Lothar Bertsch

The Parametric Aircraft Noise Analysis Module - status overview and recent applications

The German Aerospace Center (DLR) is investigating aircraft noise prediction and noise reduction capabilities. The Parametric Aircraft Noise Analysis Module (PANAM) is a fast prediction tool by the DLR Institute of Aerodynamics and Flow Technology to address overall aircraft noise. It was initially developed to (1) enable comparative design studies with respect to overall aircraft ground noise and to (2) indentify promising low-noise technologies at early aircraft design stages. A brief survey of available and established fast noise prediction codes is provided in order to rank and classify PANAM among existing tools. PANAM predicts aircraft noise generated during arbitrary 3D approach and take-off flight procedures. Noise generation of an operating aircraft is determined by its design, the relative observer position, configuration settings, and operating condition along the flight path. Feasible noise analysis requires a detailed simulation of all these dominating effects. Major aircraft noise components are simulated with individual models and interactions are neglected. Each component is simulated with a separate semi-empirical and parametric noise source model. These models capture major physical effects and correlations yet allow for fast and accurate noise prediction. Sound propagation and convection effects are applied to the emitting noise source in order to transfer static emission into aircraft ground noise impact with respect to the actual flight operating conditions. Recent developments and process interfaces are presented and prediction results are compared with experimental data recorded during DLR flyover noise campaigns with an Airbus A319 (2006), a VFW-614 (2009), and a Boeing B737-700 (2010). Overall, dominating airframe and engine noise sources are adequately modeled and overall aircraft ground noise levels can sufficiently be predicted. The paper concludes with a brief overview on current code applications towards selected noise reduction technologies.

Integration and application of a tool chain for environmental analysis of aircraft flight trajectories

The German Aerospace Center (DLR) is currently developing a tool chain for the environmental analysis of aircraft flight trajectories. The presented tool chain consists of tools for aerodynamic analysis, engine cycle modelling, flight simulation, and aircraft noise prediction. The aircraft geometry is not modified within the process but provided as an input. The implemented tools come from specialized DLR institutes, are harmonized in input/output format and are integrated into one fully automated analysis process. The PHX ModelCenter framework allows for a DLR-wide accessible server/client architecture. The new tool chain is applicable to evaluate arbitrary three dimensional flight trajectories. The focus of the presented work lies on the environmental analysis and optimization of approach and departure procedures. The predicted ground noise levels are compared to results from a dedicated DLR flyover noise campaign in 2009. A conventional approach, a steep approach, and a new three dimensional approach procedure have been flight tested with DLR’s flying testbed ATTAS. The new procedure is referred to as Helical Noise Abatement Procedure (HeNAP) due to its helix shape. The ground noise measurements confirm the predicted noise concentration and relocation along the steep approach and the HeNAP compared to the conventional approach procedure.

