Introduction
The term Biomimicry first emerged during the mid-1900’s. Though, the first example of biomimicry known to mankind can be traced back to 4000BC and it is still present today: Silk. Biomimicry is an ancient practice rather, it is the art of recreating nature’s intricate processes and integrating these reproductions into innovations. The importance of biomimicry was not acknowledged when humans first began to practice it, but it rightfully gained tremendous popularity in the late 1990’s and mid-2000’s when engineers became inspired by the multitude of ideas nature had to offer. Today, biomimicry plays such a fundamental role in our lives in several ways that we may not have thought of before. One such example of its importance can be seen in the way the bullet train has been designed – The chief engineer behind the Shinkansen’s design had a simple past time: birdwatching. It was from there that the engineer saw inspiration in a Kingfisher’s beak. This breakthrough would go onto solve the significant noise issues posed by the train at the time, which caused problems for all the inhabitants in the area. Perhaps, one of the most fascinating aspects of biomimicry is the notion that engineering problems are often solved by looking in the least unexpected places. Whilst countless examples of biomimicry exist, however in this essay we shall focus on the use of biomimicry specifically in the aviation sector.
Figure 1 shows the resemblance between the Kingfisher’s beak and the nose of the Shinkansen bullet train. © Scraplabs Blog
What is the issue being faced by the aviation industry today?
The aviation industry has long seen a history of issues related to fuel efficiency, fuel consumption and emissions ever since the very first powered aircraft created by the Wright Brothers flew in 1903. Presently, the issue of greenhouse gas emissions from aircraft has environmentalists and biologists around the world holding the aviation sector accountable for contributing significantly towards climate change. This goes hand in hand with the statement made by the Civil Aviation Authority (CAA) in 2014: greenhouse gases move throughout the atmosphere and so do not respect international boundaries. This means that they are an international issue regardless of where the emissions were released.[6]. Aviation contributes a fair share of air pollution and this brings about changes in air quality. Lack of air quality is now considered a violation to human rights by the United Nations [18] and this issue could worsen uncontrollably considering the demand for aviation is continually increasing. Presently, not only is air quality considered a human rights issue but potentially noise pollution too, which can provoke cases under Article 8 in the list of rights. [7]
Industry Emissions (million tonnes per annum)
- Power generation 150
- Road transport 108
- Industry 105
- Aviation 34
- Waste 22
- Shipping 11
Table 1 shows the distribution of yearly emissions in the UK among the 6 principal sectors in 2012. Source: CAA, Committee on Climate Change
According to the same research paper by the CAA, aviation contributes to almost 6% of total emissions in the UK and aviation emissions have increased two-fold since the 1990’s. However, since the 1990’s the aviation industry has progressively become more energy-efficient, by means of advancements to engine and airframe technology. Although the aviation industry had progressed, it did not progress swiftly enough to tackle the rapidly growing amounts of emissions due to augmentation in air traffic. Given that there was a deficiency of low-carbon substitutes for fuel and renewable energy sources, this placed the aviation industry at a risk of accounting for a greater share in total UK emissions if it did not devise newer, effective methods for reducing emissions.
The culpable greenhouse gas CO2 has been associated with an aircraft’s fuel consumption in such a way that the CO2 emissions released are proportional to the amount of fuel combusted by a certain aircraft – this relationship is supported by these figures: every tonne of aviation fuel burned produces between 3.15 and 3.18 tonnes of CO2. [6] Therefore, it is only logical to reduce fuel consumption and increase fuel efficiency in order to reduce greenhouse emissions. Fuel efficiency (also termed fuel economy), is a concept worth introducing at this stage; fuel efficiency is a quantitative, relative measure of the amount of fuel needed by a particular aircraft in order to carry out a service, such as passenger or freight transport and the distance flown between points in the route.
There are numerous factors contributing to how much CO2 is released by an aircraft, listed below are the principal factors known to impact emissions released directly. Nevertheless, this list is not exhaustive since there may be many other minute factors that can still have an impact on an aircraft’s emission:
The type of aircraft being flown (design and weight of aircraft, payload, quadruple/twin engine)
- Power setting: altitude and airspeed (also known as a Flight Profile)
- Operational methods (Continuous Climb Operation, Continuous Descent Operation (CCO & CDO))
- Navigational route configuration
- Climate during take-off, flight and landing
Although biomimicry may not hold the answer to solving all of these factors contributing to emissions, engineers have still found ways to make biomimicry useful in their designs. We will see in the subsequent section how biomimicry has been integrated into engineer’s designs. We will also revisit these factors at a later stage when considering how the issue of greenhouse emissions was dealt with before engineers turned to biomimicry for answers during the mid-2000’s.
How does Biomimicry solve the issue?
The words of the author of the world-renowned book Biomimicry: Innovation Inspired By Nature befit the general thesis of this section: “encourage biologists and engineers to collaborate, using nature as model and measure” [3].
