GRAHAM WARWICK / WASHINGTON DC
In the second of a monthly series marking the centennial of flight, we look at how commercial airliners will develop over the next 100 years
Breaking the sound barrier was once as impossible as travelling faster than light. Beginning with the Wright brothers' first flight, it took aviation 44 years to reach Mach 1, six more to exceed M2, and another 14 to reach the M6.7 speed record that still stands, 36 years later. Today only one civil aircraft can exceed the speed of sound and only a handful can fly faster than M0.9. Has civil aviation hit the speed barrier?
The new century opened to the debate over speed versus size. After Airbus launched the A380 large airliner in 2000, Boeing abandoned its competing 747-X and tried to interest airlines in the smaller, but faster Sonic Cruiser. The US manufacturer argued that the projected growth in international point-to-point services would favour speed over size.
Late in 2002, Boeing bowed to the increasingly inevitable and shelved the M0.98 Sonic Cruiser in favour of the Super Efficient Airplane, the dark horse "reference" design that had stalked the Sonic since its unveiling in 2001. While Boeing was confident the Sonic could match the 767's operating costs, applying the same technology to the slower Super Efficient resulted in operating costs 15-20% lower than the 767's. For airlines struggling to cut costs, the choice was obvious.
Cost over speed
It was not the first time that lower cost had won out over higher speed, nor is it likely to be the last. Even before 11 September, the air transport industry was heading in the direction of lower production and operating costs, and lower noise and emissions. Speed, for the foreseeable future, is not a priority.
For speed freaks, the writing may have been on the wall as early as the first generation of jet airliners. Convair's 880/990 could cruise at M0.87, but was outsold by the slower Boeing 707 and Douglas DC-8. The Aerospatiale/BAC Concorde first flew in the same year as the Boeing 747, but airlines voted for the mass-transit mathematics of the jumbo jet over the elitist economics of the supersonic airliner. The 747 did at least raise the speed bar for subsonic airliners, to M0.85, where it remains.
The USA's attempt to outdo the Concorde - the 234-passenger, M2.7 Boeing 2707 - was cancelled in 1971 following the withdrawal of US government funding in the face of environmental objections. Research continued, led by NASA, and evolved into the High Speed Civil Transport (HSCT) - a 309-seat, M2.4 airliner that was intended to have the economics that would bring supersonic travel within reach of the mass market.
But Boeing, after taking over McDonnell Douglas and reducing the USA to one commercial jetliner manufacturer, turned its back on the HSCT and NASA cancelled its high-speed research programme in 1999. Even after years of work, significant question marks remain over the environmental acceptability and economic viability of supersonic airliners. Research continues in Europe and the USA, but at a level far below that required to make a second-generation supersonic transport a reality any time soon.
So size and efficiency have won the latest round. They combine in the 550-seat A380, which Airbus calculates will burn 12% less fuel per passenger than a 747-400. The largest part of that saving comes from the latest generation of high-bypass turbofans, but the four-engined A380 also benefits from advances in fly-by-wire flight controls and composite primary structures.
The A380 will be the first airliner with a carbonfibre centre wing box, while the rear pressure bulkhead and everything aft will also be carbonfibre. A glassfibre/aluminium laminate, Glare, will be used for upper-fuselage panels, while the lower fuselage panels will be laser beam-welded aluminium. Airbus calculates the materials and structures advances in the A380, compared to the 747, will reduce weight by up to 15t while improving durability.
Electric actuation
Details of Boeing's 7E7 are still sketchy, but the 210- to 250-seat twinjet is expected to incorporate several of the technologies originally envisaged for the Sonic Cruiser. These include composite fuselage and wing structures, electric flight-control actuation, all-electric environmental control system and integrated vehicle health management. Boeing is projecting a 17% lower fuel burn per passenger than the Airbus A330, and a 10% lower operating cost, with 80% of the savings coming from the engines, which will be new very-high bypass ratio (10:1) turbofans.
Airliners have production and operating lifespans of 20-30 years, so the conventionally configured A380 and 7E7 will be around for a long time. The A380 is scheduled to enter service in 2006 and the 7E7 in 2008 and could be flying well into the middle of this century. Proponents of unconventional configurations, like the blended wing body (BWB), will have to wait for the next generation of airliners - and possibly longer, as there is much that can be done to improve the efficiency of the traditional airliner layout.
Boeing may have ditched the sleek Sonic Cruiser in favour of the super-efficient 7E7, but its long-running BWB studies continue. The aerodynamically and structurally efficient blended wing body is widely regarded as the most promising new configuration for an environmentally friendly airliner, although there are concerns over airport compatibility and passenger acceptance.
Work to date has concentrated on very large airliners, with up to 1,000 seats, but Boeing's Phantom Works has schemed out a modular family of long-range airliners ranging in capacity from 200 passengers to 550. NASA research suggests a 2015-timeframe BWB would be three times as fuel-efficient as a 747, and generate 10% of the NOx emissions. Boeing calculates that a 480-seat trijet BWB would be almost 20% lighter, and burn more than 30% less fuel per passenger, than the A380.
