A balancing act
Future air transport is subject to major requirements, resulting in considerable challenges for research. The European Union has stipulated a target of reducing aircraft noise by 65 percent by 2050. At the same time, aviation needs to be made more climate friendly. Is it possible to reconcile the two? Michael Mößner is a noise researcher at the DLR Institute of Aerodynamics and Flow Technology in Braunschweig. He is currently addressing these challenges together with his colleagues.
How many people are actually affected by aircraft noise?
In Germany, approximately one percent of the population lives in the immediate vicinity of an airport, so we are talking about 820,000 people who are exposed to noise levels of more than 55 decibels every day. They need rapid relief. If we want to achieve the level recommended by the World Health Organization (WHO), we must reduce aircraft noise by at least 15 decibels. Our research focuses primarily on noise from aircraft and around airports.
What can be done to make aircraft quieter?
In our SIAM project, we tested a number of options – different flight routes, higher altitudes and changes to the aircraft itself, to name but a few. On an aircraft, the engines are among the loudest elements during many phases of flight. At present, the trend is towards larger engines.
Won't larger engines mean more noise?
Actually, the opposite is true. While the thrust remains the same, air flows more slowly through the engine, making it quieter. By using liners, a kind of damping mat with small holes, we can also absorb sound within the engine itself. However, for people living near an airport, the most effective way of reducing noise is to place the engines above the wings. This means that the sound cannot radiate downwards unhindered, and the residents are essentially in the acoustic shadow of the engine.
I imagine that kind of aircraft would look a bit unusual.
Yes, that is true. As part of SIAM, we have virtually designed a quiet, cutting-edge aircraft that fulfils the above-mentioned and other noise reduction aspects. It manages to strike a balance between feasibility and maximum noise reduction. We have also carried out extensive aerodynamic and structural analyses, which allow us to make reliable statements about as many aspects as possible. However, new approaches to solutions often come at the expense of efficiency and climate compatibility.
So, creating a low-noise yet climate-friendly aircraft is not so easy. Why is that?
To give one example, the T-tail, which is necessary for safety reasons, is heavy. It increases the surface area, causes greater drag and ultimately increases fuel consumption. This is a major challenge in our research – identifying the optimal combination of technologies, aircraft configurations and flight procedures while meeting policy requirements.
Noise and noise pollution
The point at which a sound is deemed to be 'noise' depends on many factors. Above all, duration and volume are decisive factors. Aircraft noise is not a constant sound, but it is usually perceived as more irritating than sounds that remain at a constant level. To realistically assess and compare overall noise pollution, a special value is calculated based on a special value based on a specific period of time. This is called the equivalent continuous sound level (LAeq) and is measured in dB(A). In its Night Noise Guidelines (NNGL), the WHO recommends a guideline value for external noise of 40 dB(A). If the value is over 33 dB, the likelihood of waking up increases. A study by the DLR Institute of Aerospace Medicine showed that sleep disorders can occur at levels as low as 35 dB indoors.
That sounds like a real dilemma. How are you applying your findings?
The focus is rightly on climate compatibility, which does not allow for uncompromisingly quiet aircraft like ours. New development and market acceptance can take decades. For a quick and cost-effective solution, we focus on modifications such as covering various landing gear elements on existing aircraft. This is why, in addition to the futuristic aircraft designs in SIAM, we also carried out wind tunnel tests with an Airbus A320 half-model, with added noise reduction measures from the previous Low Noise ATRA (LNATRA) project. A few years ago, we carried out flight tests involving conversions on an A320, with very good, practicable results. All of this is now being incorporated into our follow-up project, LU(FT)² 2030. Since tests that involve additions to real aircraft are extremely expensive and time-consuming, we are applying the findings from LNATRA to a virtual environment. By conducting additional wind tunnel experiments and flight simulator tests, we are aiming to make our numerical methods more reliable and improve noise reduction measures on conventional and future aircraft.
Looking in your crystal ball, will we be able to meet the EU's requirements by 2050?
If we were to disregard climate compatibility, our SIAM design would have largely met the requirements by then. However, this would mean upgrading entire aircraft fleets, which would take decades. We cannot wait that long. We are keen to implement noise reduction measures on existing aircraft as quickly as possible, namely by 2030.
The interview was conducted by Vera Koopmann, public relations at the DLR Institute of Aerodynamics and Flow Technology. An article from the DLRmagazine 175.