An Analytical Assessment of NASA's N+1 Subsonic Fixed Wing Project Noise Goal

The Subsonic Fixed Wing Project of NASA's Fundamental Aeronautics Program has adopted a noise reduction goal for new, subsonic, single-aisle, civil aircraft expected to replace current 737 and A320 airplanes. These so-called 'N+1' aircraft - designated in NASA vernacular as such since they will follow the current, in-service, 'N' airplanes - are hoped to achieve certification noise goal levels of 32 cumulative EPNdB under current Stage 4 noise regulations. A notional, N+1, single-aisle, twinjet transport with ultrahigh bypass ratio turbofan engines is analyzed in this study using NASA software and methods. Several advanced noise-reduction technologies are analytically applied to the propulsion system and airframe. Certification noise levels are predicted and compared with the NASA goal

Towards an Airframe Noise Prediction Methodology: Survey of Current Approaches

In this paper, we present a critical survey of the current airframe noise (AFN) prediction methodologies. Four methodologies are recognized. These are the fully analytic method, CFD combined with the acoustic analogy, the semi-empirical method and fully numerical method. It is argued that for the immediate need of the aircraft industry, the semi-empirical method based on recent high quality acoustic database is the best available method. The method based on CFD and the Ffowcs William- Hawkings (FW-H) equation with penetrable data surface (FW-Hpds ) has advanced considerably and much experience has been gained in its use. However, more research is needed in the near future particularly in the area of turbulence simulation. The fully numerical method will take longer to reach maturity. Based on the current trends, it is predicted that this method will eventually develop into the method of choice. Both the turbulence simulation and propagation methods need to develop more for this method to become useful. Nonetheless, the authors propose that the method based on a combination of numerical and analytical techniques, e.g., CFD combined with FW-H equation, should also be worked on. In this effort, the current symbolic algebra software will allow more analytical approaches to be incorporated into AFN prediction methods

The HART II Test - Measurement of Helicopter Rotor Wakes

A key parameter to reduce rotor noise is a wake location relative to a blade. During the data analysis in the years after the HART test of 1994 it was found that comprehensive wake data are needed to understand wake development, vortex creation and aging, their motion and their re-development after an interaction with a rotor blade. Therefore, DLR, ONERA, NASA Langley, US Army AFDD and DNW again came together and generated a follow-on program named HART II. In this program the rotor wake was intensively measured with a novel double-stereo PIV (Particle Image Velocimetry) system. This provides instantaneous flow field data in both a large observation area and in a smaller close-up view to the vortex core. Additionally, rotor blade deformation, airloads, and noise radiation were measured to form a complete, consistent, and comprehensive data base. Another first time was the application of the blade position measurement technique SPR (Stereo Pattern Recognition), which provided instantaneous optical measurement of the blade bending in flap, lead-lag and torsion. In this paper, the HART II test is described in detail, including a description of all measurement techniques applied and with some results from each of them

Blade Wake Interaction Noise for a BO-105 Model Main Rotor

Fundamental characteristics of Blade Wake Interaction (BWI) noise are determinated, and a prediction method is developed, using acoustic and blade pressure data from a 4-bladed BO-105 model helicopter main rotor. BWI is a broadband noise source caused by the blade interactions with turbulence in the rotor wake, particularly about the tip vortices. The data are from the rotor aeroacoustic test program called HART which was jointly conducted by European and American government agencies in the German-Dutch Wind Tunnel, DNW. The test datainclude simultaneous measurements of instantaneous blade pressures and acoustic time histories as well as averaged data. The statistical character of the BWI is determined from the instantaneous surface pressures. A BWI noise metric is developed and BWI noise directivity contours (similar to those typically presented for BVI noise) are presented and shown to be a function of rotor tip-path-plane angle. Similarly, BWI

HHC Aeroacoustic Rotor Tests in the German-Dutch Wind Tunnel: Improving Physical Understanding and Prediction Codes

Within an international research program jointly conducted by US Army's Aeroflightdynamics Directorate (AFDD), NASA Langley, DNW, ONERA and DLR, the effects of a higher harmonic blade pitch angle on the rotor noise and vibrations were internsively studied. The program, denoted with Higher Harmonic Control Aeroacoustic Rotor Tests (HART) comprised both theoretical and experimental activities,the latter ones being represented by a comprehensive wind tunnel test campaign. Besides aeroacoustic measurements it comprised flow visualization tests and flow field measurements which were applied along with optical methods to determine the blade deflections. The measurements were preceded by extensive simulations performed with the analytical models of the individual participating organizations. Within these simulating different models for the rotor downwash, the blade aerodynamics and the rotor dynamics were used, leading to a very comprehensive theoretical data base