Tag Archives: cockpit windows

The blind leading the blind

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Three years after it started, the MOTIVATE project has come to an end [see ‘Getting smarter’ on June 21st, 2017].  The focus of the project has been about improving the quality of validation for predictions of structural behaviour in aircraft using fewer, better physical tests.  We have developed an enhanced flowchart for model validation [see ‘Spontaneously MOTIVATEd’ on June 27th, 2018], a method for quantifying uncertainty in measurements of deformation in an industrial environment [see ‘Industrial uncertainty’ on December 12th, 2018] and a toolbox for quantifying the extent to which predictions from computational models represent measurements made in the real-world [see ‘Alleviating industrial uncertainty’ on May 13th, 2020].  In the last phase of the project, we demonstrated all of these innovations on the fuselage nose section of an aircraft.  The region of interest was the fuselage skin behind the cockpit window for which the out-of-plane displacements resulting from an internal pressurisation load were predicted using a finite element model [see ‘Did cubism inspire engineering analysis?’ on January 25th, 2017].  The computational model was provided by Airbus and is shown on the left in the top graphic with the predictions for the region of interest on the right.  We used a stereoscopic imaging system  to record images of a speckle pattern on the fuselage before and after pressurization; and from these images, we evaluated the out-of-plane displacements using digital image correlation (DIC) [see ‘256 shades of grey‘ on January 22, 2014 for a brief explanation of DIC].  The bottom graphic shows the measurements being made with assistance from an Airbus contractor, Strain Solutions Limited.  We compared the predictions quantitatively against the measurements in a double-blind process which meant that the modellers and experimenters had no access to one another’s results.  The predictions were made by one MOTIVATE partner, Athena Research Centre; the measurements were made by another partner, Dantec Dynamics GmbH supported by Strain Solutions Limited; and the quantitative comparison was made by the project coordinator, the University of Liverpool.  We found that the level of agreement between the predictions and measurements changed with the level of pressurisation; however, the main outcome was the demonstration that it was possible to perform a double-blind validation process to quantify the extent to which the predictions represented the real-world behaviour for a full-scale aerospace structure.

The content of this post is taken from a paper that was to be given at a conference later this summer; however, the conference has been postponed due to the pandemic.  The details of the paper are: Patterson EA, Diamantakos I, Dvurecenska K, Greene RJ, Hack E, Lampeas G, Lomnitz M & Siebert T, Application of a model validation protocol to an aircraft cockpit panel, submitted to the International Conference on Advances in Experimental Mechanics to be held in Oxford in September 2021.  I would like to thank the authors for permission to write about the results in this post and Linden Harris of Airbus SAS for enabling the study and to him and Eszter Szigeti for providing technical advice.

For more on the validation flowchart see: Hack E, Burguete R, Dvurecenska K, Lampeas G, Patterson E, Siebert T & Szigeti, Steps towards industrial validation experiments, In Multidisciplinary Digital Publishing Institute Proceedings (Vol. 2, No. 8, p. 391) https://www.mdpi.com/2504-3900/2/8/391

For more posts on the MOTIVATE project: https://realizeengineering.blog/category/myresearch/motivate-project/

The MOTIVATE project has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 754660 and the Swiss State Secretariat for Education, Research and Innovation under contract number 17.00064.

The opinions expressed in this blog post reflect only the author’s view and the Clean Sky 2 Joint Undertaking is not responsible for any use that may be made of the information it contains.

Designing for damage

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Eighteen months ago I wrote about an insight on high-speed photography that Clive Siviour shared during his 2016 JSA Young Investigator Lecture [see my post entitled ‘Popping balloons‘ on June 15th, 2016].  Clive is interested in high-speed photography because he studies the properties of materials when they are subject to very high rates of deformation, in particular polymers used in mobile phones and cycle helmets – the design requirements for these two applications are very different.  The polymer used in the case of your mobile phone needs to protect the electronics inside your phone by absorbing the kinetic energy when you drop the phone on a tiled floor and it needs to be able to do this repeatedly because you are unlikely to replace the case after each accidental drop. A cyclist’s helmet also needs to protect what is inside it but it only needs to do this once because you will replace your helmet after an accident.  So, the kinetic energy resulting from an impact can be dissipated through the propagation of damage in the helmut; but in the phone case, it has to be absorbed temporarily as strain energy and then released, like in a spring.

Of course there is at least an order of magnitude difference in the consequences associated with the design of a phone case and a cycle helmet.  We can step up the consequences, at least another order of magnitude, by considering the impact performance of the polycarbonate used in the cockpit windows of airplanes.  These need to able absorb the energy associated with impacts by birds, runway debris and other objects, as well as withstanding the cycles of pressurisation associated with take-off, cruising at altitude and landing.  They can be replaced after an event but only once the plane as landed safely.  Consequently, an in-depth understanding of the material behaviour under these different loading conditions is needed to produce a successful design.  Of course, we also need a detailed knowledge of the loading conditions, which are influenced not just by the conditions and events during flight but also the way in which the window is attached to the rest of the airplane.  A large and diverse team is needed to ensure that all of this knowledge and understanding is effectively integrated in the design of the cockpit window.  The team is likely to include experts in materials, damage mechanics, structural integrity, aerodynamic loading as well as manufacturing and finance, since the window has to be made and fitted into the aircraft at an acceptable cost.  A similar team will be needed to design the mobile phone casing with the addition of product design and marketing expertise because it is a consumer product.  In other words, engineering is team activity and engineers must be able to function as team members and leaders.

I wrote this post shortly after Clive’s lecture but since then it is has languished in my drafts folder – in part because I thought it was too long and boring.  However, my editor encourages me to write about engineering more often and so, I have dusted it off and shortened it (slightly!).

Image: https://commons.wikimedia.org/wiki/File:Airbus_A350_cockpit_windows_(14274972354).jpg