Analysis
Thiking blue-sky

Thinking blue-sky - future aircraft designs

What will passenger aircraft designed 20 years from now look like? Is supersonic flight likely to make a come back to commercial aviation? How will jets be powered and what fuels will they use? Chris Kjelgaard investigates for ATE&M what the future may hold.

Beyond the new generation of commercial aircraft (which will continue to enter service until 2020) much more efficient, automated and passenger-friendly aircraft designs could start appearing. Together with government researchers and institutional scientists, aerospace companies are focusing on developing new materials, manufacturing techniques, propulsion technologies, fuel feedstocks and aerodynamic design methods that could create aircraft which revolutionise air transport as we know it today.

This body of research — which is being conducted throughout the world, in universities, government research institutions, corporate laboratories, airfields and even agricultural plantations — is vast and it is growing rapidly. Perhaps, one of the most well-known of these research efforts is the work being undertaken by Future by Airbus. This is an in-house organisation which Airbus formed to look out to 2050 and imagine what commercial aircraft then — and, given current rates of advance in existing technologies, perhaps as early as 2030 — might look like, what they might be made of, how they might function, and what they might offer passengers in terms of onboard entertainment and productivity.

This imaginative effort excited the world in 2011, when Future by Airbus released computer-generated images and videos of an Airbus Concept Plane which was both revolutionary-looking and offered revolutionary capabilities. The ability of its cabin walls to turn transparent at a touch of a button was one such development, the holographic projection of any programmed scene (including live video-conferencing) on to interior walls was another.

Also familiar to the public is the sustained effort by the US National Aeronautics and Space Administration (NASA) under several major research programmes to advance commercial-aircraft materials, aerodynamics and propulsion technologies. NASA is performing this research to promote the creation of advanced, highly environmentally efficient commercial aircraft by 2035.

Most of the research being conducted into future commercial-aircraft designs and technologies is concentrating on aircraft that will cruise at the same speeds (within a few per cent of Mach) as those flying today, but will do so while burning less fuel. However, among the most noteworthy features of NASA’s wide-ranging research is that it also seeks to encourage development within the next 20–25 years of a new generation of environmentally friendly supersonic commercial aircraft.

Supersonic flight

Coupled with the extensive wind-tunnel and flight-test research that NASA has conducted (and continues to conduct) i
n minimising the creation and propagation of sonic booms, the administration’s research into the design of supersonic airliners has proved highly encouraging. So encouraging, in fact, that NASA is now a great deal closer than many in the industry might realise to helping make noise-unobtrusive, fuel-efficient supersonic commercial transport a reality.

NASA’s continuing progress on technologies that can produce a low-noise, fuel-efficient supersonic airliner leads Peter Coen, manager of the high-speed fundamental aviation project within NASA’s Fundamental Aeronautics Programme, to say: “I’m quite optimistic that we can get to a point within [NASA’s] N+3 [target time frame, which is 2030–2035] where we could get a 200-passenger aircraft of 70 perceived-level decibels flying [supersonically] over land,” at speeds of Mach 1.4 to 1.8. “Over water, it might fly faster,” he confirms, which will make the aircraft more commercially attractive to operators.

The design of any commercial aircraft which would fly at supersonic speeds over land would need to minimise the amplitude of the sonic booms the aircraft caused. NASA’s aim is for such aircraft to create booms of such low amplitude that they don’t even reach the ground. But to achieve this goal, supersonic jets would need to be “very long and slender”, according to Coen. This is partly because of the well-known “area rule”. More properly known as the Whitcomb area rule, this rule of aerodynamics plays a sizeable role in determining the shape of an aircraft’s fuselage and wings for designers when they are seeking to minimise the formation and amplitude of shock waves at transonic and supersonic speeds.

This mean that there is some bad news for airlines: supersonic jets will always be less fuel-efficient than subsonic jets, Coen says. Given foreseeable technological development it will be not be feasible within the next 20–30 years to create a supersonic aircraft large enough to carry more than 200 passengers, because the sonic booms the jet would create would be too loud for it to fly supersonically anywhere near land. If a new generation of commercial supersonic travel begins by 2035, Coen believes it will be a high-fare, niche offering.

Subsonic aircraft
Subsonic aircraft entering service in 20 to 30 years’ time on the other hand won’t necessarily need to be long and slender. Aircraft might have different fuselage and wing shapes, as well as different engine configurations, depending on the missions for which the type is optimised. Designers are already thinking imaginatively about unusual, non-traditional designs which might make very efficient commercial aircraft, particularly large ones.