The most evident use of biomimicry in aircraft is seen in their airframe design; despite the numerous changes and developments made to the shape of the nose for previous and currently existing aircraft models, these changes proved to be insufficient to tackle the ever-increasing emission levels, as mentioned in the previous section (Addressing the issue). Otto Lilienthal, a German civil engineer, conducted one of the first studies of bird wing shapes, around 1890 [10]. For more than a century, aircrafts have continuously undergone refinements to their airframe, using biomimicry as a basis to improve aerodynamics. We will shortly see how advanced biomimicry has contributed to better aerodynamic design.
Figure 2 shows an interesting variation to the shape of the cargo carrier’s nose, resembling that of the Beluga whale. © Reddit
After undergoing numerous challenges to reduce emissions using conventional methods (use of conventional methods prior to biomimicry will be elaborated in the following section), engineers and biologists decided to collaborate and exchange ideas on how to implement nature’s works into new aircraft designs, using more advanced methods of biomimicry. We will now analyze an advancement that stems from the study of the Shortfin Mako Shark’s denticles1 conducted by a team of biologists and engineers at Harvard University and University of South Carolina.
1 Denticle: microscopic, sharp scales found on the shark’s skin
The fact that both sharks and aircrafts are designed to move through their respective fluids is what lead these engineers and biologists to deduce the similarity between aircrafts and sharks.
Upon inspecting an electron microscope image (Figure 4, Photograph A) of the denticles, the team concluded that there were in fact thousands and thousands of these structures present on the shark’s skin and that they also differed in size in different areas of the shark’s body. Hypotheses about the function of these denticles were made and the team questioned whether they could serve for drag reduction purposes, but also possibly providing components of lift by acting as vortex generators underwater. The team then went forward to reproducing these microscopic denticles using CAD, and 3D printing these structures onto the airfoil of an aircraft. The team tested the 3D printed denticles in 20 various arrangements on airfoils and placed these into a water flow tank.
Figure 4 Photo A shows the electron microscope image of the shark’s denticles. Photo B shows the 3D design of a singular denticle. Photo C shows the arrangement process of the denticles onto an airfoil. Photo D shows one of the 20 various arrangements of the denticles on the airfoil. © Harvard University
The results that emerged from this study proved the team’s hypotheses true: They [team] found that in addition to reducing drag, the denticle-shaped structures significantly increased lift, acting as high-powered, low-profile vortex generators. [5] According to the first co-author of the study, August Domel, these bio-inspired vortex generators have attained lift-to-drag improvements of more than 320%.[5] If the lift-to-drag ratio increases, this effectively reduces the amount of fuel consumed by an aircraft, especially during climb. When cross-checking with another source named The Shark’s Paintbrush: How Nature is inspiring Innovation, a concise description of the denticle’s function was found to be parallel with the hypotheses made by the research team: it [shark] gains drag resistance thanks to its cleverly evolved skin, made of tiny vertical scales known as […] dermal denticles [8].
Due to the evidence provided by this conceptual design based off biomimicry, the team are confident that these airfoils with nature-inspired vortex generators are likely to outdo conventional airfoil designs without vortex generators. On the other hand, a similar concept had been developed by Fraunhofer Institute, the difference is that instead of 3D printing denticles onto airfoils, the institute has created a paint modelled on shark skin. The Institute has estimated that up to 4.5 million tonnes of fuel can be saved per annum due to this innovation. [14] These statistics prove to be rather promising for the aviation industry whose constant struggle is to reduce emissions whilst trying to meet the demands of increasing air travel.
How was the issue solved before biomimicry became popular?
In the previous section Addressing the issue (P. 6), the principle factors known to impact emissions directly were listed. In this section, we will focus on the current standard methods used to mitigate the effects of some of these factors on the amount of emissions released.
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Type of aircraft flown
The type of aircraft flown can greatly alter its fuel efficiency/economy, fuel consumption and so the amount of emissions released, and below we will analyze how they affect these aspects of air travel. When mentioning an aircraft type, this can refer to the aircraft’s design, airframe weight, its passenger/freight load capacity and whether it has quadruple or twin-engines. Other attributes can also contribute to the description of an aircraft type but for simplicity, we will focus only on the attributes mentioned in this paragraph.
Winglets are devices that come under the design category for an aircraft. Sometimes, the winglets can be blended in for a gradual change in the wing geometry, creating what is known as a blended wing-body aircraft, which we will look at in the following factors. The purpose of these wingtip devices is to reduce the vortex drag (also known as lift-induced drag) due to vortices formed around the wing tips causing turbulent flow; to increase the lift-to-drag ratio without increasing the overall wingspan since this parameter is limited for all aircrafts. By lowering drag, they can reduce emissions by 6% [11].
Figure 5 shows a diagrammatic comparison between two aircrafts, with and without winglets. © NASA
One of the more evident disadvantages that larger aircrafts have is their heavier airframes in comparison to smaller counterparts. The use of materials science has enabled engineers to discover alternatives to traditional aluminium sheet metal to reduce the overall weight of the aircraft. In 2011, HRL laboratories on behalf of Boeing developed and synthesized a lightweight metal structure named Microlattice; the aerospace company has predicted its use in future airframes: At 99.99 percent air, it's light […] while its structure makes it strong. Strength and record-breaking lightness make it a potential metal for future airplanes and vehicles. [4] As a guideline, it is assumed that a 1% reduction in weight results in 0.75% less fuel consumption. [2] Considering economics in 2018, the global airline industry’s fuel bill can be approximated to be around $180 billion [9], the cost of which can be significantly reduced by decreasing the weight of various parts and components, livery paints using chemical engineering and installations such as the in-flight entertainment system, for example.
