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Biomimicry has been used by human beings for very long to solve their issues using designs from nature. Life on Earth has existed for more than 3 billion years and the process of evolution due to natural selection has given rise to the most simple, robust and economic designs. Using these designs to improve, innovate and design products or implementing them on a molecular level to solve human problems would best describe the term biomimicry. Various examples are showing the use of biomimicry in engineering, healthcare, and architectural applications. A few examples would be: the rib network of Amazon water lily to make the leaf strong without increasing its thickness; using hollow tubular structures mimicking bamboo or even human bones to improve strength; mimicking the beak of a kingfisher to improve the aerodynamics of bullet trains to reduce noise pollution and fuel consumption; using the design of the Stenocara beetle's shell to collect water even in desert conditions; inspiring the fin of a wind turbine from whales' fins. The rate of changes in engineering designs to improvise a product's design and performance is not so high, but biomimicry shows a promise of a rapid increase in this rate. In this essay, we will be looking at the biomimicry of airplanes from birds. Humans have been attempting to fly for a thousand years; some vigorous attempts involve tying themselves to kites and jumping off cliffs, hurting themselves. Airplanes can also be said as one of the earliest examples of biomimicry. Starting from their wings, construction, the principle behind flying, early aviators have been inspired by the well-evolved birds and bats, trying to replicate them to fly.
Though in history there has been a depiction of vehicles capable of air travel from as early as 400 BC in the Indian epics known as the Vimana, a clear idea with engineering details have not been recorded until 1502, in the Codex on the Flight of Birds by Leonardo da Vinci. In this, he has studied the wind design of birds and designed a man-powered aerial vehicle (Volstad & Boks, 2012). Since then there have been various aviators trying to achieve the dream of transportation through flight. The current aircraft would not have been in their current forms and shapes if not for these aviators and their sustained attempts even after devastating failures causing fatal injuries to the pilots (Hendrickson III & Kenneth). To name a few: George Cayley, Jean Marie Le Bris, Otto Lilienthal, Percy Pilcher and a few more. Some early approaches where passenger-carrying gliders, powered by horses followed by controlled gliders, steam-powered aircraft with propellers which were hard to control. In 1903 the Wright brothers made the first sustained and controlled heavier-than-air powered flight (FAI News, 2003). The World War One has a huge role in the expansion of the aviation industry as aircraft were started to be used as lethal weapons. World War Two was also influenced by the use of aircraft, which added strength to the German, Britain, American and the Japanese armies. Bell X-1 (Hallion & Richard, 2014), the jet aircrafts exceeding the speed of sound was first used in 1947. Today the leading aircraft manufacturers are Boeing and Airbus. The various design features of the aircraft are inspired by birds like Eagle or Falcon.
To understand more let’s try to understand how an airplane sets itself into flight. Figure 1 shows a 2-dimensional representation of an aerofoil with the movement of air around it. The jet engine when turned on propels the airplane forward and the forward-moving motion of the airplane results in a strong current of air to flow around the aerofoils or wings of the airplane.
Figure 1: 2 Dimensional representation of aerofoil and air movement around it (Source: https://www.explainthatstuff.com/howplaneswork.html)
During take-off, the inclination of the aerofoil and its aerodynamic shape as shown in Figure 1 splits the air hitting its front end. The split air travels over both the top and bottom of the aerofoil. As shown in Figure 1 the top of the aerofoil is curved and hence has a greater length compared to the bottom of the aerofoil which is considerably flatter. The same amount of air travels over the top and the bottom of the aerofoil. Hence the air traveling over the top must cover a huge surface area and hence increases its volume but with the same amount of air molecules. This, in turn, affects the pressure at the top, making it lower than the pressure at the bottom. This pressure difference is the major cause of lift and can be attributed to the Bernoulli’s theorem. The air leaving the aerofoil is forced towards the bottom due to its shape and is the second reason for the lift force developed. This mechanism is similar to the taking off of an Eagle, where the air moves in a similar manner around its wing (Woodford, 2019). The thickness of the eagle’s wings also varies; decreases as it moves away from the body. This is also mimicked in the airplanes.
The movement of the legs of the eagle while landing is also an example of biomimicry which inspires the movement of the landing gears in the modern airplanes which have retractable landing gears. Figure 2 shows the image of a commercial flight with retractable landing gear at the instance of landing. Figure 3 shows the image of a bald eagle at the instance of landing (Techtainment, 2019).