Aerodynamic and Acoustic Design of Silent Leading Edge Devices

The high lift system noise of current transport aircraft is dominated by slat noise under certain operating conditions. Suitable means to reduce the noise impact in the vicinity of airports are (i) to increase the distance between the source and the observer and (ii) to reduce source noise levels. Both objectives can only be achieved by means of a multi-disciplinary aerodynamic and acoustic development since the slat is at the same time a very important element to achieve the necessary high lift performance and the dominant noise source of a high lift system1. First attempts to reduce slat noise by means of a slat setting optimization were conducted at DLR in the mainframe of the project Leiser Flugverkehr2. This purely acoustically driven study revealed that a slat gap reduction results in a local flow speed decrease at the slat trailing edge and thus to remarkable noise reductions of up to 10 dB, the latter of course depending on the magnitude of the slat gap reduction. The drawback of this approach was that at the same time the aerodynamic performance of the high lift system was degraded by a non-acceptable level. However, this study was the starting point of the DLR project LEISA (Low noise exposing integrated design for start and approach) that combined activities in the research areas of high lift system design and aero-acoustic design, which were carried out rather independently up to this point in time. In the project LEISA different types of high lift configurations were addressed and investigated in a 2-dimensional approach. The first one is a long chord slat that provided a source noise reduction of about 6 dB while maintaining the aerodynamic performance of the reference slat system. The second, and more radical concept was to omit the slat and apply a droop nose system in order to reduce the aerodynamic losses as much as possible. The finally achieved source noise reduction with the droop nose system was about 8 dB while from the aerodynamic point of view about 50% of the losses were recovered. Based on these promising results the transposition of these high lift systems to a real 3-dimensional wing was carried out in the follow-up project SLED (Silent Leading Edge Devices). The final outcome of the project SLED can be summarized as follows. From the aerodynamic point of view the performance of the 3-dimensional long chord slat compares very well to the reference slat system. The final droop nose design was capable to recover about 40% of the lift loss due the omitted slat. The final acoustic results in terms of source noise levels are an overall 4 dB noise reduction for the long chord slat and about 6 dB noise reduction in case of the droop nose. The obtained aerodynamic and acoustic characteristics were finally transposed to flight in order to assess the effect on community noise which can be expressed in terms of noise iso-contour areas. Regarding the 60 dB(A) and the 65 dB(A) noise iso contour areas the achieved benefit is a reduction of up to 40% of the respective area’s size.

Analysis of landing gear noise during approach

Airframe noise is becoming increasingly important during approach, even reaching higher noise levels than the engines in some cases. More people are affected due to low flight altitudes and fixed traffic routing associated with typical approaches. Formost air- craft types, the landing gear system is a dominant airframe noise source. However, this element can only be modeled in an approximate manner in wind tunnel experiments. In this research, flyovers of landing aircraft were recorded using a 32 microphone array. Fun ctional beamforming was applied to analyze the noise emissions from the landing gear system. lt was confirmed that for some aircraft types, such as the Airbus A320 and the Fokker 70, the nose landing gear is a dominant noise source du ring approach. The correlation between the noise levels generated by the landing gear and the aircraft velocity was found to be significant, explai ning about 70% of the varia bility found in the noise levels, which is in good agreement with all known theory. Moreover, the experimental resu lts for the Airbus A320 measurements were compared with those obtained using the DLR system noise prediction tool PANAM. Whereas the total aircraft noise levels were in good agreement. the measurements indicate a higher contribution from the nose landing gear noise compared to the predictions.

Application of an Aircraft Design-To-Noise Simulation Process

System noise has been integrated as an additional design objective within conceptual aircraft design. The DLR system noise prediction tool PANAM accounts for individual noise sources depending on their geometry and operating conditions. PANAM is integrated into the existing aircraft design framework PrADO from the Technical University of Braunschweig in order to realize a design-to-noise simulation process. In addition, a ray-tracing tool from DLR, SHADOW, is incorporated into the simulation framework in order to account for structural engine noise shielding. The overall simulation process is then applied to identify promising low-noise aircraft concepts. The presented application aims at fan noise reduction through shielding. For the selected reference aircraft, the fan is a major noise source during both landing and takeoff. It is demonstrated, that the aircraft designers influence on the environmental vehicle characteristics is significant at the conceptual design phase. Usually, a trade-off between extensive engine noise shielding and economical flight performance is inevitable. The new design-to-noise process is well suitable to assess all four measures of ICAOs balanced approach.

Validation strategies for comprehensive aircraft noise prediction methods

This contribution deals with validation and verification issues in aircraft noise prediction. We use two different comprehensive simulation software: PANAM, developed at DLR and FLIGHT, developed at the University of Manchester. The comparison is done on the basis of extensive flight data taken on an Airbus A319-100 operated by Lufthansa. The flight recorder data have been synchronized with noise measurements on the ground, at 25 different observer locations. This paper aims to contribute to the establishment of rational validation standards, as well as realistic accuracy margins on integral noise metrics. The computer codes are briefly described. Results are shown for a variety of microphone positions, located sideways and directly along selected departure and approach flight ground tracks.

