Werner Dobrzynski

Almost 40 Years of Airframe Noise Research: What Did We Achieve?

With the advent of low noise high bypass ratio turbofan engines airframe noise gained significant importance with respect to the overall aircraft noise impact around airports. Already around 1970 airframe noise, originating from flow around the landing gears and high-lift devices, was recognized as a potential “lower aircraft noise barrier” at approach and landing. Since then, the outcome of extensive acoustic flight tests and aeroacoustic wind tunnel experiments enabled a detailed description and ranking of the major airframe noise sources and the development of noise reduction means. In the last decade advances in numerical and experimental tools led to a better understanding of complex noise source mechanisms. Efficient noise reduction technologies were developed for landing gears while the benefits of high-lift noise reduction means were often compensated by a simultaneous degradation in aerodynamic performance. The focus of this paper is not on the historical sequence of airframe noise research but rather aims to provide a concise survey of the achievements in airframe noise source description and reduction over the last 40 years worldwide. Due to the vast amount of work focused on a variety of airframe noise problems, this review can only provide examples but does not claim to be complete.

Experimental Investigations In Low-Noise Trailing-Edge Design

Within a parametric study on brush-type trailing-edge extensions, the noise reduction potential of several design concepts was determined. The obtained database represents the first phase of an ongoing project with the long-term objective to develop scaling laws for a future application of such devices as add-on solutions for todays aircraft components. The experiments comprised both acoustic and aerodynamic measurements on a zero-lift generic plate model (Re = 2.1 x 10 6 to 7.9 × 10 6 ) in DLRs open jet Aeroacoustic Wind Tunnel Braunschweig. Noise data were taken by means of a directional microphone system. Measurement results indicate a significant source noise reduction potential in excess of 10 dB, depending on the configuration. Two relevant noise reduction mechanisms were identified: 1) the suppression of narrowband bluntness noise, as well as 2) the reduction of broadband turbulent boundary-layer trailing-edge noise.

Airframe Noise Studies on Wings with Deployed High Lift Devices

Employment of very quiet high-bypass-ratio engines to propel current and future very large aircraft has caused airframe noise to become a significant contributor to the overall radiated noise from an aircraft in landing approach. This has brought about a worldwide resurgence of airframe noise studies, to try and understand the aeroacoustics of, and to ultimately control the aerodynamically caused noise from aircraft components deployed during the final approach leg, such as landing gears and high-lift devices (HLD) on wings. In view of European aviation industry to design and build a very large commercial aeroplane, a substantial and dedicated German National Research Project was initiated, culminating in a series of model- und full-scale wind tunnel experiments on HLD. This paper discusses initial results from HLD-studies in the DLR Aeroacoustics Wind Tunnel Brauschweig (AWB) where an acoustic mirror was employed on a 1/10 scale-model wing section to identify the aeroacoustic source mechanisms of slat-noise and flap side-edge noise. Tests were performed for different flow velocities and wing angles-of-attack, indicating that the very slat tracks constitute sources of excessive flow noise. Moreover, both slat-noise and flap side-edge noise, respectively, were found to each be a result of specific combinations of different unsteady-aerodynamics mechanisms. Several, though still preliminary noise reduction techniques were tested, showing nevertheless significant promise to enable containment of otherwise excessive slat-noise and flap side-edge noise, such techniques being quite feasible for future full-scale application.

Full-Scale Noise Testing on Airbus Landing Gears in the German-Dutch Wind Tunnel.

The acoustic flyover signature of modern aircraft in their approach configuration - i.e. with slats, flaps and gears deployed - are often dominated by airframe noise contributions. To study relevant source and radiation characteristics, airframe noise tests were performed in the German-Dutch Wind Tunnel employing- for the first time ever - full-scale landing gears of an A320 aircraft. Farfield noise characteristics were determined for different gear configurations (starting form the base configuration and subsequently covering ever more gear elements with streamlined fairings) at wind speeds ranging from 40 to 78m/s. Aerodynamically generated noise from landing gears turnes out to be of broadband nature with constant levels (in 1/3-oct. bands) up to several kHz. One most essential result therefore is that landing gear noise, is not at all a low-frequency phenomenon. Moreover, levels increase with the 6th power of flow velocity.

Slat Noise Source Studies for Farfield Noise Prediction

Todays low-noise high-bypass-ratio engines have made airframe noise for large commercial aircraft in the approach configuration to be compatible to that of the engines. As a consequence, flow noise from landing gears and from high lift devices (HLD) on wings - and its control - will become ever more important. In a study, dedicated to HLD flow noise, parametric wind tunnel experiments were performed on scaled wing sections in the Aeroacoustic Wind Tunnel Braunschweig, on a complete scale model aircraft and a full scale wing-section in the German-Dutch Wind Tunnel to identify relevant airframe noise mechanisms and develop noise prediction schemes. As one essential result of these tests deployed slats were identified as prominent noise contributors. Based on an extensive set of farfield noise data and on first results from measurements of the unsteady local flow properties in the slat area the effects of flow velocity and aircraft angle of attack on slat noise radiation characteristics were determined. The test results support the assumption that slat noise originates from the upper slat trailing-edge and scales with the slat cove vortex dimension. As a consequence the transposition of slat noise data from scale model tests towards a full scale situation can approximately be based on the geometric scale factor. Based on the findings of this experimental study a simplified source model is considered for slat noise prediction.

