Which parts of an aircraft cause noise and what sort of noise is it? For the first time, scientists from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and GE Aviation in Munich have succeeded in measuring the noise emitted by a turboprop aircraft synchronously on the ground and on the aircraft. With the knowledge gained, future aeroplanes can become quieter.
The aviation industry is working hard to reduce its impact on the environment and communities, in terms of emissions and noise pollution. Aircraft noise is one, if not the most obvious, environmental impact of aviation and the need to lower aircraft noise has never been more in focus. While turboprop aircraft remain the most efficient way to travel by air, they are still perceived as noisy by communities and passengers.
"This is why it is important to improve the understanding of how noise is generated and of how new technologies can reduce turboprop aircraft noise" explains Davide Giacché, Senior Engineer for GE Aviation’s Advanced Technologies team based in Munich, who is leading the German team’s efforts within the FusionProp project.
FusionProp is a three-and-a-half-year research project which started in April 2018 under the German Federal Aviation Research Programme, called LuFo, supported by the German Ministry for Economic Affairs and Energy. This technology project represents a unique opportunity to advance the state of the art in turboprop acoustics, thanks to the collaboration between GE Aviation Munich and several Institutes of the German Aerospace Center (DLR), which has extensive experience and unique capabilities in flight testing and noise measurement.
The primary source of noise in propeller-driven aircraft is the propeller, followed by the noise generated by the airframe and engines. Propeller noise is caused by their special aerodynamic features and is the result of the relative motion between the propeller blades and the surrounding air. In particular, propeller noise is made up of both a tonal and a broadband component: the tonal component, which is most likely to be perceived as a disturbance to the human ear, is due to: the displacement of air caused by the blade motion (thickness noise); the pressure field around the blades moving in the air (loading noise); and the unsteady periodic variation of loading on the blades due to, for example, the aerodynamic interaction with surrounding structures. Broadband noise, on the other hand, occurs as a result of turbulence in the boundary layer of the blade, and the interaction of the blade with turbulence in the air.
The FusionProp project is following two main paths of investigation: one experimental with the execution of two flight test campaigns, and one numerical with the validation of methods to predict turboprop noise. “Parameters that affect the noise generated by a propeller are the rotational speed (the faster the blades rotate, the more noise is generated), flight speed, the amount of thrust required and details of the aerodynamic blade design” said Lorenz Drack, Lead Engineer from GE Aviation Munich. "Under real flight conditions, the propeller noise generated is further complicated by aircraft attitude, turbulence, wind gusts, and effects resulting from the integration of the engine within the aircraft. Flight testing helps us understand the complexities of the noise generated by installed propellers, which in turn allows us to optimise our models"
In the summer of last year, two flight tests were performed as part of FusionProp. This type of test requires meticulous preparation and coordination among all the teams involved and is also subject to weather conditions being favourable to acoustic measurements. "The level of cooperation between the GE and DLR teams before, during and after the two tests was and remains exceptional, and the amount of data gathered is testament to that" says Giacché, praising DLR's unique knowledge and experience as a valuable asset in such an intensive test campaign with very high levels of innovation and complexity.
Both tests represented a full range of typical flight conditions including take-off, climb, cruise and approaches for landing. In addition to instrumentation on and inside the two aircraft, – DLR's DO 228 and a De Havilland Aircraft of Canada Dash 8-400 – over 200 microphones were installed on the ground at Magdeburg-Cochstedt Airport - in the German state of Saxony-Anhalt - where all the flyover tests were conducted. Noise measurements on the ground were performed with the goal of improving noise prediction methods in areas surrounding airports and to enable noise source localisation using DLR's large microphone array. More than 50 test points were investigated in numerous flights that were conducted with flyovers above the microphones, at several heights depending on the flight condition.
For the first time, synchronous flyover noise measurements were performed with a microphone array mounted on the fuselage of DLR's DO 228 research aircraft, and two microphone arrays on the ground. The fuselage array and the ground arrays were developed at DLR with the goal of localizing the sources of noise. "This is the first time we are able to correlate the noise arriving on the ground with the noise sources on the aircraft," says Carsten Spehr from the DLR Institute of Aerodynamics and Flow Technology in Göttingen.
The ability to accurately predict the noise levels for a variety of installations is also important for future next-generation turboprop platforms. "With the emerging introduction of hybrid and all electric propulsion solutions in the industry, attention on the propeller as the main source of noise will increase. Developing new technology to ensure that the lowest possible noise is produced by a particular application has been a focus of Dowty Propellers' research for a number of years." says Jimmy Barnard, Product Strategy and New Technology Leader at Dowty Propellers.
“The FusionProp programme is seen as the culmination of a series of projects where technologies and tools undergo flight test evaluation to provide a platform for the validation of advanced noise prediction methods for propellers," added Barnard. "For certain flight conditions and with modern propeller configurations, installation effects can play an important role in the noise generated by propellers," says Arne Stürmer, project manager at DLR. The evaluation and use of the measurement data can help make future aircraft quieter. "In computer simulations, for example, we can check how to position of the propeller relative to the wing influences noise," says Stürmer. DLR and GE are currently working on the evaluation of the large amount of data obtained during the tests.
DLR is the Federal Republic of Germany's research centre for aeronautics and space with 9000 employees across 30 locations. Four DLR institutes and facilities from three locations were involved in the investigations within the framework of FusionProp: The Institute of Aerodynamics and Flow Technology in Braunschweig and Göttingen, the Flight Experiments facility in Braunschweig, and the Institute of Propulsion Technology in Berlin.