Flying high with carbon fibre
Although unmanned aerial vehicles save lives, their development and manufacture can be costly. Andrew Mills, Principal Research Fellow in Composites Manufacturing, from Cranfield University, UK, proposes solutions.
Piloted aircraft may have less of a role as fighter planes in future, due to their high acquisition, design, development and airforce support costs and the risks of putting someone in the front line. It is expected that some future combat and reconnaissance missions would be carried out by unmanned aerial vehicles (UAV).
To address the challenge of large UAV design, BAE Systems and the UK’s Engineering and Physical Sciences Research Council (EPSRC) set up a Grand Challenge programme in 2004 called Flaviir, funding many aspects of UAV technology at 10 UK universities. Many UAVs will be small, low-cost systems, but for long-range operation, large aircraft will be required.
A major challenge has been manufacture. The aim is to optimise weight to match that of high cost structures, but at far lower construction costs and with increased robustness. Minimal maintenance enabling low-cost support are also key requirements.
Carbon fibre wing and fuselage structures have been in service since the end of the 1970s for large commercial aircraft, such as the Airbus A310 fin box in 1978, and the BAE Systems Experimental Aircraft Programme and the Beechcraft Starship in 1986. For these early certified structures, the initially expected weight savings were rarely achieved owing to damage tolerance, and the joint design and materials performance issues arising during the airframe certification process.
Furthermore, the introduction of radically different materials, manufacturing technology and design caused the cost of these structures to far exceed the initial estimates. This was due to the extensive testing required, and the labour cost of producing complex designed materials, lay-ups and joints.
More recently, production costs of aircraft, such as the Airbus A380, have been significantly reduced. Nonetheless, to meet certification requirements and allow for repair, the cost per kilogramme of laminated, cured and assembly structure is around £500 for a large commercial aircraft, with the purchased carbon fibre, epoxy resin pre-preg costing around £100 per kilogramme. This factor of five between raw material and finished structure reflects the complexity of the moulding and assembly processes.
The composites manufacturing activity at Cranfield University, UK, was set up to conceive, investigate and demonstrate a range of novel materials and manufacturing technologies. It has targeted areas such as materials costs, detailed parts elimination by integrating smaller components into larger mouldings. The team has also explored replacing fasteners with bonded joints and jigless assembly.
Integrated take off
The concept of parts integration has resulted in detailed components being cured together, avoiding the need for separate mould tooling, edge trimming and bonding, and thereby reducing the cost of assembly. The project resulted in a new tooling and moulding technique to integrate the wing skins, stiffeners, spar and rib flanges, so that a rapid jigless, fully adhesive bonded, slotted joint assembly can be used.
Carbon fibre composite (CFC) airframes use high-cost carbon fibre tapes and woven fabrics pre-impregnated with polymer adhesives. Flaviir, in conjunction with the materials supplier partners Hexcel Composites, in Lyon, France, and Eurocarbon, in Sittard, The Netherlands, has developed low-cost textile tapes and braided sleeves.
When combined with low-cost liquid epoxy polymer provided by Huntsman, Cambridge, UK, through infusion and resin transfer moulding, these tapes result in high stiffness structures using low-cost materials.
For fighter CFC technology, a huge cost problem is the need to attach many of the individual components with titanium bolts to prevent disbonding after impact or ballistic damage. Flaviir was able to eliminate all of the fasteners within the wing-box by using a novel sewing process called tufting.
The components most vulnerable to disbonding have been co-cured with the skin laminate after being tufted with a novel high-toughness carbon fibre thread provided by Toho Tenax Fibers, Wuppertal, Germany. Even after major damage, the tufted parts remain attached, preventing catastrophic failure and allowing low-cost repair. The less critical parts are co-cured using low-cost resin, but with highly toughened joints created by embedding toughening layers.
These technologies have been demonstrated through the design and manufacture of a two-metre long-wing box representing a section of a combat UAV. This showed a 55% reduction in manufacturing cost with equivalent weight, compared to structures manufactured using pre-impregnated materials and autoclave processing of the latest Eurofighter Typhoon type aircraft. This is the first wing-box manufactured worldwide using low cost materials and processing technology that provides equivalent weight to current structures.
For the implementation of these lower cost materials and manufacturing technologies, there are several challenges to overcome. Quality assurance with infusion moulding is more complex since the components have more variability in the resin and fibre distribution. Certification of bonded joints is more complex than for bolted ones since each bond line cannot be inspected to guarantee its strength.
The durability and damage tolerance of the structure must be ensured through careful joint design and adhesives selection, as demonstrated in the 1980s by the fully bonded, boltless wings of the Beechcraft Starship.
Alongside the manufacturing technology research, a flying demonstrator has investigated aircraft control without moving control surfaces. This has been achieved by disrupting airflow using variable flow rate through the wing’s trailing edges.
The aircraft has a novel configuration and uses advanced composite construction in a vehicle three metres long and weighing 80kg. It is powered by a small gas turbine and uses sophisticated thrustvectoring devices. A new feature is the use of an auxiliary gas turbine to provide compressed air for the fluidic control devices.
The all-CFC airframe has been designed and built by the Cranfield Composites Centre using standard prototyping pre-preg materials. The manufacturing process was hand lay-up using 60°C curing pre-pregs supplied by the Advanced Composites Group, and low cost mould tooling. The approach features a completely adhesive bonded airframe with no mechanical fastenings. A feature for assembly cost reduction is the use of one-piece wing-to-wing tip spars. The completed ultralight airframe weighs 17kg.
Andrew Mills, Principal Research Fellow in Composites Manufacturing, Composites Centre, Cranfield University, Bedfordshire, MK43 0AL, UK. Tel: +44 (0)1234 750111 x2405. Email: A.R.Mills@cranfield.ac.uk Website: www.cranfield.ac.uk