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.

Flying in a V

The lorry pulls up very slowly, en route from the Faculty of Aerospace Engineering to Schiphol. The cargo on board is precious: the 3-meter-wide flying scale model of the revolutionary Flying-V aircraft. Chief engineer Malcom Brown: “The Flying V will use far less energy thanks to its aerodynamic V-shape.”

Bringing silent aviation closer

Hundreds of millions of people living near airports are still at increased risk of developing hearing problems cardiovascular diseases. Roberto Merino-Martinez, who himself lives near Rotterdam Airport, is researching a relatively new topic in the field of aerospace engineering: Aeroacoustics. An annoying whistle Although aircraft engine noise has decreased substantially over the past decades, air flow around the airplane structure itself is still a considerable source of noise. In wind turbines, this mechanism causes the typical swooshing sound. During the first part of his PhD, in which Merino-Martinez focused on flyover measurements at Schiphol airport, he was surprised to find that many types of planes produced a huge tonal peak at a frequency of 1720 Hz. That is a sound about as annoying as a heart monitor flatlining. “It was especially loud for the Airbus A320, one of the most popular passenger aircraft of all time,” Merino-Martinez says. “It turned out to be caused by a cavity in the nose landing gear, acting like a whistle. It is my most cited research paper to date, and I believe Airbus may fix this in their newer models.” An array of van microphones “The noise models used when designing new planes are simply not good enough. They lack sufficient detail and often predict noise levels to be lower than they actually are”, explains Merino-Martinez. “Improvements in acoustic imaging and its algorithms are essential. A single microphone suffices to measure a general noise level. Add a second and some directionality can be obtained. But to really understand aircraft noise, to make a high-resolution image of all its sound sources, you need to use a microphone array.” Noise created by aircraft wings As it turned out, Merino-Martinez worked most of his PhD on integrating a versatile microphone array into the existing TU Delft laminar flow anechoic wind tunnel, in order to be able to perform the desired measurements. “The solution we came up with allows for the microphones to be rearranged depending on the details of the experiment,” he says. “Sometimes you want to move them further apart for a higher resolution image, or closer together to improve the array’s frequency response.” He also spent a lot of time improving the algorithm for reconstructing the sound sources. “I wanted to accurately measure the sound levels created by airfoils, such as the wings of an airplane or the blades of a wind turbine. A lot of noise can be created by instabilities in the airflow at the trailing edge.” Most acoustic imaging algorithms are based on point sources because they are mathematically simpler. Merino-Martinez rewrote the algorithm so that it was also possible to accurately reproduce line sources of sound. "Our approach gives better results than existing solutions, even those developed by NASA." The silent flight of owls Using the new array and algorithm, Merino-Martinez and his colleagues performed measurements on variations of two noise-reduction techniques. “They are both inspired by the silent flight of owls,” he says. “Their feathers create a saw-tooth profile, called serrations, which interferes with the scattering of the airflow at the trailing edge.” The researchers also investigated the insertion of porous materials or metal foams at the trailing edge, causing a similar interference. “It’s a great solution from a scientific point of view,” he says, “but real-world application is difficult, as these materials can get dirty or attract lightning.” Whereas serrations are already being commercially used by some wind turbine manufacturers, it may take another ten years for the conservative airplane industry to follow suit. “Safety first.” Roberto Merino-Martinez +31 (0)15 27 81736 r.merinomartinez@tudelft.nl This is a story from Aerospace Engineering An annoying whistle Although aircraft engine noise has decreased substantially over the past decades, air flow around the airplane structure itself is still a considerable source of noise. In wind turbines, this mechanism causes the typical swooshing sound. During the first part of his PhD, in which Merino-Martinez focused on flyover measurements at Schiphol airport, he was surprised to find that many types of planes produced a huge tonal peak at a frequency of 1720 Hz. That is a sound about as annoying as a heart monitor flatlining. “It was especially loud for the Airbus A320, one of the most popular passenger aircraft of all time,” Merino-Martinez says. “It turned out to be caused by a cavity in the nose landing gear, acting like a whistle. It is my most cited research paper to date, and I believe Airbus may fix this in their newer models.” An array of van microphones “The noise models used when designing new planes are simply not good enough. They lack sufficient detail and often predict noise levels to be lower than they actually are”, explains Merino-Martinez. “Improvements in acoustic imaging and its algorithms are essential. A single microphone suffices to measure a general noise level. Add a second and some directionality can be obtained. But to really understand aircraft noise, to make a high-resolution image of all its sound sources, you need to use a microphone array.” Noise created by aircraft wings As it turned out, Merino-Martinez worked most of his PhD on integrating a versatile microphone array into the existing TU Delft laminar flow anechoic wind tunnel, in order to be able to perform the desired measurements. “The solution we came up with allows for the microphones to be rearranged depending on the details of the experiment,” he says. “Sometimes you want to move them further apart for a higher resolution image, or closer together to improve the array’s frequency response.” He also spent a lot of time improving the algorithm for reconstructing the sound sources. “I wanted to accurately measure the sound levels created by airfoils, such as the wings of an airplane or the blades of a wind turbine. A lot of noise can be created by instabilities in the airflow at the trailing edge.” Most acoustic imaging algorithms are based on point sources because they are mathematically simpler. Merino-Martinez rewrote the algorithm so that it was also possible to accurately reproduce line sources of sound. "Our approach gives better results than existing solutions, even those developed by NASA." The silent flight of owls Using the new array and algorithm, Merino-Martinez and his colleagues performed measurements on variations of two noise-reduction techniques. “They are both inspired by the silent flight of owls,” he says. “Their feathers create a saw-tooth profile, called serrations, which interferes with the scattering of the airflow at the trailing edge.” The researchers also investigated the insertion of porous materials or metal foams at the trailing edge, causing a similar interference. “It’s a great solution from a scientific point of view,” he says, “but real-world application is difficult, as these materials can get dirty or attract lightning.” Whereas serrations are already being commercially used by some wind turbine manufacturers, it may take another ten years for the conservative airplane industry to follow suit. “Safety first.” Roberto Merino-Martinez +31 (0)15 27 81736 r.merinomartinez@tudelft.nl This is a story from Aerospace Engineering Read more stories of Aerospace Engineering Related stories The responsibility gap of self-driving cars Roboats in Amsterdam The flying V

