Sustainable MarketsSustainability of aviation

Authors:Thalina PrinsReinder ReindersFolkert Nobels

Introduction

The attraction of aviationIn the last century, the aviation industry has grown exponentially. Since 1960, passenger traffic has increased with 9 percent per year. [1] In the same period, freight traffic has increased a lot as well. This increase in demands induced a rapid growth of aviation industry. Aviation has become one of the mean ways oftransport and the industry is expected to keep growing even larger.The high use of aviation is partly caused by nowadays relatively low prices for flights. Budget companies like Ryanair or Easyjet offer flights for a few bucks within Europe. This way, using other means of transportation like trains is becoming more and more expense in comparison to flying to your destination. One of the most important advantages of aviation is the high speed of this transportation method. Because of this rapidity, travelling by aircraft is usually favourable compared to other means of transportation, even if those other options are less expensive. This way, aviation will remain popular as long as it stays affordable. As there are still enough fossil energy sources nowadays, the prices can remain low until these sources start to diminish. The use of high-speed transportation like aircrafts increases when the gross domestic product of a developing country increases. Nowadays more and more countries are prospering and their welfare increases, which will lead to a higheruse of aviation. Moreover, human networks are growing larger and larger. With the development of fast communication methods like the internet, it has become very easy to communicate and keep in touch with people on the other side of the world. Meanwhile, taking an airplane to visit people or go on vacation is also becoming more attractive as aviation prices decrease. Through these vast networks, people have a high influence on each other. When everybody in your environment makes high use of aviation for holiday or business purposes, you are also more inclined to show this kind of behaviour. More and more countries are developing and their populations are expanding their networks. This way, commercial aviation will continue to grow in the coming decades (see image 1).

Image 1: Worldwide annual air passenger traffic and forecast [1]

Unsustainability Though it is one of the major means of transport, aviation is far from sustainable yet. Airplanes might even be one of the least sustainable means of transport on this planet. More sustainable airplanes have been developed, like the Boeing 787, which uses 20 percent less fuel than any other airplane of its size.[2,11] Aircrafts like this one may be more sustainable and use less fuel than fifty years ago, but with the increased use of aviation, they still cause large adverse effects for climate. These consequences willonly become larger as theaviation industry continues toexpand. Nowadays, kerosene is one ofthe main fuel sources forairplanes. Tons of this energysource are being used per flight.This causes a tremendously highemission of carbon dioxide. Thecarbon dioxide emission causedby transportation is much largerthan the emission caused byindustry (see image 2). The largeamounts of carbon dioxide,which accumulate in theatmosphere, cause problems forthe environment. Thegreenhouse effect causes anincrease of the mean temperature of the earth. This results in major consequences like the rise of the sea level and the melting of ice. Many ecological systems might be the victim of these effects, with loss of biodiversityas a severe consequence. For people, too, the greenhouse effect carries tremendous problems, for example by causing drought and hereby decreasing fertile soil to provide necessary food resources. Moreover, tropical diseases like malaria can spread to larger areas when the temperature in these areas rises. This could cause large epidemics, because these diseases might spread to places where the population is more vulnerable to the disease.Furthermore, nuisance produced by airplanes can cause problems for environment as well. Taking this into consideration, it is of very high importance to reduce the carbon dioxide emission of aviation. If this does not happen soon, the consequences might be severe. A sustainable aircraft or aviation method has tobe produced in order to prevent these consequences from happening.

Image 2: Carbon emissions of transportation and other sources [2]

How does an airplane fly?Who has never flown with an airplane today? Millions of passengers fly every day, to vacation destinies, for business, and for many other purposes. Nowadays it is normal to fly with an airplane, but how does an airplane work from a physical perspective?

