04 AeroSHARK: Flying more sustainably with a new surface technology

Intro 

AeroSHARK is the world´s first series application of sharkskin-inspired surface technology, which helps to reduce aircraft fuel consumption. We´ve come a long way until we were able to spread the news in 2021. No one knows that better than Dr. Kai-Christoph Pfingsten, who is not only Head of Innovation at Lufthansa Technik, but moreover an aerodynamicist from heart. He earned a PhD in the field of fluid dynamics and takes now a deep dive into his and our company’s manifold approaches from the past until today.

When we announced AeroSHARK in 2021, our publication was mostly met with overwhelmingly positive feedback, but there have also been skeptical voices like: “We have heard that so many times before – already decades ago – and still no commercial aircraft features this miracle microstructure”. There is truth in this.

So let’s have a look at the challenge of sharkskin technology which is twofold:

So let’s have a look at the challenge of sharkskin technology which is twofold: The first challenge is the material. The surface structure consisting of riblets, measuring around 50 micrometers, imitates the properties of sharkskin and therefore optimizes the aerodynamics on flow-related parts of the aircraft. Microstructures are very small and a commercial aircraft is very big. In addition, the durability of the material is very important. If you are able to get the microstructures on the surface, you want them to endure in this demanding environment as long as possible. The impact of high velocity particles could just remove the riblets; strong UV radiation could destroy the material or change its properties. The latter could also happen because of the extreme temperature difference between cruise which is -60°C on high flight levels and a hot apron which is +45°C in some world regions.

So let’s have a look at the challenge of sharkskin technology which is twofold: The second challenge is to be able to simulate the effect of the microstructures on the flow around the aircraft. For each position on the surface, there is an optimal design and orientation for the microstructures to get the highest reduction in skin friction. To make it even more complicated, the optimum also depends on the speed, flight level and weight of the aircraft. Thus, to find the overall optimum in respect to drag reduction, it is of paramount importance to be able to simulate the effect of the microstructures. But to simulate something as big as an aircraft in so much detail that you also have the riblets in your geometry model, makes the necessary computational power ridiculous. One design point would run for months on a computer cluster – and we would need to run hundreds of design points to find the optimum.

So let’s have a look at the challenge of sharkskin technology which is twofold: Over the years, a huge number of very different parties including universities, research institutions, and industry players such as our own company itself had already investigated sharkskin technologies and time and again praised its qualities as a means of making air travel more sustainable ... in theory. However, the two aforementioned obstacles in the way to a widespread practical application couldn’t be conquered – until now.

But before we look at the most recent developments, let’s go a few years back again:

Already in 2011, we entered a cooperation with Fraunhofer IFAM and Airbus to investigate the application of riblets on aircraft. After the so far rather unconvincing experience with applying adhesive riblet films in the aviation community, we tried a completely different approach. Instead of adhering sharkskin films onto the painted aircraft skin, we went with a new technology: The aim back then was to literally emboss the riblet structure into an additional layer of coating.

Already in 2011, we entered a cooperation with Fraunhofer IFAM and Airbus to investigate the application of riblets on aircraft. After the so far rather unconvincing experience with applying adhesive riblet films in the aviation community, we tried a completely different approach. Instead of adhering sharkskin films onto the painted aircraft skin, we went with a new technology: The trick here is, that the coating is cured during the embossing process, thus keeping its micro-structured surface. The focus of the project “Multifunctional Coating" was to test the longevity and durability of the microstructures in regular aircraft operations. Therefore, riblets were applied on small spots on the fuselage and the upper surface of the wing to monitor the degradation.

Already in 2011, we entered a cooperation with Fraunhofer IFAM and Airbus to investigate the application of riblets on aircraft. After the so far rather unconvincing experience with applying adhesive riblet films in the aviation community, we tried a completely different approach. Instead of adhering sharkskin films onto the painted aircraft skin, we went with a new technology: With additional funding, we were able to continue the research already in 2014 within the project FAMOS. Here, the focus was on the automatic application of the microstructures. The major objective was to apply riblets directly on a full- size wing segment.