PREFER project aims to help engineering graduates become more employable

Engineering graduates learn plenty of skills during their studies. But what happens when your skills and interests don’t fully match with what industry desires? The PREFER project aims to reduce that mismatch and help engineering graduates become more employable.

Unpredictable training helps pilots manage surprise and stress in the cockpit

Unexpected events in the cockpit can provoke a startle and surprise response in pilots, sometimes resulting in fatal crashes. So, can pilots be given more effective tools to respond to these situations? Dr Annemarie Landman set out to investigate this topic in her recent doctoral thesis.

Visualizing aircraft noise – bringing silent aviation closer

Aircraft engine noise has been reduced substantially, but the growth in air traffic still puts the hundreds of millions of people at an increased risk.

TU Delft expanding in space

The Delfi space program is preparing to launch its third satellite, and TU Delft satellite technology may even go beyond the far side of the moon.

A precision mirror positioning system for compact earth observation space telescopes

Sean Pepper has won the Heinz Stoewer award for his thesis work on a nanometre-scale positioning system for mirrors used in a Deployable Space Telescope.

TU Delft’s team Silverwing reaches the finals of the GoFly Prize

By Heather Montague As one of five finalists in the Boeing sponsored GoFly Prize, TU Delft’s own team Silverwing aims to win with its S1 design, a personal flying motorcycle. Although it might seem like something out of a science fiction movie, human flight may soon become a reality. By founding the GoFly Prize, CEO Gwen Lighter set out to stimulate innovation in the development of personal flying devices. The three-phase global competition, announced in November 2017, aims to foster the development of safe, quiet, ultra-compact, near-VTOL (vertical take-off and landing) personal flying devices capable of flying twenty miles while carrying a single person. Making people fly TU Delft’s Silverwing came into being when two aerospace students put together a small team to submit a concept for the first phase of GoFly. When their idea was selected as one of the top ten out of hundreds of entries, Technical Manager Victor Sonneveld, a master’s student, and Team Manager James Murdza (BSc 2018) quickly pulled together a larger multi-disciplinary team. It has since grown to 34 students representing ten nationalities and nearly every faculty at TU Delft. In March, team Silverwing learned they had been chosen as one of the top five designs by GoFly. During this second phase of the competition, teams had to present a more detailed design and built various test set-ups, including a half-scale prototype. The S1, a tailsitter aircraft, rotates 90 degrees to take off and land on its tail, requiring no runway and less space than a car. “It’s basically a flying motorcycle, but what makes it special is that it takes off and lands vertically,” said Ruben Forkink, an aerospace graduate and Silverwing Chief of Partnerships & Business Development. “At the desired altitude you transition from vertical to horizontal flight and then you basically ride it like a motorcycle and transport yourself from A to B. Then you transition back from horizontal flight to land vertically on your tail.” Going beyond the call Although it was not a requirement of the competition, the S1 is battery powered, fully electric and able to fly autonomously. “We’re one of the few teams that opted to go completely electric,” said Nisarg Thakrar, an aerospace master’s student and member of the Silverwing structures team. “From our point of view, to be successful, it has to achieve the modern requirements, being electric and autonomous.” And although the limitations of battery technology make it challenging to carry the required amount of weight, making it autonomous means that the S1 could be used to transport packages, not just people. Students bring a lot to the table As competitions go, the GoFly Prize is unique in that it is open to anybody from anywhere in the world. “What’s really great and what makes us stand out is that we are the only undergrad team in the finals,” said Forkink. The other finalist teams include PhDs, researchers and companies with industry experience. “We have a lot of bachelor’s students on our team so it’s a challenge, but that’s what we really like,” noted Forkink. “We are able to combine the knowledge from all of these faculties and that makes us a real multidisciplinary team.” Putting hands-on education to work The aerospace faculty has played an integral role in Silverwing’s design and development process. According to Forkink, the team has members from all of the different tracks at aerospace, and knowledge gained in the classroom has been useful. “We have students that go to their lecture and 30 minutes later they come here and start working on what they just learned.” And Thakrar believes the master’s programme structures track has been a huge asset in the design of the S1. “We do most of our own work, but we do have limitations and at that point we can consult with professors as well as external parties that help us,” he said. “In my opinion, one of the best ways to be a good engineer is through application, not only through books and this project has been a great way to support that idea.” The final stage Team Silverwing is currently finalising the S1 design to prepare for manufacturing and they hope to have a test flight by the end of this year. In early 2020, they will participate in a final fly-off in the United States. During this last stage of the competition, the aircraft must take off vertically, cover an eleven-kilometre course and then land again vertically. Points will be awarded for low noise levels, size (the smaller the better) and speed. The winning team will receive a US$ 1 million grand prize. "GoFly is excited to see Team Silverwing-- a young team made up of many undergraduates-- competing with established companies and veterans of the industry, and holding their own,” said Lighter. “Team Silverwing brings novel innovation and out-of-the-box thinking to their personal flyer. We look forward to seeing Team Silverwing at the GoFly Final Fly Off next year, and we can’t wait to see them change the world." On April 30, 2019 Silverwing will unveil their award-winning design for the S1 to the general public. Want to see it? Click here .

Can ancient algae help replace chromium-6 in coatings?

