015 and Boeing’s 737 MAX rolled out, but these are derivatives of in-service aircraft.
Embraer’s E-Jet E2 will roll out in February but this is a further development of today’s E-jet and Airbus A350-1000 is a variant of the in-service A350-900.
It will be a long time before we see so much new in a year, so it can be instructive to look at to what extent did these new aircraft bring the state of the art of airliners forward.
The CSeries, MRJ and C919 are all similar in the overall airframe architecture. They are all classical tube with wings with the engines slung under the wings for highest aerodynamic and structural efficiency. The counterweight effect of the engines on the wings and the focusing of the propulsive and landing gear forces close to the wing’s center wingbox is an efficient way of building an airliner.
The fuselage is clean of forces except those from pressurization and the control forces from the empennage and nose gear. It was not long ago that we had engines in different locations, including at the root of the horizontal tailplane.
While the overall architecture of the aircraft is similar, the detail technology is not. None of the projects followed the lead from Boeing’s 787 and Airbus A350 of using Carbon Fibre Reinforced Plastic (CFRP) for the fuselage. Ten years ago this would have been unthinkable. 2005 was the year of the drug-like rush of the virtues of CFRP construction. It made things lighter, easier and therefore in the end cheaper to build and virtually maintenance free.
Since then a lot of practical experience has been gathered and the picture is more nuanced. While the initial promise was for a 20% lighter structure (that is the ideal gain for CFRP versus aluminium construction), it is today debated if there was any weight gain in making the core fuselage tube in CFRP.
While there might be gains for larger aircraft, they are harder to achieve for smaller types. The reason is that the CFRP fuselage skin of a smaller airliner gets so thin that is can’t stand up to everyday handling hazards. CFRP fibres are so strong in tension that the pressurization and bending forces for the fuselage can be countered by so few CFRP tape layers that the skin gets sensitive to forces from the side, such as hail, birds or sideways bumps from dropped tools, loose containers or catering trucks that don’t brake in time.
As a consequence, so much of the fuselage will be dimensioned from damage tolerance that a lot of the gains from CFRP’s strength gets lost. While there might be some weight gain for a larger dual aisle aircraft, the gain for smaller single aisles are dubious. As for ease of manufacturing, this seems today to be a pipe dream; Boeing is still wrestling with high manufacturing costs for the all-CFRP Dreamliner.
Airbus is keeping the cost of building the A350 to itself (the program accounting makes the Dreamliner build costs public whereas the standard accounting practice of Airbus does not reveal costs for building the A350). There has been no boasting about its all CFRP construction making it easier or cheaper to build.
This leaves maintenance as the final advantage for CFRP fuselages. CFRP allows fuselage tube inspections up to the 12 year heavy check to be made as external ultrasound de-lamination checks, instead of the internal inspections that require that seats and interior is taken out. So there are real benefits from CFRP design from a maintenance perspective.
The only problem is that BBD CSeries achieves the same 12 years heavy check schedule with its AlLi- based fuselage. BBD has furthered AlLi’s corrosion and fatigue resistance to the level where it has the same maintenance intervals as CFRP.
When one looks carefully at the CSeries, its AlLi fuselage seems to have gained the advantages of CFRP in most areas, yet stayed away from its unknowns in damage tolerance, electrical structural network (CFRP can’t work as a lightning protective cage nor as a return path for electrics without metal meshes added to the structure) and production costs. Its nominal 5% weight gain from making the aircraft’s fuselage tube in AlLi is what a CFRP based fuselage could have brought.
BBD chose CFRP for the empennage and wings, where CFRP offer real benefits. The flight loads demand skin thicknesses that make the surfaces stand wear and tear without extra weight for damage tolerance. Mitsubishi and COMAC stayed with classical aluminium wings but this was more to not wandering into the unknown with their clean sheet designs. It had more to do with prudence (it was a first ever for COMAC and Mitsubishi hasn’t made an aircraft for decades) than aluminium being a better choice for a wing for a 100 to 150 seater in 2015.
Both also stayed classical aluminium alloys for the fuselage rather than AlLi for the same reasons. Mitsubishi is venturing new territory only in the vertical tail wingbox; here it employs a novel “out of auto-clave” CFRP production method, something United Aircraft has announced as well for their up-and-coming MS-21 wing.
We have looked at the airframe technologies used in the clean sheet projects that passed major milestones during 2015. The common thread is: use CFRP where it has undoubted benefits (empennage, wings) if you know how to do it. Otherwise stay away and take the weight and maintenance hit. For the fuselage tube, make it with aluminium. If you have the time and knowledge, make it an AlLi design.
The reason is that CFRP does not offer any real benefits for the tube; there are simply too many other dimensioning requirements which are not playing to CFRP’s strengths. There are huge needs of electrical conductivity, yet CFRP cannot conduct. CFRP skins are not strong in the traverse direction, yet this is the attack angle of the catering truck.
So much for what happened on the airframe side; next week we will have a look at 2015 engine technology progress.