The Aircraft Structural Layout Engineering Essay
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Published: Mon, 5 Dec 2016
The A380, when first produced was one of the newest and most technologically advanced civil airliners in the world, hosting the use of new materials, new manufacturing techniques and overcoming many engineering problems that the sheer size of the aircraft procured.
The requirement for an ultra high capacity aircraft, ‘UHCA’ , came about at the conclusion of the cold war; international conflict and tensions were dissipating and international travel became more viable. Airports were becoming more and more congested and Boeing was monopolising the civil air transport market Airbus needed something to new and preferably big to combat these issues.
The A3XX was conceived in 1988 and developed over the years until what we know now as the largest passenger airliner in the world; the A380. The eight was chosen to reflect the structure of the airframe, the two floor arrangement and the fact that in many Asian countries the number eight is considered as a ‘lucky’ number.
At the time, the aircraft was the latest feat of engineering for the aviation industry; if not for engineering in general. Its technological advances are something to admire but there were several problems to face this pioneering project. Many new manufacturing practices and techniques had to be developed; along with the logistical problems of combining over one hundred international partners contributing parts. Then the huge task of transporting the various oversized parts to the assembly line in Toulouse, France.
Designers were confronted with countless questions, how to transfer the immense loads the aircraft would come under? How will the undercarriage distribute the weight of such and still be manoeuvre efficiently in existing airports? How much power would an aircraft of such size require? Noise levels to comply with newly implemented restrictions? The 3E’s were imposed during the design process; environment, economy and energy.
This report will discuss the options and solutions that were chosen to overcome the many problems mentioned above.
DESCRIPTION OF THE STRUCTURE OF THE AIRFRAME
The complex number of airframe components were all put together – as one part – and were analysed and optimised using very advanced computational fluid dynamics. The A380 is quite a blunt aircraft and this helps by cutting drag by 2% as opposed to other desings. The designer enhanced many aspects of the A380 such as the wing body fairing and the wing. This enabled weight to be saved without creating drag consequences. The airframe of the A380 was built to last 25 years. On Page 5 of Appendix F a detailed cutaway can be seen of the A380. Also in Appendix A Figure 3 an overview of the components of the A380 can be seen. Full specifications of the A380 can be seen in Appendix E.
The nose section was difficult to engineer as the double deck design created a deep profile requirement. There is a large pressure bulkhead which is above and behind the nose gear bay. It is made from a double curved panel stiffened by longitudinal stringers which are locally welded. This bulkhead panel is unique as it is designed to form part of the cock pit floor. The unpressurized nose gear bay is in the forward lower part.
The fuselage of the A380 is the biggest in the world and the most complex ever made by Airbus. For example, just taking the middle section of the fuselage, there is a large amount of components. There are three doors, the belly fairing, main landing gear bay, centre wing box, two wing gears and the body gears.
Although the structure of the A380 is relatively traditional new materials were used. This will be discussed in more detail later in the report, here is a summary; the semi monocoque structure of the fuselage is formed from very advanced aluminium alloy.
There are plastic frames in the tail cone section reinforced with CFRP, carbon fibre reinforced polymer. The skins of the upper and lateral fuselage, forward and aft, are made from GLARE, glass fibre reinforced aluminium laminate. The centre fuselage however is made from aluminium alloys and glass fibres with imbedded adhesive. This is a very advanced way to save weight. This was not the only reason advanced composites were used; they also have better damage tolerance and fatigue.
The fuselage is 230ft long and has two main types of cross section. It is spherical until frame 31 then aft of this it becomes ovoid. This can be seen in figure 3 of appendix B where four possible cross sections for the A380 are compared. For interest only there is a sketch in Appendix C which shows what the A380 could have looked like with the horizontal double bubble fuselage cross section. Also in Appendix A Figure 2 a comparison can be seen of the A380 (marked A3xx) and the Boeing 747 cross section.
The rear section is unpressurized. The tail section and the forward unit section form this. The tail section is separated from the rest of the fuselage by a CFRP reinforced plastic rear pressure bulkhead. The rear section is attached to aft section of the fuselage. This runs from frame 74 to 95.
