Analysis of nose landing gear noise comparing numerical computations, prediction models and flyover and wind-tunnel measurements

Abstract

The noise emissions of a full-scale nose landing gear, measured in a wind tunnel and obtained from computational simulations, are compared with those of three regional aircraft types recorded in flyover measurements. The results from these three approaches are also compared with the predictions of two airframe noise models (Fink and Guo). The geometries of the nose landing gears in all cases were similar. Microphone arrays and acoustic imaging algorithms were employed to estimate the sound emissions of the nose landing gears. A good agreement was found between the overall trends of the frequency spectra in all cases. Moreover, the expected 6 th power law with the flow velocity was confirmed. On the other hand, strong tonal peaks (at around 2200 Hz) were only found for the flyover tests and computational simulations and are not present in typical noise prediction models. As the frequencies of the tones did not depend on the flow velocity, they are likely to be caused by cavities found in structural components of the nose landing gear. Removing these tones would cause overall noise reductions up to 2 dB in the frequency range examined. The noise emissions in the side direction did not present tonal peaks. The acoustic source maps showed that the dominant noise sources were located in the middle of the wheel axle, followed by the main strut and the bay doors. It is, therefore, recommended to further investigate this phenomenon, to include cavity-noise estimations in the current noise prediction models, and to eliminate such cavities where possible with the use of cavity caps, for example.

Sound quality metrics applied to aircraft components under operational conditions using a microphone array

“…This aircraft type was selected because its NLG system is known to be one of the dominant noise sources during landing [1,18,21,22], due to its strong tonal signature (see Figs. 2a and 2b), which is most probably caused by the presence of open cavities [27][28][29][30].…”

“…The atmospheric absorption coefficient α was calculated considering the measured ambient temperature, relative humidity and sound frequency, as stated in [48]. The wind speeds and background noise levels were relatively low [22], so it was decided to use the full CSM for the calculations, i.e., without removing its main diagonal, since functional beamforming is quite sensitive to this approach [49].…”

“…The obtained source maps were integrated over different regions of integration (ROI) [50,51] to exclude the influence of other sources outside of these areas. This technique is analogous to the source power integration (SPI) [17,22,[50][51][52][53][54][55][56][57][58][59] but considering the formulation of functional beamforming [21,22]. This integration process provides the sound emissions for each source.…”

“…In case multiple microphones are employed simultaneously for the recordings, such as with the so-called phased microphone arrays [3,14], acoustic imaging methods [15] can be applied in order to localize the different sound sources on the flying aircraft and analyze them separately [1,[16][17][18][19][20][21][22]. These devices enable the study of the noise emissions of specific aircraft elements, such as the landing gear system [1,21,22] or the turbofan engines [23,24].…”

“…In the current study, two different noise sources are analyzed due to their expected importance during landing [24,26]: the nose landing gear (NLG) and the turbofan engines. The NLG system of this aircraft type (and other commercial aircraft) is known to emit a strong tonal component [1,18,21,22], which is most likely caused by the presence of open cavities interacting with the incoming flow [27][28][29][30].…”