The basic principle why airplanes fly is the fact that the wings of an airplane generate lift. This lift produces an upwards force which is greater than the gravitational force from the earth. Because lift is larger than the gravitational force, an airplane can fly. But to generate a large enough lift, an airplane needsto have a speed which can generate this lift. This speed is obtained by propellers or jet engines on the wings of the airplane. In the next section the aerodynamic force, propellers and jet engines will be further explained.[3]

Aerodynamic forceThe aerodynamic force is the net force which isgenerated on a wing. This aerodynamic force canbe divided in 2 components, lift force and dragforce. As is shown in figure 1, the lift force isperpendicular to the direction propagationdirection and the drag force is parallel to thedirection of propagation. The lift and drag forcecan be calculated with equations 1 and 2. Inequations 1 and 2, ρ is the density, v the velocity,S the surface area and

Object 2and CL are the lift

and drag coefficient[4].

FL=12

ρ v ² SCL (1)

FD=12

ρ v ² SCD (2)

Immediately can be seen from equation 1 and 2, that there is symmetry between the lift and drag force. This symmetry is the fact that both the lift force and the drag force have the same dependence on the density, velocity and surface area. This means that changing one of these quantities will not change the ratio of the lift and drag force. The only parts of these equations which can change the ratio are the lift and drag coefficients. So on which physical quantities do these coefficients depend?

The lift coefficientThe lift coefficient is mainlydependent from 5 physicalquantities. The 2 quantities whichcan be manipulated by a pilot arethe angle of attack ( α as shown infigure 1) and camber of the wings.The quantity which themanufacturer can change is theaspect ratio (this depends on theshape of the wing). The remaining

Image 3: The aerodynamic forces[4]

Image 4: Graph of the lift coefficient[5]

quantities depend on the environment, and are some difficult physical properties of the gas called viscosity and the compressibility of the gas [4].

Because the lift coefficient is a very complicated quantity, it is determined empirical as a dependence on the angle of attack. This is done because the lift coefficient depends mostly on the angle of attack. When this is done a graph like figure 2 is obtained[4,5].

The drag coefficientLike the lift coefficient, the drag coefficient isdetermined empirical because it is such acomplicated coefficient. The drag force of anairplane will never be zero if it moves because thereis always resistance. The drag force can be dividedin 2 kinds of drag force, the drag force generated bythe lift and the drag force which is not generated bylift. Drag generated by lift is highly dependent onthe resistance of the wing and the shape of thewing. Wings which are more aerodynamic have alower drag. Drag which is not generated by lift has amajor impact at the end of the wings, where itgenerates vortices. Because the most impact on thedrag coefficient is due to the angle of attack, a plotlike 2 can be determined for the drag coefficientshown in figure 3.[4]

It can be seen from figure 2 and 3 that at some point there needs to be an optimal ratio for the lift coefficient and the drag coefficient. This is shown in Image 6[4].

Image 5: Graph of the drag coefficient [4]

Image 6: Ratio of the drag and lift coefficient[4]

PropellersPropellers are used a lot when airspeed is needed for an airplane. Propellers have the advantage that it is possible to make propellers which work on electricity. A disadvantage of proppellers is that the propellers need a minimumair density to work properly. So airplanes cannot fly at high altitude[3].

Jet EnginesAn alternative for propellers are jet engines. Jet engines work properly at high altitude, because jet engines compress the air to a higher density. A disadvantage of jet engines is the fact that jet engines cannot work on electricity[3].

Innovations in airplane technologyIn the next part, several ways to innovate airplane technology will be discussed.