Figure 2: Image of a commercial flight with retractable landing gear while landing (Source: https://www.youtube.com/watch?v=c5FG-7dJ0NU)
Figure 3: Image of a bald eagle while landing (Source: https://www.wallpapers13.com/bald-eagle-landing-on-a-dry-tree-sky-blue-desktop-wallpapers-mobile-phones-and-computers/)
Apart from the landing gear, the shape of the aircraft is also inspired by the streamlined body of the birds which helps them tackle the huge drag force created due to air resistance. The streamlined shape helps the airflow smoothly over the body. Figure 4 shows the biomimicry of the streamlined body of an eagle in an aircraft. The birds groom their feathers in order to have a smooth flight. In the same way, the flights have polished smooth surfaces which help them avoid air resistance (Benyus, 2002; Lakhtakia & Martín-Palma, 2013)
When migrating in flocks the birds tend to fly in a V-Shape, each row slightly above the prior row. Behind every bird, there is a wingtip vortex, which seems like a little tornado, and at the tip or outermost part of the wing, there is air moving up a little bit. If the following row of birds flies in this sweet spot where the air is moving up the effort required to keep the bird flying is much lesser. The birds tend to interchange their spots in the V-profile so that all the birds would have enough energy during migration over long distances.
Figure 4: Biomimicry of the streamlined body (Source: https://weburbanist.com/wp-content/uploads/2016/04/bird-inspired-b2-falcon.jpg)
The jet aircrafts used for military operations fly together. These aircraft are observed to save 15% energy when they mimic the V-shaped flying profile of the migrating birds. Figure 5 shows the V-shaped profile followed by the birds while migrating. Figure 6 shows the wingtip vortex and the air moving up at the outer end of the wing. While there have not been a lot of scientific studies done on this, an experiment conducted by an aeronautical engineer, Bernardo Malfitano at Seattle’s Museum of Flight proves this phenomenon (Honey, 2016). When flying his flight at a sweet spot just outside the outer wing region of another flight in front of his, he observed an increase in the speed of his flight without himself applying any changes to his flying conditions.
Hollow structures like the bones of birds tend to have high stiffness. This principle is used while building the body of the aircraft, especially the wings of the aircraft. These hollow structures can also be used to carry the fuel that is necessary for the flight. Similar principles have been followed during early architecture. Bricks made of clay were reinforced with straws and were found to have more strength compared to the bricks consisting of 100% clay. The modern age lightweight composite materials that are used in the aviation, automobile and other industries also use the same concept but on a microscopic level. Fibers are reinforced matrix materials, which give a composite material that has better mechanical properties than the fibers and the original matrix materials themselves.
Figure 5: V-Shaped profile of birds while migrating (Source: https://media.mnn.com/assets/images/2016/10/plane-birds.jpg.1000x0_q80_crop-smart.jpg)
Figure 6: Depiction of wingtip vortex and upward moving air (Source: https://www.youtube.com/watch?v=lQEatfYOGCI)
Another scope in the future for improvising the aircraft design and perfecting biomimicry would when the wings of the aircraft could be moved or stretched in the following manner. While an Eagle is in the air it spreads its wings the maximum which helps it glide without a lot of effort. If there is an upward moving air then the eagle can glide at the same altitude without any effort. This is called soaring. This is already mimicked by gliders. When the eagle spots prey and wants to dive in faster, it folds its wings closer to its body making it move much faster. This is mimicked during skydiving and daredevils who dive along cliffs with winged jumpsuits. If this could be implemented in aircraft it could improve fuel consumption and reduce costs. This concept is called wing morphing. Mechanical Engineer Professor Shaker Meguid (Airplanes.com, 2008) is developing morphing techniques to achieve this goal. His first approach is to use shape memory alloy (SMA) which is a material that contracts when heated above a temperature. The next approach is to use piezoelectric materials that contract or expand when an electrical field is applied to it. A combination of these two approaches can also be used to change the shape of the aerofoil which would lower the noise, pollution, and costs. Currently, there has not been a lot of implementation of wing morphing in commercial aircraft; only the flaps and ailerons have been developed to partially achieve the goals of wing morphing.
The aviation industry is a huge consumer of non-renewable sources of energy. In 2013 the monthly passenger-kilometers were recorded to be around 500 Billion, which means 16.5 Billion passenger-kilometers per day. The modern aircraft like Airbus 380 and Boeing 787 uses approximately 3 to 3.5 liters per 100 passenger-kilometers. Considering an average aviation fuel density of 0.81kg/l we arrive at the number 466,000 metric tons of fuel per day. Adding some more as waste, training, cargo, etc. we can expect this number to be between 500,000 to 600,000 metric tons per day (IEA, 2019). Any improvement in design reducing the fuel consumption would save a lot of fuel while considering these big numbers. The aviation industry has almost reached its saturation level in terms of using modern engineering techniques like using lightweight materials for construction, improved aerodynamics, avoiding unnecessary parts, simple designs, etc. Biomimicry has played a huge part in this saturation, but with so many innovative researches taking place, one can never say when a breakthrough would arrive. Biomimicry has huge potential and can affect these numbers if it can be implemented to even the smallest parts in an aircraft. Wing morphing is now being developed for UAVs and maybe in the future can be applied for commercial flights also.
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