Recently NLR has tested a new method to measure the noise of aircraft engines during a flight. NLR’s Cessna Citation research aircraft has been equipped with a so-called nose boom, a pole on the nose of the Citation, which catches the noise of another aircraft nearby. With this method the noise of an aircraft engine can be measured more extensively compared to flyover tests. Through the more extensive measurements engine manufacturers can focus in the development of new propulsion with less emissions and less noise. The research is part of the PropMat project in the European Clean Sky 2 programme, which is supporting Airbus with the maturation of future propulsion concepts. See also this video.

Novel engine technology can deliver a step change in the reduction of fuel consumption and noise. The development of innovative propulsion concepts and their integration in aircraft is therefore a key contributor to achieve the carbon-neutral growth and specific community noise reduction targets for aviation. Full scale flying demonstrators of the new propulsion concepts enable accurate acoustic measurements in cabin (at one side of the engine) and on-ground (below the aircraft). Following a need expressed by Airbus, NLR aims to provide an innovative acoustic in-flight measurement method that provides accurate measurements in all directions and at various distances from the flying demonstrator and in various operational conditions. A special focus is on the assessment of noise generation of Open Rotor engines (including CROR – Counter Rotating Open Rotors- engines) in all directions and in cruise.

NLR Cessna Citation research aircraft - detail nose boom

NLR Cessna Citation research aircraft equipped with nose boom

Novel chase aircraft acoustic in-flight measurement method
The innovative acoustic in-flight measurement method is based on acoustic measurement from the nose boom of a chase aircraft, which is flying close to the flying demonstrator. The challenge is to be able to measure the noise of the flying demonstrator while not being disturbed by the noise of the chase aircraft itself, by the boundary layer noise of the air passing over microphones or by reflections on other chase aircraft parts before reaching the microphones. Putting the acoustic sensors on a nose boom of a tail-engined chase aircraft provides the maximum distance between the acoustic sensors and the chase aircraft engines and minimises reflections. Combined with advanced acoustic measurement techniques and post-processing of the acoustic data this should provide an accurate and widely applicable in-flight acoustic measurement method.

Recent flight test of the novel acoustic measurement method
The feasibility of the chase aircraft acoustic in-flight measurement method has now been investigated in a small experimental test campaign in which NLR’s Cessna Citation research aircraft is measuring the noise of a propeller aircraft. Two types of acoustic sensors have been tested and measurements have been taken from several directions and at several distances in different flight conditions. Synchronisation between the chase aircraft and the propeller aircraft has been established to relate the acoustic measurements with the propeller position. The synchronisation technology is based on the technology developed for wind tunnels in the Smart Fixed Wing Aircraft ITD of Clean Sky. The first results indicate that the noise of the propeller aircraft is detectable and not disturbed by engine noise. This indicates that the chase aircraft acoustic measurement method is feasible. The experience gathered paves the way for further improvements.

Project context and disclaimer
The chase aircraft acoustic in-flight measurement method is developed in the Clean Sky 2 project PropMat for Platform 1 of the Large Passenger Aircraft IADP. This project has received funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 research and innovation programme under grant agreement No 680954. This message doesn’t necessarily reflect the views of the JU Clean Sky.