Sustainable fuel sourcesIn the future, airplanes with hydrogen fuelled tanks can be made. Hydrogen has many advantages compared with ordinary kerosene. But there are still challenges in realizing this technology. The advantage of using hydrogen lies in the reduction of carbon emissions. Moreover, the use of pure hydrogen technology does not have cause any emission at all. Another advantage is the lower weight of hydrogen per energy unit, which means that a hydrogen powered airplane is lighter than conventional airplanes, which reduces the amount of fuel needed. The hydrogen technology also works well with jet engine technology. Challenges that need to be solved, however, are the fact that hydrogen tanks are 4 times as large as conventional fuel technology and the fact that the tanks of hydrogen technology are more complicated and heavier than conventional fuel tanks[6].Another innovation which focuses on the fuel technology is the use of bio jet fuel in airplanes. This technology is promising but brings challenges and obstacles as well. The big advantage is the reduction of carbon emission but the challenges lie in producing enough plants to make enough bio jet fuel. An obstacle in realizing this is the ethical perspective on food production. If peopleare starving, is it allowed to produce bio jet fuel from plants which also can be eaten? From an ethical point of view we would have to follow the precautionaryprinciple, which means bio fuel in general would not be allowed[6].

Using the body for liftInstead of wings being the only part of the airplane that generate lift, it is also possible to let the body of the airplane generate lift. This can be done by changing the shape of the body of the airplane to an aerodynamic lift generating body. In the past it has been proven by NASA that lifting bodies are possible and that this technique can be used at high speeds. If this technology is applied to passenger airplanes this can reduce the total amount of fuel to gain the same amount of lift, thus this reduces the total amount of emission of an airplane[7].

Reduction of resistanceAs discussed earlier, the drag force depends on the resistance in the medium. In other words, higher resistance for air means more drag force. Thus the drag force can be lowered by reducing the resistance of the complete airplane. How can this be done? This can be done with the help of a different paint than is used nowadays. This paint is smoother at the micro scale, and thus reduces thedrag. This reduces the total carbon emission. Moreover this new paint is also better for the environment compared to the old paint[8].

Formation flightAirplanes which fly in formation are seen a lot in military use of airplanes, but this also can be performed with passenger airplanes. When airplanes fly in formation, the drag force is greatly reduced. This means that when airplanes flyin formation 25% less fuel is used[6].

Reducing vorticesToday a lot of airplanes produce vortices when they fly. Whenvortices are made, power is spoiled to produce these. So howcan we reduce vortices? Vortices can be reduced byintroducing a wing with a wingtip. When wingtips are used,this reduces fuel consumption and thus carbon emission.Other advantages of wingtips are the increase of safety whenplanes fly in formation. Without wingtips flying in formationcan be dangerous and unpredictable[9].

Image 7: Airplanes flying in formation[6]

Image 8: Vortices of wings with and without wingtips[9]

Use of nano materialsIn the past decade, there has been a tremendous amount of new developmentsin nanoscience. In the future, these materials with special properties can be used in airplane industry. These nano materials are designed in such a way, that they have the optimal properties compared to other materials. Nano materials like carbon nanotubes are very strong compared to the composites used nowadays in airplane industry. This means that in the future airplanes canbe made of nano materials. This way, airplanes become lighter but retain their strength (or their strength even increases).

Different distribution of luggageNowadays, the luggage in airplanes is distributed at random. Because of this, the mass of the luggage is hardly ever distributed equally. This means that a random distribution of luggage causes the centre of mass of the airplane to be off centred. Because the centre of mass is off centred, an important symmetry is broken in the airplane. The symmetry which is broken implies that the part ofthe airplane which is heavier needs to be correct for this off balance situation. The only way to do this, is by correcting, using jet engines or propellers. In other words, this means that an airplane needs more fuel if luggage is distributed unequally. To reduce this problem luggage can be distributed equally, which means fuel can be saved.

Optimizing of routesNormally airplanes fly from A to B in a straight line. This seems to be the best way, but simplicity deludes. In reality, the atmosphere is a very complicated system. The atmosphere has what is called wind zones. In these zones, there isa certain wind direction. To use the route with the lowest fuel consumption, a complicated optimizing problem needs to be solved. So in some cases it might be better to fly ten kilometres to the east before going to the north, for example. Once the optimal situation is attained, percentages of the fuel can be saved.

