With the recent announcement by Airbus concerning the end of production of the A380, there’s been much discussion from many outlets that its size was what made it a game changer. Further, pundits claim that despite all of its shortcomings, the A380 pointed an important way forward, much as the introduction of the Boeing 747 did fifty years ago.
Ever larger aircraft, however, are nothing new in the history of aviation, at every point since the Wright Brothers made those first leaps skyward, there was always the engineering challenge of a bigger aircraft right around the corner. Successful or not, the Airbus A380 is a part of that trend and undoubtedly, there will be a time again in the future when an aircraft as large, if not larger, will make its appearance in our skies again, challenging established paradigms in the airline industry.
What makes the A380 truly remarkable isn’t its size or passenger capacity, but its wing. The trends in modern commercial aircraft design have been towards long slender wings, or wings of high aspect ratio.
Quite simply, a wing’s aspect ratio is the ratio of its span to mean chord – or to think of it another way, aspect ratio is the square of the wingspan divided by the wing area. In aircraft design, high aspect ratio wings tend to have aerodynamic advantages in that they have a better lift to drag ratio – and the best way to think of the lift to drag ratio is to consider what correlates closely to lift to drag – the glide ratio. How much altitude is lost for a given distance forward in an unpowered state? Think of high performance sailplanes that can easily travel 30-60 units of distance forward yet only lose 1 unit of altitude in that same distance. Those sailplanes have very high aspect ratio or long and slender wings.
Aspect ratio is also important when considering the major source of drag on aircraft wings, the wingtip vortices. The higher the aspect ratio of the wing, the weaker the wingtip vortices, the less the drag caused by those vortices. This is why winglets are so effective – by attenuating the strength of the wingtip vortex, they get a wing to act like it has a higher aspect ratio than it actually does.
Amongst the current generation of commercial aircraft, the Boeing 787 has highest aspect ratio at 9.58. The Airbus A350 has an aspect ratio of 9.49, while the Boeing 777 is at 8.68. The Boeing 747-400 has an aspect ratio of 7.91 while the 747-8 is at 8.4. The Airbus A380, however, has a relatively low aspect ratio by comparison – only 7.5, which is at odds with the general trends in modern commercial aircraft.
And here’s what makes the A380’s wing so remarkable – despite an aspect ratio that’s significantly lower than most of its commercial aircraft contemporaries, it’s nearly as efficient.
The A380’s wing design had two big constraints. The first one was that the aircraft had to fit within an 80 meter “box” to be compatible with most airport infrastructure. This imposed a limitation on the wingspan, which at 79.75 meters, just barely fits in that “box.” Ordinarily, the best approach would have been a long slender wing, but that would have clearly exceeded the 80 meter limitation. Interestingly, the upcoming Boeing 777-9X gets around this 80 meter limitation by having the outermost portion of the wings fold upward, something that was once considered for the Boeing 777 during its development.
If we return to our earlier discussion of lift to drag ratios, you can improve the efficiency of a wing by either increasing its lift or decreasing its drag. A drag decrease was accomplished on the A380 wing by using what essentially enlarged versions of the wing tip fences used on the A320 family. While not offering the full effective increase in aspect ratio (remember, winglets attenuate the strength of the wingtip vortex), the vertically-oriented wingtip fences still offered drag reduction while not breaking the 80 meter restriction.
Increasing lift would have been a simple matter of increasing the chord of the wing, but this was the second big constraint on the A380 wing, the location of the emergency evacuation slides. A wing with more chord would have made the evacuation slide deployment geometry unnecessarily complex for the forward upper deck passenger door (U1L and U1R) and the middle upper deck passenger door (U2L and U2R) in particular, especially if the trailing edge flaps were deployed. The slide deployment geometry would determine where the doors could be located and it was paramount that the U1 and U2 doors not exceed the FAA maximum distance between emergency exits.
