Raising the limits

The modelling and successful design and manufacture of a new generation of cargo lifting airships poses far bigger problems than those involved in the design of fixed wing aircraft. But finite element analysis technology has reached the point where it can now help designers achieve this goal. In particular, it is now possible to design composite components and devices for aerospace applications that were previously considered too safety critical to be made out of anything other than metal. It also allows designers to delve deeply into the possible effects of defects and how to design round them. Addressing a recent Simulia customer conference in London, Frank Smith, associate technical fellow at Boeing, described his progress with tackling the design problems associated with the SkyHook HLV aircraft – a 137m long airship which is 62m high and 43m wide with four helicopter rotors. The idea is that a non rigid gas bag will support the static weight of the machine, with the rotors applying additional lift to allow up to 40 tonnes of cargo of cargo to be transported for up to 100 nautical miles at speeds of up to 50 knots. The target application is the transport of oil industry and mining equipment in the North of Canada, where proper roads and airstrips are non-existent. Currently, heavy items have to be moved over frozen ground during a winter season that is becoming increasingly shorter, owing to global climate change. According to Smith, the 100,000m3 airship will be the largest non rigid airship ever built and the first machine is scheduled to fly in 2012. The non rigid design is likely to be lighter than the large, rigid lighter than air machines constructed between the World Wars and more robust. The lifting agent will be helium and the construction should be able to resist buffeting by sudden storms over the prairies. It does, however, lead to other problems that have to be solved by modelling. The pressure distribution inside the envelope varies from zero at the bottom of the envelope to a maximum value at the top, providing lift. This lift is balanced by gravity loading and there are rotor and thruster forces to consider, as well as an inertia relief force and moment. In the first design iteration, it was found that if the centre of gravity was just 6mm off the centre line of the aircraft, the inertia relief moment would rotate the aircraft. This would then rotate the total lift vector, deforming the envelope so that a powered hover would 'end up looking like a banked turn'. The solution was to use Abaqus software to model hydrostatic pressure loading on the inside and outside faces to obtain a differential buoyancy pressure. These hydrostatic pressure loads were then updated with large displacements to produce a buoyancy force that is always turned upwards, despite large deformations. The analysis is run repeatedly, first to apply overpressure, then 'other loads'. Smith said: "The assembly sequence must also be considered." Typically, an airship envelope is inflated first, before the other components are added. In the case of the SkyHook, these will include the nose cone and possible envelope mounted thrusters. If all the parts were modelled and connected in their undeformed positions, and overpressure then applied, the external components would restrain the envelope from expanding and would be stressed by the envelope when it did expand. Another important aspect that has had to be studied is what happens when the airship is moored. Spectacular accidents happened in the 1920s and 1930s during such operations, because of wind gusts and when they were being moved into hangars. "There's still plenty of things we have left to do," Smith continued. When asked about modelling the possible effects of wind shear produced by line storms, he responded: "It's definitely in the plans." When asked if there would be any rigid reinforcements to the envelope, he indicated there might be, as well as the possibility of adding ballonets, into which air is pumped under pressure to control buoyancy. On the subject of analysing composites for more conventional aircraft, much work has been undertaken to enhance Abaqus itself and by third party developers and users, especially with regard to the effects of cracks. Dr Alison MacMillan, an engineering specialist at Rolls Royce, explained there will always be imperfections in composite fabrications, particularly in the resin. She has been engaged in a research project, supported by a Royal Society Industry Fellowship, to model multiple cracks in composite resin using Abaqus. The aim of the research, she added, was to discover 'when we need to start worrying about [cracks]." Abaqus 6.9 now includes the Extended Finite Element Method (XFEM) that provides a tool for simulating crack growth along arbitrary paths that do not correspond to element boundaries. Dr John Klintworth, ceo of Simulayt, explained the capabilities of its composite software, which sits on top of Abaqus, Catia and other software packages to allow the effects of draping and laying up multiple plies to be modelled. As well as allowing Formula 1 teams to take even more weight out of their constructions, advances in composite modelling now permit aircraft engineers to design critical structures with confidence that previously had to be fabricated from metal. One such part was described by Martin Kuessner, general manager of Simulia, who presented a paper on behalf of Dr Tamas Havar, a project engineer with EADS Innovation Works. The paper explored the redesign of a load introduction rib (LIR) for Airbus, a device that transmits forces from aircraft flap tracks to flaps. Kuessner said that changing from a metal to composite not only allowed reduction in weight, but also cost, because it is now possible to go from a Double C profile to an Omega profile, with local reinforcement. The resulting design is easier to fabricate using composite construction as compared with adding pieces of metal. It is also possible, he pointed out, to have directional strength where required. Kuessner said that a particular capability of the software was that it allowed a whole structure to be modelled, based on the modelling of a single rivet, as opposed to a structure involving the modelling of all the 324 rivets within it. He said: "Simulation is even more important for composite structures than for metallic. If a part buckles and buckles back, there may be damage inside." Physical testing of a manufactured composite LIR, he concluded, will 'come soon'. Pointers * Modelling is key to the successful re-introduction of large airship based machines * It also allows the designing of composite parts with greater confidence, saving both weight and cost * Crack modelling for composites is advancing.