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NASA NTRS2011

Core Noise Reduction

This presentation is a technical summary of and outlook for NASA-internal and NASA-sponsored external research on core (combustor and turbine) noise funded by the Fundamental Aeronautics Program Subsonic Fixed Wing (SFW) Project. Sections of the presentation cover: the SFW system-level noise metrics for the 2015, 2020, and 2025 timeframes; turbofan design trends and their aeroacoustic implications; the emerging importance of core noise and its relevance to the SFW Reduce-Perceived-Noise Technical Challenge; and the current research activities in the core noise area. Recent work1 on the turbine-transmission loss of combustor noise is briefly described, two2,3 new NRA efforts in the core-noise area are outlined, and an effort to develop CMC-based acoustic liners for broadband noise reduction suitable for turbofan-core application is delineated. The NASA Fundamental Aeronautics Program has the principal objective of overcoming today's national challenges in air transportation. The reduction of aircraft noise is critical to enabling the anticipated large increase in future air traffic. The Subsonic Fixed Wing Project's Reduce-Perceived-Noise Technical Challenge aims to develop concepts and technologies to dramatically reduce the perceived aircraft noise outside of airport boundaries.

Hultgren, Lennart S.↗

NASA NTRS2012

Uncertainty Analysis of the Grazing Flow Impedance Tube

This paper outlines a methodology to identify the measurement uncertainty of NASA Langley s Grazing Flow Impedance Tube (GFIT) over its operating range, and to identify the parameters that most significantly contribute to the acoustic impedance prediction. Two acoustic liners are used for this study. The first is a single-layer, perforate-over-honeycomb liner that is nonlinear with respect to sound pressure level. The second consists of a wire-mesh facesheet and a honeycomb core, and is linear with respect to sound pressure level. These liners allow for evaluation of the effects of measurement uncertainty on impedances educed with linear and nonlinear liners. In general, the measurement uncertainty is observed to be larger for the nonlinear liners, with the largest uncertainty occurring near anti-resonance. A sensitivity analysis of the aerodynamic parameters (Mach number, static temperature, and static pressure) used in the impedance eduction process is also conducted using a Monte-Carlo approach. This sensitivity analysis demonstrates that the impedance eduction process is virtually insensitive to each of these parameters.

Brown, Martha C.↗

NASA NTRS2015

A Fully Nonmetallic Gas Turbine Engine Enabled by Additive Manufacturing, Part II: Additive Manufacturing and Characterization of Polymer Composites

This publication is the second part of the three part report of the project entitled "A Fully Nonmetallic Gas Turbine Engine Enabled by Additive Manufacturing" funded by NASA Aeronautics Research Institute (NARI). The objective of this project was to conduct additive manufacturing to produce aircraft engine components by Fused Deposition Modeling (FDM), using commercially available polyetherimides-Ultem 9085 and experimental Ultem 1000 mixed with 10% chopped carbon fiber. A property comparison between FDM-printed and injection molded coupons for Ultem 9085, Ultem 1000 resin and the fiber-filled composite Ultem 1000 was carried out. Furthermore, an acoustic liner was printed from Ultem 9085 simulating conventional honeycomb structured liners and tested in a wind tunnel. Composite compressor inlet guide vanes were also printed using fiber-filled Ultem 1000 filaments and tested in a cascade rig. The fiber-filled Ultem 1000 filaments and composite vanes were characterized by scanning electron microscope (SEM) and acid digestion to determine the porosity of FDM-printed articles which ranged from 25 to 31%. Coupons of Ultem 9085, experimental Ultem 1000 composites and XH6050 resin were tested at room temperature and 400F to evaluate their corresponding mechanical properties. A preliminary modeling was also initiated to predict the mechanical properties of FDM-printed Ultem 9085 coupons in relation to varied raster angles and void contents, using the GRC-developed MAC/GMC program.