Timelapse: corrosion protection of the letters ‘TUDelft’ What seemed like a wild idea in 2014, using the external skeletons of algae to prevent corrosion, has now been shown to provide long term protection of aluminium used in airplanes. In a few years’ time, it may provide a safe and environmentally-friendly replacement for the use of chromium-6. “Because of its toxicity, the European Commission has forbidden the use of chromium-6,” says Paul Denissen, PhD researcher in the Novel Aerospace Materials group at the faculty of Aerospace Engineering. “Use of chromium-6 is only still tolerated in situations where good alternatives are lacking, for example to protect airplanes against corrosion.” He explains that the aluminium alloy most used in aviation is especially susceptible to corrosion because of the copper that has been added to increase material strength. Typically, multiple boundary layers are applied to protect this aluminium against weathering. One of these layers is a primer coating loaded with chromium-6. “Our research focusses on using the external skeletons of a sort of algae to develop an environmentally friendly alternative for the use of chromium-6 in this layer.” Challenging chromium-6 Chromium-6 is a so-called active corrosion-inhibitor. When a treated surface is damaged, for example by scratching, the chromium-6 atoms will be released from the primer layer. They will create a thin layer of chromium oxide on the exposed metal surface, preventing further corrosion. After their release, chromium-6 atoms can continually redistribute themselves, providing continuous protection of the damaged area. “There are a number of alternative corrosion-inhibitors that are also very good at creating a protective barrier,” Denissen explains. “Unlike chromium-6, however, they can oxidize only once, and the protective layer they create is not permanent. Long-term protection therefore requires the continuous release of these inhibitors. More importantly, these alternative inhibitors may already chemically react with the primer coating at the time of its fabrication or application, thereby weakening their anti-corrosive power.” Quite some challenges to overcome, with a possible solution coming from the world of algae. Various shapes of the external skeleton of diatom algae Source : https://paleonerdish.files.wordpress.com/2013/06/diatoms.jpg Pill-box protection Diatoms are a group of microalgae that have been roaming the earth for more than 100 million years. These single cell organisms come in various sizes, ranging from one to a few tens of micrometres. They have a hard, inorganic shell to protect them from the environment. This cell wall is made out of silica, the same material as glass, and contains many nanometre-sized pores. Inspired by the pill-box shape of these shells ( see figure ), it was Santiago Garcia, associate professor in the same group and the supervisor of Denissen, who came up with the idea to use them for active corrosion protection in coatings. Garcia explains that “my idea was to fill these shells with alternative corrosion-inhibitors, and then add these loaded shells to the primer coating. I envisioned the pill-box structure to prevent the unwanted chemical reaction between inhibitors and coating.” He also imagined the pores to allow the immediate and sustained release of these inhibitors when the protective layers are damaged, and the metal surface is exposed. “And these algae shells are easily available at low-cost,” Denissen adds. Rapid development Denissen explains that his 2015 master’s thesis was merely a feasibility study, to see if this approach could be successful. “Now, we are three years into my PhD and despite limited resources we have just shown corrosion protection potentially equalling that of chromium-6. We still use our first pick of algae shells, but we have substantially increased their filling with inhibitors as well as their release efficiency, leading to a much-improved protection.” 