A good example is the “flying doughnut” design which Airbus designers recently patented and which made headlines all over the world. Airbus was quick to say that its engineers come up with 6,000 design ideas a year, most of which are never put into production, and that it patented the flying-doughnut design merely to protect its intellectual property. Nevertheless, the design — which makes use of a flying-wing shape, something like that used today by the B-2A Spirit stealth bomber — is recognised by aeronautical engineers as offering various efficiency benefits for a subsonic aircraft. Among these would be better distribution of the stress loads caused by pressurisation of cabin air, allowing the aircraft’s structure to be lighter and more efficient than those of conventional airliner designs.

The patented design’s most unusual feature is that, for an unspecified reason, it would have a circular hole in the centre of the wide fuselage. All the passengers would sit around this hole, in circular, concentric seat rows — business class being in the innermost circle and economy in the outermost. However, this arrangement could potentially present problems for passenger disorientation — most if not all passengers would not have views through windows (in fact, the aircraft might not have any windows at all) — as well as for loading, unloading and servicing the aircraft. Nevertheless, Airbus noted that such a design would allow more passengers to be seated within a given aircraft size than in today’s tube-shaped fuselages.

Tube-and-wing designs have been around since the early days of aviation. Today’s mighty widebodies represent an evolutionary development in design from that of the 1909 Blériot monoplane and the mid-1930s Douglas DC-3, for example, while tomorrow’s commercial-aircraft designs might provide a revolution. Phil Webb, Royal Academy of Engineering and Airbus professor in aerostructures design at Cranfield University, suggests, however, that from the OEM viewpoint, there will remain much to commend the traditional tube-and-wing arrangement.

Laminar flows

For one thing, argues Webb, tube-and-wing aircraft are (relatively) easy to make and the aerospace industry has an enormous amount of experience in making them efficiently. Another factor is that, as wing designs become increasingly long and thin and aerofoils become more complex, manufacturing tolerances are becoming increasingly important. This is all the more true as designers seek to create commercial-aircraft designs which offer laminar airflow over the wings, a highly desirable quality because it would substantially reduce the amount of drag on the aircraft.

Laminar flow is achieved when the boundary layer of air flowing directly over the top surface of the wing stays closely bound to the wing (and thus flowing smoothly) along most of the wing’s aerofoil cross-section. When the boundary layer of air breaks away from the wing surface — as it must do eventually because of the curved cross-section the wing needs to create lift — it creates turbulent vortices which result in drag. Experimenters have achieved laminar airflow over as much as 60 per cent of a wing’s aerofoil cross-section for about 40 per cent of the wing’s overall length. Such results produce a 24 per cent reduction in drag from the wing.

Researchers are using several methods to try to improve wings’ laminar-flow qualities. Two methods show particular promise. One is to blow air into the boundary layer from a series of tiny holes (fractions of millimetres, or even less, in diameter) placed at regular intervals along the top of the wing near its leading edge. The other is to place a series of very tiny dimples at regular intervals in a similar location on the wing.

The sizes, shapes and areas of these holes or dimples, as well as the distances between them, are of critical importance in inducing laminar flow and so they require very exact manufacturing tolerances. Webb argues that such tolerances might not be easy to create — or repeat from aircraft to aircraft — in large wings with critically shaped aerofoils, if manufacturers adopt radical new aircraft designs.

This can also be said for fuselages and tails, over which laminar airflow can also reduce the amount of drag on the aircraft. One researcher in the US is currently studying a surface treatment which could potentially allow designers to make vertical stabilisers smaller. Meanwhile, Lufthansa is painting the fuselages of some of its A320-family jets with a new sharkskin-like ribbed coating, which sister company Lufthansa Technik has demonstrated can reduce by several percentage points the amount of fuselage-produced drag (see more at www.mro-network.com/sharkskin).

Laminar flow and other critical design features, such as the aerofoil and fuselage surface shape, are going to require a heretofore unseen level of surface accuracy to achieve the drag reduction that commercial aircraft designers are seeking. Webb says: “The size and tolerances are the problem. How well is [the design] going to scale up? Can manufacturers deliver to these exacting tolerances?” Webb does, however, note that new laminar-flow techniques could potentially mitigate the manufacturing challenge.

A design revolution

Despite legitimate concerns regarding the scalability and repeatability of manufacturing tolerances in radical aircraft designs, many researchers are experimenting with designs that move well away from the traditional tube-and-wing concept. NASA and the various research institutions and companies to which it has allocated study contracts have produced a welter of different 2035-era designs, some of them very unusual — including a design for a large aircraft with a 3D box-wing-and-tail arrangement, devised by Lockheed Martin.

