Leonard V. Lopes

Design of the Next Generation Aircraft Noise Prediction Program: ANOPP2

The requirements, constraints, and design of NASAs next generation Aircraft NOise Prediction Program (ANOPP2) are introduced. Similar to its predecessor (ANOPP), ANOPP2 provides the U.S. Government with an independent aircraft system noise prediction capability that can be used as a stand-alone program or within larger trade studies that include performance, emissions, and fuel burn. The ANOPP2 framework is designed to facilitate the combination of acoustic approaches of varying fidelity for the analysis of noise from conventional and unconventional aircraft. ANOPP2 integrates noise prediction and propagation methods, including those found in ANOPP, into a unified system that is compatible for use within general aircraft analysis software. The design of the system is described in terms of its functionality and capability to perform predictions accounting for distributed sources, installation effects, and propagation through a non-uniform atmosphere including refraction and the influence of terrain. The philosophy of mixed fidelity noise prediction through the use of nested Ffowcs Williams and Hawkings surfaces is presented and specific issues associated with its implementation are identified. Demonstrations for a conventional twin-aisle and an unconventional hybrid wing body aircraft configuration are presented to show the feasibility and capabilities of the system. Isolated model-scale jet noise predictions are also presented using high-fidelity and reduced order models, further demonstrating ANOPP2s ability to provide predictions for model-scale test configurations.

Near Real-Time Simulation of Rotorcraft Acoustics and Flight Dynamics

In this paper, a near-real-time rotorcraft flight dynamics-acoustics prediction system is presented. The highfidelity PSU-WOPWOP rotor noise prediction code is coupled with the GENHEL flight simulation code, which provides low-fidelity blade loading and motion. This system is an initial step intended to investigate the feasibility of real-time rotorcraft noise prediction and to demonstrate the utility of such a system. Limited acoustic validation is shown for a contemporary-design four-bladed main rotor in level flight. A complex 80-s maneuver was used to demonstrate the potential of the coupled system. This realistic maneuver includes a climb, coordinated turn, and level flight conditions. The noise predictions show changes in main rotor noise radiation strength and directivity caused by maneuver transients, aircraft attitude changes, and the aircraft flight—but do not include the effect of blade-vortex-interaction noise. A comparison of the total noise with the thickness and loading noise components helps explain the noise directivity. The computations for a single observer were very fast, although the algorithm is not currently organized as a real-time computation.

System Noise Assessment and the Potential for a Low Noise Hybrid Wing Body Aircraft with Open Rotor Propulsion

An aircraft system noise assessment was conducted for a hybrid wing body freighter aircraft concept configured with three open rotor engines. The primary objective of the study was to determine the aircraft system level noise given the significant impact of installation effects including shielding the open rotor noise by the airframe. The aircraft was designed to carry a payload of 100,000 lbs on a 6,500 nautical mile mission. An experimental database was used to establish the propulsion airframe aeroacoustic installation effects including those from shielding by the airframe planform, interactions with the control surfaces, and additional noise reduction technologies. A second objective of the study applied the impacts of projected low noise airframe technology and a projection of advanced low noise rotors appropriate for the NASA N+2 2025 timeframe. With the projection of low noise rotors and installation effects, the aircraft system level was 26.0 EPNLdB below Stage 4 level with the engine installed at 1.0 rotor diameters upstream of the trailing edge. Moving the engine to 1.5 rotor diameters brought the system level noise to 30.8 EPNLdB below Stage 4. At these locations on the airframe, the integrated level of installation effects including shielding can be as much as 20 EPNLdB cumulative in addition to lower engine source noise from advanced low noise rotors. And finally, an additional set of technology effects were identified and the potential impact at the system level was estimated for noise only without assessing the impact on aircraft performance. If these additional effects were to be included it is estimated that the potential aircraft system noise could reach as low as 38.0 EPNLdB cumulative below Stage 4.

