Stories of Aerospace Engineering

Read the stories of researchers and students at the Faculty of Aerospace Engineering, and discover the scientific questions they are working on and the solutions they come up with.

What flying robots can teach us

Fruit fly is not the first association that comes to mind when looking at the 30cm wingspan of the Delfly Nimble. The aerial acrobatics of it do, however, very much mimic fruit fly behaviour. This allowed its developers to test important assumptions about how these flying insects perform their evasive manoeuvres. Their recent publication in Science is only the beginning of what flying robots can teach us about insect flight. The Nimble is the newest in a line of micro-air vehicles developed by researchers at the TU Delft MAVLab. “It is our first insect like robot that doesn’t have a plane-like tail,” said Dr Matěj Karásek, post-doctoral researcher at the MAVLab. “Just like insects, our robot uses only its wings to initiate rotations around any of its three body axes.” According to Dr Guido Croon, scientific director of the MAVLab, “Nimble is an important step towards our ultimate goal of lightweight, safe and smart drones. Because of its agility, Nimble can operate effectively in much harsher wind conditions than our previous tailed drones.” Inspired by nature Imagine the Nimble standing up, like a person. For rotations around its head-to-foot axis (known as yawing), the Nimble pushes the ‘bottom’ of both wing pairs in opposite directions, angling both wings pairs with respect to each other. For rotations around its left-to-right axis (known as pitching), it pushes both wing pairs towards its back or belly. “Both strategies are similar to the method used by fruit flies and many other insects,” Karásek said. “Rolling is an exception. For a sideways rotation (around their front-to-back axis), a fruit fly varies the amplitude of its wing motion, while we change the flapping frequency on one side of Nimble’s body. We don’t copy nature, we are inspired by it.” The Nimble is controlled by a tiny 2.8-gram programmable autopilot unit that also houses sensors for estimating body orientation and rate of rotation. According to Karásek, “the control algorithms are comparable to what we believe insects use.” At the press of a button Unless instructed to do otherwise, the Nimble will return to a hovering position, its body hanging vertically in the air and its wings flapping about seventeen times per second. But it can also use its sensors and wing motion adaptation to follow pre-programmed commands, initiated through a remote controller. “These pre-programmed manoeuvres include aerial tricks such as 360-degree flips, but also turns inspired by the escape manoeuvres of real fruit flies,” Karásek said. “We press a switch to initiate the motion, and only when the Nimble recovers do we get back control.” Nature’s code “Fruit flies can drive us crazy when they manage to repeatedly escape our attempts at swatting them,” Karásek said. “Scientists have tried to explain how fruit flies perform these manoeuvres, based on observations made with high-speed cameras. But for confirmation one would need to look into the animal’s brain. Our insect-like Nimble provides an alternative approach.” In a collaboration with Florian Muijres, biologist and assistant professor at Wageningen University & Research, MAVlab researchers programmed the robot to replicate the fruit fly manoeuvres. When they compared its flight trajectories to those of fruit flies, there were remarkable similarities. Capturing insect flight “Our most important finding may be the discovery of a new aerodynamic effect assisting the fly in making turns,” said Karásek. “For both fruit flies and our robot, we observed rotations around the head-to-foot axis. But for our robot we were certain this was a passive phenomenon as we had disabled its yawing mechanism. This rotation is initiated by a combination of body translation and the adjusted wing motion, necessary to initiate rotations around the other axes.” The researchers were able to capture this complex mechanism, assisting the fly in turning its body into the escape direction, in a surprisingly straightforward equation. Nimble as a bee Their research with Nimble has already resulted in a publication in Science. But, according to Karásek, “there is so much more that our robots can teach us about insects. And there is so much more that we can learn from insects to improve our robots.” In their NWO-funded project, “To be as nimble as a bee”, they are again collaborating with Wageningen University & Research. They will look at how insects handle sudden wind gusts. They also want to understand why different insects have different sensory systems. “Fruit flies use so-called ‘halteres’, biological gyroscopes, to provide information about their body rotation during flight. The gyroscopes in our autopilot unit are actually based on the same principle. But certain flying insects do not have halteres, and yet they are flying perfectly fine. What sensors are involved and how do they use them?” asked Karásek. Smaller and smarter The current Delfly Nimble has only limited autonomy. Sure, it flies wirelessly, but its behaviour is completely governed by a few hundred lines of code and by external commands. The Nimble is also blind. It has to be steered or it will bump into objects. “Nimble will have the onboard intelligence to fly autonomously using only a single camera,” said Croon. “And we have already developed the wireless technology to have our drones operate in swarms.” Karásek expects to be able to scale down the size of the robot. He explained that “at small scales, flapping wing robots fly more efficiently compared to traditional quadcopter drones.” When asked for a futuristic application for such agile, small and smart drones, Karásek said, “imagine a swarm of miniaturized Nimbles, flying autonomously in a greenhouse, pollinating flowers.” More information: - Here you can see a movie of Delfly Nimble . - The publiciation in Science is available here.