The rear fuselage section is a very complex set up. This is because it has to support the fin and horizontal tailplane. The substructure is reinforced with highly loaded aluminium frames as well as resin frames. The panels that attach to the substructure are chemically milled CFRP skins. Welded stringer panels are used in the lower fuselage. They are machined with integrated stringers in tandem. This creates stronger panels. The upper shell is strengthened by GLARE.
There are areas of high stress within the fuselage such as around the wing root area and the frames that run the length of the fuselage. These areas are machined. Areas which are less loaded are extruded, for example the upper shell.
The upper deck and the main deck are constructed in a similar manner, a floor grid supported by cross beams and frames in turn supported by vertical struts, but use different materials. The upper deck uses CFRP crossbeams which connect to the frames by a shear joint. The main decks cross beam is made from advanced aluminium lithium alloy.
There are two stairways within the fuselage. By door one is the double width stairway for in flight activity. In the curvature of the rear pressure bulkhead is another stairway made wide enough for service equipment and a stretcher.
The belly fairing has an aluminium substructure which supports panels made of a nomex honeycomb and hybrid epoxy skin sandwich. Deformation occurs between the fuselage structure and the belly fairing and this means loads from the fuselage are transferred to the fairing. Also there is a metallic strip in the rear section of the fairing it allows bending loads to be absorbed as it enables the composite shell to flex.
Due to the above wings of the A380 are the biggest ever made. They cover 9104sqft with a chord of 13ft and a 261ft6inch span and have an aspect ratio of 7.52. The sweep angle is 33.5 degrees at the 25% chord mark. The dihedral is 5.6 degrees at the tip.
The wings are very complex. The leading edge has six slat sections and two droop noses. The trailing edge has three single slotted fowler flaps. The ailerons, outer flaps and spoilers are made from composites. The inner flap is metallic. The wing itself has three ailerons and eight spoilers. Also the wing supports two engine pylons and the wing landing gear. A kruger flap would have been on the inboard leading edge but because of the huge depth of the A380 wing a droop nose device was included. It is completely sealed which means it makes the wing stall inboard and it pivots around a fixed point.
The wings frame is made from a CFRP and aluminium alloy hybrid centre box and a metallic outer part. The centre box consists of a root rib, rear, front and centre spars with skins above and below. The centre box is joined to the fuselage by frame fittings. Diagonally orientated struts support the floor structure above. The wing frame can be split into two parts the inboard frame, from ribs 1 to 17, and outboard frame, from ribs 17 to 49.They are differentiated by the fact that the outboard from has no centre spar, only front and rear. The spar material changes from aluminium at the middle point between the engines as a weight saving device. Twenty three of the forty nine ribs are made partly from CFRP.
The winglets take aerodynamic loads in roll. The A380 winglets have a 13ft chord, are at a length of 119ft away from the fuselage and are an optional item in the specification.
The tailplane in totality is a height of 79ft5inches and has a 99ft span. To put this into perspective this is almost the span of an A320 wing and the same chord as the A340. The tailplanes support from the fuselage has been spoken about previously. The tailplane is attached to the fuselage using two rows of lug and shear bolts. This is a similar concept the A340 design of this area. The frame that supports the tailplane is before the first row of lug bolts at frame 108. There is also double curvature of the skin at the root of the tailplane. This is because the airflow locally around the stabilizer will be greater than mach 1. The double curvature enables any drag rise to be dispersed. Between frames 99 and 100 is the single trim screw.
Rear of the tail cone is the rear fairing made up of titanium firewalls and is home to the APU (Auxillary Processing Unit) exhaust. The APU itself sits just forward of this within the tailcones CFRP frame and stringers. A revolutionary design that utilises a single torsion box with a lower and upper rudder has been used for both the fin and rudder. The fin box for example consists of a rear and front spar that span the whole fin with a framework of ribs made from CFRP and resin. The end fittings are made from aluminium alloy. The vertical stabiliser is 48ft high and has a chord of 39ft6inches with a taper ratio of 0.39 and an aspect ratio of 1.74. The horizontal stabilizer has an integral fuel tank. The lateral loads of this stabilizer are taken by a heavily reinforced structure.
