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Latest composite technologies (ATL, RTM, automated drilling) are applied to a stabilizer that is larger than a 120-passenger aircraft wing. With a fuel capacity of 23,700 litres this is the airliner’s largest airborne composite pressurised structure.
(Published on June-July 2005 – JEC Magazine #18)
BY DANIEL CLARET VIRÓS,
DAVID ALFONSO CEREZO ARCE, AIRBUS ESPANA SL SPECIAL ACKNOWLEDGEMENTS TO JOSÉ MARÍA BLANCO SAIZ AND PEDRO NOGUEROLES VIÑES FRANCISCO ESCOBAR BENAVIDES
In 1971, CASA, which is now split into Airbus España S.L. and EADS CASA, joined the Airbus venture to supply the horizontal stabiliser for the Airbus A300. Parts delivered – mainly on the Horizontal Tail Plane (HTP) – were already made out of composite materials. Composites technology was emerging at that time for airliner structural applications, as it promised better performance for lower weight compared to traditional metal alloys. In the late 1960s, CASA started supplying most of these composite components to Airbus and Boeing (wing movables and elevators for the 737, 757 and 777 families), and also to other companies such as Dornier or Dassault. Today, 50% of Airbus’ CFRP structures come from Airbus España.
Since then, the utilisation of composite materials in aircraft structures has become more and more extensive. In this regard, the world leadership of Airbus Spain in the field of commercial jets is unquestionable. It was the first company to introduce composite parts on the HTP, then the first full composite HTP and, finally, the first composite HTP with fuel tanks inside. It was the first to apply CFRP to a jetliner fuselage section (A380), and it is also a pioneer in the application of the latest automated technologies – e.g. automated fibre placement or Resin Transfer Moulding (RTM) – to civil aircraft, among others.
Part of Airbus’ success is due to the latest technologies applied to structures. Its expertise in composites has made it possible to increase the use of CFRP for primary structures on the A380: HTP and elevators, vertical tail plane (VTP) and rudder, belly fairing, upper-deck floor beams, main landing gear doors, flaps, spoilers and ailerons. Now, for the first time in a jetliner, Airbus is using carbon fibre composite for fuselage sections 19 and 19.1 (rear cone), the centre wing-box covers and spars, and many of the wing rib webs.
In the modern history of commercial aviation from the first A300 on, the empennage components have always been the first to benefit from advanced composite materials. Since then, the original CFRP structural concepts and materials introduced on some A300 series parts (VTP, elevator, and rudder) using hand lay-up methods have evolved into the current automated technologies (automated tape lay-up, or ATL, fibre placement and infusion moulding) and latest high-strength (HS) and intermediate-modulus (IM) materials used on the all-composite HTPs of A340-500/600s.
In this context, the A380 HTP (fig.2) is a step beyond. For the first time, the latest composite technologies are being applied to a stabilizer that is larger than the wing of a single-aisle commercial jet: the A320 wingspan is about 15m, and the A380 HTP halfspan, about 19m. With a surface area of 205m2, the A380 HTP is the world’s largest airborne composite tank, equivalent in size to a 120-passenger aircraft wing and with all the complexity involved in large control surfaces and fuel carrying and management (fig.3).
Fig 3.: A380 horizontal stabilizer geometry.
The covers of the HTP torque box involve the manufacture of 18-m-long integral CFRP parts. The basic prepreg material is an IM unidirectional (UD) 180º curing tape. Large parts are co-cured or co-bonded.
Automation has been a key issue. For the HTP box, a fully automated ATL process is used for skin, stringers, spars and ribs, along with automatic drilling. The elevators feature a monolithic structure for improved maintenance, and the latest RTM techniques have been used successfully for the leading edge ribs and trailing edge fittings, among others.
The A380 HTP structural design uses a double torsion box concept with a centre joint. Each torsion box is configured as a multirib box designed for an extensive use of advanced composite materials. Torque box covers (fig.4) consist of integrally stiffened skins, with T-section stringers at optimised spacing chordwise.
Both skins and stiffeners are stacked using automated tape lay-up for fast, low-cost manufacturing. Fresh stringers with optimised spanwise section (variable feet width and web height and feet and web thicknesses) are placed on a pre-cured skin for autoclave consolidation in a co-bonding process.
The torsion box spars follow the A340-600 design concept: optimised design for toughened epoxy-resin material with a C-shape concept, pointing inwards for the rear spar (RS) and outwards for the front spar (FS). The integral spars include pre-cured T-profile stiffeners: vertical ones on the flat spar surface (wet surface for FS and dry surface for RS), and horizontal ones on the inner side of the FS.
A flat ATL process prior to hot-forming was chosen for this part. The curing cycle is performed in female tooling to reduce tolerance problems. Finally, the pre-cured stiffeners are bonded to the pre-cured spars using an adhesive qualified at temperature for structural aeronautical applications.
Integral C-shaped ribs for the torsion box are also manufactured with advanced composite materials. The ATL process is used for both ribs and stiffeners, while T-clips, tees, angles and fittings are made of titanium and advanced aluminium alloys.
