Safety first in air travel

“From the point of view of technology, we have three parameters to work with: the propulsion, the form and design of the aeroplane (aerodynamics), and structural optimisation (by using, for example, new lightweight materials). In addition to this, we have parameters associated with the way in which the plane is operated: its altitude, speed, climb pattern, descent and approach profiles, etc.” The words are from Ingo Staack, aviation reseacher at the Division of Fluid and Mechatronic Systems, Linköping University.

Between the 1990s and today, technical research and development has led to a reduction in fuel consumption for global aviation of 2.6% per year. In 2016, this corresponded to a saving of more than 20 million tonnes of carbon dioxide, according to an opinion piece published in the Swedish national daily Dagens Nyheter signed by five Swedish aviation researchers, one of them Professor Petter Krus from LiU.

Increase in air travel

However, improvements from technology are more than counterbalanced by an increase in the amount of global air travel. The number or air passengers is rapidly increasing, and aeroplane manufacturers Boeing and Airbus expect an increase of 4-5% per year, between now and 2035. Travel in the Middle East accounts for the greater part of this, but flying is also increasing in Sweden, not least as a result of the entry of no-frills airlines into the market.

Technical development, however, continues unabated. At EU level, for example, the CleanSky programme, part of Horizon 2020, has a budget of EUR 4 billion and participants from 24 countries. The most recent innovation, in which Sweden and Saab have played an active role, is an improved design of the wing. By using laminar air flow around the wing profile, friction from the wing can be reduced by 50% and carbon dioxide emissions by 5%.

Photo credit Anna NilsenAircraft engine technology is also undergoing rapid development.
“We use a parameter known as the ‘bypass ratio’, BPR. A higher BPR results in a higher (outer) propulsion efficiency, but at the same time the engines become more sensitive. Current BPRs for passenger planes are up to 12”, says Ingo Staack.

This means that for each kilogramme of air that passes through the engine, 12 kilogrammes are accelerated by the fan in the front of the engine. The higher the BPR, the lower the fuel consumption, and the lower the noise level. On the other hand side, the higher the BPR, the larger and heavier the engine becomes.

New generation of engines

A new generation of engines with higher BPRs and thus lower fuel consumption has been developed. These engines also have higher internal temperatures and pressures, which increases the thermodynamic efficiency. This in turn, however, places higher demands on the materials used – not least for the turbine blades, which are subject to high stress. Materials development for turbines is another area in which both LiU and the Chalmers University of Technology have excellent research environments.

“We have achieved a 15-20% reduction in fuel consumption in recent years, simply by exchanging the engines of old aircraft. All technological change is subject to extremely high safety requirements: the new engine technology, for example, has undergone thousands of hours of testing before being certified”, says Ingo Staack.

Another possibility is to use laminar air flow in the most efficient manner, not just around the wings. Such research, in a field known as “active flow control”, is possible due to advanced simulations in supercomputers and wind-tunnel testing.

“If you look at the engine used in the B787, you will see that it is full of small holes in the part that surrounds the engine inlet. Air is drawn into these small holes and contributes to a laminar flow that also gives lower fuel consumption. We will see this technology in the future applied also on the tail section and wings”, he says.

Complex interactions

An aeroplane has essentially a single source of power – its engines. But power is needed not only for propulsion, but also in form of electricity for the cabin, kitchen, cockpit instruments, lighting, ventilation, the onboard maintenance systems, etc.

In Boeing’s most recently developed aeroplane, the B-787 Dreamliner, some of the pneumatic systems (driven by pressurised air) have been exchanged for systems where electricity is taken directly from the engine. This is not only in order to test the long-term viability of the technology, but also to give higher efficiency and simpler maintenance.

“The electrical systems turned out to be expensive, and similar electrical systems have not been built after the Dreamliner. Airlines, however, appreciate the system since it increases the comfort and safety of the passengers”, Ingo Staack concludes.

Photo credit Anna NilsenAnother idea that the researchers are working with is to build aircraft with a flexible wing shape, known as “morping” or “smart structure”. Aeroplanes have had pretty much the same appearance for many years, and this is a compromise between the optimal design for take-off and landing and the form that is most efficient when the plane is at its cruising altitude. If the plane could change its configuration, re-form itself, while in the air, this could give higher efficiency.

Electrical aircraft – not yet

Aviation researchers agree that aeroplanes powered solely by electricity are far in the future, apart from relatively small planes used over short distances. Battery technology that can cope with flights longer than around an hour is not available yet. Doubts have also been raised about safety and charging infrastructure.

In contrast, biofuel may be a short/mid-term alternative that can reduce carbon dioxide emissions.

“From a purely technical point of view, it’s not a problem to power the current generation of engines with biofuels. There are, however, considerations of price, availability and certification. Fossil fuels are much cheaper”, says Ingo Staack.
The economic association FlyGreenFund last year delivered approximately 500 tonnes of biofuel-based aviation fuel to Swedish airports, financed by donations. This corresponds to 0.05% of aviation fuel consumption in Sweden. Furthermore, not only price but also availability are limiting factors with respect to biofuels. (More information...)

More than just technology

Ingo Staack also gives examples of concrete measures that would reduce air travel fuel consumption: one is to introduce speed limits for air travel, while another is to fly in
fMigrating birds. Photo credit Anagrammormations as migrating birds do. A third is to use aircraft of lower range. While it is true that such planes cannot fly as far on a single refueling, they have lower fuel consumption since it is not necessary to carry fuel for the final part of the journey. Refuelling in midair is another solution of the same problem – a technology that is state-of-the-art for military applications.

All measures, however, involve increased costs that someone must be prepared to pay, and the measures will require international agreement.

SARC

Swedish aeronautic research is a world-leader in several fields, and has now been collected in the Swedish Aeronautics Research Center, SARC, hosted by LiU. It is intended that the resources now gathered within SARC will help to stake out the future of Swedish aeronautic research, in unison with the Swedish aircraft industry and international research.

LiU researchers offer expertise not only within materials science, but also within issues that are critical for safety and studies of systems of systems (SoS).

“An aeroplane contains dozens, maybe hundreds of subsystems that all must work together. Furthermore, the aeroplane itself is part of a larger ever-changing eco system that includes airport infrastructure, air-traffic control, fleet and route management, investors, certification, political decisions and acceptance by society (e.g. noise).

If one of them is changed or exchanged, or if a modification is made to one of them, it influences many others. This is one of the areas where our expertise comes in: we work with different scenarios and analyse which systems work together and which ones don’t”, says Ingo Staack.

It is impossible to introduce measures to make air travel more environmentally sensitive if they place safety at risk.

“Safety has the highest priority in aeronautics research”, Ingo Staack emphasises.

SARC – the Swedish Aeronautic Research Center

Innovair, the research investment body of Vinnova and the Swedish Air Force, is financier of SARC, and operator of the National Aeronautics Research Programme, NFFP. This is now in its seventh stage, covering the period 2017-2022. The first call for proposals led to SEK 180 million being distributed.

Translation George Farrants