Solar powered aviationOne of the largest opportunities for making aviation more sustainable seems tolie in solar powered aircrafts. An aircraft flying solely on solar power has already been developed.[4] With 11000 energy-gathering solar cells, this

Image 9: Carbon nanotube [10]

airplane is even able to fly during the night by storing energy in batteries. However, solar powered aircrafts are still a long way from being useful for commercial aviation. The planes which have been developed so far are very slow and cannot carry much weight yet besides the pilot’s, let alone the weightof many passengers and a lot of luggage. The technology, however, is promising and solar power could be of major impact in establishing a sustainable aircraft.

Space travelAnother innovative idea would be to use spaceships or planes for commercial aviation. In space, less energy would have to be used because of lower air resistance. However, this technology is far from developed yet, as launching a spaceship into space usually costs a lot of energy. Large innovations will have to be made in order to make this technology sustainable and successful.

Gliding flightGliding planes or sailplanes are mainly used for recreational purposes. However, this technology might be of use for establishing a sustainable airplane. Most gliders do not have an engine, so after launch, they do not need energy sources like fossil energy, which makes these planes far more sustainable than present commercial aircrafts. Using gliders in commercial aviation might therefore reduce energy consumption vastly. However, gliders have some disadvantages that complicate use for passenger transportation. Carrying large amounts of passengers and luggage might make such a plane too heavy to fly, and even if this problem can be overcome, flying large distances could be difficult. However, using small glider planes on short distances might be a good start in initiating sustainable aviation.

Alternatives for transporting luggageTo reduce weight of airplanes, for example to make commercial glider planes orsolar planes possible, baggage could be transported in different ways, for example by train. However, this might cause severe logistical problems when large distances have to be covered and both passengers and luggage have to arrive at the same time at the destination.

Image 10: Solar powered aircraft

Alternatives for aviationIn addition to making aviation itself more sustainable, other sustainable ways of transportation can be developed. An example would be to make a tube through the earth, to travel rapidly and sustainably from one place to another by using gravitation. However, technology is far from advanced enough to establish this kind of transportation system, but in the future these kinds of innovations might be possible. As long as aviation is not sustainable, encouraging passengers to use more sustainable ways of transportation, like trains, might also be beneficial. This could for example be carried out by making unsustainable aviation more expensive and travelling by train less expensive.

InnovationWe have given some examples of innovations in the branch. But in the end, it will never be sure where the innovation in the airplane business will start. Hence we must be prepared for innovations when they come and where they come.

When innovations are being developed and being brought on the market, the most problems they usually have is with law. There are a lot of rules and regulations which they have to deal with, from a lot of different parties. In Europe alone there is the European Union, the countries and several government agencies, who all issue rules independent from each other. And then there are local parties like municipalities, local inhabitants and pressure groups, who all enforce conditions. Think for example about a plane in the formof one big wing. There are countless problems to think of, which it will encounter when trying to get it 'in the air' from Dutch soil. It will probably never even meet the current requirement.

As an example, we take the newly expanded airport of Groningen-Eelde. There is a union of residents, the VOLE. This union has the right to defend the residents in legal procedures. Further growth can very well be limited because of this union’s actions. Usually these pressure groups enforce a limit on the noise nuisance. If a big innovation is made, which makes a plane (more) sustainable, but increases the sound it makes, it could very well be forbidden toland on Eelde-Groningen. The interests of flight companies and residents are almost constantly in conflict, and of course the problems at busier airports like Schiphol are much bigger.

Short said: 'rules limit innovation.' And it's a depressing thought that a big innovation could be about to happen, but probably won't be able to meet the requirements and hence will not be able to actually fly commercially. Roughly there are two types of rules. The ones that secure the rights of people, and the ones that deal technical procedures, like security, mandatory devices and safety rules. The first category is unchangeable. Around airports there is alwaysconflict of interests. Changing this is in the advantage of airplanes is a political issue, and doesn't solve problems but only moves them. The second category is changeable, in the benefit of innovative airplanes.

