Airbus Industrie's planned A3XX will be the ultimate expression of the classic airliner configuration, representing the end of the road for the layout of cylindrical fuselage, swept wing and podded engines so familiar today. That is the belief of Professor Ian Poll, head of blended wing-body (BWB) aircraft studies at the UK's Cranfield College of Aeronautics.
Cranfield is undertaking a three-year research programme, supported by British Aerospace and Rolls-Royce, to look at BWB configurations. The first design uses today's technologies, which allows direct comparison with the A3XX. A second design will look at emerging technologies and novel methods of installing engines and systems.
The UK study echoes work under way elsewhere in Europe and in the USA which suggests that flying-wing configurations are being considered seriously for future large airliners.
"The A3XX will remain our dominant design priority for years," says Dominique Gentili, head of future projects at the Toulouse-based design office of Aerospatiale Matra, "but we're beginning to look seriously at flying-wing designs as a potential solution for long-range transports". While it is a "revolutionary idea that will not see the light of day until 2020 at least", he remembers the concept was once mooted for the A3XX, but was quickly rejected because of risk, cost and technology arguments.
"Flying-wing-type designs offer a potential for a 30% improvement in aerodynamic efficiency, which would translate to about the same reduction in direct operating costs," says Gentili. This improvement results from distributing the weight of the aircraft throughout the airframe, all of which becomes a lifting surface. "You win a lot," he says, "but there are a great many questions yet to be answered."
US work on flying-wing designs centres on the Boeing/NASA Blended Wing Body programme. NASA programme manager Frank Cutler says the BWB configuration eliminates interference drag and lowers induced drag. "Instead of the fuselage being along for the ride, it contributes to the lift and, with the resulting large internal volume, [the BWB] would be highly efficient."
Changes to the BWB configuration, for passenger evacuation and production efficiencies, have encouraged Boeing to take another serious look at the concept for a future large airliner. Passengers would be seated on a single, wide, "theatre" deck, with freight and fuel located further outboard. An 80m (270ft)-span BWB would accommodate between 500 and 600 passengers and have transpacific range. It would probably be powered by three turbofans mounted on the aft upper surface. In military guise, the long endurance and high capacity of the BWB will make it a likely candidate for a future long-range tanker/transport.
The Cranfield BWB would carry 656 passengers in three classes (960 single-class) over 14,200km (7,650nm) at a cruise speed of Mach 0.85. The aircraft must be compatible with existing airport infrastructure, says Dr Howard Smith, leader of the design study. Initial work has highlighted two main areas of interest: human factors and the potential for more integrated systems because of the vast internal space available.
The first area is looking at issues such as passenger emergency evacuation from a cabin that, internally, would be more akin to a large sea-going ferry. Current certification rules specify that passengers must be able to leave the aircraft in a maximum 90s - but this could change if it were shown that a flying-wing design was more crashworthy, or that the extra seating space made leaving easier.
Pressurising a complex shape such as the BWB, and the resultant non-linear stresses and fatigue loads, has led Smith and his team to study two types of internal pressure vessel - an all-composite structure, or one containing internal aluminium tubes. Composites are more complex and weigh slightly more, he says, but the internal volume is greater. It would also be possible to put the emergency exits in the outer skin, directly accessible from the cabin.
With internal aluminium cylinders, escape becomes more complicated. The team has looked at linking cabin exits to doors in the outer skin by unpressurised and pressurised tunnels. Such methods are complicated by airworthiness rules, which stipulate that an emergency exit must open with a single lever stroke - so the two doors would need to be linked or the rules would need renegotiating.
The Cranfield study has also found there is extensive potential for improved integration of the engines and boundary layer control devices. A blended wing, for example, need not necessarily have podded engines on top of the fuselage. Instead, an engine core buried in the fuselage could drive a remote fan. This would enable the engine to be fully integrated with a laminar flow system, the internal volume of the BWB allowing space for the complex plumbing.
Gentili believes a BWB would probably be made largely from new aluminium/composite materials now under test and likely to be used in the upper fuselage of the A3XX. The flying-wing aircraft, because it is inherently unstable, would demand a fly-by-wire flight control system.
Boeing and NASA plan to conduct flight tests using a subscale remotely piloted BWB. "We will look at stall and general handling characteristics and see what it does in a spin or a stall, to find out whether it has a 'tumble' mode," says NASA's Cutler. Tests of the multiple-surface flight control system will focus on development of control laws to protect the aircraft from exceeding its flight envelope. Initial tests have indicated a need for wingtip "rudderlets" that combine the function of blended winglet and vertical fin tail for increased directional stability. "Being fairly small, and the benefit very large, they will probably be part of the final design," says Cutler. The BWB will have split elevons for roll control and speed braking.
To get the best out of a flying-wing aircraft, Cranfield's Smith says, it should have neutral or slightly negative stability. He predicts the all-up weight of an unstable aircraft could be 20% lower than its stable equivalent. The College's BWB will have neutral directional stability, with no fins. Split elevons will provide yaw control, but safety will be crucially important, he says.
Pitch inertia
A simulator is being used to assess how the aircraft could be controlled in the event of flight control failure. The BWB's size means that pitch inertia "is enormous. Nothing happens fast", says Smith. Reconfigurable controls could be used to ensure an elliptical pressure distribution - the most efficient way to produce lift. All the trailing edge surfaces could be used as elevators while those inboard would be used for roll control to reduce manoeuvre loads.
Boeing's current design effort will culminate with the building of a 10.7m-span remotely piloted vehicle powered by three 240lb-thrust (1.1kN) Williams turbofans. This will initially be tested in the large windtunnel at NASA Ames before flight testing at NASA Dryden in late 2002. If the tests succeed, it is likely that the BWB will be developed for military or commercial service by the second decade of this century.
Gentili says there have been no talks with airlines yet on a flying-wing airliner, "but Airbus and its partners are beginning to be interested They see this as the only solution for a future very large, long-range aircraft. We're trying to analyse the concept and see if there are any show stoppers. We're looking at the passenger cabin, engine integration, flight controls and pressurisation. It's going to cost several billion dollars [to develop], so we have to look at it carefully."
Source: Flight International
