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AIRBUS STUDIES FUTURE AIRLINER CONCEPTS

EADS Airbus's appearance at the SITEF transport fair in Toulouse was quite spectacular, as development engineers who had previously belonged to Aérospatiale-Matra presented their design visions for the future. Their latest studies, be it of an ecological version of the Airbus, a flying wing or a successor to Concorde, have produced development suggestions with unusual aircraft designs aimed at increasing efficiency and environmental acceptability. Today these ideas are little more than intellectual exercises. But soon they could form the basis of future Airbus types. These the European manufacturer needs if it is to maintain its image as an innovator of high technology solutions in the decades to come.

In an exclusive interview with FLUG REVUE, Jean-Jacques Mirat, chief engineer for new ideas in the subsonic area from the Future Projects Department at EADS Airbus, talked through the designs.

A whole family of new Airbus concepts - the Low Noise Aircraft (LNA) - is concerned with the goal of noise reduction. Here the development engineers are using relatively conventional fuselage-wing structures from the existing Airbus range as the starting point. However, the unusual feature is the positioning of the jet engines, which have been moved from their traditional position underneath the wings to locations above the wings or even above the fuselage where less noise is deflected downwards.

The first Airbus LNA, substructure type "Rear Fuselage Nacelles", uses in the "tandem fin" configuration an engine layout which is already familiar from the American A-10 military aircraft. However, in the "wart-hog" the primary concern is not so much engine noise but to conceal the thermal signature of the engines against the threat of heat-seeking missiles. On the other hand if the Airbus LNA were designed so that the elevator unit was directly below the jet orifice, this would reduce the deflection of noise towards the ground. Another advantage of such a design is that the engines are very close to each other. In the event of an engine failure, the equalising yawing moments about the normal axis would be relatively low, so that the two-part split rudder unit only needs to be small. Again, a wing without engine struts and their associated services is easier to design.

The disadvantages of this design are that the engines are more difficult for maintenance engineers to access and there is the possibility of "uncontained engine failures", i.e. where pieces of debris become detached from a failed engine, causing damage to the fuselage which is very close. Again, the entire tail structure needs to be strengthened to accommodate the additional mass and engine vibration. A further drawback is that the heavy engines are a long way from the aircraft's centre of gravity.

The next variant of the Low Noise Aircraft utilises a similar layout, in which once again the fuselage and tail assembly shield the nozzle exit, thereby reducing the downward deflection of noise. However, on the "LNA V-Tail" there are only two control surfaces at the rear as the tail assembly is designed in a V shape. This means that not only is one control surface with its associated drag and weight eliminated, but the tail assembly structure can have a simpler design. This is because the structural joining assemblies between the vertical and horizontal tail surfaces, of which, in the first version there would need to be two, are completely dispensed with. Although there have been a few attempts to build aircraft with a V-shaped tail assembly - the most well-known examples are the Fouga Magister and the original Beech Bonanza - in practice the V-tail has not yet caught on. However, Mirat is hoping to obtain a significant improvement in the control characteristics of his 20% lighter V-shaped tail assembly using electronic fly-by-wire systems.

As the final variant in the series of noise-reducing LNA's, Airbus is studying a layout which in some respects is reminiscent of the German VFW-614 jet transport. This time the two engines have been positioned above the wing so that the wing structure itself shields the engine noise deflected towards the ground. But in the "LNA OWN" (On-Wing-Nacelles), the engines would be mounted a little further forward. This position has the advantages that the wing is protected against uncontained engine failures and the engines are also easier to access for maintenance. The most significant benefit, however, is probably the positioning of the jet engines close to the centre of gravity, compared with the rear-powered LNA variants, although the latter do provide slightly better noise insulation. In the OWN concept, a T-tail is used in order to avoid the exhaust gas streams from the high-positioned engines flowing onto the elevator unit, as would be the case with this nozzle layout in a conventional Airbus tail assembly.