New aerospace MOOC has launched

After the success of its first MOOC (massive open online course), the TU Delft Faculty of Aerospace is preparing to launch a brand new one. Introduction to Aerospace Structures and Materials , a seven week interactive course, went live on 28 August 2018. Making quality education accessible The first TU Delft aerospace MOOC, Introduction to Aeronautical Engineering , went online in 2014. It was based on an existing in-house course that Assistant Professor Mark Voskuijl helped to create. “Originally, the vision was ‘we want to educate the world’, which is a bit of a bold statement,” he said. “But the general idea is that everybody should have access to high quality education, and with MOOCs you can provide that on an introductory level.” MOOCs, by definition, are online courses that are open to anyone at no cost. The process of creating a MOOC is quite different from an in-person course. There is a lot of up-front development work, according to Voskuijl, but once it’s done the material can be used for a long time. “You don’t have a 90-minute lecture, you have to create short pieces of video followed by exercise material,” he said. “And we get thousands of students through the course which makes communicating difficult. We develop a lot of interactive exercises so they can check whether or not they have mastered the material.” As for the quality of the faculty’s first MOOC, Voskuijl thinks it’s quite good. “In some ways the videos are much better than a traditional lecture because you really thought about it well in the preparation and you have all of the information condensed in a good way,” he said. In fact, he now uses some of the videos from the MOOC in his in-house course to include things like interviews with pilots or tours inside an aircraft, which you can’t do in a live lecture. Broad reach Since its inception, more than 60,000 people have registered for Introduction to Aeronautical Engineering. Although not all of them complete it, that number certainly reflects a high level of interest and the potential reach of courses like this one. Lecturers can access an interactive map during the course to see where participants are located. And Voskuijl noted that some students in places like India actually meet up and sit together during the MOOC. “It’s really all over the world. I once went to a scientific conference and someone from MIT came up to me saying he knew me,” said Voskuijl. “I was quite surprised that he took my course. He was a professor in a slightly different field, but needed to know a bit more about aeronautical engineering.” Voskuijl also noted that an increasing number of international master’s track students have taken the course prior to coming to Delft. “It’s good for them to see our style of teaching and how we do things, because it can be quite different from other countries.” A new MOOC with a long history The second MOOC, which focuses on how aircraft and spacecraft are designed and manufactured, started on 28 August. According to Associate Professor Gillian Saunders-Smits, one of the course creators, it fills a gap in the online course offerings for the Faculty of Aerospace. She noted that about five years ago, the Aerospace Structures and Materials department started a pilot developing online master’s courses. “But we always felt that there was something missing, the introductory step,” she said. “Plus we really liked the idea of making a MOOC, the fact that you can share your passion for aerospace structures and materials with anyone, no matter where in the world they are based or what their previous education is.” The origin of the course itself dates back much further than five years. Introduction to Aerospace Structures and Materials, one of the oldest courses at the Faculty of Aerospace, was introduced in 1945 when the first professor in aerospace structures was appointed at TU Delft. The content of the course has obviously evolved over the years and is about the meat and bones of the aircraft, as Saunders-Smits describes it. “We really feel that it is should be sort of a showcase of everything we do here at aerospace with structures and materials,” she said. “We want to explain why airplanes work and why they break, and safety is also a part of it.” Making online learning engaging Giving MOOC participants a conceptual understanding and a foundation for the topic is important to the course creators. And there are many differences between learning in person and online. “The most important one I think is to ensure you keep the attention of the participants,” said Saunders-Smits. “You make sure that the videos are engaging and varied. We don’t want just talking heads or voiceover PowerPoints. We try and intermix many different things like movies, animations, and activities that are different all the time.” There will also be some unique learning approaches used during this online course. Saunders-Smits explained that they will include a variety of experiments that anyone can do individually using materials they already have at home. “This will help them have a better