30-day protection by algae coating with corrosion-inhibitors Testing in Paris After intensive work in Delft to proof the concept, the researchers travelled to Paris for a challenging experiment. “We were curious as to the long-term protective power of our coating for large damages, as required by several companies,” Denissen says. Together with their collaborators from the group of Polina Volovich at Chimie ParisTech, they applied a 1 mm wide scratch to samples of aluminium used for airplanes, covered in a variety of their test ‘algae-coatings’. These samples were subsequently immersed in large volumes of a highly corrosive environment. The researchers got what they bargained for ( see figure ). “We were astounded,” continues Denissen, “what we saw was full protection against corrosion, even after thirty days of immersion. Only a couple of alternative solutions come this close to the results obtained with chromium-6. It’s an amazing result after only such a short period of development.” Visualising corrosion protection Denissen and Garcia have also developed a novel method to study the onset and development of corrosion. It allowed them to gain a detailed understanding of the results they obtain with their algae shells, guiding further optimisation. “It is relatively simple technology, using a basic optical camera,” Garcia explains. “Optical techniques have traditionally been used to obtain qualitative information or to make beautiful pictures. What we have shown is that optics can be used to monitor and quantify local corrosion processes at a very high resolution, in real time. It is mature technology, allowing us to analyse any coating, commercially available or still in development.” Optimal protection “We use our experimental findings to build a computer model for further optimization of our coatings,” Denissen says. This can prove very beneficial as these algae shells come in more than 100.000 sizes and shapes. And there are more variables to tune, such as the type of corrosion-inhibitor used, whether or not to add an outside layer to the algae shell to even better regulate inhibitor release, or the optimal concentration of shells in the coating. “We may for example want to use disc-shaped shells to reduce our protective layer to the thickness currently used by the industry,” Denissen explains. “We are also looking into using combinations of inhibitors and shells in our coatings, further improving corrosion protection.” A small revolution It is not an easy task to replace chromium-6. “There are many barriers, resulting in a lack of good alternatives,” Denissen says. “For example, the Dutch Ministry of Defence wants proof that alternatives will provide twenty-year protection of their military equipment. But there are no good methods to accelerate this evaluation, to validate it in only a limited time-span.” More importantly, he explains, many of the tests used to validate the efficiency of new coating materials are designed specifically for chromium-6. “It is not a level playing field. It means that you have to prove your alternative coating to behave similar to chromium-6, rather than prove that it provides adequate protection.” Nevertheless, a small revolution has recently taken place. Rather than waiting for coating manufacturers to replace chromium-6, airplane manufacturers are now actively developing their own solutions as well. “At the moment, we are already talking to both.” Future perspective Despite very promising results, Denissen stresses that “we need a few more years to develop and demonstrate our algae-based coating before it can be used on planes, bridges or any metal surface that needs protection against corrosion. Does our coating protect sufficiently against scraping and scratching? Can it withstand frequent variations in outside temperature? Will it bond well with the other protective layers?” Garcia adds that “our main commitment is to find solutions to societal problems. We are currently talking to several industry partners about collaboration. Together we can speed up the development and launch of our technology and we expect to be ready for operational experiments by 2022, on an airplane.” Until completion of those experiments and passing the required certifications, the airplane industry may require the European Commission to again extend its leniency, tolerating the use of chromium-6 for the time being. You can find scientific publications, related to this research, here in Corrosion Science and in Electrochimica Acta .

Gas turbines: essential for the transition to renewable energy sources

Gas turbines are best known as the jet engines that power aircrafts. But they also are the work horses of large power plants generating electricity for our industry and homes.