Biomimetics is the science (or art) of engineering natural phenomena. Sharks are known to be extremely efficient swimmers and this is helped, in part, by their skin, which is covered in denticles, or small riblets. These smooth the flow of water and reduce drag. As the science of hydrodynamics and that of aerodynamics are similar, this has been of interest to aviation for some time but has taken on a new lease of life with the increased push towards the reduction of CO2 emissions.
Lufthansa Technik has been playing with the idea for a number of years. In 2011, along with Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) and Airbus, it started the three-year Multifunctional Coating project funded by the German Federal Ministry for Economic Affairs and Energy. This involved embossing a riblet pattern into a coating, the process also curing the coating to give rigidity. Small patches were then attached to the fuselage and the upper surface of the wing of various Lufthansa aircraft to test the longevity and durability of the coating in regular aircraft operations.
With additional funding by the Federal Ministry for Economic Affairs and Energy, 2014 saw the three partners joined by bwm, a specialist machine tool manufacturer, in the FAMOS project, which set out to develop a method of automatic application of the riblets. This was achieved in 2017 in a demonstration at the ZAL Center for Applied Aeronautical Research in Hamburg. However, the conclusion was that embossing was too complex.
After discovering some teams in the Red Bull Air Race were successfully using sharkskin film to boost the performance of their aircraft, it was decided that this was the better solution. Two Austrian concerns had developed the sharkskin film. Bionic Surface Technology, a small high-tech start-up specializing in high-fidelity flow simulations, had used a self-developed code to design the optimal size for the microstructures, while Joanneum Research, a material sciences research institute, could print the microstructures on a very thin adhesive film.
Although Joanneum Research was able to develop the riblet film and produce enough for testing and certification, it would be unable to do this on an industrial scale, so chemical giant BASF became involved as it had already started to work on a process to produce films with microstructures for various applications and had developed and lab-tested a prototype aviation-grade riblet film for Lufthansa Technik, the first version of AeroSHARK. The next steps were to resume testing with more than 100 small patches again being fitted to aircraft in service to monitor durability and to test application and removal methods on a Boeing 737-500 used as an apprentice training tool in Hamburg.
In October 2019, during a C-check, almost the entire lower fuselage of a Lufthansa Boeing 747-400 was covered in 500 m2 of the film, the first large-scale application made to a commercial aircraft. With the modification certified by EASA, the aircraft has remained in service and has accumulated more than 6,500 flight hours, demonstrating that emissions were reduced by up to 0.8%.
Jens-Uwe Mueller, manager AeroSHARK, says the main reason for selecting the Boeing 747-400 back then was to promote the benefits of retrofit to customers while taking advantage of many years of experience with the aircraft to accelerate the testing and certification process. He adds that 0.8% may not sound much but, combined with other emission reduction measures, it plays a small but important part in making aviation cleaner. It also means that the technology is mainly aimed at long-range, widebody aircraft, which spend the greatest part of each flight in cruise.
Unfortunately, plans to retrofit the entire Lufthansa 747-400 fleet were cancelled by the pandemic but, encouraged by the test results, the partners designed a new, and even larger, AeroSHARK modification that uses the same adhesive films with riblets measuring around 50 µm in size and a maximum weight of 180 g/m2.
Having looked at possible alternative platforms to the 747-400, the Boeing 777 was selected, not just because there are plenty of aircraft in service (about 1,100, or 25% of the world widebody fleet) but also because they are operated by two Lufthansa Group airlines: Swiss International Air Lines (SWISS) flies the 777-300ER, carrying passengers, and Lufthansa Cargo has the 777F freighter.
The AeroSHARK film is able to withstand strong UV radiation as well as temperature and pressure fluctuations at high altitudes. Resilience against cleaning procedures and icing and flammability were also tested. One result was that dry washes are not yet permitted for AeroSHARK-equipped aircraft.