Flight testing of noise abating required navigation performance procedures and steep approaches

To test different types of noise abatement approach procedures the Institute of Flight Guidance and the Institute of Aerodynamics and Flow Technology performed flight tests on 6 September 2010 with a Boeing 737-700. In total, 13 approaches to the research airport in Brunswick, Germany (EDVE) were flown while the approach area of the airport was equipped with six noise measurement microphones. Brunswick airport is equipped with an experimental ground based augmentation system which allows the implementation of 49 instrument landing system (ILS) look-alike precision approach procedures with different approach angles simultaneously.

Comparison of Aircraft Noise Models with Flyover Data

This contribution deals with validation and verification issues in aircraft noise prediction. We use two different comprehensive simulation software: FLIGHT and PANAM. The comparison is done on the basis of extensive flight data taken on an Airbus A319-100 operated by Lufthansa. This paper aims to contribute to the establishment of rational validation standards, as well as realistic accuracy margins on integral noise metrics. The computer codes are briefly described. Results are shown for a variety of microphone positions, located sideways and directly along selected departure and approach flight ground tracks.

Simulation process for perception-based noise optimization of conventional and novel aircraft concepts

Noise from civil air traffic affects millions of people worldwide. Aircraft noise management should be addressed by different principal elements, such as noise reduction at the source and noise abatement operational procedures. To date, usually conventional noise metrics are applied towards the acoustical optimization while noise effects are usually only indirectly accounted for. The objective of this contribution is the optimization of conventional and novel aircraft concepts with respect to their evoked aircraft noise annoyance. The optimization will be based on the perceived sound and the associated annoyance. To do so, virtual aircraft flyovers are auralized based on noise level predictions, i.e. they are artificially made audible. The auralization is accomplished by parametric sound synthe- sis and a 3D spatial audio technique. Short-term noise annoyance is measured through controlled listening experiments in which participants rate the level of annoyance for each auralized flyover. The aircraft design and the flight path are evaluated according to the associated annoyance. The subsequent ranking can be compared to a conventional ranking based on standard noise metrics. The results of such a study will help to identify parameters describing aircraft and flight path parameters that have an impact on noise annoyance. Consequently, these parameters can then be selected for further optimization to reach even lower levels of noise annoyance and not simply reduce standard noise metrics. Ultimately, the main goal of the research is optimizing the noise annoyance of (novel) aircraft along tailored flight paths. This contribution documents the status quo of the joint DLR and Empa activities, i.e., the structure of a pilot study. First results that were obtained while developing the methodology and the test cases within the pilot study are presented.

Assessment of the Noise Immission along Approach and Departure Flightpaths for Different SFB 880 Vehicle Concepts

The number of flight movements is further increasing in the future and some major airports are already at their capacity limit. Therefore, it becomes beneficial for short range aircraft to operate on regional airports as well. Short range aircraft with conventional high-lift systems, however, are not able to safely operate on the comparatively short runways of regional airports. Instead, new aircraft concepts are required that are equipped with an active high-lift system. Such an active high-lift system offers high lift coefficients and thus the ability for short take-off and landing. In order to ensure a sustainable growth in aviation, such new aircraft concepts also have to offer reduced fuel consumption and low noise on the ground. The Coordinated Research Center (SFB) 880 focuses on such an active high-lift system. This active high-lift system is comprised of a flexible leading edge device, referred to as Droop nose, and an internally blown flap at the trailing edge, referred to as Coanda flap. Within the SFB880, the active high-lift system is applied to several aircraft concepts. These aircraft concepts are equipped with different propulsion systems, that is, turbofan engines of different bypass ratio or a turbine-driven propeller engine. In this study, the noise prediction methodology for the noise assessment within the SFB880 is summarized and applied to the aircraft concepts. The assessment includes the noise at the three noise certification points as well as along a line of observers. The study also includes a preliminary uncertainty assessment in order to evaluate the reliability of the predicted noise on the ground. The results show that the SFB vehicles can provide significant noise reduction compared to a reference aircraft with a conventional high-lift system and turbofan engine. Most noise reduction can be achieved with the aircraft that is equipped with the ultra-high bypass ratio turbofan engine.