idea of what we’re talking about,” she said. “They can not only see it, but they can have something tangible to learn from.” In addition, students will be asked to identify their favourite aircraft or spacecraft, and weekly assignments during the course will be centered around that as much as possible. Participants will have to think up new design options or explain why or how a certain design issue can be solved, which will help them understand more about their favourite aircraft for spacecraft. Enrollment opportunity Although the content of Introduction to Aerospace Structures and Materials will be available for many years to come, the first offering is unique as it allows for more interaction between participants and instructors. And with the start date approaching, enrollment activity has been very encouraging. “Since registration opened in February there has been a steady increase and we have about 4000 so far,” said Saunders-Smits. “That’s more than I’ve taught in 10 years together here at aerospace!” Although this is only the second MOOC from the Faculty of Aerospace, there are many more paid courses available for both course credit and professional development. Click here for or more information on the entire portfolio of online learning opportunities at TU Delft.

A summer of space

This summer, space experts and professionals from across the globe will be gathering in the Netherlands for the 31 st annual International Space University (ISU) Space Studies Programme (SSP). The Netherlands Space Office, in collaboration with TU Delft, Leiden University and the European Space Agency, will be hosting this prestigious event from 25 June to 24 August. A one of a kind programme Founded in 1988, the Space Studies Program (SSP) is a nine-week graduate level professional development programme held in a different country each year. This year the programme will take place at different locations across the Netherlands, with a significant portion being held at the Faculty of Aerospace Engineering. The interdisciplinary curriculum covers both technical and non-technical space related fields, including policy and law, business and management, engineering, physical sciences and space applications. Around 140 participants, the largest group to date, are expected to attend and will be housed on campus at TU Delft. “Most of the teaching and project work during the first weeks will take place here at the faculty,” said Ineke Boneschansker, communications manager for the Faculty of Aerospace Engineering and a member of the local organising committee. “So people who work here or study here and are around during the summer will get to experience the energy that it brings.” Learning from a wide range of experts The SSU programme is intensive and gives participants a broad experience including various lectures, group projects and visits to industry. According to Boneschansker, about 225 lecturers are brought in from all over the world. “These are really interesting people,” she said. “They are astronauts from NASA, space entrepreneurs, artists, science fiction writers, film makers and directors, but also doctors and space law people. There are all these disciplines that we don’t have in house and they will all be here teaching. I think that’s fantastic.” One of those lecturers is TU Delft’s own Dr. Daphne Stam, Associate Professor of Planetary Sciences. She said it is an honour to be invited to give a core lecture about solar system planets and exoplanets and it’s also a benefit to the faculty. “The space department at aerospace has been growing during the last years, partly because of the increased interest in space due to commercial companies, for example Elon Musk launching his Tesla into space,” she said. “We already have quite a good reputation internationally, but I think by hosting this we put ourselves on the map.” She also noted that this event is helping to build and strengthen collaborations, especially with the astronomers in Leiden. Something for everyone Luckily, it’s not just SSP participants who get to benefit from the summer programme. Local organisers have also launched the Sizzling Summer of Space with numerous activities all open to the public. Events for space enthusiasts of all ages will be taking place in the cities of Delft, Leiden, The Hague and Noordwijk. “I know that people are busy and it’s a summer holiday period, but this is going to be really special and it’s worth it,” said Boneschansker. And there really is something for everyone. Activities include exhibits throughout the summer at the Delft central library entitled The Afterlife of Satellites and Food for Mars. On 26 June, former NASA astronaut Jeff Hoffman will give a lecture about his five space missions, including his work to repair the Hubble space telescope. The Science Centre will host an event on 30 June for Asteroid Day, a day organised to raise awareness about asteroids. And local film house Lumen is hosting the Unlimited Space Film Festival with screenings such as the documentary Orphans of Apollo on 12 July, introduced by the director Michael