Ducted propellers are an interesting design choice for unmanned aerial vehicle (UAV) concepts due to a potential increase of the propeller efficiency. In such designs, it is commonly assumed that introducing the duct also results in an overall noise reduction. The objective of this work is to experimentally analyze and quantify noise of a ducted propeller suitable to be installed on a medium size UAV (wingspan 5–10 m). A microphone array is used for recording the noise levels at each microphone position and used collectively to localize noise sources with beamforming. Different types of noise sources are considered (an omni-directional source and a propeller). In addition, the effect of the presence of an incoming airflow is assessed. With no incoming airflow, it is found that the duct significantly modifies the noise radiation both in the frequency and the spatial domain. With an incoming airflow, the effect of the duct on the frequency content of the signal is almost eliminated. The fact that for this case the harmonics become lower results in a reduction of the received noise levels. Also the directivity changes. These insights are of importance in efforts towards modeling the effects of ducts for complex noise sources such as propellers.@en

The contribution of the engine and the airframe to the total noise generated by an aircraft varies with the operating conditions. Semi-empirical models are able to account for such variations but require detailed engine and airframe data as input that is not readily available for most aircraft types and operations. This hinders the validation of these models through comparison between predictions and experimental data. This work investigates the sensitivity of semi-empirical models of engine and airframe noise to slight variations of the input data, representative of uncertainties in geometrical parameters and variability of the aircraft operating conditions during flyovers. In addition, the predictions are compared to measurements of A320, A330, and B777 landings and departures. This, together with the sensitivity analysis, indicates frequency regions where a mismatch between measurements and predictions exists. The deviation between predictions and measurements for landings can be partially explained by the underestimation of the sound pressure level of the higher harmonics of the fan. For takeoff, the models predict lower levels than measured. This is hypothesized to be associated with jet-installation noise, which is not accounted for in the semi-empirical models. The predicted spectra of the Airbus A320 and A330 were adjusted to account for jet installation noise, using levels available in the literature. This resulted in a better agreement between modeled and measured spectra at low frequencies.@en

The shielding of engine noise by the aircraft wings and fuselage can lead to a significant reduction on perceived noise on ground. Most research on noise shielding is focused on BlendedWing Body (BWB) configurations because of the large dimension of the fuselage. However, noise shielding is also considered relevant in conventional tube and wing configurations when the engines are mounted above the wings. Therefore, it is important to have a noise shielding method adaptable to different aircraft geometries without compromising the accuracy of the predictions or resulting in very slow computations. In this work two methods are used to calculate noise shielding and compared in terms of accuracy and computational time, the Diffraction Integral Method (DIM) and a method built on the Modified Theory of the Physical Optics (MTPO). Both methods are based on the Kirchhoff integral and are considered accurate for sharp-edged objects. It was verified that the two methods are comparable in terms of their predictions, but the MTPO-based method is more efficient computationally. Noise shielding predictions for different frequencies are presented for a flyover of the Fokker 70 considering a realistic geometry and flight conditions. These predictions indicate significant values of noise shielding, which demonstrates its importance in current aircraft.@en

