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Aircraft engineers turn to biomimicry for greener designs

Birds do it. Bees do it. And now, increasingly, aircraft engineers are falling in love with the idea of studying the natural world to find solutions that can be adapted and applied to the design of more fuel-efficient airliners.

The science of biomimicry is taken seriously by aircraft manufacturers, and there is the potential for some quite mind-boggling tricks of nature to be emulated and used in aviation to reduce drag and better enable aircraft to adapt to changing conditions during flight.

Eagles, © Pekka Fagel
 © Pekka Fagel
Engineers are still finding inspiration from birds in the quest for more efficient aircraft

"Biomimicry is looking at how you can use Mother Nature as a measure. It is looking to the biological system and seeing where you can find guidance and inspiration," says Airbus senior manager of flight physics research David Hills. It involves studying how nature solves problems and formulating solutions that are "free from the trappings of civil aeronautical design", he adds.

DRAG

The design and development of compliant surfaces is one area of aircraft design that could have significant environmental consequences and will need to draw its inspiration from nature. On today's commercial airliners, about 40% of drag can be attributed to the turbulent boundary layer - a thin sheet of air just above the aircraft's skin that creates friction. A continuously adapting compliant surface that destroys the turbulence in this layer could virtually eliminate this skin friction drag.

"There is an opportunity to reduce this 40% to close to zero, which would be a massive step towards reducing the environmental impact, but you're talking about manipulating very small, fast dynamic fluid structures, so it's a significant challenge," says Hills. "If you look to the biological world, marine mammals such as dolphins have a compliant surface which senses the state of the water and ripples the skin to suppress turbulence."

But while Hills believes compliant surfaces will be "a big area for the future", a simple engineered solution does not appear imminent. "To copy and paste what dolphins do in the water to our situation in the air is very challenging. It's many years off," he says.

Aircraft engineers have managed to come up with a solution that reduces drag by a few percent and which takes its inspiration from the structure of shark skin, but it has its drawbacks. Riblets - a series of small grooves on an aircraft's surface that are aligned to the direction of the air flow - "can demonstrate a 4-7% reduction in skin friction", says Hills. But because riblets are "very easily damaged", this creates a "significant engineering problem".

However, the Fraunhofer Institute in Germany has designed a paint modelled on shark skin that incorporates grooves similar to riblets and can be applied using a stencil as the outermost coating on an aircraft. The paint contains nanoparticles, which ensure that it can withstand ultra-violet radiation and changing temperatures. The Fraunhofer Institute says that if this paint were to be applied to every aircraft in the world it could save 4.48 million tonnes of fuel annually.

Yvonne Wilke, one of the Fraunhofer scientists who developed the paint, says the idea for inventing a shark skin-like aircraft surface came up 20 years ago. Five years later an experiment was carried out that involved coating aircraft with "a shark skin foil to evaluate its fuel-saving effects". While this produced "good results", it had several disadvantages. "The first problem was applying the foil on curved surfaces. If you think of a ball that you want to cover with foil you will understand the problem quite soon," explains Wilke. "Another problem was the additional weight of the foil and the removing of it for maintenance work. So the idea came up to integrate the shark skin surface into the topcoat."

Wilke believes the paint could be ready for commercial use three to eight years from now. "There is interest and there are projects on this paint and its application in the aircraft industry and the ship-building sector. All of these projects are in the phase of carrying out technical experiments," she says.

FISH SLIME

Another possibility for making surfaces more compliant is to draw inspiration from fish slime, as Hills explains: "Certain fish have slime close to their bodies which introduces long chain polymers to reduce drag. I wouldn't suggest that aircraft will exude slime, but we can think about long, hairy surface structures as a possible means of engineering a technical solution from something you see in nature."

Boeing is also following the science of biomimicry closely - so much so that it has sent teams of engineers on field trips to the rainforests of Costa Rica to take inspiration from their surroundings for use back at the design table. The trips were organised by the Montana-based Biomimicry Guild, a consultancy which specialises in guiding companies towards finding solutions to their engineering problems by studying the natural world.

"We have done a couple of different things with Boeing," says Dayna Baumeister, co-founder of the Biomimicry Guild. "We teach a workshop every year and we had several Boeing engineers at a workshop in Costa Rica and one in Peru. The group that came down from Boeing was interested less in the flight of the plane itself and more in the interior and functions within the plane."

Heidi Kneller, a mechanical design engineer at Boeing's Payloads Concept Center and a member of the Boeing team that attended the Biomimicry Guild workshop in Costa Rica, says the week-long trip "gave me a new lease on being creative in my job".

"I went with two other Boeing folks, one from the Systems Concept Center and one who is working in production. We went to learn the concepts of applying biomimicry in our respective disciplines," says Kneller. "Biologists speak a very different language to engineers and so the crux of the matter as an engineer was to learn how to ask my engineering questions and seek answers to my engineering challenges in a way that biologists would be better able to understand."

The purpose of Kneller's trip was to look at noise management. "I don't know if I can say anything specific, but I can say that the rainforest is full of mechanisms for managing, dampening or even amplifying sound," she says. "We came away with some good ideas that we documented, but our organisation is at the level where we pass it off to another group. A lot of the ideas that we came up with require a lot more development."

Baumeister of the Biomimicry Guild points to examples from nature that could be applied to the design of more fuel-efficient aircraft, one of which is the beak of a kingfisher. This example has already been used by engineers working on the design of Japan's bullet train. In this instance, the engineers were looking for a way of preventing the sonic boom that occurs when the fast-moving train exits a tunnel after creating a pressure wave on the way in. The solution was to develop a new nose for the train based on the design of a kingfisher's beak.

"The kingfisher's nose is elongated so it's like a knife cutting through the air," says Baumeister. "I could see a commercial airplane getting more of a kingfisher-inspired nose." She also believes there are "huge opportunities" to cut the weight of an aircraft by examining lightweight materials found in nature.

"For example, you need softness on an airplane chair so you add cushioning, but then you also need it to be waterproof so you add a coating, but then you also need durability so you weave in fabrics. You keep adding layers on but you want to keep the plane as light as possible," Baumeister says. "Nature would use a single material that would provide the cushioning and the waterproofing and the durability. It would probably be heavier than one of the layers [on an aircraft seat] but it would certainly be lighter than all three."

The science of biomimicry itself has "come in and out of favour for a long time", says Hills of Airbus, but it is having a resurgence as pressures mount on the aviation industry to reduce its carbon dioxide emissions. "I don't think it's flavour of the month, but given the environmental pressures on the planet, maybe it's being put to the forefront," he notes.

Baumeister is sure that biomimicry will play an increasing role in changing the future design of aircraft. "I'm almost 100% certain that flight as we know it today will not be flight as our children know it," she says.

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