Potter. You can also visit the Science Café on 5 July at the central library in The Hague, where three scientists will give short, but inspiring lectures sharing the latest insights into their fields. And on 10 July, Pete Worden of Breakthrough Initiatives will be giving a lecture about the Starships project. And of particular interest is a unique robot building competition. As part of their coursework, SSP participants will be building and programming Lego robot landers. Students from local schools are also taking on the challenge and the adults and youngsters will compete against each other at the grand finale on 14 July, with astronaut Andre Kuipers in attendance. These are just a few of the many upcoming events, but for a detailed schedule and full descriptions click here . An opportunity not to be missed It is indeed a unique opportunity for the Faculty of Aerospace Engineering and the local community to be involved with both the SSP and the Sizzling Summer of Space. “It will make the faculty visible as an essential partner in the Dutch space sector,” said Boneschansker, “and it will make us visible to the international space world.” But it is also a chance to show what the aerospace faculty does to the general public. With that in mind, Boneschansker thinks everyone should try and take part. “This is so big we will probably never do it again at the faculty and it would be a shame if people don’t benefit from it,” she said. “Try to join the public events and take your family. Support your colleagues who are a part of this and be visible.”

Women changing the game in aerospace

Despite the fact that more and more women are pursuing careers in STEM – science, technology, engineering and mathematics – the gender gap is still significant. The good news is that at the Faculty of Aerospace Engineering, the number of women has consistently increased over the last years. But as of 2016, women still represented just 13 percent of students (bachelor’s and master’s), 20 percent of PhD candidates and 13 percent of academic staff. Those numbers reflect the paradigm of the past, but what does the future look like? Meet three women of the aerospace faculty, game changers who have ignored the paradigm and are inspiring not just other women, but current and future generations of aerospace engineers. Dr. Sofia Teixeira de Freitas Assistant Professor Dr. Sofia Teixeira de Freitas took an unconventional path to her career in aerospace. After completing a master’s degree in civil engineering in her native Portugal, she worked in the construction industry for two years. “In the first year of practice I was already missing the challenge of learning and exploring new fields so I decided to do a PhD.” That desire to learn brought Teixeira de Freitas to Delft to where she earned a PhD in civil engineering in 2012. With research focused on bridges, after her PhD she wanted to explore new structures. “I thought, why not try something that is even more challenging,” she recalled. So she decided to apply for a postdoctoral position in aerospace, trying to apply her previous knowledge and expertise on bonded structures in aircraft. “I thought, they are the pioneers in bonded structures so why not try it. I think it was the sake of the challenge that drove me to change.” Taking that leap resulted in good things for Teixeira de Freitas. In 2014, she received the prestigious Delft Technology Fellowship, which awards tenure-track positions to outstanding female academic researchers. As a fellow, she has had the unique opportunity to establish her own research group. And in 2015, she was also the recipient of a Veni grant from the Netherlands Organization for Scientific Research (NWO). Teixeira de Freitas is using her position to pursue research interests in the understanding of the failure mechanisms and degradation of adhesively bonded structures, using the principles of structural mechanics. Although she acknowledges that aerospace is a male-dominated field, especially in higher level positions, Teixeira de Freitas believes things are changing. With regards to her fellowship and her Veni grant, she said, “All these initiatives do contribute enormously to pushing females to a higher level. I’m enjoying that for the moment and I think we are heading in the right direction.” Looking to the future, Teixeira de Freitas thinks there need to be more female teachers and professors. “Delivering knowledge to students, showing that there are females in scientific and engineering careers, will be a model for girls who are currently in secondary school and thinking about university,” she said. “You can look at women who are successful, who are passionate about their jobs in STEM. Having those role models I think is one of the most important things.” Dr. Irene Fernandez Villegas Dr. Irene Fernandez Villegas always liked science. “I was really interested in chemistry because it was fun just to mess around with things and I always loved math,” she said. “When I finally understood it, I also really liked physics. I’ve always been a nerd!” Early on she also realised she