There is a compelling push from the aviation industry to reduce its impact on the environment, and communities, in terms of emissions and noise pollution. Aircraft noise is 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 perceived as noisy. “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, funded 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 Centre (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 aerodynamic by its nature and is caused by 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 by far the most piercing to the human ear, is due to: the displacement of fluid caused by the blade motion (thickness noise); the pressure field around the blades moving in the air (loading noise); 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, is random in nature and occurs as a result of turbulence in the boundary layer of the blade, and interaction of the blade with turbulence in the air. The FUSIONProp project is following two main paths of investigation: first one is experimental, with the execution of two flight test campaigns, and the other one is 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 the effects of aircraft installation. Flight testing helps us understand the complexities of installed propeller noise better, which in turn improves our models.”
FUSIONProp is part of a broader commitment by the GE Aviation Munich team in European R&D programs, aimed at quieter and more efficient next-generation propeller-driven aircraft. During the last few years, the team in Munich has been involved in several EU funded programs, such as Clean Sky 2, aimed at reducing the environmental footprint of turboprop aircraft, with emphasis on low-noise propeller technology development and in close collaboration with colleagues from the leading propeller manufacturer Dowty Propellers.
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, 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 community noise prediction methods and to enable noise source localization 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. “As the industry accelerates towards the introduction of hybrid and all electric propulsion solutions, 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, ensuring that Dowty in collaboration with GE Aviation in Munich maintains its position as a world leader in the supply of propeller solutions for all turboprop applications” added Barnard.
At present, Giacché and the teams are working on the vast amount of data acquired during the tests: “We are currently evaluating the data in detail to validate the physical understanding of turboprop acoustics under realistic flight conditions, and the preliminary results so far are proving incredibly valuable in enhancing our predictive capabilities, and paving the way to technological innovation for tomorrow’s high-efficiency and quiet turboprop/propeller-driven aircraft”.