Figure 6 displays a bar chart of net profits gained by the global airline industry and the fluctuations in the total industry fuel costs in the last 14 years. © IATA
Fuel efficiency relies on the total weight of passengers/freight (also known as payload) and the distance flown between points in the route. Generally, the greater the number of passengers flown over a long haul, the more fuel efficient an aircraft is said to be. Although one may think that larger aircraft have greater passenger capacity hence better fuel economy, this is not always the case: aircraft with four engines […] tend to be less fuel-efficient than twinjets due to inherent design factors such as a higher wing weight and a smaller engine fan diameter. [14] Therefore, to tackle this issue on a grander scale, airlines are encouraged to increase their passenger capacity for smaller aircrafts and to purchase smaller twin-engine aircrafts instead of larger aircrafts.
Flight Profile
Information about an aircraft’s fuel expenditure can be deduced from its altitude, power setting and airspeed and using this information, pilots can effectively reduce the impact that each of these elements have on the emissions during each stage of the flight. The drag equation becomes relevant in this section; parts of the drag equation by Lord Rayleigh will be used to analyze the relationship between the different variables.
F_(D )= 1/2 ρ u^2 〖 C〗_(D ) A
Where F_D= drag force and F_(D )=skin friction+form drag
ρ=density of the fluid
u=flow velocity of the fluid relative to the object
A= Cross sectional area of interest
C_D=drag coefficent
Altitude, Power Setting and Airspeed
It is known that air density varies inversely with altitude; from the equation above, the drag force is proportional to the density of the fluid, hence the drag force also varies inversely with altitude. This means that an aircraft consumes less fuel to overcome both skin friction and form drag at higher altitudes. Aircrafts fly within a limited altitude between the troposphere and the stratosphere and the ozone layer is found in between these two atmospheric layers; the higher the altitude, greater the greenhouse effects: emissions of NOx [nitrogen oxide] at high altitudes result in greater concentrations of ozone than ground-level emissions. [12] The greenhouse effects of flying at high altitudes are inevitable, nevertheless, the principle motive is to reduce fuel consumption, and one of the methods to achieve that is flying at higher altitudes.
The altitude and airspeed depend on the power setting of the aircraft. To elaborate, the power setting determines whether an aircraft is climbing, in level-flight or in descent. When an aircraft is in climb, fuel consumption is increased because the maximum cruise thrust setting is used to climb, therefore gradual increments in climb are used to save fuel. [15] This concept can also apply when an aircraft is in descent. When the power setting is increased whilst the aircraft is in level flight, the cruise speed increases since the engine thrust needed to accelerate forward and overcome the aerodynamic drag has increased. This increases fuel consumption and so pilots are advised to adopt a cruise speed which is optimal for the particular aircraft for the majority of the flight journey.
Operational methods - Continuous Climb Operation, Continuous Descent Operation (CCO & CDO)
Following on from the previous paragraph on power settings, the fuel consumption of an aircraft is affected by which power setting is used. Air Traffic Control (ATC) responsible for guiding pilots to maneuver the aircraft in ways that will optimize fuel efficiency and generate financial advantages with regards to fuel consumption [11]. The Continuous Climb/Descent Operations follows the concept proposed in the previous paragraph concerning gradual increments in climb. Abrupt changes in altitude such as those that occur in step-down and step-up operations usually increase the fuel consumption, this is once again associated with the power setting. Therefore, CDO and CCO operations are used by ATC in preference to the traditional step-operations to minimize fuel consumption.
Figure 7 illustrates the difference between typical step and CDO/CCO operation trajectories. © Toratani, Daichi. (2016) Study on Simultaneous Optimization Method for Trajectory and Sequence of Air Traffic Management.What are the prerequisites for innovations based off biomimicry?
In the introduction, biomimicry was described as the art of recreating nature’s intricate processes and implementing them into innovations. In order to recreate nature’s processes, close, in-depth study of the relevant natural processes is required. This can mean that a team of engineers and biologists spend anywhere from a few months to years investigating a natural phenomenon. Primarily in this section, we will focus on some futuristic concepts based off biomimicry and the changes that will be needed in order to realize these innovations.
Apart from imitating shark denticles to improve aerodynamic performance, aviation engineers have found another inspiration from the sea: fish slime. Some species of fish are known to possess a slime-like substance near the surface of their bodies, the slime consists of long chain polymers which play a role in reducing drag. [14] Although the engineers behind this concept do not propose to design an aircraft that discharges such a substance, they expect to create ciliated surface, which could potentially develop into a solution based off the fish slime concept.
Reiterating the fact that biomimicry requires close study of nature, this can prove to be a particularly challenging procedure for engineers because reproducing life-like conditions can be an arduous process.
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