Stories of Aerospace Engineering

Read interviews and stories of researchers and students at the Faculty of Aerospace Engineering, and discover the scientific questions on which they work and the solutions they present.

A voyage of discovery through our solar system with lasers

How can we learn more about planets and moons? Dominic Dirkx recently wrote his PhD dissertation on a new method of accurately measuring the distance between the earth and satellites orbiting or on planets and moons to within between a millimetre and a centimetre. Current radio measurements are accurate to about a metre. Dirkx took laser tracking, a technique that’s currently used for measuring the distance to earth’s satellites (accurate to within a few millimetres), and extrapolated that to interplanetary distances. His research primarily shows that it’s vital that we not only measure the distance extremely accurately. Other measurements will also need to be improved in order to make the most effective use of laser tracking. If we manage to do that, this method can potentially play an important role in the exploration of our solar system. That might sound obvious, but his research has shown that focusing on making laser measurements and measurements of elements such as magnetic fields, shape or a planet’s seismic activity more accurate can really have a significant impact on your results. Influence of the clock on earth When trying to improve laser measurements, Dirkx asked to what extent the ultimate measurements are influenced by the clock we use here on earth. Dirkx: ‘When you measure distance, you’re actually measuring movement. You use a clock on earth and a clock in space to measure how long a laser pulse is en route. Because you know what the speed of light is, you look to see where a satellite is from one moment to the next. And that means that if you’re just a nanosecond out, it can easily make 30 centimetres difference.’ If you examine how something moves, you examine how gravity works and as such, you learn about the surroundings. Imagine that you measure how a satellite orbits the planet Mars: that will allow you to find out more about the composition of the planet. Last year, for example, this method was used – alongside other measurements – to discover an ocean under the surface of Enceladus, a moon of the planet Saturn. A remarkable thesis defence Dirkx graduated nominally and with distinction. David Smith (MIT) felt it was worth the trip to Delft to sit on the committee during his thesis defence. Smith is specialised in laser ranging as well as planetary sciences. Dirkx: ‘He’d worked with colleagues from our research group in the past, and I’d met him at conferences. He’s someone who’s extremely intimidating to doctoral candidates – a major figure with an endless list of significant publications to his name.’ Dirkx: ‘What really helped during my promotion was having the opportunity to work on the European FP7 ESPaCE project. This enabled me to build up a large network, so I’ve always had lots of people to discuss my research with. My tip for other PhDs is therefore to talk to people whenever you can. Not only within your own research group, but particularly outside of it.’

Kite power: towards affordable, clean energy

How can we produce clean and renewable energy in a more affordable way? As we are confronted with climate change and global warming, this question represents one of the greatest challenges of the 21 st century. A research team in the field of airborne wind energy of the faculty of Aerospace Engineering recently kicked off their ‘Fast Track to Innovation’ project REACH, which is funded with 3.7 million Euro by the European Horizon 2020 programme. Their ambition? To ensure cost-effective renewable energy with a low environmental footprint by using kite power, or, as it is called more generally, airborne wind energy. On 31 May 2016, the official kick-off meeting of the REACH project took place at the faculty of Aerospace Engineering. After a word of welcome by coordinator Dr. Roland Schmehl (section Wind Energy), the various partners involved gave short presentations, followed by lunch and discussions. Kitepower, a recent start-up of TU Delft, is at the core of the REACH project with the mission objective to commercialize the technology. Enevate B.V. is the technical coordinator of the project and will integrate the developed kite system, market and sell it. The goal is to have the first commercial prototype of the E100 – the name of the 100 kW kite power system – within two years, and to sell it within three. Several parties have already expressed their interest. Losing weight and reducing costs Horizontal axis wind turbines with rigid rotor blades are the most common way of converting wind energy. There are now over 200,000 wind turbines operating worldwide. But these conventional wind turbines have downsides: they are heavy, expensive, make noise and detract from the visual quality of the landscape (and so does the needed infrastructure such as high-voltage lines). The concept of a kite power system has the upper hand when it comes to these downsides. ‘Wind turbines are very robust. This means they have to deal with immense structural forces,’ Schmehl explains. ‘The tower and rigid rotor blades that take all the aerodynamic loads are heavy and expensive. The essence of airborne wind energy is to replace this heavy structure by lightweight cables and membranes. This way, we might use a tenth of the material – and thus weight – to produce the same amount of energy. This means the cost of energy would turn out dramatically lower if we use kite power.’ How does the system work? Functional components of the 20 kW technology demonstrator developed at Delft University of Technology The ground station holds an integrated ground control centre and incorporates the cable drum with a cable routing mechanism. The generator holds a 20 kWh battery. Because it houses also the central control computer, the ground station functions as the ‘brain’ of the system. The kite control unit (or KCU) determines how the kite flies by steering and depowering of the wing. The KCU is commanded by the ground control centre via several redundant wireless communication channels. Eventually, the goal is to be able to have the KCU function as a ‘brain’ as well, because it would closer to the kite and thus more reliable. Also, it would then be able to communicate with KCUs of neighbouring kite systems to avoid collision. The kite has 25 m 2 surface area and holds an on board sensor platform to continuously measure the position, orientation and velocity of the wing. The wing creates the aerodynamic lift force as turbine blades on conventional wind turbines do. Challenges ahead In the coming months, the team is set on showing the feasibility of certain features. One of the first milestones to pass, is flying during the night. This involves illumination, because aircraft need to be able to see the kite. Although every flight is registered, a back-up plan is needed, should communication fail. A second milestone would be to fly for 24 hours straight by the end of this year. The advantage the kite system has over the wind turbine – its low weight – poses a challenge. The system must be lightweight, but also strong enough to be reliable and durable. One particular difficulty in this is securing automatic launches and landings for the system: ‘If there is a thunderstorm coming, or troubles occur with a wind turbine, you can simply push a button and it will stop rotating. A flying system is very different, as you will need to land it. You cannot just stop mid-air.’ Lastly, there is currently an ongoing debate on whether a kite power system should be considered an aircraft or an obstacle. Certification of the E100 is therefore also something that will be handled in the near future. Schmehl: ‘By the end of 2017, we want to have a commercial prototype of the 100kW system.’ The 20 kW kite power system of TU Delft in operation at the former naval airbase Valkenburg, The Netherlands Dr. Roland Schmehl Johannes Peschel TU Delft : general coordinator, contributes research Kitepower : associated start-up, technical coordinator Dromec B.V. : ground station Maxon Motor GmbH : control drive trains Genetrix : kite development and production

