The animal kingdom as a template for aerodynamics in industry
When technology learns from nature, it is referred to as bionics. The methods by which lotus flowers repel water, geckos’ feet offer extraordinary adhesion, or dragonflies manage to fly through the air are some of the best examples of innovations that humanity has copied from the animal kingdom. In the field of aerodynamics, engineers are now working to use nature in their quest to optimise the behaviour of aircraft, vehicles and other objects as air flows over them.
Aerodynamics and how wind resistance is measured
The term “aerodynamics” is widely understood in the context of developing racing cars that cut smoothly through the air, aircraft wings that offer low wind resistance, or particularly energy-efficient trains. But what exactly is aerodynamics and what does it have to do with the often-cited Cd value?
The word “aerodynamics” originates in the Greek language and combines the words for air and force. As a discipline within the field of physics, aerodynamics describes the behaviour of currents in gases, which means that it has a significant impact on many everyday activities. One example should help to make the effect of aerodynamics clearer: if you hold your hand beneath a running tap, first vertically and then horizontally, you feel the different forces that have an effect on the hand in different ways, depending on the angle of attack. The same thought process can be applied to the way that air currents flow around bodies: resistance forces on a body in motion substantially depend from the shape and the nature of the body’s surface.
The Rumpler Tropfenwagen achieved a drag coefficient of 0.28 way back in 1921 – which is still considered good today
The so-called coefficient of drag was developed to provide a quantitative measurement of the aerodynamic efficiency of bodies in forward motion. This value, also known as Cd and which does not have an associated unit of measurement, is a way of measuring the drag on an object as air flows around it. It is expressed as a quotient of resistance forces and is force produced by the dynamic pressure times the area. While this may sound complicated, it can be explained in very simple terms: the lower the Cd of a body, the better its aerodynamic qualities are. While a long, right-angled surface could achieve a highly un-aerodynamic Cd of 2.0, a droplet that tapers in line with the airflow has the perfect shape, with a Cd of 0.02. As a comparison: modern cars generally achieve a Cd value of between 0.25 and 0.40. However, as far back as 1921, Edmund Rumpler had the idea of shaping the bodywork of a car to resemble a falling droplet of water, as a result of which the Rumpler Tropfenwagen managed to achieve a Cd value of 0.28, which is still considered good by contemporary standards.
In industrial settings, the Cd value is generally calculated using special wind tunnels. From normal cars for use on the streets, through to Formula 1 racing cars, or aircraft wings, engineers are always highly interested in ensuring that air resistance is as low as possible. The costly, time-intensive measures to optimise aerodynamics have paid off, with cars consuming less fuel and emitting less CO2, while racing cars achieve higher top speeds and aircraft glide more silently through the air. Above all, against a backdrop of ambitious energy efficiency goals, it quickly becomes clear how much aerodynamics has to contribute to environmental protection.
Practical applications of aerodynamics
Industrial aerodynamics is particularly closely associated with automotive design. That is hardly surprising, as hardly any sectors have to deal with such stringent requirements for greater energy efficiency in the future as the automotive industry. The politically-set goal to reduce the specific emissions values of vehicle fleets to 95g/km by 2020 demands everything that engineers have to offer in terms aerodynamics, lightweight construction, engine development, and tyre development. Success in this area justifies the high levels of cost and effort involved: despite the trend for greater comfort, larger engines and a wider track, the automotive industry has continually succeeded in reducing the drag of its products over the last few years. Nevertheless, experts believe that further aerodynamic progress for mass production vehicles will only be achieved in very small steps in future.
As an aside, the drag coefficient of Formula 1 racing cars is a surprisingly poor 1.2. The reason for this is that extreme cornering speeds are only possible with extreme downforce, which goes hand in hand with high levels of drag.
Many vehicle characteristics are tested in motor sports and feed through to ordinary road vehicles: its wings and its body shape help to generate downforce and better control when cornering
Unlike Formula 1 racing teams, aircraft manufacturers do not aim to maximise speed. Instead, they aim to reduce fuel consumption and noise emissions while optimising aircraft’s flight dynamics. Scientists and engineers therefore use modern computer simulations and enormous wind tunnels to perfect the aerodynamics of aircraft.
Aerodynamic applications are, in fact, in no way limited to the automotive industry, aviation, and space sectors – and this is demonstrated by examples from light and heavy mechanical engineering, such as air compressors or aerodynamic bearings.
What can we learn from the animal kingdom?
The past has shown that the animal kingdom offers a vast pool of technologies that have inspired people and led to innovation. In the field of aerodynamics, we have the most to learn from animals that spend their time in the air or in the water.
A particularly striking example can be shown with a look at marine biology: with a Cd value of 0.06, the boxfish – although it looks like a smooth-edged box – is actually much more slippery than a Porsche. Mercedes-Benz has recognised the potential of the boxfish and, using it as a template, has built a Bionic Car with a highly advanced drag coefficient of 0.19. This vehicle, which was presented in Washington in 2005, consumes fuel at a rate of 4.3l/100 km (65.6 mpg imperial) on the EU measurement cycle according to its manufacturer’s figures, as well as achieving a top speed of 190 km/h.
The Mercedes Benz Bionic Car apes nature by copying the boxfish
During its study, Mercedes-Benz originally had a completely different animal in mind: the penguin. As its body shape is not, however, a suitable inspiration for a car body, it is now used by the aerospace industry as a model of efficient aerodynamics. The model of an aircraft with a laminar fuselage, designed by Heinrich Hertel in the 1960s, is based on the aerodynamically efficient shape of a penguin. The “penguin plane” has not yet been produced, due to the high costs involved; however, it serves as an impressive demonstration of the opportunities represented by optimised aerodynamics. Other animals have been more successful in regard; for example, the stork serves as a model for optimising air currents as they flow over aircraft wings, sharks’ unique skin offers impressive characteristics, while owls are particularly silent in flight.
The fact that measures to optimise aerodynamic performance do not always sit easily with a visually appealing design is shown by the example of the duck-billed shape of high-speed trains. This bionically optimised design does, nevertheless, enable extremely low air resistance at high speeds.
The 700-series Shinkansen is a high-speed train with a design inspired by a duck’s bill
Nevertheless, it is often the case that nature fails to strike the optimal balance between of technical efficiency. As such, researchers do not always view birds as nature’s greatest teachers. Instead, the focus should be on drawing inspiration from nature and applying this inspiration to the problems that humanity faces.
Aerodynamics in industry – a view of the future
The intense effort that goes into optimising wind resistance in the automotive industry demonstrates the enormous importance of aerodynamics. In particular, legislative requirements set as a result of energy policy force manufacturers to develop their vehicle fleets to be as aerodynamic and as fuel-efficient as possible. Nevertheless, expert opinion suggests that development of production cars is slowly approaching its optimal limits, and further improvements will soon only be possible by significantly reducing practicality – as changes to achieve a teardrop shape will reduce interior space. Nevertheless, automotive manufacturers continue to invest a great deal of time to squeeze out the last percentage point of aerodynamic potential, because every last gram of CO2 emitted per kilometre driven makes a difference to the environment.
ARTS is fascinated by the potential that the animal kingdom has to offer our pursuit of technological excellence. Our experts are inspired by bionics every single day, and work to make high-tech industries more efficient, more sustainable, and better overall. As a technology-oriented business, we actively work with industry to build the future and give our customers a competitive edge with our expertise and experience. Consequently, we are constantly seeking highly motivated experts who share our goals and who are as enthusiastic about bionics as we are.