The next Airbus design, designated the "Joined Wing Concept", whose aerodynamics are particularly complex, is concerned with a quite different primary objective, to achieve a significant reduction in the weight and structure of the wing. A "joined wing" would be able to manage with a small wing span and space requirement due to the relatively short, but now double design of its wings. However, the double wing could result in an increase in drag. Airbus is hoping with this modern variant of an idea that was first developed in the 1930s to achieve a significant reduction in fuel consumption. Moreover, the altered and significantly taller fuselage cross-section is not necessarily aimed at later holding hydrogen tanks, but at preventing unwanted interference between the pairs of wings through their spatial separation.

The Airbus Three Surface Aircraft (TSA) is quite striking with its unusual canard wing. An additional pair of canard wings behind the cockpit is intended to act as a "partner" to the elevator unit and support its operation. This might make it possible to reduce the size of the conventional elevator unit on the tail, thus saving weight. This "naturally unstable" aircraft concept would be artificially stabilised with computer support. Airbus is hoping that the TSA, with its diverse control surfaces and trimming possibilities, will have very good cruising characteristics and low fuel consumption. Moreover, with this design it would be possible to generate additional lift at a later date by enlarging the canards without having to increase the wing span of the main wing. Handling during slow flight would significantly improve also.

The biggest departure from commercial aircraft as we know them today is Airbus's giant flying wing. The four-engined longhaul passenger aircraft for around 1,000 passengers combines fuselage and wings into a single low-drag, light structure. This design, which might some day succeed the A3XX, holds out the promise of radically reduced fuel consumption compared with conventional wing-fuselage concepts. The main aerodynamic challenge in a flying wing is to obtain a clean flow of air over the thick midsection of the fuselage in which the payload areas are located. As in Boeing's "Blended Wing Body" concept, the jet engines are located above the trailing edge. From a design point of view, this approach poses some special challenges, as it would entail combining "wing design" and "fuselage design", whereas up to now these have been two distinct and separate design disciplines. In addition, the complex flying wing structure also has to bear cabin pressurisation loads.

Another drawback is the limited scope for modification of a flying wing from a technical viewpoint: whereas a conventional aircraft type can simply be "stretched", it is likely that every model of a flying wing aircraft, and every size of every model, would require a dedicated design, with the associated cost implications. Due to the complex structure, it would not be possible to insert or remove segments of the fuselage. Nevertheless Jean-Jacques Mirat is optimistic that Airbus will be able to offer several versions of this fuel-efficient model in 30 years' time at the latest.

Finally, the development engineer took the opportunity to round off the interview by presenting another strange creation from his area, the basic concept for a modern and enlarged Concorde replacement from Airbus. This four-engined design for 250 passengers which would travel at twice the speed of sound must, however, satisfy stringent noise and environmental standards. No further details were offered, for example, on the question of powerplants.

Even if not every one of these ideas eventually turns into a real aircraft type, these studies are assisting the engineers to move closer towards the future performance requirements in aircraft construction.

In the early phase of aircraft design, as seen here, the layouts presented are not the finished outcome of a series of compromises between the conflicting desires of the development, production and sales departments, as is the case with a real aircraft. Instead, the studies should be viewed more as radical solutions to specific problems, for example noise reduction, which are addressed more or less in isolation. The design features used in such a solution could have distinctly adverse effects on a different aspect of the aircraft and hence on the overall aircraft concept, but in this preliminary phase these are deliberately ignored. That is, after all, precisely the aim of a feasibility study, to ultimately enable the aircraft manufacturer to take a first look at the theoretical advantages and disadvantages of novel design features.

That the conceptual studies shown are highly sensitive and not just dabbling on the part of the development engineers is suggested by the ease with which Airbus shows signs of nervousness. Since the company's amazing presentation at SITEF, the EADS Airbus headquarters has firmly pulled the shutters back down on its fascinating futuristic ideas.

From page 22 of FLUG REVUE 1/2001