Polymer↗

NASA NTRS2015

Additive Manufacturing of Ultem Polymers and Composites

The objective of this project was to conduct additive manufacturing to produce aircraft engine components by Fused Deposition Modeling (FDM), using commercially available polyetherimdes Ultem 9085 and experimental Ultem 1000 filled with 10 chopped carbon fiber. A property comparison between FDM-printed and injection molded coupons for Ultem 9085, Ultem 1000 resin and the fiber-filled composite Ultem 1000 was carried out. Furthermore, an acoustic liner was printed from Ultem 9085 simulating conventional honeycomb structured liners and tested in a wind tunnel. Composite compressor inlet guide vanes were also printed using fiber-filled Ultem 1000 filaments and tested in a cascade rig. The fiber-filled Ultem 1000 filaments and composite vanes were characterized by scanning electron microscope (SEM) and acid digestion to determine the porosity of FDM-printed articles which ranged from 25-31. Coupons of Ultem 9085, experimental Ultem 1000 composites and XH6050 resin were tested at room temperature and 400F to evaluate their corresponding mechanical properties.

Polymer↗

NASA NTRS2015

Additive Manufacturing and Characterization of Ultem Polymers and Composites

The objective of this project was to conduct additive manufacturing to produce aircraft engine components by Fused Deposition Modeling (FDM), using commercially available polyetherimides - Ultem 9085 and experimental Ultem 1000 mixed with 10 percent chopped carbon fiber. A property comparison between FDM-printed and injection-molded coupons for Ultem 9085, Ultem 1000 resin and the fiber-filled composite Ultem 1000 was carried out. Furthermore, an acoustic liner was printed from Ultem 9085 simulating conventional honeycomb structured liners and tested in a wind tunnel. Composite compressor inlet guide vanes were also printed using fiber-filled Ultem 1000 filaments and tested in a cascade rig. The fiber-filled Ultem 1000 filaments and composite vanes were characterized by scanning electron microscope (SEM) and acid digestion to determine the porosity of FDM-printed articles which ranged from 25-31 percent. Coupons of Ultem 9085 and experimental Ultem 1000 composites were tested at room temperature and 400 degrees Fahrenheit to evaluate their corresponding mechanical properties.

Polymers↗

NASA NTRS2015

Flexible Micropost Arrays for Shear Stress Measurement

Increased fuel costs, heightened environmental protection requirements, and noise abatement continue to place drag reduction at the forefront of aerospace research priorities. Unfortunately, shortfalls still exist in the fundamental understanding of boundary-layer airflow over aerodynamic surfaces, especially regarding drag arising from skin friction. For example, there is insufficient availability of instrumentation to adequately characterize complex flows with strong pressure gradients, heat transfer, wall mass flux, three-dimensionality, separation, shock waves, and transient phenomena. One example is the acoustic liner efficacy on aircraft engine nacelle walls. Active measurement of shear stress in boundary layer airflow would enable a better understanding of how aircraft structure and flight dynamics affect skin friction. Current shear stress measurement techniques suffer from reliability, complexity, and airflow disruption, thereby compromising resultant shear stress data. The state-of-the-art for shear stress sensing uses indirect or direct measurement techniques. Indirect measurements (e.g., hot-wire, heat flux gages, oil interferometry, laser Doppler anemometry, small scale pressure drag surfaces, i.e., fences) require intricate knowledge of the studied flow, restrictive instrument arrangements, large surface areas, flow disruption, or seeding material; with smaller, higher bandwidth probes under development. Direct measurements involve strain displacement of a sensor element and require no prior knowledge of the flow. Unfortunately, conventional "floating" recessed components for direct measurements are mm to cm in size. Whispering gallery mode devices and Fiber Bragg Gratings are examples of recent additions to this type of sensor with much smaller (m) sensor components. Direct detection techniques are often single point measurements and difficult to calibrate and implement in wind tunnel experiments. In addition, the wiring, packaging, and installation of delicate micro-electromechanical devices impede the use of most direct shear sensors. Similarly, the cavity required for sensing element displacement is sensitive to particulate obstruction. This work was focused on developing a shear stress sensor for use in subsonic wind tunnel test facilities applicable to an array of test configurations. The non-displacement shear sensors described here have minimal packaging requirements resulting in minimal or no disturbance of boundary layer flow. Compared to previous concepts, device installation could be simple with reduced cost and down-time. The novelty lies in the creation of low profile (nanoscale to 100 μm) micropost arrays that stay within the viscous sub-layer of the airflow. Aerodynamic forces, which are related to the surface shear stress, cause post deflection and optical property changes. Ultimately, a reliable, accurate shear stress sensor that does not disrupt the airflow has the potential to provide high value data for flow physics researchers, aerodynamicists, and aircraft manufacturers leading to greater flight efficiency arising from more in-depth knowledge on how aircraft design impacts near surface properties.