One of the most promising areas of NASA’s research has focused on the hybrid wing body (HWB), which closely resembles the earlier blended wing body concept devised by Boeing’s Phantomworks research organisation. Now flying in a reduced-scale, experimental, unmanned form in NASA flight tests as the Boeing X-48C, the HWB blends a flying-wing design with a delta-shaped, wide fuselage that produces a considerable amount of lift in its own right.

Ernest Arvai, president of aerospace consultancy The Arvai Group and a principal in the AirInsight group of aviation consulting firms, thinks the HWB concept — and similar lifting-body concepts such as the flying wing — will become more acceptable to the airline industry, despite its strong tendency to be conservative about design.

Arvai argues that the move towards more computerised, more automated commercial aircraft, together with the compelling environmental and economic need for OEMs to produce aircraft which are substantially more fuel-efficient than those they are making today, will gradually make airlines receptive to new ideas. Although such designs will have to be “sold carefully to airlines”, because they will represent “a radical change … and [airlines] tend not to like radical change”, Arvai thinks lifting-body-shaped commercial jets will become acceptable within 20 to 30 years, “particularly for intercontinental flights”.

The tendency by aircraft designers to move toward more laminar-flow, critical wing aerofoils which require longer and thinner wings also makes Arvai think that tomorrow’s designers might opt to create aircraft which effectively have two fuselages. Such “twin-boom” designs would have two parallel fuselages separated by a central section of wing, with the outer wings protruding from the outer sides of the two fuselage booms. However, the flight deck might only be located within one of the two fuselages.

Researchers at the Massachusetts Institute of Technology contracted by NASA under its environmentally responsible aviation project devised another arrangement which uses similar thinking. However, instead of creating two separate fuselages, their designs used a “double-bubble” concept which effectively placed two fuselages together to create a single, double-width fuselage considerably wider in cross-section than today’s widebodies.

The researchers combined this double-bubble body with long, thin, unswept wings mounted high on each side of the aircraft’s fuselage and engines positioned on top of the rear fuselage. Such an aircraft would be relatively slow — commercial aircraft need swept wings to be able to achieve near-supersonic flight — and thus probably would be most effective if used for short-haul flights. In those circumstances, it would be highly fuel-efficient, because the wide fuselage would create a substantial amount of lift in its own right, complementing the lift created by the nearly laminar-flow wings.

Whether future commercial aircraft are designed with wide fuselages and wings, have twin booms, double-bubble fuselages or some other exotic arrangement these aircraft almost certainly won’t carry their engines under their wings. Instead, the engines will be mounted on top of the aircraft’s rear fuselage or else semi-embedded or fully embedded within it. Nor are such future-aircraft designs likely to have a single, vertical tail or horizontal stabilisers. Instead, they will either boast twin, angled tail surfaces which act as both vertical and horizontal stabilisers (the Airbus Concept Plane has such an arrangement, see right), or else they may do without tail-mounted control surfaces at all.

Future thrust

Despite the fond wishes of many in the industry for a cheap, widely available replacement for fossil fuel-derived jet fuel, it is extremely unlikely to appear within the next 20 years. This is not, however, stopping airlines from seeking ways to mitigate the environmental and economic effects of using fossil fuels, and for a means of replacing fossil fuels completely by 2050.

The industry’s biggest hope — albeit a remote one — is that Lockheed Martin’s Skunk Works in Palmdale, California can deliver on its remarkable announcement last autumn that it thinks it can produce a workable, commercially feasible fusion reactor within the next 10 years. Although this model would weigh 1,000 tonnes or more, the Skunk Works also thinks it could eventually produce a portable fusion reactor which would fit on the back of a lorry. Assuming the weight of such a device wouldn’t preclude the lorry from actually transporting it, it might be possible for someone to design and produce a fusion reactor which would fit on a commercial aircraft.

Should fusion reactors be developed in the next 50 years, the world’s overall energy picture will be revolutionised, because fusion uses a fuel which is available in virtually inexhaustible amounts: an isotope of water. For commercial aviation, practical concerns might remain: the issues of reactor portability and radioactivity (even though fusion produces little if any radioactivity, it usually requires nuclear fission to get the process started), and the means of converting large amounts of heat during flight to enough electricity to keep the aircraft airborne.

