Drawing from the expertise, knowledge and technology of an extensive network of partners, including the UK’s Civil Aviation Authority, Rolls-Royce, Boeing and Luton Airport, the Silent Aircraft Initiative has resulted in some truly innovative approaches to noise reduction. With a structured plat form that allows progressive improvements and stakeholder involvement, the total solution is divided into short-, medium- and long-range components, culminating in the development of a concencept plane that is virtually inaudible outside an airport’s perimeter.
In designing the concept plane, the SAI team focused on acoustically sound principles. This meant abandoning the traditional design conventions of the “tube and wings” construction and embedding the engines, the largest single noise source, above the airframe in order to shield people on the ground from the engines’ roar as the plane takes off and lands. While the most obvious source of aircraft noise is the plane’s engines, constant improvements n engine design has brought other noise culprits to the forefront, such as the airframe .Alone, the drag needed to slow down a plane during landing generates enough turbulence around the plane’s body that the resulting sound has become a considerable noise source.
To address design issues related to the airframe, a dedicated Airframe Team was established within the SAI. Through the use of specialised prediction tools, aero acoustic testing, and innovative applications, the team has gained unique insight into how airframe noise is produced and what changes can be made to reduce the sound levels to their target levels, while maintaining the necessary aero dynamics. In one of the SAI’s latest test scenarios, Brüel & Kjær was present to provide technical support and cutting-edge solutions.
The tests were headed by SAI researchers Dr. Ho Chul-Shin and Andrew Faszer and centered on Andrew’s research projects, one of which was a unique aerofoil with interchangeable trailing-edge plates with ventilation slots of varying size (from no slots to completely perforated). Andrew, a PhD candidate in Engineering at Cambridge University, graduated from the University of Alberta with a BSc in Mechanical Engineering. He has worked within the field of acoustical engineering since his undergraduate years, and was drawn to Cambridge by the aggressive noise target set by the SAI project. As a member of the SAI Airframe Team, Andrew’s research will help determine the lowest aero acoustic parameter of the airframe at approach and landing, namely trailing-edge noise, and establish if the noise levels could be manipulated, even reduced. Andrew explains, “As trailing-edge noise is the smallest element of noise produced by an aircraft, and is very low level, it is hard to measure it. However, we need to be able to do so, as it will set the benchmark for the minimum level of noise that the ‘silent’ aircraft will produce”.
Taking place at the Whittle Laboratory in Cambridge, the test employed the lab’s “Markham” open jet wind tunnel to produce wind conditions proportionally similar to those present during typical aircraft approach and landing. By placing the aerofoil in-flow, measurements could be taken of the noise generated by the air turbulence scattered at the farthest end of the aerofoil, the trailing edge.
By changing the plates at the trailing edge and manipulating the air passing through these plates, one could possibly manipulate the boundary layer, the layer of airflow closest to the aerofoil’s surface, and its effect on the noise levels generated.
Using Brüel & Kjær’s state-of-the-art Combo Array system for combined Beam forming (for far-field measurements) and Statistically Optimised Near-field Acoustic Holography –SONAH (for near-field measurements), a wide range of frequencies was covered without changing the array configuration. “The array focuses on the test object, negating any outside sources such as the wind tunnel, people and surfaces”, explained Claus Blaabjerg, the Brüel & Kjær Innovations specialist on hand during testing.
With foam set up to help minimise reflections and the array positioned out-of-flow, far-field measurements were taken using PULSE Beamforming software. For near-field measurements, the software’s integrated SONAH algorithm was used. The complete system provided the researchers with a single-array solution with no microphone reconfiguration and high-resolution source maps over a wide frequency range (200Hz – 10kHz). An accurate mapping of the sound sources was immediately ready for analysis, which for Andrew was amazing, “I was pleased at how quick the measurement was – only 5 seconds per measurement.
With the raw data stored in PULSE Data Manager, Andrew could retrieve measurements for post-analysis in PULSE Beamforming and create graphs and reports for review and comparison. Andrew’s analysis showed that “the trailing-edge plates were successful in manipulating the trailing-edge noise,” and that “trailing-edge noise can be decreased slightly through suction of the boundary layer”. He could then happily conclude that “the measurements were successful in identifying and measuring trailing-edge noise generated by the aerofoil model”. As a result, “A better understanding will be gained in the relationship between the boundary layer(noise source) properties and the propagating trailing-edge noise. By manipulating the boundary layer (changing plates and suction/blowing of air through the plates) as well as observing the effects on trailing-edge noise (be it increasing the noise or decreasing it), a better understanding of the trailing-edge noise generation mechanism will be obtained”.
For SAI Project Manager, Paul Collins, “The collaboration with Brüel & Kjær is a great example of two-way knowledge exchange across the University-industry interface, as they bring world-class expertise and measurement systems to help develop our advanced experimental techniques. We look forward to continuing our collaboration with Brüel &Kjær throughout the project and beyond.”