The simplest change would be to cancel a lot of rules regarding the way of building planes, the shape and mandatory materials. If governments declare those rules to be not applicable to innovative planes, it would become more attractive to develop such a plane. Deregulation, specifically for innovative airplanes, is an easy way to promote innovation.

However, there is a problem which occurs when rules would simply be removed. And that is the ones regarding safety. Deregulating safety regulationsisn't an option due to the fact that no one would travel with a plane of which safety they aren't sure. Also it isn't a good start for an innovative plane, it would simply never come off the ground, figuratively speaking. You cannot tearon safety regulations.

Then, there is the problem which concerns the fact that there are a lot of different countries, which all issue different rules. These different rules are confusing for innovating companies. It is also ineffective if only one country, especially small countries like the Netherlands, issues rules which stimulate innovation. Companies would simply not innovate, because they aren't forced worldwide. It is important to have rules which apply to the whole of Europe, or even more.

Still, a lot of gain can be added by deregulation. In the end, it could be easier, and maybe even easier than with conventional planes to develop planes and enter the market with them. In theory, simply changing regulations is a costlessway of stimulating innovation. And as stated before, we never know from whichcorner innovation will come. Therefore it is important for the legal system to beas prepared as possible.

Conclusion Altogether, making aviation sustainable is of great importance to avoid environmental consequences of carbon dioxide emission. However, realising a sustainable aircraft seems to be a tough problem. Aircrafts have to fulfil a lot ofrequirements in order to be able to fly. First of all, the physics have to be right. Without the right drag and lift force, an airplane would not be able to fly. Furthermore, several legislation requirements have to be attained, mainly concerning safety measures. Safety for passengers is of great importance, as accidents with airplanes are usually fatal. Aspects like noise are also important for this matter, because for example ear damage would also be an unwanted effect of aviation. Travelling by plane should also not take too much time, because this could cause unpleasant effects, so the airplane should be fast. When a passenger has to sit in one place for a very long time, thrombosis or blood clotting can occur in the legs, which can cause fatal lung embolism or even a heart attack. These varying kinds of issues make it hard to develop a aircraft useable for commercial aviation.Many innovative ideas have been developed so far, but they all have disadvantages that complicate use in commercial aviation. Some of the innovations, however, can be used already, like using paint with lower resistance, distributing luggage more equally, or reducing vortices. By changing these relatively small parts, airplanes already use lower amounts of fuel. Other, more revolutionary ideas, like using solar powered aircrafts, are notyet developed enough to use in commercial aviation. However, with the right stimulation these new technologies might develop rapidly.

We can, however, also approach sustainable aviation on a more positive note. Airplanes are amazing works of technology, able to transport many people overvast distances within a short time. If we can make a sustainable aircraft, this mean of transportation would become even more attractive. In order to make aviation sustainable, large changes have to be carried trough. Of the innovations that have already been done, with further development one, or perhaps a combination of several, might be the right innovation to make aviation sustainable. On the other hand, large innovation sometimes comes from an unexpected quarter.

References

[1]Paul Upham et al.. Towards Sustainable Aviation. Earthscan; 2003 [2]http://aa.stanford.edu/events/50thAnniversary/media/Kroo.pdf (consulted on 21-7-2014)[3] L.A. Bloomfield. How things work. Wiley, 4th edition 2010.[4] Navial Aviation schools command. Fundamentals of aerodynamics. 2008[5] NASA images. June 2014[6] I. Kroo. Sustainable Aviation: future air transportation and the environment. May 2008[7] NASA. Lifting bodies. June 2014[8] Clean Sky. Grooved paint surface for aerodynamic drag reduction. June 2014[9] L. Neal, N. Harrison, D. Mujezinovic. Wingtip device. March 2004[10] Carbon nanotubes. June 2014. phys.org[11]http://www.etatrust.org.uk/2013/04/are-solar-powered-planes-the-future-of-sustainable-air-travel/ (consulted on 21-7-2014)[12] http://www.boeing.com/boeing/commercial/787family/787-8prod.page (consulted on 21-7-2014)