Heino Buchholz

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.

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.

In-flight Sound Measurements: A First Overview

A series of flight tests were carried out in June 2011 with more than 250 sensors in a cabin cross section upstream of the wings. The main purpose of the flight test was to qualify and quantify the main sources of cabin noise as well as the transfer paths to the passenger under real flight condition. The sensor set consists of pressure transducers installed in three dummy windows, accelerometers on the fuselage and the cabin and microphones inside the cabin. While varying the flight conditions by changing flight altitude, thrust and speed the main noise sources were distinguished and qualified.

Installation Effects of a Propeller Mounted on a High-Lift Wing with a Coanda Flap. Part I: Aeroacoustic Experiments

In this contribution, we present aeroacoustic experiments concerning installation effects of propellers. Such installation effects are important as they can significantly alter the sound radiation as compared to an isolated propeller. For this purpose, detailed experiments have been conducted in the NWB aeroacoustic wind tunnel in Braunschweig, Germany. The considered geometry is a nine-bladed propeller installed in front of a high-lift wing (employing a Coanda flap). The results illustrate the influence of propeller rotational speed, blade pitch angle, wind tunnel velocity, and angle of attack variations on the sound radiation. Furthermore, with a source localisation technique insight is gained in the dominant sound sources, and reveals the importance of periodic as well as broadband noise for the considered geometry.

Contributions of Different Aeroacoustic Sources to Aircraft Cabin Noise

The turbulent boundary layer (TBL) on the fuselage, jet noise and the air conditioning system (ACS) are considered as three important aeroacoustic sources of aircraft cabin noise. To improve current cabin noise prediction approaches as well as to investigate the different noise sources and their respective noise transfer paths, flight tests with DLR’s A320-232 research aircraft ’D-ATRA’ were carried out within the German national (LuFo IV) project SIMKAB. Extensive measurement data were collected using microphones inside the cabin, unsteady surface-pressure sensors for the characterization of the external TBL- and jet noise induced fuselage excitation, and accelerometers mounted at the frame structure, fuselage skin fields and cabin panels. Flight speed and -level as well as engine and air conditioning system operating conditions were varied to separately evaluate their parametric effects on cabin noise. The analysis of this extensive data base is still ongoing; in the current paper the focus is set on the results from microphone measurements at various longitudinal positions inside the cabin. Both TBL- and jet noise induced contributions increase towards the rear, reflecting the natural growth of the TBL thickness and typical jet noise radiation characteristics. Contrary to that the air conditioning system noise is of minor importance.

Characteristics of Noise from Aircraft Ground Operations

Noise impact in the surroundings of airports is not only governed by noise from departing and approaching aircraft but is also strongly affected by noise from ground operations, i.e. noise from decelerating or accelerating aircraft on the runway in particular, for which only few data is available in the open literature. Therefore a noise data base was acquired for different commercial aircraft coming in and leaving Munich Airport International according to the daily normal schedule. Noise spectra and directivities were determined for aircraft during take-off or landing with reverse thrust, respectively. In addition noise data were taken from aircraft during taxiing and for APU operation on the hub. While also noise directivities were acquired for different aircraft with either the APU operating alone or together with the air-condition system pass by noise levels from taxiing aircraft were just documented as level-time traces. From these records maximum overall A-weighted noise levels were determined for comparison with those from take-off and reverse thrust operation of landing aircraft, the latter being about 10 dB noisier compared to taxiing or APU operation.