Covered in tiny structures known as dermal denticles, the ribbed pattern of shark skin reduces drag across their surface, allowing some species to swim at 50km/h. Others can dart forward even faster over short distances.
Such a useful property has not gone unnoticed by engineers. Projects have replicated the hydrodynamic effect in air using ‘riblets’, with companies including 3M and Airbus trialling products.
Now, a riblet film known as AeroShark is being rolled out on to operational aircraft. Developed by German multinationals Lufthansa Technik and BASF, the technology was installed on passenger Boeing 777-300ER planes at Swiss from October 2022, and Lufthansa Cargo's entire Boeing 777F freighter fleet at the beginning of 2023.
This week, Japanese airline ANA began scheduled cargo flights with a modified 777F treated with the film, with plans to start using it on a passenger aircraft by next spring.
On the Lufthansa Cargo fleet, the technology has reportedly reduced drag by more than 1%. While that may sound small, for a fleet of 10 aircraft that equated to annual savings of about 4,000 tonnes of kerosene fuel and 12,000 tonnes of carbon dioxide (CO2) emissions – the equivalent of 53 freight flights from Frankfurt to Shanghai.
As pressure to decarbonise grows ever stronger, ANA and other operators hope to enjoy similar benefits. And, experts say, riblet technology could bring even more advantages.
Functional film
Described as a “functional surface film inspired by the drag-reducing structure of shark skin”, AeroShark is covered in 50-micrometre riblets. Separated into patches for straightforward application to the surface, the film has millions of the prism-shaped structures. The patches are aligned with the airflow to reduce drag and boost efficiency.
The film will cover nearly the entire fuselage of ANA’s 777s, with several hundred square metres required. Although the airline described the modification as “almost invisible”, it expects annual savings of approximately 250 tonnes of fuel and 800 tonnes of CO2 emissions per aircraft.
“The introduction of AeroShark technology on our Boeing 777 aircraft marks a significant milestone in our sustainability strategy, in support of our broader goal of reducing carbon emissions across our fleet,” said executive vice-president Kohei Tsuji.
The airline aims to validate the effectiveness of the technology in daily operation, with plans to expand its use across other aircraft of the same type. Initial model calculations suggest that sharkskin technology could potentially reduce CO2 emissions by up to 3%.
“Every percent that you can achieve in reducing the friction is really important for aircraft, for sustainable transport,” said Professor Christoph Bruecker, an expert from City, University of London, who was not involved in AeroShark development.
The Lufthansa and BASF technology is focused on reducing turbulent friction. The boundary layer next to the surface is already turbulent during flight, Professor Bruecker said, but the airflow is channelled through the riblets to reduce fluctuations, and therefore the turbulent friction.
Professor Bruecker’s own research, working with BAE Systems, takes a different approach. He aims to reduce drag by instead focusing on keeping the airflow alongside an aerofoil in the laminar state for as long as possible, delaying the transition to turbulent boundary layer flows.
The main challenge with shark and fish-inspired aircraft skins is keeping them clean, he said, which has held back previous projects. “The contamination with dust and everything, the cleaning cost to keep these very thin, tiny riblets clean, was more than the benefit that you achieved by reducing the friction drag,” he said.
The additional weight from the coating is another factor to consider, he added. Laser structure coating could be another “ideal” way of applying it to surfaces without extra mass, he said.
Nature-inspired solutions
Technologies like AeroShark will be used on all commercial aircraft in the years to come, Professor Bruecker said. “If the technology is ripe, if it is there, it will be taken rapidly… because it's so easy to apply,” he said.
Shark skin-like coverings could even be used on trains or cars, according to Professor Bruecker. “You can save a lot of turbulent friction drag on trains using the same coating principles,” he said. “Everything you can think of where drag is an important contribution to the thrust… can profit from nature-inspired solutions to reduce the turbulent friction drag.”
The technologies might even have other benefits. Research from Cranfield University, which is not yet published, has explored the possibility that the surfaces could have a de-icing effect, potentially helping solve a major problem for aircraft.
“What we've shown is that because they reduce the turbulent skin friction, it means that it also reduces the ability of water droplets to stick on… to the surface,” said Professor Simon Prince, who supervised the work by Xiangning Zeng alongside colleague Dr Craig Lawson.
“If you measure the shear stress that you need to apply in order to break the ice off and get rid of the ice, then it is significantly less with the shark skin or fish surfaces than it is with pure aluminium.”
The surfaces could also change the way in which noise radiates from aircraft. By applying them to turbine blades, for example, engineers could reduce noise signatures or shift them to more desirable frequencies.
In the meantime, BASF and Lufthansa Technik hope AeroShark can help more airlines achieve their sustainability goals. Current efforts include gaining approval for additional aircraft types and covering larger surface areas.
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