Already in 2011, we entered a cooperation with Fraunhofer IFAM and Airbus to investigate the application of riblets on aircraft. After the so far rather unconvincing experience with applying adhesive riblet films in the aviation community, we tried a completely different approach. Instead of adhering sharkskin films onto the painted aircraft skin, we went with a new technology: Parallel to the development of the necessary robotic system, the durability of the sharkskin made with different coatings was tested on a Lufthansa Airbus A330 under real service conditions. The FAMOS project ended in 2017 with the automated application of riblets on a wing segment in Hamburg’s TechCenter for applied aeronautical research.

Already in 2011, we entered a cooperation with Fraunhofer IFAM and Airbus to investigate the application of riblets on aircraft. After the so far rather unconvincing experience with applying adhesive riblet films in the aviation community, we tried a completely different approach. Instead of adhering sharkskin films onto the painted aircraft skin, we went with a new technology: On one hand, FAMOS answered many questions regarding the automated application of sharkskin. On the other, it also showed that, even with robotic support, the further advancement towards a series application using the embossing approach would remain pretty challenging.

Already in 2011, we entered a cooperation with Fraunhofer IFAM and Airbus to investigate the application of riblets on aircraft. After the so far rather unconvincing experience with applying adhesive riblet films in the aviation community, we tried a completely different approach. Instead of adhering sharkskin films onto the painted aircraft skin, we went with a new technology: Based on this conclusion, Dr. Matthias Nolte, the project manager of FAMOS and Multifunctional Coating, and myself started to look for alternatives...

Already in 2011, we entered a cooperation with Fraunhofer IFAM and Airbus to investigate the application of riblets on aircraft. After the so far rather unconvincing experience with applying adhesive riblet films in the aviation community, we tried a completely different approach. Instead of adhering sharkskin films onto the painted aircraft skin, we went with a new technology: ...and were inspired by Red Bull Air Race

In spring 2015, after leaving no stone unturned, we discovered this: Certain teams in the Red Bull Air Race were successfully using sharkskin films to boost the performance of their racing planes. That made us curious and we started to investigate further. What we found surprised us even more. Behind that technology was an unusual couple from Graz in Austria: Bionic Surface Technology, a small high-tech startup specializing in high-fidelity flow simulations, and Joanneum Research, a research institute for material science. 

In spring 2015, after leaving no stone unturned, we discovered this: Bionic Surface Technology used a self-developed code to model the sharkskin effect in their flow simulations to design the optimal size for the microstructures; Joanneum Research was able to print the microstructures in a special process on a very thin adhesive film. If this could be transferred to transport aircraft, you get the very interesting option of changing size and orientation of the riblets and thus making them much more effective.

In spring 2015, after leaving no stone unturned, we discovered this: As material experts and as an aerodynamicist we were very skeptical, but for all the good questions we had, the Austrians came up with even better answers. And at least for racing planes it seemed to work!

So, on the flight back, we had some thinking to do:

So, on the flight back, we had some thinking to do: Could that mean riblet film isn’t the bad idea everybody thought it is?

So, on the flight back, we had some thinking to do: Does the advance in material science allow us to succeed, where we failed two decades earlier?

So, on the flight back, we had some thinking to do: Could the material be adapted to also endure the much more demanding environment of transport aircraft?

So, on the flight back, we had some thinking to do: Would the printing process still work with the new material we might need?

So, on the flight back, we had some thinking to do: Would the capability to take account of the riblets in the flow simulations allow us to find the optimum riblet patch for each position on the surface?

So, on the flight back, we had some thinking to do: The short answer was yes – it just took us a little longer to prove.

So, on the flight back, we had some thinking to do: Let’s explore the next step which was industrializing the production

So, on the flight back, we had some thinking to do: Believing that we would be successful, we started to think about how we could industrialize the production. Joanneum Research was able to develop the riblet film and produce enough film for testing and certification. But obviously, they would not be able to produce enough to supply the complete aviation industry, which we would have to aim for if we really want to considerably reduce aviation’s global CO2 footprint.

So, on the flight back, we had some thinking to do: As the largest chemical producer in the world, with extensive experience in industrializing surface technologies, BASF seemed to be the perfect match for this challenge. Luckily for us, they had already started to work on a roll-to-roll process to produce films with microstructures for various applications.