excelled most under pressure so she opted to study aerospace, which she saw as the most challenging degree. “I think I was raised to believe that you could do anything you wanted. I was encouraged from the beginning.” During her studies at Universidad Politécnica de Madrid, Fernandez Villegas noted that only around 10% of students and almost no teachers were female. “But I never felt that people looked down on me or that I couldn’t do things because I was a woman,” she said. “I never felt treated less, but I was always getting very good grades so that helps.” After completing her studies, Fernandez Villegas worked for Spain’s National Institute of Aerospace Technology (INTA), where she focused on materials. While working at INTA, she also earned a PhD in aerospace engineering from Universidad Politécnica de Madrid. In 2008 she came to TU Delft as a researcher and became an assistant professor four years later, where she now works on welding processes for composites. During the last year, thanks to her research and her self-described stubbornness, some game changing things have happened for Fernandez Villegas. When she started at TU Delft ten years ago she began working on a process called ultrasonic welding. She saw the potential so she went to Airbus and told them about her results. “They said no, we are never going to use that process because we don’t believe in it. I said fine, I will prove you wrong.” Last year Airbus brought a list of the welding technologies they want to use for their new aircraft and her technology was right at the top. Fernandez Villegas now manages a project to industrialise that technology. On the idea of changing the paradigm, Fernandez Villegas thinks we don’t necessarily have to encourage girls to pursue careers in science. “I think we have to let them know that they can do whatever they want to, that they can be as smart as or smarter than guys and not necessarily just in science.” But for those women who do want to pursue a career in aerospace, she has some sound advice. “Don’t forget that you are women. This is a male dominated field and I’ve seen a lot of women, including myself, who tried to act like guys. I think we shouldn’t lose the fact that we are women and we are different. That doesn’t mean that we are less.” Foto: Henri Werij Tineke Bakker-van der Veen Tineke Bakker-van der Veen has a photo when she was about two years old sitting with her mother and grandmother next to the airstrip at Schiphol airport watching the planes land and take off. “I think that’s where my interest in the magic of aerospace started,” said the TU Delft aerospace alum (MSc, 2005).That passion never faded and eventually led to her current role as Managing Director of Boeing Benelux & Nordics. The Dutch native credits her parents with inspiring her interest in engineering. “Even from a really young age, they always explained things to me, made it possible to help them with projects,” she said. Though she considered studying mechanical engineering, she chose aerospace because of the focus on innovation and what she describes as “pushing the boundaries on a daily basis.” During her studies, Bakker-van der Veen recalls there wasn’t a female aerospace leader who she could look to as an example. That, in part, inspires her active involvement in mentoring programmes and promoting engineering and technology with young people. “We have to open up the door for future generations of innovators, entrepreneurs and engineers, to be available if they want to reach out, if they want to have a chat,” she said. “But I don’t want to just focus on the gender issue, it’s also about inclusion, that we make every child really enthusiastic about STEM education and show them that STEM is something that’s exciting, that you can have an amazing opportunity and career in aerospace where everyone can be a game changer. By doing that, we also open up the door for the next female leaders.” Before joining Boeing, Bakker-van der Veen worked for well-known companies including Rolls-Royce, Airbus and Fokker. Her roles have ranged from being on the operational shop floor to management positions and she has always felt like part of the team. But she acknowledges the fact that women do stand out in this male-dominated environment. “What was important for me is that I always wanted to be judged on my performance, not my gender,” she said. “Yes, as a woman, you do stick out in a male-dominated field. That has some advantages when it comes to presenting things because you do draw attention. But you’d better perform well because it’s like you have a spotlight on you.” As for inspiring the next generation of aerospace engineers, Bakker-van der Veen believes it’s important to stay true to yourself. “There is no way that you can have a successful career by pretending and acting upon what other people might expect,” she said. She also encourages being open to feedback and finding people you know you can trust who are honest with you. “Try to determine what you can bring to the table and what strengths you have that can really make a difference for the team.”