Development of a Low-Cost, Multi-Disciplinary Rotorcraft Simulation Facility

A low-cost rotorcraft simulation facility was developed for use in university-based multidisciplinaryresearchprograms.Theobjectivewastodevelopaflexibleandeffectiveresearch facility with a low initial cost and minimal recurring costs. The simulation facility was constructedentirelyfromcommerciallyavailablecommodityhardwarecomponents.Several computers are linked together via a local area network to form a “graphical cluster,” which allows for interaction between multiple computing nodes and multiple displays. The facility uses the open source FlightGear flight simulation code and the U.S.Army/NASA GENHEL flight dynamics model. In addition, a graphical user interface and a suite of data processing and analysis tools were developed using MATLAB. The system is being used for advanced research programs in the areas of flight control design, advanced rotorcraft flight dynamics modeling, and near real-time acoustics simulation.The use of open source software provides needed flexibility and cost-effectiveness, although incomplete or nonexistent documentation presented challenges. Overall, the simulation facility fulfills the needs of the university research environment in the areas of control design and rotorcraft acoustics. Some weaknesses related to control feel were noted, but were found to be acceptable relative to the low-cost nature of the facility.

Complex Landing Gear Noise Prediction Using a Simple Toolkit

This paper describes the initial development of a method for the prediction of the noise radiated by aircraft landing gear. Called the Landing Gear Model and Acoustic Prediction (LGMAP), it will eventually include all the geometric complexity of a realistic landing gear. This will be achieved by dividing the gear into a number of elements or objects. The noise from each of these elements is described by a simple acoustic model. Each object has three attributes; its geometry and location, and an upstream and downstream environment. This enables the ∞ow or noise from one element to interact with any other. The method is designed to allow improved element acoustic models to be introduced as they become available. This paper contains some initial examples for two objects; a cylinder element and a wheel model. The landing gear is divided into assemblies made up of these elements. The radiated noise is calculated in the time{ domain using a source{time{dominant solution to the Ffowcs Williams{Hawkings equation. This initial, rather crude, model is calibrated by comparison with experiment and existing noise prediction methods. The purpose of this paper is primarily to introduce the modeling philosophy rather than make extensive predictions. Though more than one example is given. The model is still in its early development stages and many important acoustic mechanisms are not included. Some of the future plans and necessary extensions to the model are discussed.

Noise Scaling and Community Noise Metrics for the Hybrid Wing Body Aircraft

An aircraft system noise assessment was performed for the hybrid wing body aircraft concept, known as the N2A-EXTE. This assessment is a result of an effort by NASA to explore a realistic HWB design that has the potential to substantially reduce noise and fuel burn. Under contract to NASA, Boeing designed the aircraft using practical aircraft design princip0les with incorporation of noise technologies projected to be available in the 2020 timeframe. NASA tested 5.8% scale-mode of the design in the NASA Langley 14- by 22-Foot Subsonic Tunnel to provide source noise directivity and installation effects for aircraft engine and airframe configurations. Analysis permitted direct scaling of the model-scale jet, airframe, and engine shielding effect measurements to full-scale. Use of these in combination with ANOPP predictions enabled computations of the cumulative (CUM) noise margins relative to FAA Stage 4 limits. The CUM margins were computed for a baseline N2A-EXTE configuration and for configurations with added noise reduction strategies. The strategies include reduced approach speed, over-the-rotor line and soft-vane fan technologies, vertical tail placement and orientation, and modified landing gear designs with fairings. Combining the inherent HWB engine shielding by the airframe with added noise technologies, the cumulative noise was assessed at 38.7 dB below FAA Stage 4 certification level, just 3.3 dB short of the NASA N+2 goal of 42 dB. This new result shows that the NASA N+2 goal is approachable and that significant reduction in overall aircraft noise is possible through configurations with noise reduction technologies and operational changes.