Today's globalized world depends on civil air transportation, which has been continuously growing over the last decades. Nevertheless, the sustainability of this expansion is a challenge due to environmental problems. Along with greenhouse gas emissions, noise represents a severe hazard for human health, and consequently, noise regulations limit the airport capacity and impose night curfews. Noise is therefore an important design driver for future aircraft, and accurate noise predictions are essential at an early design stage. The total noise emission of an aircraft poses a complex problem, as the distinct components emit noise with different characteristics. High fidelity methods are computationally demanding and time-consuming at an early design phase and less complex solutions, such as semi-empirical methods, are often considered to be more suitable. This thesis focuses on aspects that can improve noise predictions for a new generation of silent aircraft. The concept of noise shielding is present in many future aircraft designs, in which engine noise is partially shielded by the airframe, resulting in a noise reduction on the ground. The noise shielding predictions presented in this work use a theory based on the Kirchhoff integral and the Modified Theory of Physical Optics. This method was extended to consider other noise source radiations patterns than the monopole and to calculate the creeping rays originated by smooth edges. Experiments in the wind tunnel were used to validate these methods and showed that the values of noise shielding are strongly dependent on the source directivity and the shape of the obstacle. Flyover measurements of rear engined aircraft were compared with predictions of noise shielding. The good agreement obtained considering a sharp-edged wing in the predictions was further improved by considering the curvature of the leading edge. A low-noise variation of the Boeing 747-400 is explored using noise shielding predictions, and the optimal engine positions were found to be different when considering the wing leading edge as sharp and with a curvature. This analysis shows how the design of an aircraft is affected by the approximations adopted in the noise shielding predictions, therefore also affecting its performance. For conventional aircraft, the noise emission is commonly estimated using semi-empirical methods. These models are based on experimental data and require detailed input of the aircraft geometry and engine settings. This work uses experimental data to test the limitations of such empirical methods during take-off and landing. The efforts to reduce aircraft noise are only meaningful when resulting in a decrease of annoyance. Traditional metrics such as the Effected Perceived Noise Level are used to assess the annoyance caused by aircraft flyovers but do not provide information about the sound characteristics, such as tonal content and fast and slow amplitude oscillations. Sound quality metrics provide a more complete characterization of a sound and can be combined in psychoacoustics annoyance metrics. Flyover measurements of different aircraft types during take-off and landing were used to investigate the correlation between the sound quality metrics and the aircraft geometry and propulsion system. Strong correlations were found between the sound quality metrics and a number of aircraft characteristics, indicating that psychoacoustic metrics can be used to drive the design process, similarly to existing methods that apply traditional metrics for the same purpose. The variability of the sound quality metrics and psychoacoustic annoyance within the same aircraft type was also investigated. This variability was attributed to the aircraft operating conditions. @en

Engine noise shielding is an important measure towards low-noise aircraft configurations. Such designs are supported by prediction tools that indicate high values for shielding of engine noise. Most prediction models approximate the complex nature of engine noise to simple noise sources such as monopoles or dipoles. This work compares predictions of noise shielding with experiments using different noise sources and shielding body geometries. The experiments considered in this work concern a monopole source shielded by a flat plate and a NACA 64-008 A wing, and a propeller shielded by the same wing. Comparisons between models and measurements are made by analysis of noise levels at individual microphones and using conventional beamforming. Results show that for the monopole cases the model predictions are in agreement with the experimental data, with an average deviation of 2-3 dB. The curvature of the leading edge of the wing influences the noise shielding results. The measured values of noise shielding of propeller noise are lower than those measured for the omni-directional source. Different types of source directivity are used to approximate the propeller in the predictions: monopole, dipole and a multi-source. The dipole approximation shows the best agreement with the experiments for the case of the propeller.@en

The shielding of engine noise by the airframe of an aircraft is considered an effective way of reducing noise levels on the ground. Noise shielding in conventional aircraft is mainly due to the presence of the wings and most model predictions of full-scale aircraft neglect the effect of the airfoil curvature. The engine is typically simpliffied as a point source. The objective of such approximations is to reduce the complexity of the model implementation and to decrease the computational time. Measurements of noise shielding of a model wing took place in an anechoic facility using a microphone array. Two noise sources are considered: a point source and a model propeller. These measurements assess differences in noise shielding between using a point source and a source with strong directivity as a propeller. The comparison of experimental data with model predictions ascertain whether the simpliffications commonly used in noise shielding problems are realistic. The noise shielding predictions use a method based on the Kirchhoff integral and the Modiffied Theory of Physical Optics (MTPO). This work aims to understand, using experimental data, possible limitations of noise shielding predictions when adopting typical simpliffications. @en

Aircraft noise is an increasingly important issue that causes annoyance and complaints for the communities living in the vicinity of airports. The conventional sound metrics (such as the A–weighted sound pressure level) typically used for assessing the impact of aircraft noise often fail to conveniently represent the actual annoyance experienced. More sophisticated sound quality metrics (such as loudness, tonality and sharpness) can be used to determine the psychoacoustic annoyance perceived by the human ear. In this paper, an Airbus A320 landing flyover under operational conditions recorded with a microphone array is analyzed. The application of functional beamforming to the acoustic data allows for the separation of the emissions of different noise sources on board of the aircraft. For this case, the nose landing gear (NLG) and the turbofan engines were selected, due to their expected dominance during approach. It was found that, despite being more quiet than the turbofan engines, the NLG system emits a strong tonal sound that makes it overall more annoying than the noise of the engines. Airframe noise prediction models (Fink’s and Guo’s methods) do not consider this tone and considerably underpredict the noise levels and the annoyance of the NLG. Thus, it is highly recommended to employ these sound quality metrics to study aircraft noise and to evaluate the potential improvement in annoyance of future low–noise aircraft concepts, rather than just a difference in the sound pressure level in decibels.@en