That rocket is/isn't going to fly

Space is no longer only accessible to governments. Nevertheless, developers of small satellites, such as universities and small businesses, must depend on the unused space in large rockets to launch them. This consequently means that they have no control over the timing of the launch and that their choice is limited when it comes to the satellite's orbit. The commercial development of rockets, specifically designed for smaller amounts of cargo, offers a solution. For his graduation project, Nigel Drenthe improved and validated a cost estimation model for this that won the first Heinz Stoewer Space Award. “It's no longer the major satellite projects which you hear about in the media," says Nigel Drenthe. "Start-ups for developing new rockets offer fascinating technical innovations and everyone is quoting much lower prices for each kilogram of cargo". As a young boy, Nigel already had a fascination for rockets. So naturally, he chose to study Aerospace Engineering. It was during his studies that he learned exactly which technical systems were needed to shoot a rocket into the sky in a controlled way. But he also learned that a rocket will never fly without proper groundwork and cost management. Brilliant, but not feasible “Engineers are full of brilliant ideas," says Nigel Drenthe, "but they have absolutely no idea what these mean for the production of the rocket. On the other hand, managers and marketing people have no idea about technical feasibility". A visit to the Cost Engineering section of the European Space Agency (ESA) taught him that a good cost estimation model is needed, particularly in the early stage of the development process. This allows choices to be made, for example, between buying the necessary technology or developing it yourself, or pulling the plug on the whole project. Because he wanted to focus on the interface between these topics for his thesis, Nigel needed two supervisors: Barry Zandbergen (MSc Engineering) of the Space Systems Engineering Group (SSE) became his daily supervisor while Prof. Ricky Curran of the Air Transport and Operations Group (ATO) was chiefly consulted about the methodology of cost estimates. A complex cost model Nigel’s graduation project elaborates on the earlier work of Dietrich E. Koelle, a luminary in the field of space travel who has devoted himself to the costs of space transportation since 1970. Koelle suggested that commercial parties can reduce costs because they outsource work less frequently, which yields them a greater profit. As a result, they also need to employ fewer people to maintain all the contacts with their suppliers. It is these aspects that Nigel Drenthe added to Koelle’s existing cost estimation model. What’s more, he examined the costs involved in the development and production phases in much greater detail. So instead of providing an estimate for the cost for an entire rocket stage, he detailed the costs of all of the components, such as a navigation subsystem, a fuel tank and a bypass fuel system. Twenty percent accurate It is the number of launches that especially proved to be a major factor in the costs. Nigel Drenthe: “It is only at four or more launches that the contribution of the fixed costs drops below the generally accepted standard of 35% of the launch costs. For example, think about the costs of facilities, the maintenance team and mission control". He says there is scarcely any publicly available information about the development cost of rockets. Nevertheless, Nigel was able to test his model on the basis of partially classified data from two small launch vehicles: the Falcon 1 from SpaceX and the Pegasus XL from Orbital ATK. His model proved to be accurate to within twenty percent. “This is sufficient for the early stage of development", says Nigel Drenthe. “We’ll see whether this also applies to the rockets that are currently being developed. Incidentally, my model applies to both small and large launcher vehicles". A graduating student knows what to do While he was in the process of completing his studies, Nigel already put his model to use when a group of people from a certain project were considering launching a rocket from underneath a fighter jet. This would mean they could use an airport instead of an expensive launch pad. “Unfortunately, it turned out that sufficient thrust could only be provided by toxic rocket fuel. And safety requirements, mean an airport simply could not be used for this purpose. My model also showed that the costs of such a design were extremely high". The chicken and the egg Space start-ups quote extremely low prices of between five to ten million euros for transporting a few hundred kilos of cargo into Low Earth Orbit. As the price is largely determined by the number of launches, this also makes for an interesting marketing aspect. “The commercial development of small launch vehicles involves the classic chicken and egg dilemma", says Nigel Drenthe. “If the given prices become reality, I’m convinced that many small parties will be eager to make use of them. And if there is enough demand for small launch vehicles, my model demonstrates that these prices are achievable ". In addition to the technical-commercial aspect, Nigel would also have liked to have constructed the price elasticity model. “Now that’s the perfect topic for another student’s research". Click here if you would like to view Nigel’s entire graduation project and click here if you would like to read about the award ceremony.