That’s the background on the constraints of the A380’s wing design- it was limited in wingspan and chord yet had to lift a payload commensurate with a double deck fuselage that was bigger than that of the Boeing 747. How Airbus solved that engineering problem is what makes the A380 wing the real game changer and not its immense size. Airbus had to take an intensive multidisciplinary approach that relied heavily on computational fluid dynamics (CFD) and extensive wind tunnel testing. The aerodynamics solutions alone for the A380’s wings took seven years, going through seventeen wing designs and 25 discrete wind tunnel testing campaigns. It required a close relationship between several major groups at Airbus- aerodynamics, loads, structures, and systems.
Traditionally in aircraft design, each of those four aspects- aerodynamics, loads, structures, and systems- involved a very labor intensive processes to reconcile and validate the approaches taken by each group to arrive at a common solution. For the A380, this process was streamlined using what’s called Knowledge Based Engineering, or KBE. To put it simply, KBE was a way of automating the number crunching necessary amongst the various design disciplines so that each group worked from a common data set. Changes to wing design that would have taken weeks to circulate amongst the different disciplines could now be accomplished within a single day or less. In the A380 wing design, there were five major categories of wing design work being done- aerodynamic design, geometry configuration, load calculation, structural design, and fuel tank modeling.
Loads, structures, and aerodynamics are self-explanatory. Geometry configuration dealt with the moving parts of the wing as well as how changes in fuselage configuration, engine nacelle design and the geometric constraints affected the final wing design. Fuel tank modeling wasn’t just due to the fact that the A380, like most commercial aircraft, would hold all of its fuel in the wings. As part of a design objective to make the wings lighter, the A380 is the first commercial aircraft to use a combined load alleviation function to minimize stresses on the wing, thereby making a heavier structure unnecessary.
The load alleviation function uses two methods – the first one is deflection of the control surfaces of the wing to lessen aerodynamic loads. This is nothing new in aircraft design but where the A380 broke new ground was to combine the use of the control surfaces with the active transfer of fuel laterally in the wing. Most Airbus aircraft use a tail tank and fuel is transferred between the wing tanks and the tail tank to maintain the optimum center of gravity to minimize drag, something that was done as far back as the Concorde. In the A380, fuel is also transferred from the inner wing to the outer wing after takeoff to reduce loads on the outer wings by putting more weight outboard to offset the tendency of the outer wing to flex upward. Before landing, fuel is transferred from the outer tanks to the inner tanks to reduce the impact on ground effect on the wing at touchdown. Fuel tank modeling was necessary to determine the impacts on the other categories of wing design as fuel was moved laterally within the wing.
What Knowledge-Based Engineering tools provided the design team was not just the ability to rapidly crunch the numbers and validate the data amongst all the design groups, it also freed the design teams from laborious processes so they could explore a larger number of supercritical wing configurations.
Computational fluid dynamics coupled with wind tunnel testing also validated the optimal design of the engine nacelles and pylons. In modern commercial aircraft, the aerodynamic interface of the engine nacelles and the wing are some of the most challenging areas in wing design. In addition, the same aerodynamic tools were used to determine the optimum shape of the wing-body fairing- the top part of the fairings are tailored to exert a positive influence on the pressure distribution across the top of the wing and the lower part of the wing-body fairing was shaped to reduce drag-causing shockwaves under the wing, especially near the inboard nacelles, all while having the necessary volume to accommodate the large main landing gear assemblies when retracted. Even the shape of the large flap actuator/track fairings took into account their effects on shaping local airflow. While this sort of process is nothing new to commercial aircraft design, on the A380 it took on added priority any and all methods to improve lift and reduce drag on the wing were paramount in overcoming the constraints of a relatively low aspect ratio for a commercial aircraft.
While the active load alleviation function already mentioned allowed for a lighter wing structure, the extensive use of composites in the wing ribs was a first in commercial aircraft for the primary wing structure. In the inboard wing which is much thicker, the wing ribs for the first time have a truss structure not unlike that of a bridge which offered weight savings without sacrificing structural integrity. Large sections of the center wing box also use composites.
The inboard leading edge high lift device of the Airbus A380 is different from the slats on the rest of the wing from the inboard nacelle outward. The leading edge device on the inboard wing is called a droop nose flap and it differs from a slat in that the leading edge slides/pivots down but a gap never opens up between the droop nose flap and the rest of the wing like a traditional slat. During development, Airbus needed to improve the stall characteristics of the inboard wing.