The capacity of airports is limited due to the negative community response to noise. Traditional metrics, such as the A-weighted maximum sound pressure level (Lp;A;max), indicate the overall noise generated by an aircraft flyover but do not provide any information on tonal components or frequency variations in time that are known to affect annoyance. In this work 158 flyovers recorded at Amsterdam Schiphol Airport are analyzed in terms of sound quality metrics (SQM), loudness, roughness, tonality, sharpness and fluctuation strength. The recordings include landing and takeoff operations of 15 different aircraft types. The variability of the levels of the SQM are assessed per aircraft type. Possible correlations between the SQM and airframe and engine characteristics are investigated and empirical expressions for the loudness and roughness are formulated. The Effective Perceived Noise Level (EPNL) and the Psychoacoustic Annoyance Metric (PAmod) are calculated for each flyover. The two metrics show a high correlation between them and this result is further investigated using listening tests. The listening tests show that tonality has a high importance in annoyance, however, its influence in PAmod was found to be small. @en

The reduction of aircraft noise over the past decades has generated a growing awareness that the characteristics of a signal can be equally or more important to annoyance than the sound pressure level. Sound can be perceived as more annoying, depending on the frequency content or tonal components. The sound quality metrics loudness, roughness, sharpness, and tonality are important tools to characterize sound. Flyover measurements of landing and takeoff aircraft are investigated in terms of sound quality metrics. The experimental dataset includes 141 measurements of 14 landing aircraft types and 160 measurements of 12 takeoff aircraft types. The sound quality metrics are compared for different aircraft types, and their variability within the same aircraft is investigated. Possible correlations of the sound quality metrics with the airframe, engines, and aircraft operational conditions are investigated. This analysis provides empirical expressions that show a good agreement with experimental data for loudness, sharpness, and roughness for takeoff aircraft. For landing aircraft, empirical expressions could only be obtained for loudness and tonality.@en

The pursuit of greener aviation led to the investigation of new aircraft concepts. Disruptive aircraft configurations show great potential in reducing the ground noise impact, but extensive research is required before they can be manufactured. Tube-and-wing aircraft with over-the-wing engines, benefiting from noise shielding, are a more feasible option for midterm noise reduction targets. This work explores a low-noise version of the B747-400 considering over-the-wing engines, and a multidisciplinary procedure is used to calculate the aircraft and engine performance, the flight procedure, and the noise impact. Engine positions are found providing maximumengine noise shielding, reflected in a decrease in the sound exposure level (SEL) contours. These contours are calculated considering the wing leading edge as both a sharp and a curved edge. This work investigates whether a sharp leading-edge approximation is acceptable for first estimates of the optimal engines position and corresponding noise reduction values. From the results it is found that the maximumdecrease in theSEL obtained for the modified aircraft is 2 dB considering a sharp leading edge and 1.5 dB for a curved leading edge, for departure. For approach, the SEL contours do not show significant differences for the sharp and curved leading edge, with maximum noise reduction values of 3.5 dB for both cases.@en

The pursuit of greener aviation led to the investigation of new aircraft concepts. Disruptive aircraft configurations show great potential in reducing the ground noise impact, but extensive research is required before they can be manufactured. Tube-and-wing aircraft with over-the-wing engines, benefiting from noise shielding, are a more feasible option for midterm noise reduction targets. This work explores a low-noise version of the B747-400 considering over-the-wing engines, and a multidisciplinary procedure is used to calculate the aircraft and engine performance, the flight procedure, and the noise impact. Engine positions are found providing maximumengine noise shielding, reflected in a decrease in the sound exposure level (SEL) contours. These contours are calculated considering the wing leading edge as both a sharp and a curved edge. This work investigates whether a sharp leading-edge approximation is acceptable for first estimates of the optimal engines position and corresponding noise reduction values. From the results it is found that the maximumdecrease in theSEL obtained for the modified aircraft is 2 dB considering a sharp leading edge and 1.5 dB for a curved leading edge, for departure. For approach, the SEL contours do not show significant differences for the sharp and curved leading edge, with maximum noise reduction values of 3.5 dB for both cases.@en