A wealth of information on noise

In aircraft, noise is an unwanted side-effect of propulsion. In sonar, it is a useful tool for conducting accurate depth measurements. But irrespective of whether it involves a disruptive noise or the useful application of sound, it always tells you something about the source and the surrounding environment. As Associate Professor in the Aircraft Noise and Climate Effects (ANCE) group, Mirjam Snellen specialises in techniques for imaging sound and using sound for imaging. She is only happy when all of the information has been extracted from the measured noise, often using detectors developed by the group. With its noise research, the small and close-knit ANCE research group, led by Prof. Dick Simons, is deliberately positioned at the interface between metrics and modelling. “If all you do is measure and take readings, you will not have the knowledge to design quieter aircraft,” says Mirjam Snellen. “Other groups are specialised in calculation. They build models that use the shape and material of an aircraft fuselage to predict the noise that it will produce at different speeds. Of course, this then needs to be validated using measurements.” As far as the ANCE group is concerned, even an accurate noise reading on a scale model in a wind tunnel is not sufficient. They prefer to measure it in practice, in the most realistic conditions possible. A key factor in this process is to ensure that the source of the noise, in this case the fuselage, can be properly separated from other noise sources, such as the engines. Mirjam Snellen: “We can achieve that in ANCE, using our arrays”. Separating out sound A single microphone only enables you to measure the volume and pitch of the sound. This changes if you use an array of several microphones, spread across a surface. For example, the noise that comes from the aircraft's nose cone reaches the closest microphone first, followed shortly by the microphones slightly further away. Software can be used to reconstruct the different sources of noise. “This is called beamforming,” explains Mirjam Snellen. “We can use it to separate the measured noise over time according to source location, pitch (frequency) and volume. We then make corrections for background noises, noise reflections on the ground and the Doppler effect, if necessary.” Certain preconditions are required for an array of this kind. To achieve a good resolution of the low pitches, which have a long wavelength, the microphones need to be slightly further apart. For the higher pitches, they need to be slightly closer together, to prevent distortion. For this reason, an array generally has a mixture of greater and smaller distances between microphones. An array is also limited in size in order to enable transportation. A large white surface The researchers in this group have mastered the beamforming techniques down to the finest details. In the summer, they took their first readings using the newly-designed array of 64 microphones, over a surface of 4x4 square metres. With it, they can also distinguish between the different sources of noise on small aircraft. Before setting off to work, the researchers always call the airport. “After all, you're going to be covering a large white surface there. Even now, someone from the airport always comes along, but that's just out of interest.” Array readings of this kind produce impressive pictures that clearly show which part of an aircraft produces which noise. The airflow around the wings typically results in slightly lower sound frequencies whereas the engines and the blast behind them produce higher frequencies. “Our measurements test the predictions made by models. If we are not expecting much noise from the undercarriage and our measurements show something different, the model may need to be revised.” In other words, different types of aircraft have different ‘fingerprints’ and this calls for a different way of reducing noise nuisance. Quieter aircraft Schiphol and Lelystad airports are reaching their permitted noise limits and are committed to quieter aircraft for the future. In 2015, Prof. Symons joined forces with the German Aerospace Center (DLR) in organising a workshop with experts from across the world. The key question was to what extent can aircraft become quieter, and how can this be achieved. “It's not easy,” says Mirjam Snellen. “For example, one option is ultra-high bypass engines. They are much quieter, but also extremely large and therefore heavy. The additional propulsion required to take off and fly could potentially end up increasing noise nuisance. Using alternative materials can alleviate the problem slightly.” The group is working on this together with the wind energy group in the same faculty, which is exploring the use of porous materials for wind turbines. “They could be used to make the nose and wing flaps or the landing gear of an aircraft slightly less noisy.” New developments like this are first tested in a wind tunnel. For