And it didn´t take long until this new-found cooperation bore its first fruits: Already in 2017, BASF had developed and lab-tested the first prototype of an aviation-grade riblet film based on the strict requirements specified by my colleagues here at Lufthansa Technik. Immediately after the first successful laboratory tests we again started to test riblet patches of different promising formulations on real aircraft in service to get a realistic assessment of their degradation behavior.

And it didn´t take long until this new-found cooperation bore its first fruits: At the same time, we applied the riblet film we had developed together with Joanneum Research, to a racing plane to check if we also got the expected performance enhancement. And yes, with our newly developed material we achieved the same positive results in respect to performance. But in addition the film was way more durable as it was designed for the much harsher operating environment of large transport aircraft.

And it didn´t take long until this new-found cooperation bore its first fruits: An interesting fact here is, that, due to the lower speed of the racing plane and the lower density of the air around a transport aircraft in cruise, the aerodynamic properties of the surrounding flow for both are pretty similar and, thus, is the optimal size of the riblets. So this test showed us that we had a system that gave us the aerodynamic advantage we were looking for and the patch tests on our Boeing 747-400 proved their superior durability under real conditions.

And it didn´t take long until this new-found cooperation bore its first fruits: In 2017 we started with the first test application of BASF’s riblet films, using a Boeing 737-500 to test if a large-scale application can be manually done. This decommissioned aircraft provided the perfect platform to safely trial all different aspects regarding the trimming, positioning, application and removal of the first jointly developed riblet films. After a final rehearsal in summer 2019 and after hundreds of square meters successfully applied to and safely removed from the grounded jet, we felt prepared to move to the next step … size-wise it was more of a giant leap.

The reason is simple: Up to now, our proof of concept was entirely based on wind tunnel testing, simulations, and racing planes. The proof that sharkskin really works on a large commercial transport aircraft was still missing.

The reason is simple: So, what was next? Testing under real conditions of course!

The reason is simple: To really quantify the drag reduction, and thus the fuel and emissions savings potential, we would have to test the riblet film on a long-haul aircraft in real service and we would have to cover a substantial part of its surface. So, what better aircraft could we choose than a Boeing 747-400 – the Queen of the Skies!

The reason is simple: As we would need several days for this test application, we had to closely align the first riblet modification to the aircraft’s maintenance check schedule. The perfect chance hence emerged in Lufthansa’s Boeing 747-400 “Tango Kilo”, that was scheduled for a C-check in Frankfurt in late October 2019.

The fact that we wanted to modify this aircraft under ordinary check conditions, with no exemptions being made for the sharkskin modification, made the project management aspect quite challenging. The spectrum of challenges experienced went well beyond the availability of ground servicing equipment or tooling: Any unexpected structural finding or unscheduled maintenance requirement for the “Tango Kilo” could have easily interrupted the entire modification schedule. As these events can happen during any maintenance check, it was a welcome opportunity for us to take “real-life conditions” into account from the very beginning.

Another important aspect was plain manpower: Although our Minimum Viable Product 1 encompassed only a modification of the lower fuselage of the Boeing 747-400 with riblet films, the amount of modified surface area reached above 500 square meters. With a single riblet film patch measuring only 100 x 50 centimeters, more than 1,000 patches had to be measured, trimmed, and applied. As a consequence, the workforce needed was much greater than during the “dry runs” on the 737-500 in Hamburg. However, in a combined effort with supporting colleagues from Lufthansa Technik Budapest, Lufthansa Technik Sofia, and our colleagues from Hamburg and Frankfurt, we finally completed the modification right on time. In doing so, we paved the way for the subsequent Supplemental Type Certificate check flight. In this flight the EASA had to approve the riblet modification before the aircraft’s return to commercial service that would also mark the beginning of the validation phase.

Another important aspect was plain manpower: Our first-ever “real life” field test to quantify the saving potential of the sharkskin technology had finally begun! How this all worked out, and what other aspects we had to take into account while making 500 square meters of sharkskin airborne, will be detailed in upcoming episodes. So stay tuned!