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Martin-Baker Ltd was founded in 1929 by James Martin and Captain Valentine Baker as an aircraft manufacturer. By the end of the Second World War emergency parachute escapes from military aircraft were becoming increasingly difficult and the introduction of jet-powered aircraft made it virtually an impossibility. Around this time Martin-Baker was invited to investigate the practicability of providing fighter aircraft with assisted escape for the pilot. It soon became evident that the most attractive option would be by forced ejection of the seat with the occupant sitting in it and the ejection being carried out by an explosive charge. Since 1949, Martin-Baker has been dedicated not only to the design and supply of ejection seats, and has saved the lives of over 6,800 aircrew, but also to the design and supply of safety systems, including crashworthy seats. The photograph to the right illustrates an example of one such safety system - the Sikorsky S-92 Armoured Helicopter crew seat.
Scope of Project
The analysis work was carried out to investigate the structural behaviour of an armoured crew seat for the Sikorsky S-92 helicopter. The armoured crew seat is made up of an armoured bucket, which in turn is attached to a light alloy frame incorporating seat height adjustment and an anti-rebound, energy-absorbing system. A finite element analysis of the seat is implemented using the ANSYS program, in order to provide evidence of integrity of the design, under the various loading requirements of flight, and crash regimes. Variations in the finite element model were required to perform analyses of loading cases where the energy attenuation feature was unstroked and also where it was fully stroked. The energy attenuation feature is a shock absorber mechanism which works by means of material sacrifice. The analysis was carried out in accordance with the requirements of the Sikorsky specifications.
CADFEM UK CAE Ltd. were provided with an assembly model of the seat geometry from Martin-Baker's Unigraphics system. The individual parts in the assembly were then imported into the ANSYS program via the ANSYS Connection for Unigraphics product. Due to the size of the FE model, the substructuring method was employed to solve the problem. Subtructuring is a procedure, which condenses a group of finite elements into one element represented as a matrix. This single matrix is called a superelement. Each component making up the seat frame was modeled as a superelement, with the armoured bucket and connecting pins being modeled with shell and line elements respectively. The graphic to the left shows a plot of all the superelements. It was found that by employing superelements in the analysis the solution was carried out more efficiently, drastically reducing the total number of degrees of freedom.
The graphic to the right illustrates the finite element model used in the analyses. Seven loadcases were run in total, each loadcase comprising inertial loads and occupant loads. The occupant loads were applied directly as forces on the shell model for the crew seat armoured bucket. Two of the seven loadcases simulated dynamic loading. The dynamic loading was achieved by applying an equivalent quasi-static loading plus an imposed racking load. The racking load involved one of the seat legs being lifted by 10 degrees, this amount of lift represents the distortion of the aircraft floor in a crash situation. The seat was restrained at the helicopter mounting points, which allowed for the calculation of the helicopter floor loads.
The most challenging modelling issue was the seat connectivity and the strain relief features which existed within the crew seat. A combination of pipes, beam and spring elements were used. The graphic to the left shows a sample of the connectivity at the bottom of the leg.For each loadcase run, von Mises equivalent stress plots were extracted for each of the superelement components by expanding the results, an example of this can be seen to the right.
The loadcase giving rise to the maximum stress value was established for each component and this stress value was then used to calculate the critical reserve factors. The critical reserve factors were used to provide confidence in the structural integrity of the armoured crew seat for a Sikorsky S-92 helicopter for the load cases analysed. In addition to evaluating the stresses within the model checks were also made on deflections and these were compared with deflection criteria as stated in the specification. Mass and reaction force checks were also carried out.
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