To understand what’s behind the recent losses for the A330neo versus the 787, we will compare key characteristics of the types. Once we understand where the aircraft differ and how this affects the performance of the aircraft we can start to understand what’s happening in the marketplace.
We can compare the aircraft directly as they transport about the same passenger load. Using our apples-to-apples normalized cabins, with neither aircraft resorting to slimline lavatories nor other seat number increasing technologies, the A330-900 takes 294 passengers and the 787-9 292.
We will start the comparison with the overall design of the aircraft and their aerodynamic characteristics.
The best way to understand how the aerodynamic concepts stack up is to study a planform comparison of the aircraft, Figure 1.
The aerodynamics of the A330 were designed around 1990, about 15 years ahead of the 787. At the time, concepts like transonic wing design were well understood. There are therefore no principal differences in the aerodynamics between the A330 and the 787.
Figure 1 shows the A330 wing to have a wider span and a lower sweep than the 787 wing (30° at the quarter chord line versus 32.2° for the 787). The longer wingspan combined with a wing area which is only slightly larger (372m2 compared with the 787-9’s 360m2, both measured with the Airbus method) gives the A330-900 the higher effective aspect ratio, 10.6 versus 9.7 for the 787-9.
It’s a bit surprising, the older aircraft having the wing with the higher aspect ratio. It has to do with the history of the A330.
The A330 wing was designed for the A340, the long-range aircraft in the A330/A340 joint project. The A340-200 and -300 (the first versions) had rather weak engines (uprated CFM56-5 engines from the A320). To keep A340 performance, a slender wing with a high aspect ratio was designed (the A340/A330 classic’s effective aspect ratio was 9.7, the same as the later 787 wing)
The high span for the A340 helped with both take-off and cruise performance. The A330, which was designed as a 212 tonnes medium-range aircraft with 4,500nm range, didn’t need the advanced wing for its mission; it came free with the joint design concept.
The wing could be designed without excessive weight consequences thanks to a modern transonic (i.e. thick) profile and a low sweep (which limits cruise speed to M0.82). When Airbus decided to put the heavier 787 engines on the A330 to create the A330neo, it increased the span further.
Thanks to the aluminium construction, the A330-900 would be seven tonnes heavier empty than the 787-9. To match the fuel performance of the 787, the higher wingspan, and by it higher aspect ratio was necessary.
Both aircraft are Fly-By-Wire (FBW) designs and have concepts for protecting from stall and overspeed. How this is achieved differs, but it doesn’t affect the aerodynamic efficiency of the aircraft. With abnormal flight situations avoided by the FBW, the designers could focus the aerodynamics on an efficient normal flight.
In addition to having FBW to minimize cruise stability margins and by it trim drag, both aircraft employ further advanced concepts to increase cruise efficiency:
The fuel consumption of a long-range widebody aircraft is dominated by the generated cruise drag. The engine’s thrust must be set to combat this drag. Increased drag means higher thrust, which results in increased fuel consumption.
As the A330neo and the 787 use the same type of engines with virtually identical fuel efficiency (the A330neo uses the Rolls-Royce Trent 1000-TEN in a bleed version called Trent 7000 and the 787 the Trent 1000-TEN or GE GEnx-1B), the drag difference of the aircraft will set their fuel consumption.
The dominant drag during cruise is drag due to size, Parasitic drag, with about 55% of total drag. Parasitic drag is primarily caused by air friction against the aircraft’s wetted area. This important parameter is within 0.5% between the two aircraft, with the 787 being fractionally larger at 2011m2 compared with 2001m2 for the A330-900.
The 787 fuselage is wider and therefore has a 3.5% larger wetted area than the A330-900. This outweighs the difference in wing size for the two.
The other large drag during cruise is drag due to weight, induced drag. This drag is around 44% for the aircraft, with the 787 induced drag being slightly higher than the A330-900.
The lower weight of the 787 gains it an advantage in the climb fuel consumption to the cruise altitudes.
Overall fuel consumption
In the next article, we will fly the aircraft over typical missions to understand their fuel consumption characteristics.