Prediction of Landing Gear Noise Reduction and Comparison to Measurements

Noise continues to be an ongoing problem for existing aircraft in flight and is projected to be a concern for next generation designs. During landing, when the engines are operating at reduced power, the noise from the airframe, of which landing gear noise is an important part, is equal to the engine noise. There are several methods of predicting landing gear noise, but none have been applied to predict the change in noise due to a change in landing gear design. The current effort uses the Landing Gear Model and Acoustic Prediction (LGMAP) code, developed at The Pennsylvania State University to predict the noise from landing gear. These predictions include the influence of noise reduction concepts on the landing gear noise. LGMAP is compared to wind tunnel experiments of a 6.3%-scale Boeing 777 main gear performed in the Quiet Flow Facility (QFF) at NASA Langley. The geometries tested in the QFF include the landing gear with and without a toboggan fairing and the door. It is shown that LGMAP is able to predict the noise directivities and spectra from the model-scale test for the baseline configuration as accurately as current gear prediction methods. However, LGMAP is also able to predict the difference in noise caused by the toboggan fairing and by removing the landing gear door. LGMAP is also compared to far-field ground-based flush-mounted microphone measurements from the 2005 Quiet Technology Demonstrator 2 (QTD 2) flight test. These comparisons include a Boeing 777-300ER with and without a toboggan fairing that demonstrate that LGMAP can be applied to full-scale flyover measurements. LGMAP predictions of the noise generated by the nose gear on the main gear measurements are also shown.

Framework for a Landing-Gear Model and Acoustic Prediction

numbers. The initial landing-gear-model-and-acoustic-prediction predictions for dressed configurations show an increase in noise in the frequency range representative of the added landing-gear components. The landing-gear model and acoustic prediction is also compared with wind-tunnel measurements for much larger landing-gear geometry,measuredintheVirginiaPolytechnicInstituteandStateUniversityacousticwindtunnel.Predictionsshow the ability of the landing-gear model and acoustic prediction to predict the contribution of each component to the overall values. Although the landing-gear model and acoustic prediction is in an early stage of development, the present agreement between the calculations and measurements suggests the method has promise for future application in the prediction of airframe noise.

Open Rotor Tone Shielding Methods for System Noise Assessments Using Multiple Databases

Advanced aircraft designs such as the hybrid wing body, in conjunction with open rotor engines, may allow for significant improvements in the environmental impact of aviation. System noise assessments allow for the prediction of the aircraft noise of such designs while they are still in the conceptual phase. Due to significant requirements of computational methods, these predictions still rely on experimental data to account for the interaction of the open rotor tones with the hybrid wing body airframe. Recently, multiple aircraft system noise assessments have been conducted for hybrid wing body designs with open rotor engines. These assessments utilized measured benchmark data from a Propulsion Airframe Aeroacoustic interaction effects test. The measured data demonstrated airframe shielding of open rotor tonal and broadband noise with legacy F7/A7 open rotor blades. Two methods are proposed for improving the use of these data on general open rotor designs in a system noise assessment. The first, direct difference, is a simple octave band subtraction which does not account for tone distribution within the rotor acoustic signal. The second, tone matching, is a higher-fidelity process incorporating additional physical aspects of the problem, where isolated rotor tones are matched by their directivity to determine tone-by-tone shielding. A case study is conducted with the two methods to assess how well each reproduces the measured data and identify the merits of each. Both methods perform similarly for system level results and successfully approach the experimental data for the case study. The tone matching method provides additional tools for assessing the quality of the match to the data set. Additionally, a potential path to improve the tone matching method is provided.

An initial analysis of transient noise in rotorcraft maneuvering flight

This paper examines the additional noise produced by a helicopter rotor during a short-time maneuver, which we call “transient maneuver noise.” A coupled flight simulation/noise prediction system was developed to predict the low-frequency noise generated during transient maneuvers. The system is partially validated for steady noise predictions in this paper. A simulation of a complex 80-second maneuver flight is performed and transient maneuver noise is identified both in the entry and exit of a coordinated turn. The characteristics of the transient maneuver noise were examined in detail by studying the turn entry. The noise generated during short-duration maneuvers is due to multiple sources: aperiodic blade motions, aircraft attitude changes, and transient aerodynamic loading. More aggressive maneuvers produce higher noise levels. The primary effect of a transient maneuver on thickness noise is to change the acoustic directivity, while both loading noise directivity and levels change significantly during aggressive maneuvers.