The landing gear bays surround the cargo compartment between the aft cargo section and the centre wing box. The walls (inner and upper) are made from self stiffened panels.
DISCUSSION OF THE PRODUCTION AND ASSEMBLY PROCESS
With an aircraft of such size choosing the final assembly site was going to cause problems. The decision would have to involve a political and logistical discussion as to decide the options that were going to be best for the whole process. A collaboration between many companies throughout the airbus group and some other specialist companies was going to be vital to getting the highest quality for the aircraft. . The five largest contributors to A380, by value, are Rolls-Royce, SAFRAN, United Technologies, General Electric, and Goodrich. Airbus sized the production facilities and supply chain for a production rate of four A380s. Many newly created and some most advanced manufacturing techniques were used in the production of the A380 to allow this rate to be achieved.
Companies across Europe built the major structural sections in France, Germany, Spain, and the United Kingdom; other components came from across the world. JAMCO made the upper deck floor carbon cross beams and the stiffners and stringers for the fin centre box. Shin Maywa was contracted for the main wing root fillet fairing and the wing ramp surfaces. Yokohama made the water and waste tanks. Korean Aerospace Industries constructed the lower outer wing skin panels. Australian company Hawker de Havilland built the large wingtip fences. Chinese company AVIC make the panels of the landing gear bay. Hamilton Sundstrand produced the air generation system. Canadian Pratt and Whitney make the Auxillary Power Unit. Honeywell made the flight management system. American company Eaton provided the high pressure hydraulic system and the high pressure hoses. Parkers Aerospace’s Electronic Systems Division provides the fuel management systems. Rockwell Collins supplies the avionics full duplex AFDX Ethernet switch. A final example of the multi-corporate build is Goodrich developing the evacuation slides.
The construction of such an aircraft required huge amounts of money to be spent in order to design and build the sites that could cater for the A380 components. For example Airbus UK’s build site at Broughton received a brand new £35million building to contain the wing construction.
Each manufacture site was delegated different areas of manufacture; these were split into or aircraft component management teams (ACMT’s) then further split into combined design build teams (CDBT’s). carrying on from above the wing construction was in Broughton but the Wing assembly was delegated to Filton. ACMT’s were created for wing nose, centre fuselage, forward and aft fuselage, propulsion, empennage, landing gear, systems, interior and final assembly. The use of breaking down the ACMT’s into CDBT allowed for responsibility if components had issues, late deadlines etc.
As stated earlier, new production techniques were introduced. Laser beam welding; involves a highly accurate automated laser beam, typically carbon dioxide or solid state YAG (yttrium-aluminium-garnet) laser, which was introduced into manufacture in 2001. It has a built in inspection unit leading to much quicker and quality welds. This technique was used to attach the stringers of the lower fuselage shell skins, reducing dramatically the weight, need for fasteners and time taken of previous technique (reaching a production speed of 26ft a minute when welding the stringers it). This manufacturing process was also used for the curved bulkhead panel and lower fuselage skin.
Another manufacturing technique which revolutionised the A380 manufacture was an advanced robotic milling machine. It had a fixed axis and spun up to 24000 rpm and operated under a shower of lubricant. This was utilised in the manufacture of the aluminium alloy cockpit window frames.
Assembly of such an aircraft was going to be an epic logistical journey. Getting the parts to the final assembly hall in Toulouse was going to be the hardest part as a new technique for spatial alignment had been created. This groundbreaking feature of the assembly was vital; the positioning system aided by lasers (41/40 single station unit) aligned jigs to subassemblies with high tech optics to attach the fuselage and the wings together. It was very advanced as it calculates exact dimensions of sections and is interfaced with a CAD system which could derive structural qualities and average tolerances.
Techniques adopted by previous Airbus models used the A300-600ST Beluga aircraft to transport large parts; however this was not going to be as much use for the A380. This meant another option of travel was going to be used. Land and water was going to have to be used to get all components to Toulouse.