One way to make composite aircraft lighter: stop riveting and start bonding

Provided it’s manufactured and applied correctly, adhesive bonding (using glue for non-material scientists) is a safe and efficient way to join aircraft parts together. “Adhesive bonding has been used for several years instead of or together with rivets in conventional metal aircraft. But now that aircraft fuselages are increasingly made from composites we need to know more about how we can use adhesive bonding to join composite parts optimally." "At the moment we apply similar design methodologies used up to now in metal bonding to adhesively bond the new material composites. This penalizes significantly the weight saving potential of composites”, says structural joints researcher Sofia Teixeira de Freitas of the faculty of Aerospace Engineering, TU Delft. ”The design of bonded joints has to be re-invented in order to efficiently join composites, both in term of shape and in terms of material properties optimisation (fibre direction and layup).” In July Teixeira de Freitas was awarded a NWO Veni-grant. The grant gives her the opportunity to come up with a new design methodology with which the aircraft industry can determine the optimal properties of the composite material and the optimal geometry to join the different aircraft parts together safely and efficiently. Drilling holes and fitting rivets Since about the time Boeing introduced the Dreamliner, aircraft manufacturers have predominantly used carbon fibre composite materials for parts of aircraft as a light and strong replacement of aluminium. On aircraft leaving the production lines now, about half of the materials used are composites. It’s understandable that composites are popular with the manufacturers: lighter aircraft have lower fuel consumption and lower CO2 emissions for example. Aircraft consist of many small parts that have to be joined together. In traditional aluminium-made aircraft fasteners such as rivets were used to do this. Teixeira de Freitas: “We have only slightly adapted the old joining methods used for aluminium to fit modern airplanes that are made from both aluminium and composites. Basically, we still drill holes and fit rivets. This is far from optimal. Drilling holes cuts the carbon fibres of the composite and significantly destroys their load bearing characteristics. To compensate that more material is used, which makes the airplanes heavier again.” Joints between the wing skin and the stiffener, aircraft wing Zooming in on the adhesive bond solution Adhesive bonding – glue Using just adhesive bonding could solve this issue, but much more knowledge is needed about the ’glue’s’ behaviour in the longer term and we need to know better how to shape the composite’s fibres and what geometry is needed to connect the two parts together. Teixeira de Freitas: “Composites are not like metal that has fixed properties. The fibres that make up the composite can be placed in different directions, making it possible to adjust material properties. We need to find out how to create optimal properties to build a stronger and safer adhesive joint. Also we need to know what the optimal geometry is of connecting the two parts together, whether it’s linking composites together or whether it’s connecting composite and metals.” Teixeira de Freitas faces an interesting scientific challenge: “Composites already exist as highly efficient materials, but they need to be optimised to become very efficient structures as well. What we need to do is scale up to larger structures. Adhesive bonding plays a pivotal role in this”. Is the future made of glued composites? Will all aircraft – or any other metal structure – consist of bonded composite parts in the future? Teixeira de Freitas believes in the future every part of a structure will be made from the best material tailored to its purpose and that new solutions will be found for joining these together safely and efficiently: “Interfaces in hybrid structures will become increasingly important. In the future we will need joints that do not reduce the performance of material parts. Adhesive bonding is a very promising candidate for this, but other options are also researched (for example by my colleagues at the faculty), such as welding plastics or even using a type of Velcro.” Veni The Innovational Research Incentives Scheme Veni is a grant from the Netherlands Organisation for Scientific Research (NWO) for researchers who have recently obtained their PhD. It allows them to conduct independent research and develop their ideas for a period of three years. The researchers receive a maximum of 250.000 Euros. Teixeira de Freitas: “I am planning to use the grant mostly to collaborate with experts at other universities and build bridges between disciplines.” Offshore industry This summer Teixeira de Freitas received another piece of good news: she – and her colleagues in a broader consortium – also received a grant of 500.000 Euro from the Top Sector High Tech Systems and Materials for research on using composites in the offshore and maritime industry. Sofia Teixeira de Freitas Sofia Teixeira de Freitas is a civil engineer with a Master’s degree from the University of Lisbon in Portugal. In her PhD at the faculty of Civil Engineering at TU Delft shedeveloped adhesive bonding technology for reinforcing steel bridges. She currently holds the position of Assistant Professor in the department of Aerospace Structures and Materials at the faculty of Aerospace Engineering at TU Delft. Sofia: “My multi-engineering background gives me a broader perspective, an overview of disciplines which I find very useful. What really motivates me? To expand my knowledge and pass it on to new generations.” The banner photo pictures the composites laboratory at TU Delft