Until then, or until some means can be found of storing in portable batteries enough electric energy to keep large aircraft in the air, airliners will continue to require jet fuel. “We have to take the view that we have to use fossil fuels if we’re going to continue flying,” says Webb. Apart from any consideration of the time it will take to develop a viable alternate propulsion technology, the energy density (the amount of energy a given volume of fuel can produce) that jet fuel offers, and the infrastructure that exists to deliver it, argue compellingly that jet fuel will power commercial aircraft for the next 20 to 30 years.

In the meantime, researchers are working hard to find ways to make jet fuel from renewable, sustainable sources rather than from fossil fuels. For instance, Airbus has established six projects in different areas of the world to create regional, sustainable biofuel “value chains” connecting farmers, refiners and airlines. In each case the project is using the renewable biomass material most appropriate to local conditions.

Suitable feedstock sources include natural gas, algae, woodchip waste, used cooking oils, domestic waste, yeast, the camelina weed, the jatropha curcas plant, saltwater-grown plants such as salicornia, and other fast-growing, hardy weeds which have no agricultural utility but which could yield useful energy densities when refined into jet fuels.

The major fuel-standards bodies have already approved 50 per cent blends of biofuels and conventional jet fuels for commercial airline operations. Airbus estimates that, by 2030, up to one-third of aviation jet fuel could be derived from non-fossil sources.

Engine development

The major engine manufacturers continue to develop concepts such as counter-rotating open rotors (see also p.38), ducted counter-rotating fans and ultra-high-bypass advanced turbofans (which will almost certainly be geared, like Pratt & Whitney’s PW1000G family) to improve fuel-efficiency.

Together with the revolutionary improvement in understanding of air flows through engines that today’s computational fluid dynamics techniques have given manufacturers, new materials (such as ceramic matrix composites) and thermal barrier coatings allow the latest engines to run hotter and more efficiently, reducing the amount of pollutants they emit.

However, aircraft and engine manufacturers are collaborating with other industrial companies in looking to the future and developing revolutionary new propulsion methods. For instance, within its “eConcept” project to visualise the potential architecture and configuration of a commercial aircraft in 2050, Airbus is working with Rolls-Royce and Siemens on ideas for a propulsion system, in particular what Airbus calls “hybrid propulsion”.

Seen by Airbus as an interim step before developing fully electric propulsion for commercial aircraft, the hybrid-propulsion concept would use a conventional turbofan or turbine engine to generate enough electrical power to run six large fans mounted on the rear fuselage of an aircraft. Future by Airbus’ concept sounds like one developed three years ago by a scientist at NASA’s Glenn Research Centre in Ohio, where NASA performs most of its advanced aircraft propulsion research.

NASA’s turboelectric distributed power (TeDP) concept for a commercial aircraft of 2035 would use two turbofans (each about the size of a JT8D-217), with one engine mounted on each of an HWB aircraft’s two wingtips. Each engine would generate just enough thrust to negate the drag it caused, but it would also generate large amounts of electrical power. This power would be drawn from each engine by superconducting electrical lines to run a series of 12 or more CFM56-sized fans mounted in a shallow V-shaped formation (to cut out inlet-air interference between adjacent fans) on top of the HWB’s rear fuselage.

As consultant Arvai notes, a big plus for the TeDP concept is that “the efficiency of electrical fans is fantastic”. All things being equal, the concept could work very efficiently. However, TeDP also has one massive drawback, which the Airbus, Rolls-Royce and Siemens hybrid propulsion concept presumably shares; to conduct electrical power from a wingtip to a series of fans on the fuselage, the aircraft’s electrical lines would need to be cooled to a temperature just above absolute zero.

This would require heavy refrigeration equipment be installed on the aircraft and that the aircraft carry refrigerant. Despite aviation’s past unfortunate experiences with hydrogen in its highly flammable gas form (for instance, the Hindenburg disaster), a NASA researcher proposed that the aircraft be fitted with tanks to carry liquid hydrogen, it could serve as a refrigerant and a fuel to power the aircraft’s engines.

The researcher’s careful calculations showed that such a combination could work for a large commercial aircraft. He also noted that, at current technological development rates, within the next 20 years a way might be found to create electrical superconduction at the ambient temperatures found during aircraft flight. Were such a discovery to be made, TeDP could quickly become a reality.

In the absence of the development of a portable fusion reactor, another technology allowing fully electric propulsion of commercial aircraft could yet be found by 2035: a stable new battery technology which could provide an energy density equivalent to or better than that of jet fuel. Were such a technology to be developed, commercial aircraft could become a lot more environmentally friendly, and different looking, within our lifetimes.

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