The decision as to where the final assembly should be located caused friction within the airbus family at first. The German airbus section had to be appeased by using its Ville de Bordeaux, Roll-on Roll-off sea vessel (RORO) as seen in Figure 5, to allow the choice of Toulouse as Final Assembly. Parts from across the world where transported to Europe to respective manufacturing areas and eventually moved to France.
Two main transport systems were utilised in the assembly of the A380. Air transport used of a number of guppy aircraft, devised by Felix Kracht, to manufacturing sites. By sea the RORO vessel travels to four different countries in Europe collecting parts; then transfers them to Paupac. From there it is transported by canal and road to Toulouse.
The front and rear sections of the fuselage are loaded on to RORO in Hamburg, northern Germany, whence they are shipped to the United Kingdom. The wings; transported by barge to Mostyn from Filton in Bristol and Broughton in North Wales, where the ship adds them to its cargo. In Saint-Nazaire in western France, the ship trades the fuselage sections from Hamburg for larger, assembled sections, some of which include the nose. The ship unloads in Bordeaux. Afterwards, the ship picks up the belly and tail sections by Construcciones Aeronáuticas SA in Cadiz in southern Spain, and delivers them to Bordeaux. From there, the A380 parts are transported by barge to Langon, and by oversize road convoys to the assembly hall in Toulouse.
The original pathways that were going to be used for transportation were simply not sufficient enough; therefore new wider roads, canal systems and barges were developed to deliver the A380 parts. After assembly, the aircraft are flown to Hamburg to be furnished and painted. It takes 3,600 litres of paint to cover the 3,100 m² exterior of an A380.
DISCUSSION OF THE CHOSEN MATERIALS USED IN THE STRUCTURE
The A380 was very progressive in its design. Forty percent of its structure consisted of carbon composites and advanced metal alloys.
The wing structure for example was constructed with a carbon fibre wing box. They used monolithic CFRP as it was found to be one and a half tonnes lighter than using aluminium alloys. The fin also created in this manner with a solid laminate CFRP fin box cured in an autoclave. This was also utilised in the rudder, horizontal stabilizer and elevators.
CFRP was employed in other areas of the plane too not just external components. Pressure bulkheads and upper deck floor beams were also made of this material. The vertical tail is made of a CFRP truss structure.
The wing skin was changed from aluminium alloys to composites in the design process.
Thermoplastics were used for lots of components. For example the ribs in the fixed leading edges of the vertical and horizontal stabilisers. Also for the secondary support holding the interior furnishing and the cabin trim. Impact resistant thermoplastics were used on the wing leading edges.
The A380 as an engineering feat used revolutionary materials. For example GLARE. The acronym stands for glass fibre reinforced aluminium laminate. It was used for the upper fuselage shell. The material was tested in 1990 and consists of alternation layers of 0.015inch aluminium sheet and glass fibre reinforced bond film. This material is revolutionary because it has better corrosion, fatigue and damage resistance properties than aluminium. It is also less dense with a weight saving of between fifteen and thirty percent. The weight saving was about 500kg in construction. This material is exceptional as the glass fibre layers between the aluminium stop cracks propagating and even operate as a load path.
Aluminium alloy was utilised massively within the production of the A380. One area where it was exploited was the windows in the cockpit. The frames were made from AL7040 aluminium alloy. A strengthened variant of the alloy was used as the bird impact shield.
The windshield fairing was also redesigned to use aluminium. It consists of aluminium skins, which were chemically milled, covering machined ribs and extruded stringers.
Aluminium alloy was used as a substructure for the massive belly fairing. It supported panels which utilised modern materials. The panel core is a nomex honeycomb core which is covered in a hybrid carbon glass fibre epoxy skin. Titanium was utilised in the fairing also with a corrosion resistant variant being used for stringers and frame roots.
The intuitive designers saved weight and increased performance wherever they could. The engines pylons where mostly made from titanium, however they had a secondary structure made from thermoplastic carbon and aluminium. An epoxy CFRP was used for the nacelle cowl and fan cowl skin.
These advanced materials were used as they reduced the weight of the structural form, made it more aerodynamic and improved the performance of the aircraft. Take the centre wing box as a case study. 2,200 pounds of weight was saved by using composites, mainly carbon fibre, for 50% of the 23ft by 20ft by 7ft structure. Component weight reductions also reduce stresses on the planes structure.