Sound Radiation from Aircraft Wheel-WelVLanding-Gear Configurations

An experimental program was initiated to determine the noise radiation from landing-gear/wheel-well configurations of large commercial aircraft. Scale models of typical nose gear and main gear (synthesized from different type aircraft) were exposed to flow of typical landing approach speeds (up to 65 m/sec) on a stationary outdoor wall-jet flow facility and attached to the wings of an aerodynamically very clean glider (SB-10). Landing-gear noise is composed of sound generated by the interaction of flow with 1) the wheel-well volume, and 2) the external gear equipment. Wheel-well related sound is characterized by discrete tones whose intensity is heavily damped by the spoiling effect of the gear. Externally generated landing-gear noise is mostly of broadband character. The contributions of some dominant features of a gear (shaft, struts, actuators, doors, wheels) to the total sound signature were determined and normalized nose gear and main gear spectra were developed that predict measured full-scale landing-gear noise fairly well.

Airframe noise characteristics from flyover measurements and prediction

Aircraft noise impact around airports will increase corresponding to the predicted growth in air-traffic if no measures for aircraft source noise reduction are taken or noise abatement flight procedures are developed. During the final approach phase engine noise and airframe noise are comparable in level, the latter being governed by flow noise originating from landing gears and high lift devices. Based on the results of dedicated wind tunnel studies semi-empirical/empirical airframe noise prediction schemes were developed for both high lift devices’ and landing gear noise to support the calculation of noise impact in the vicinity of airports. Within an ongoing German national research project on the development of noise abatement procedures, co-financed by the German Ministry of Education and Research (BMBF), flyover noise measurements were conducted on an Airbus A319 aiming at the validation of DLR’s airframe noise prediction schemes. In order to distinguish between airframe and engine noise sources flyovers were performed for different aircraft configurations and operational conditions.

Design and Testing of Low Noise Landing Gears

In the approach phase of large commercial aircraft, airframe noise – and in particular that from landing gears – is one of the dominant aircraft noise components. Within a European co-financed research project entitled “Significantly Lower Community Exposure to Aircraft Noise” (codenamed SILENCER) a study in “advanced low noise landing gear design” was performed to develop operational landing gears which take into account aeroacoustic constraints early in the design stage. Airbus aircraft typical configurations of low wing with underslung engines and A340 type gears were selected as reference. RANS flow field calculations were performed and used to identify and thus avoid the impingement of high speed flow onto critical gear structure elements. The evaluation of CFD results with respect to the effects on aerodynamic noise was performed on the basis of related experimental experience and a semi-empirical landing gear noise model. Both low noise advanced nose and main landing gears were designed and manufactured at full scale for noise testing in the 8 m by 6 m open test section of the German-Dutch Wind Tunnel (DNW-LLF).

Research into Landing Gear Airframe Noise Reduction

Servere air-traffic constraints are imposed by commercial airports following public complaints about the ever increasing noise impact. This situation lead to renewed emphasis by aviation industry into aircraft noise source research. While the engine noise still dominates during aircraft full-power take-off, it is the airframe noise which represents the essential contributor to the overall flyover noise signature in the landing phase for todays high-bypass ratio engine powered large commercial aircraft. Therefore in Europe a research project was launched aiming at the reduction of landing gear noise, which is the dominant airframe noise component for wide-body aircraft and the subject area of this paper. Based on the knowledge of a state-of-the-art full scale A340 nose- and main-landing gear baseline noise test, realistic noise reduction add-on devices were developed for these gears and the devices effectiveness tested in a dedicated wind tunnel study. Both farfield noise data were taken and techniques employed for noise source localization. By removing the noise reduction devices orderly, their individual effectiveness were assessed. It was demonstrated that significant noise reduction can be achieved with add-on devices which have been designed to cause minimum interference with respect to the gears functionality and maintainability.

Model and full scale high-lift wing wind tunnel experiments dedicated to airframe noise reduction

During landing approach, airframe noise has become a significant contributor to the overall radiated noise from commercial aircraft, when propelled by quiet high-bypass-ratio engines. The major sources of airframe noise are the landing gears and the wing high-lift devices (HLD). In view of European aviation industry to design and build a very large commercial aeroplane, the A3XX, a German National Research Project was initiated, culminating in a series of model- und full scale wind tunnel experiments on HLD. This paper discusses recent results from HLD-studies in the German Dutch Wind Tunnel (DNW) on a 1/7.5 scaled complete model aircraft and the outer section of an A320 full scale wing, employing farfield microphones, unsteady pressure instrumentation and source localization techniques to quantify airframe noise levels and identify the major aeroacoustic sources. The tests provided a baseline data set for the development of noise prediction schemes. Test results obtained on the full scale wing section revealed the importance of excess noise from construction details of a real wing HLD. Tonal components in scale model flap side-edge surface pressure spectra were found to originate from scale model Reynolds number effects. The acoustic effectiveness of initial noise reduction concepts was assessed.