Research challenges in wind energy

Can supercooled generators improve the efficiency of offshore wind turbines? How can ‘big data’ from sensors mounted on wind turbines help in the maintenance of wind farms? How can we calculate the effects of waves on floating wind turbines? Is it possible to forecast wind power at the height of wind turbines? Is it a good idea to combine offshore wind turbines with wave or tidal energy generators? These are some of the questions included in this research agenda for wind energy drawn up for the European Academy of Wind Energy by TU Delft professor of wind energy Gijs van Kuik and his colleague Joachim Peinke from the Carl von Ossietzky University of Oldenburg, Germany. They are fundamental scientific questions from researchers working in 11 different fields, ranging from material sciences to energy conversion, and from environmental impacts to aerodynamics. Van Kuik: ‘Most wind energy agendas, such as that of the International Energy Agency, focus on technologies for application in the short term, for example to reduce the costs of wind energy. However, this agenda looks to the future.’ Why do we need to do this? ‘The biggest wind turbines currently in use are the largest rotating machines in the world, wind turbines are more and more often used in large offshore wind farms, and the contribution of wind energy to the global energy mix is increasing. This increase in scale means there are new scientific challenges to be solved. As scientists, we are of course interested in the answers, but the main reason for coming up with answers to these long-term challenges is that they can result in game changers – radical new technologies that make it possible for wind energy to become a significant component of the energy mix.’ Paper encourages discussion Under the banner of the European Academy of Wind Energy, TU Delft professor of wind energy Gijs van Kuik and his colleague Joachim Peinke from the University of Oldenburg have drawn up a long-term research agenda for wind energy. Researchers working in 11 different fields in Europe and the US submitted their fundamental research challenges for inclusion in the agenda. The agenda will initially be published as a scientific paper with the title ‘Long-term research challenges in wind energy’, in the new open access journal Wind Energy Science. It will also be published in book form later this year. Gijs van Kuik: ‘The aim of this paper is to show that wind energy is more than just an engineering discipline that comes up with short-term solutions. Answers to fundamental research problems can help in the future development of the wind energy sector.’The main aim of the paper is to encourage discourse amongst colleagues. Van Kuik: ‘I hope that many people who read the paper disagree with our views, so that we encourage a lively scientific debate.'The EAWEwind energy research agenda will also be published in book form by Springer later in 2016. Further information For further information please contact professor Gijs van Kuik at +31 15 27 84980 or via . Article published in Wind Energy Science 9 February 2016: van Kuik, G.A.M., J. Peinke ‘Long-term research challenges in wind energy’ . The new Wind Energy Science journal