The film comes in standard panels measuring 100 cm x 50 cm, which are cut to size and currently applied to about 40% of the overall surface area of the aircraft, covering large parts of the fuselage and the engine nacelles. For the 777 modification, this involves more than 2,000 individually trimmed parts. The shapes and riblet pattern are determined using a 3D model of the aircraft developed from very precise laser scanning and CFD computation to establish the airflow around the aircraft. To include the wing bending geometry in the 3D model, Fraunhofer IOSB carried out a photogrammetric procedure on a SWISS flight from Zurich to San Francisco in the summer of 2021. A single camera looking through a cabin window took a photo every few minutes, mapping the positions of special markings on the wing to provide a 3D model of the wing flex. The accuracy of Lufthansa Technik’s CFD simulations was validated, in August 2021, with a series of tests in the low speed wind tunnel at the Deutschen Zentrum für Luft- und Raumfahrt (DLR) in Brunswick, using a scale model of the cancelled Dornier 728 regional jet.
With the CFD model verified, work turned to where the panels might be applied. No-go areas included sensors, heated areas and doors as well as where they might be at risk from ice build-up or foreign object damage or could disrupt laminar flow. In addition, some panels required holes to be cut so that they could fit round access panels. Preferred areas were those that were easy to certify and, thanks to CFD, where drag reduction would be greatest. This is a complex exercise, as the airflow alters direction along the aircraft, which is also, typically, in a 4° nose up attitude in cruise. That means the direction of the riblets must subtly change from panel to panel. The panels are applied in such a way that they do not overlap. This means, in the unlikely event that a panel detaches, it will not take other panels with it.
Having said that careful alignment is needed for maximum results, the effect of not being aligned in climb and descent is negligible. Similarly, the weight penalty can be offset after a couple of hours in cruise. For the 777-300ER, the basic saving is 80 kg of jet fuel per hour, which means a little less fuel can be loaded, making the aircraft lighter. Taking a 10 hour flight as an example, that means a saving of 800 kg. This is reduced to 640 kg by the deadweight of 160 kg for AeroSHARK. However, the lighter aircraft weight realizes additional fuel savings of 180 kg for that flight, giving a total reduction of 980 kg.
Annual savings for the same aircraft are expected to be around 400 tons of kerosene and more than 1,200 tons of CO2, while the Boeing 777F will save around 370 tons of fuel and 1,170 tons of CO2 each year. The differences arise from the routes, world regions and utilization for each airline. As SWISS and Lufthansa Cargo have twelve 777-300ERs and eleven 777Fs respectively, these are meaningful reductions, more than 25,000 tons of CO2 annually.
In the future, Lufthansa Technik plans to use a software algorithm in its AVIATAR system to calculate the savings by using consumption data from the fuel flow rate sensors on the engines. Currently, the company uses a different method based on full flight data. This delivers precise measurements to within +/– 0.1%.
The first 777 application was to a SWISS 777-300ER in August last year, with successful test flights in early September resulting in a temporary Permit To Fly from the Swiss Federal Office of Civil Aviation (FOCA) for that particular aircraft, which entered revenue service in October. The test flights were followed by several weeks of evaluation of the collected data and other documents, such as measured values from flow simulations. After completing its review of all submitted documents, EASA granted the STC for both types in December, allowing fleet modifications to begin. The first 777F was fitted with AeroSHARK in February in Frankfurt during a scheduled maintenance layover. The FAA STC was received in April. Incidentally, although the two types have different fuselage lengths, they share many aspects relevant for the design and certification process, for example the same wing geometry and structure, so each STC covers the 777-300ER and the 777F.
To date, ten 777-300ERs of SWISS have been modified so far, with completion of remaining two expected in late summer/early fall this year. This is because the airline is very keen to advance their sustainability efforts and willing to take aircraft out of service just to receive the modification. In contrast, Lufthansa Cargo is integrating the AeroSHARK modification into the standard scheduled base maintenance layovers of their 777Fs, with the second aircraft planned for July and completion of the remaining nine aircraft not expected before 2026. However, experience has brought about a reduction in downtime for the modification to about five days (less than usually needed for a C-check on this aircraft type). The eleven 777s currently modified with AeroSHARK have also already accumulated several thousand flight hours in recent months.
The plan is to expand coverage, for example, to the Airbus A330 and A350 and Boeing 787. There is also an exception to the widebody application and that is the Airbus A321XLR, with its 4,700 nm range making AeroSHARK a viable option. Of course, each of these would require a new CFD model and a separate STC.
Finally, Mueller says the reaction from the industry has been very positive, with airlines showing interest in the 777 having more than 250 of these aircraft in their fleets. He also notes that the interest is now coming from airline personnel involved in sustainability, not engineering.