this research, the group placed a microphone array in the relatively quiet vertical wind tunnel. The walls were covered with absorbent material to minimise sound reflection. Mirjam Snellen: “The challenge is to only measure the object. The array itself cannot be allowed to cause sound reflections and the microphones need to be positioned to enable precise measurements and beamforming.” Virtual noise It is easy to forget that the listener also plays an important role in noise nuisance and this also points the way towards new developments. If a group of test subjects are not disturbed by the hum from the fuselage, nothing needs to be adapted. Dick Simons was closely involved in the development of the Virtual Community Noise Simulator by the Netherlands Aerospace Centre (NLR). In it, you can experience the noise of a landing aircraft in a completely simulated natural environment. This requires carefully measured or modelled sound, something that the ANCE group can provide. Mirjam Snellen: “In a simulator, you can switch off the noise from the wings and engines only or change the pitch, for example.” Although the latter may seem futuristic, groups are already trying to achieve it using smart modifications in shape and material. Safety in shipping Beamforming also plays an important role in using echo sounding to determine the depth of water, which can improve safety in shipping. For example, precise information about the current water depths provides useful input for the frequency of dredging in and around the port of Rotterdam, ensuring it remains accessible even to the largest ships. Identifying water depths in this way can be done efficiently using multi-beam echo sounders (MBES). An MBES is an echo sounder that emits several narrow beams of sound simultaneously (a ‘ping’). Each of the beams propagates through the water, reflecting on the bottom before being recorded by the MBES again. Combined with the noise profile as a function of the depth, half of the time between sending and receiving a ping gives you the distance from the MBES to the bottom. Each ping identifies the bottom across a wide line, perpendicular to the direction of travel of the ship being studied. The entire bottom of the channel of water can be charted by using several parallel movements. Mirjam Snellen: “in the Netherlands, we are one of very few groups with a good knowledge of all the technical details of MBES; the system, its impact on the environment and the algorithms.” The MBES systems are complicated and expensive but achieve the level of precision required by government of just a few centimetres at depths of up to several tens of metres. Any new systems and cheaper alternatives will also need to prove they can meet these criteria before they will be permitted for use by a dredging company, for example. Mirjam Snellen: “We have developed a model that determines whether measurements meet these standards, given a set of measurement systems.” In addition, it is possible to obtain more information from the current systems, for example by also looking at the intensity of the echo signal alongside its two-way travel time. Mirjam Snellen: “In recent years, we have developed several methods to not only determine the water depths, but also the composition of the seabed using MBES measurements. For example, would it be a suitable site for a wind farm?” Using all of the data It was her determination to obtain all of the information available from sound that resulted in a method for improving the accuracy of MBES depth measurements. The raw data from an MBES system has to be corrected for many variables, including the underwater sound speed profile. This depends on the pressure (depth), temperature and salt content. In estuaries or tidal areas in particular, local variations in the sound speed can result in inaccurate readings. Mirjam Snellen: “This lack of precision increases towards the edges of the strip being measured. In the past, this was simply accepted or the measurements were taken again. But that is not necessary if you realise that the strips measured partly overlap.” She developed an algorithm that uses the extra data from this overlap to calculate the sound speed profiles. “It's not an estimate, it's physics,” says Mirjam Snellen. “With the occasional re-measurement, of course,” she adds with a laugh. Measurement is the key to knowledge Her algorithm for the underwater sound speed profile was recently implemented in commercial hydrographic software, but actually originates from 2009. “As in other fields, it can take time for solutions to be applied in practice,” says Mirjam Snellen. Another current topic to which this applies is the calculation of aircraft noise impact at airports. For this, so-called noise power distance tables are used. “No one knows how accurate the use of these tables is. This is 2018, why not position some arrays at the airport and actually measure this noise experience?” Find more information about the ANCE group here.