READ MORE: To learn more about ‘Aerodynamic Design of Airbus High-Lift Wings in a Multidisciplinary Environment’, click here to download the PDF.
It was too thick for a Krueger flap to be effective and while a sealed slat would have been ideal (a slat that had a flexible seal to cover the gap and maintain the continuity of the wing surface), Boeing held the design patents on such a slat. British Aerospace came up with the droop nose flap that acts like a sealed slat- droop nose flaps were used on the Hawker Siddeley HS.121 Trident in the 1960s, so the idea was essentially scaled up to A380 size. The trailing edge flaps are highly efficient- they are continuous to the ailerons with no discontinuities like thrust gates behind the inboard engines like that of the Boeing 747. A continuous trailing edge flap generates a lot of lift for a simpler geometry, not unlike the continuous trailing edge flaps seen on the MD-80 series or the A320 family.
The degree of aerodynamic refinement and weight savings of the A380 wing had a dramatic effect on its efficiency despite having an aspect ratio significantly less than its contemporaries. While engine performance is an important part of the overall fuel burn, compared to the Boeing 747-8 Intercontinental, the 747-8i has a 6% higher fuel burn rate. The Boeing 777-300ER, the type that is replacing many older 747s worldwide, has an 11% higher fuel burn rate. And quite impressively, the Boeing 747-400 has a 21% higher fuel burn rate.
When the Airbus consortium was first set up to design and build the A300, wing design was given to the British on account of the wing design done on the Hawker Siddeley Trident jetliner – in those early days of the nascent consortium, wing design work was done at at Hatfield and assembly of the wing sets was done at Broughton in North Wales. In 1977, the Hatfield facility was closed and design work now done for Airbus is performed at Filton. Broughton is still the assembly site for Airbus wings like that of the A380 and are shipped by sea to France for A380 final assembly. The Broughton facility for Airbus also builds not just the A380 wings but also the A320, A330/A340 and A350 wings. To most, places like Hatfield, Broughton, and Filton are just locations on a map. But in the history of British aircraft design, these are locations that bear witness to the storied legacies of the British aircraft industry that goes back to the dawn of flight.
Geoffrey de Havilland, one of the pioneers of British flight, selected the Hatfield Aerodrome as the location for his nascent aviation enterprise that by the 1930s was building the Moth training biplane and the small Dragon and Dragon Rapide airliners. By the Second World War, the famed de Havilland Mosquito was being built at Hatfield with the Vampire jet fighter making its appearance int the waning days of the war. Postwar, it was the site of manufacture for the pioneering Comet jetliner and the Trident. The de Havilland Aircraft Company was acquired by Hawker Siddeley in 1960 with the de Havilland name passing into history by 1963. When Hawker was merged with BAC to form British Aerospace in the 1978, Hawker’s design for an STOL airliner became the BAe-146. Hatfield closed in 1994 and design work for Airbus was moved to Filton.
Filton was the birthplace of the Royal Flying Corps which later became the Royal Air Force. Filton Aerodrome was the home of Bristol Aircraft where the Blenheim and Beaufort light bombers were built along with the Beaufighter. After the war, Filton was where the giant Brabazon was built and tested. In 1960, Bristol was absorbed by BAC and Filton became the UK production site for the Concorde. Airbus UK now operates the facilities at Filton after BAE Systems, the successor to British Aerospace, divested itself of its holdings in the Airbus consortium.
Broughton in Wales, the site of Airbus wing manufacturing, was the site of Vickers during the Second World War where Wellington and Lancaster bombers were built. Postwar, in 1948, Vickers’s Broughton facilities were acquired by de Havilland and for forty years, was the site of production for the DH.125 business jet before it was sold to Raytheon. Airbus wings have been built at Broughton since the days of the A300.
There is a history and legacy that underlies the technological achievement of the Airbus A380 wing that echoes with the great names of British aviation design and manufacturing history. And more so not just the A380’s remarkable wing design but the wings of every Airbus aircraft since the A300 are a flying testament to the excellence of British aircraft design.