However the designers did not always go for the composite option. They considered the benefits and disadvantages carefully. For example the wing. There were many drawbacks to having a fully composite wing which were not balanced by being 1500 pounds lighter. For example the huge structural join now required would weigh more than the composite weight loss. This additional weight then required the wing to be strengthened as it had lost the bending relief moment. A more obvious drawback is the greater manufacturing costs of using composites.
DISCUSSION OF THE WEIGHT GROWTH WITH SERIES DEVELOPMENT
Throughout out the design phase the aircraft had been considered as passenger aircraft, with a freighter option considered as a sustainable use for the aircraft as well. However this was put on hold being postponed in March 2006 after both launch customers cancelled their orders.
With an aircraft this size small component changes were going to have huge effects on weight. An example of this is when the engines had to comply with the QC/2 noise regulations many changes occurred to the engine. This had a knock on effect with the wings, fuel volume, control surfaces and the structure that holds the wing; greatly affecting the weight of the aircraft.
The passenger variants of this aircraft are very versatile, but having the ability to carry up to 555 passengers and luggage, as is the case with the main A380-800 model, into an aircraft is going to significantly increase the weight. Other variants of the passenger are an extended range model with a shorter fuselage and only 481 passengers is the A380-700. Also a VIP aircraft has been ordered by HRH Prince Alaweed bin Talal bin Abdulaziz Alsaud which is known as the A380-Flying Palace. Also the United States Air Force is looking at the A380 as a replacement for the Presidential Air Force One aircraft; which is a Boeing 747 at present. A proposed stretch version the A380-900 has been proposed; it would be 12 frames longer be able to carry 656 passengers (three class layout) and would have an increase MTOW.
The A380-800C11 a passenger/cargo plane is an in between variant which can carry 11 cargo pallets as well as passengers.
The other main variant is the A380-800F dedicated freighter, a shelled out A380-800 model, with a capacity of 25 pallets on the upper deck, 33 pallets on the main deck, and 13 pallets on the lower deck. With a total available volume of 948.m3 it allows for a payload up to 150,000kg over 10,371km; this is almost double the capability of the Boeing 747-400F. The large range means no need for stopovers when crossing large oceans meaning quicker delivery times.But this required some of the composites to be replaced by aluminium-lithium alloys to allow for this greater loading; inevitably increasing the weight of the aircraft. More structural strengthening was undertaken fortified frames, more substantial skins and stronger landing gear. Figure 9 shows a comparison between the weights of the two main variants mentioned above.
The A380 is a world leading aircraft in not only in the ability to carry more passengers than any other aircraft it the market but because of the ground breaking technologies, new manufacturing techniques and the use many new materials. It is a very unique aircraft.
The airframe structure is an engineering feat; the high loads and stresses due to the sheer size of the A380 were dealt with by an innovative airframe made viable by the use of advanced computational fluid dynamics. These allowed for big decreases in weight due to the design and helped reduce the effects of drag.
The production and assembly process was a fairly political global project which involved the coordination of more than 100 companies. New techniques such as 41/40 single station unit used in assembly along with the laser beam welding. Collaboration of all the companies brought together by the Ville de Bordeaux the RORO ship, overland road convoys and fleet of guppy aircraft.
Mad from 40% composite materials with a host of new materials being utilized across all areas of the aircraft. They reduced the weight dramatically and aided in aerodynamic properties. These are the main reasons that allow the A380 to be such a mammoth.
The undercarriage design consists of a two wheel forward retracting nose bogie a six wheel rear retracting body bogie and two four wheel sideways retracting wing bogies. Extremely clever in design creating highly ground maneuverable aircraft.
At the moment only the A380-800 model is in production and being used to travel to many worldwide destinations. The A380-800F model is still on hold due to companies cancelling.
In this report I have discussed the structure of the airframe. I have also considered the influences of the production and assembly process, the structural materials used and the reasons behind them, the undercarriage design in terms of position and retraction and the weight growth with series development.
Figure 1: Airbus A380 Cutaway
Figure 2: A380 Cutaway (Flight Global)
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