The centre joint is designed with multiple load paths in a fail-safe case. It consists of CFRP rib 1 with titanium milled tees (interface with skins) and front and rear titanium fitting supports (interfaces with spars) (fig.5).
The HTP structure is supported at three main points on the rear fuselage: two Al alloy pivot fittings attached to the rear spar (HTP trimming axis) and the Al alloy screwjack fitting at the aircraft centreline (trimming actuator).
The leading edge is designed with RTM ribs (fig.6) that support an aluminium D-nose skin with false vertical and diagonal metallic spars and upper and lower sandwich panels.
The RTM process consists in the introduction of a dry preform into a closed mould. Later, the resin is injected to impregnate the dry preform and the mould is heated to perform the curing cycle. Using RTM technology allows manufacturing integral complex parts with outstanding tolerances and surface finishing. With RTM, the manufacturing and assembly operations are also shorter. RTM parts are near to net-shape due to in-mould consolidation (reduced material scrap), with a high fibre content that is easily regulated since the amount of fibre and resin injected is totally controlled.
The diagonal spar structure is the result of a new design concept to optimise leading-edge stiffness against bird strikes (fig.7). Al 2024 alloy was chosen to design the HTP tip, both for lightning-strike requirements and for the trade-off performed between CFRP and machined aluminium with a chemical milling process to reduce thicknesses.
On the trailing edge, the large and complex fittings of the inboard elevator are also manufactured in dry carbon fabrics using an RTM process (fig.8). Prepreg curing technology was chosen for the outboard elevator fittings. CFRP low-density Nomex-coresandwich upper and lower panels complete the structure, together with aluminium fittings at hinge line to guarantee its alignment.
The elevator is divided into an outer and an inner elevator. The skin design concept for elevators is also innovative: the typical sandwich panels have been substituted by a monolithic design based on intermediate-modulus CFRP and co-cured U-stringers. This design weighs the same as the traditional one, but presents many advantages in terms of maintenance, reparability and inspection, avoiding potential water ingress problems. The elevator ribs and spars are also made out of carbon-fibre tape. The leading edge ribs are made in a RTM process from a dry carbon fabric.
Trim-tank fuel system
Airbus wide-body commercial jets fly with a trim tank inside the HTP (fig.9). The objective of this strategy is twofold: to counterbalance the fuel consumption during the flight transferring fuel from the wing tanks, thus moving the centre of gravity as desired, and to increase the aircraft range.
Fig 9.: Airbus trim tanks summary.
Again, the A380 HTP implies an increase in magnitude and performance as it hides the world’s largest composite airborne tank. The fuel system consists of 2 refuelling galleries (between rib 8 left hand and rib 8 right hand) capable of accepting fuel from pressurised sources on the leading edge of the wings; transfer pumps; valves; a tank venting system to maintain pressure within the design limits (between ribs 8 and 9 of the right hand); waterdrainage and scavenge systems (to keep the water in suspension); and different control devices.
The composite HTP design combines airframe load cases with internal fuel pressure and thermal loads to obtain a proven structure that prevents any fuel leakage or fuel flow to the fuselage pipe under crash landing (critical load condition for the trim tank).
The certification of the A380 horizontal empennage is based on the requirements set in FAR 25 and JAR 25, change 15. State-of-the-art commercial software (MSC Nastran / Patran,…) and internal methods and tools supported by tests were used in the sizing of this HTP, as the structure is analysed from many different approaches: resistance, buckling, crippling, fatigue, dynamic, modal… (fig.10).
Catia v5 role has been very relevant as a design, digital mock-up and assembly tool, allowing integrated work for the whole aircraft, which is very important in an international company.
Sizing of the structure was performed according to design allowables, considering the worst realistic damage scenarios and environmental conditions.
The A380 horizontal stabiliser development and certification followed the building-block concept (pyramidal test programme), in which final proof of the structure was achieved by hundreds of coupon and element tests, dozens of subcomponent tests and one full-scale aircraft component to verify the result of the analytical studies.
All materials used on the A380 HTP (HS and IM fibres, UD and fabric tapes, dry fabrics and RTM resins) have been qualified and some of them were already used in previous programmes (A340-500/600, EF-2000,…).
Coupon, element and subcomponent tests were performed to adjust the available design-allowable values to the parts and laminates used on this HTP. Additional element and subcomponent tests were carried out to validate the new design concepts applied on the A380. The tests performed covered the worst scenarios the HTP could come across in service: static loading, fatigue cycling, internal fuel pressure (for boxes) and impact strength, among others.
Subcomponent and full-scale specimens were tested with artificial defects representing potential manufacturing imperfections (porosities and foreign objects) and impact damages (delaminations). Those, after fatigue and damage tolerance cycling, underwent ultimate load.
The full-scale HTP is being tested under static loads, fatigue cycling phase oriented to metallic parts and damage tolerance cycling. This test for certification will validate structural design solutions and applications of the largest airborne composite pressurised structure.