Boeing's latest work has also pushed up the cruise speed of the blended wing body, from the originally envisaged M0.85. Computational fluid-dynamics analysis indicates a cruise speed of M0.93 is feasible, but the "sweet spot" in terms of improved speed with minimal penalty is M0.9. In the eyes of the BWB's many proponents, this additional speed capability further improves the design's appeal.
The BWB's big advantage over conventional designs is that, as a spanloader, the configuration is extremely efficient. There are aerodynamic and structural challenges to be overcome, mainly involving aeroelasticity and flight control, but the biggest barrier to development is market demand. While there remains room to improve the efficiency of traditional configurations, there is little incentive to pursue unconventional designs.
The incentive could yet come in the shape of the environmental targets civil aviation will be expected to meet. Airlines have doubled their fuel efficiency over the last 20 years, but will be required to reduce their fuel burn by a further 50% over the next 20 years if their CO2 emissions are to remain constant while aircraft numbers double. At the same time, according to the European Union's Vision 2020, NOx emissions will have to be reduced by 80%, and perceived noise cut by 50% to allow round-the-clock airport operations.
The majority of those improvements are expected to come from reductions in aircraft drag and weight, reducing the engine thrust required and, as a consequence, the emissions produced and the noise generated. While new low drag and low weight configurations could play a role in meeting the objectives, there are a host of technologies under development that could dramatically improve the efficiency of conventional designs.
Airbus believes a large part of the 50% cut in fuel consumption will come from drag reductions, with active and passive flow control expected to reduce drag by 30% or more over the next 20 years. Today, airflow over aircraft is controlled through shaping and devices such as flaps and slats. Tomorrow's wings will combine active and passive flow control technologies to reduce drag and increase lift.
Active flow control
Europe is pursuing hybrid laminar flow control, combining suction on the leading edge with shaping of the aerofoil to delay the laminar-turbulent transition and dramatically reduce skin-friction drag, which accounts for half the total drag. NASA and the Europeans are also enthusiastic about the transonic cruise drag reductions possible with adaptive shock control - variable-height "bumps" on the wing which weaken the shockwave.
The USA, meanwhile, is charging ahead with research into other areas of active flow control. NASA is experimenting with pulsed jets as a way to delay flow separation. A row of jets just aft of the leading edge and along the forward edge of the flap could produce an effective high-lift system that, in place of today's leading-edge slats and multi-slotted flaps, consists of a simple droop and plain flap. Instead of pulsed jets powered by pressurised air or fuel detonation, it may prove possible to use electrically driven synthetic jets.
This would tie in with plans to make aircraft first "more electric" then "all-electric", by moving first to power-by-wire electric flight-control actuation - to be introduced into airline service with the A380 - and then to electric auxiliary power, environmental control, landing gear and other systems. Engines will also become more electric, with shaft-integrated starter/generators and electric-driven accessories.
There is long-term potential that the engines will be electrically driven, using hydrogen fuel cells. Europe continues work on the Cyroplane concept, which burns environmentally friendly liquid hydrogen in turbine engines, but concerns over the ground infrastructure and high-altitude contrails are pushing the USA towards electric propulsion. The ultimate goal - eliminating CO2 emissions - is the same.
Ample electric power will allow further advances in active flow control, using arrays of micro-electromechanical systems (MEMS) or even phased plasma fields to manipulate the airflow around the aircraft. "Smart wing" control using shape-memory alloy and piezoelectric actuators has been successfully demonstrated in the USA, and Europe believes a MEMS-based flow control system could be a reality within the next 10-15 years.
Eventually it is envisaged that sensors, processors and actuators will be integrated into the structure, allowing the aircraft to change its shape. Adaptive structural morphing, as NASA calls it, has enormous potential to improve aircraft efficiency. While the far-term vision is of an aircraft that can change its wing shape with the ease of a bird, there is nearer-term potential for flexible, hingeless and seamless control surfaces that reduce drag and increase lift.
It is likely that these technologies will be applied first to conventional subsonic aircraft, but the potential performance improvements offered by active flow control and adaptive multifunction structures could eventually rekindle interest in high-speed aircraft. Reducing airport noise, cruise drag and high-altitude emissions are all keys to making a supersonic transport viable.
Improving efficiency and reducing weight would also help minimise the sonic boom, which is essential if aircraft are to operate supersonically over land. The US Defense Advanced Projects Research Agency's Quiet Supersonic Platform programme has the potential to demonstrate airframe and propulsion technologies that could make the USA's next attempt at developing a supersonic transport more successful than its past efforts. But until then, or until Boeing chooses to dust off the Sonic Cruiser, airline passengers will have to make do with travelling the world at M0.85 or slower.
Source: Flight International
