Will the widebody airframe change substantially in the next 20 years or will the proven aerodynamic concept of the jet age simply evolve?
Poor single-aisle aircraft. Compared to the innovative leaps lavished on a diverse crop of new widebody models over the last decade, the straightforward re-engining programmes reserved for single-aisle aircraft – such as the Airbus A320neo and Boeing 737 Max – seem almost neglectful, albeit appropriately so, judging by the superlative size of their respective order backlogs.
By any comparison, three all-new widebody models – the Airbus A380 and A350, plus the Boeing 787 – have benefited from a historic influx of new technologies, as their designers traded metallic for composite structures and hydraulic and pneumatic for electric power systems. The models also raised the standard for comfort inside the cabin, in terms of new lows set for pressurization altitude and noise levels.
These radical innovations set such a high bar that Airbus can afford itself the luxury of a strategic pause. As Boeing executives speak of “harvesting” a decade’s worth of innovations in projects such as the 777X, Airbus is looking far ahead to beyond the 2030s, when the next cycle of clean-sheet widebody aircraft designs opens opportunities for another historic leap in technological progress.
“With the A320neo sold out, it means we have more time to reopen the door to emerging technology in terms of configuration,” says Charles Champion, executive vice-president of engineering. “If we had to launch a new product between maybe now and 2020, maybe we would have done something similar to the A320neo. But now that we have the breathing space to look further out, we can keep the door open to considering breakthrough new configurations and technologies.”
In aviation, technology usually advances in increments. Innovations are introduced cautiously in the beginning, then scaled up or expanded with the next generation of aircraft that enters service. The transition to carbonfibre-based composite materials offers a guide. Airbus first introduced the material with the rudder of the A310 in the 1970s. The A340 features a rear bulkhead made using carbonfibre reinforced plastic. The same material was then adopted for the centre wing box of the A380. Finally, Airbus’ production system and engineering skills had matured enough to make CFRP form the fuselage structure of the A350.
“In our business you have to make step-changes,” says Champion.
Another example is how Airbus is gradually introducing a new, “more-electric” system architecture on widebody aircraft. Any innovation must be proved at least as safe as the technology being replaced. The A340 used a standard triple-redundant hydraulic power system. For the A380, Airbus cautiously entered the trend towards “more-electric” aircraft systems by replacing one of the three hydraulic systems with two electric systems. The so-called “two-plus-two” systems architecture was then repeated on the A350.
But that represents only the beginning of Airbus’s plans for replacing hydraulic power systems with electric generators on widebody aircraft. The company’s commitment to electric power is clear with the E-Fan programme. The twin-engined demonstrator is expected to lead to the introduction of a four-seat aircraft optimised for pilot training in a few years. Along the way, Airbus is gaining more experience and confidence with electric power systems.
The move to the “two-plus-two” systems architecture on the A380 and A350 represented a major change, and more are possible in future.
“Basically, if you could go one step further and remove another hydraulic circuit, imagine what you could gain? You could potentially get rid of hydraulics and just use electrical power,” Champion says.
Replacing gas-powered turbines with electricity as a means of propulsion remains decades beyond the reach of current technology, but there are logical progressions between now and that point.
One example is replacing hydraulic power onboard aircraft with electrical circuits. Another step towards a fully electric-powered commercial widebody aircraft is auxiliary power. Currently, a gas-fuelled auxiliary power unit (APU) is located in the tail cone or aft section of modern widebody aircraft. This mini-turbine engine is needed on the ground to start the engines and sometimes provide onboard power to the aircraft systems while the engines are shut down. In flight, the APU can serve as a back-up power generator in the extremely rare chance that both engines fail.
As a step toward electric-powered flight, replacing the APU with high-power battery systems may be possible within the next two decades. Airbus and Boeing already use batteries to start the APU. The next step may be replacing the APU itself.
“If we could improve the density of energy in the batteries this then will open up our hybrid-electric systems,” Champion says. “If you could get rid of the APU – it’s a bit of dead weight [in flight]. It’s only there to provide power on the ground.”
Another area ripe for improvement is the aircraft wings and stabilisers. The last decade of widebody aircraft innovation included the switch to composite wing structure, allowing aircraft designers to significantly improve the aerodynamic efficiency of the lifting surfaces.
Having upgraded the structural material, the next step is to subtly improve how those structures are shaped. One cause of aerodynamic drag is the airflow over the wing itself. The airflow is laminar as it comes in contact with the top of the wing. At some point, this airflow becomes turbulent, which reduces the wing’s aerodynamic efficiency. Aerodynamicists dream of designing an outboard section of a wing that is naturally laminar for at least two-thirds of the surface.
For many years, Airbus and Boeing have experimented with a slightly different approach, which is known as hybrid laminar flow control. Rather than designing a naturally laminar wing surface, this approach requires embedding a mesh with thousands of tiny holes in the wing surface, with differential pressure used to draw the air into a laminar flow. Boeing, in fact, took the concept one step further with the 787-9 and 787-10, embedding a hybrid laminar flow control system in the leading edge of the vertical stabiliser. The same system was originally planned to be used on the 777X, but Boeing decided to remove it as the design was finalised.
According to Champion, Boeing’s decision confirms Airbus’ research that hybrid laminar flow control systems sometimes produce marginal results.
“It shows there are some borderline trade-offs on that one,” he says. Nonetheless, it could still prove useful depending on the application.
“We’re looking at hybrid also,” he says. “Particularly on the vertical tailplane, it’s clear it could make sense.”
More promising is adopting new wing shapes that encourage natural laminar flow. But such shapes are easily compromised by slight tolerance errors during manufacturing, or even the residue of dead insects in operation. The benefits, however, are clear.
“It depends on the aircraft configuration, but you could get probably 4-5% [reduction in fuel burn],” Champion says, though cautioning: “That’s to be demonstrated.”
Indeed, the EU-backed Clean Sky initiative is planning exactly such a demonstration, using an A340 and two natural laminar flow (NLF) airfoils, supplied by Saab and GKN Aerospace. Airbus is removing the outboard sections of each wing on the A340. Saab and GKN are each installing their NLF design on the outboard section. The project is scheduled to achieve first flight in 2017.
“It will be a big asset to see how far we can go,” Champion says.