AEROSPACE Biomimicry

Engineering nature

Is it a bird? Is it a plane? Is it the best of both? BILL READ FRAeS reports on how aeronautical engineers have been learning from nature to create new aircraft designs, concepts and modes of operation to optimise performance.

Aeronautical engineers are looking to nature for inspiration on new ways to make aircraft lighter and more fuel-efficient. Not surprisingly, the focus on biomimicry has begun with studying birds. When birds are in the air, they extend their wings to reduce air drag and help them to stay high – in a similar way to a glider attempting to increase lift and reduce drag. When birds want to move faster, they close their wings – as birds of prey do in an attack dive to catch prey.

Learning from birds

There could be many efficiency advantages if aircraft could replicate the flight of birds and change their wing shape in different stages of flight. Adaptive wings could provide a significant increase in performance, including fuel saving, longer range and reduced noise. Different wing shapes could also assist aircraft experiencing changes in weight and weight distribution as fuel is used up during flight to fly more efficiently.

In addition to morphing wings, engineers have also been inspired by other avian characteristics. In 2017, researchers at the University of Denmark used computational morphogenesis to design an alternative ‘organic’ interior structure for a Boeing 777 wing, based on the structure of a bird’s wing which was 5% lighter than a conventional wing structure.

In 2019, Airbus produced the ‘Bird of Prey’ conceptual airliner design inspired by the eagle. The theoretical design was a hybrid-electric, regional turbo-prop which mimics the eagle’s wing and tail structure and features individually controlled ‘feathers’ that provide active flight control.

Airbus has also studied the wing design of the long-eared owl to see how it can fly so silently. Most birds generate noise when flying through turbulence created when air flows over the surface of their wings. However, the long-eared owl has primary feathers, serrated like a comb, which muffle the sound by enabling the air to pass through easily. Airbus is designing a retractable, brushlike fringe to mimic the owl’s serrated feathers on wings, as well as a velvety coating on aircraft landing gear.

There has also been research into the development of aircraft with beating wings. A paper published in Science Robotics in July 2020 described how students at Nanyang Technological University in Singapore had designed a micro ornithopter which flies like a bird and could be used for monitoring crops or crowds. Engineers have also developed micro aerial vehicles (MAVs) which look like birds to observe birds. Developed by engineering students at the University of Delft in 2007, the 5cm wide morphing wing RoboSwift is a small propeller-driven MAV, fitted with cameras designed to observe swifts in their natural environment.

Colour perception and flight patterns

Biomimicry research is not restricted to the way birds fly but also to how they see. The Lund Vision Group in Sweden has designed a camera that recreates how birds distinguish colour which could have applications for aircraft navigational systems and pilot enhanced vision systems, as well as improved sensor and guidance systems for UAVs.

The RoboSwift MAV developed at the University of Delft. University of DelftHowever, biomimicry of birds does not end with bird shapes but also with flight patterns. Airbus has worked on the fello’fly demonstrator project which looks at how aircraft might learn from the way in which flocks of snow geese fly in a ‘V shape’. According to Airbus, this is because the follower geese expend less energy by surfing on the wakes (ie left-over kinetic energy of moving air in the sky) created by the leader bird. When flying in this way, geese immediately benefit from free lift, which enables them to stay aloft with minimal fatigue over long distances. The Airbus project is looking at how a commercial aircraft could save 5-10% of fuel from ‘wake-energy retrieval’ by following the air upwash from a leader aircraft.

The swarming flight patterns of insects have also inspired military UAV researchers into the potential of using swarms of miniature drones to overwhelm enemy defences.

Bees, fish and bones

Insects have also provided a source of inspiration with the development of insect-sized nano UAVs to be used for military reconnaissance and surveillance missions. Researchers at the University of Arizona have studied the aerodynamic characteristics of a bumblebee micro air vehicle while Oxford-based Animal Dynamics has created miniature drones with flapping wings based on dragonflies which it claims can hover in 20mph winds.

Airbus has also produced 3D printed metal cabin dividers based on cell and bone structures which are 45% lighter than conventional partitions.

Not all biomimicry research is based on structures. In 2003, researchers at the State University of New York and University of Missouri were reported to be studying the vibrational communications used by treehopper insects with a view to locating the source of sounds – which could be used to locate a problem on an aircraft. NASA also conducted a project with Boeing to create a windscreen coating based on the exterior surface of a lotus leaf which would reduce the effect of dirt, dust and water on aircraft windshields.

Sharkskin paint

Nor is biomimicry limited to animals which fly. Another area of biomimicry research inspired by marine animals has been into ‘compliant surfaces’. It has been calculated that around 40% of drag is caused by the turbulent boundary layer – a thin sheet of air just above the aircraft’s skin which creates friction. Marine animals, such as dolphins, are able to suppress this turbulence when swimming through water by continuously rippling their skin. Researchers believe that fitting aircraft with a continuously adapting compliant surface could virtually eliminate skin friction drag.

Top: Animal Dynamics’ 8in Skeeter drone was inspired by the double wings on a dragonfly. Animal Dynamics. Lower: Original designs for Airbus’ Concept aircraft, ‘Bird of Prey’. AirbusWhile a way of replicating a continuously moving surface on an aircraft fuselage has not yet been solved, aircraft engineers have developed ‘riblets’, a series of small grooves on the surface of an aircraft aligned to the direction of the air flow – which are claimed to achieve a 4-7% reduction in skin friction. In 2010, a team at The Fraunhofer Institute in Germany was awarded the 2010 Joseph von Fraunhofer Prize for a paint, modelled on shark skin, incorporating grooves which was tested in 2013 on two Lufthansa A340-300s and has since been commercialised and is being marketed by German laser specialist company 4JET and aircraft paint manufacturer Mankiewicz as the Laser Enhanced Air Flow (LEAF) system.

Researchers have also been learning from the human body. Scientists from the University of Illinois have worked on a USAF-funded project on autonomic materials systems which mimic the body’s processes – including the potential of self-healing polymers which can enable composite aircraft structures to ‘heal’ themselves, if damaged by, releasing resin into cracks. Work is also being carried out on composite materials that can bend with the application of low-voltage charges – in the same way that muscles contract and expand. Such materials have many potential applications, including the operation of actuators in space vehicles.

Such is the interest in the inspirational engineering possibilities offered by biomimicry that the US-based Biomimicry Guild organises field trips to wildlife sites for groups interested in learning from nature. Two workshops in Costa Rica and Peru were attended by engineers from Boeing which inspired new ideas on aircraft seat design and suppressing aircraft noise.