Wohl, Christopher J.↗

NASA NTRS2015

System Noise Assessment of Blended-Wing-Body Aircraft With Open Rotor Propulsion

An aircraft system noise study is presented for the Blended-Wing-Body (BWB) aircraft concept with three open rotor engines mounted on the upper surface of the airframe. It is shown that for such an aircraft, the cumulative Effective Perceived Noise Level (EPNL) is about 24 dB below the current aircraft noise regulations of Stage 4. While this makes the design acoustically viable in meeting the regulatory requirements, even with the consideration of more stringent noise regulations of a possible Stage 5 in the next decade or so, the design will likely meet stiff competitions from aircraft with turbofan engines. It is shown that the noise levels of the BWB design are held up by the inherently high noise levels of the open rotor engines and the limitation on the shielding benefit due to the practical design constraint on the engine location. Furthermore, it is shown that the BWB design has high levels of noise from the main landing gear, due to their exposure to high speed flow at the junction between the center body and outer wing. These are also the reasons why this baseline BWB design does not meet the NASA N+2 noise goal of 42 dB below Stage 4. To identify approaches that may further reduce noise, parametric studies are also presented, including variations in engine location, vertical tail and elevon variations, and airframe surface acoustic liner treatment effect. These have the potential to further reduce noise but they are only at the conceptual stage.

Guo, Yueping↗

NASA NTRS2017

Propulsion Noise Reduction Research in the NASA Advanced Air Transport Technology Project

The Aircraft Noise Reduction (ANR) sub-project is focused on the generation, development, and testing of component noise reduction technologies progressing toward the NASA far term noise goals while providing associated near and mid-term benefits. The ANR sub-project has efforts in airframe noise reduction, propulsion (including fan and core) noise reduction, acoustic liner technology, and propulsion airframe aeroacoustics for candidate conventional and unconventional aircraft configurations. The current suite of propulsion specific noise research areas is reviewed along with emerging facility and measurement capabilities. In the longer term, the changes in engine and aircraft configuration will influence the suite of technologies necessary to reduce noise in next generation systems.

Van Zante, Dale↗

NASA NTRSDate not supplied

Fan Acoustic Flight Effects on the PAA & ASN Flight Test

Accurate and reliable prediction of noise emitted from turbofan aircraft engines is an ongoing challenge. For continued progress, it is necessary to assess the performance of the most recent prediction methods in the NASA Aircraft NOise Prediction Program (ANOPP), and to examine the effects of flight condition on fan noise. The assessment is based on an extensive acoustic dataset acquired as part of the Propulsion Airframe Aeroacoustics and Aircraft System Noise (PAA&ASN) Flight Test conducted on a Boeing 787-10 aircraft. Engine operating condition was varied to study the characteristics of fan noise in flight. In a subset of the flight test matrix, the influence of the aft duct acoustic liner was studied by covering the lined area with aluminum tape to simulate a hardwall duct. The combination of test data at different flight conditions, along with rigorous predictions of jet noise, is used in this paper to extract fan noise from the measured total aircraft noise levels. The measured fan noise levels are compared with those predicted using the latest methods for fan source noise, liner attenuation, and PAA effect prediction. Detailed analyses are undertaken to understand agreement between data and various aspects of the prediction models, including spectral shape, directivity, and response to throttle setting. An additional investigation was undertaken to study the effect of aircraft speed on fan noise, as current models do not directly account for this parameter.

Aircraft System Noise↗

NASA NTRSDate not supplied

NASA’s Quiet Electric ENgines (QUEEN): Summary of the Acoustic Tests of the QUEEN V1

Noise produced by an electric ducted fan system was measured in tests at the NASA Glenn Research Center Acoustical Testing Laboratory. Main components of the system include a Commercial-Off- the-Shelf (COTS) fan and motor, an Electronic Speed Controller, a custom-designed inlet bellmouth, and several experimental inlet duct acoustic liners. Fan speed, thrust, and noise were measured in this static ground test of the propulsor. This propulsor prototype is one of NASA’s Quiet Electric ENgines (QUEENs) and is designated the ‘QUEEN V1.’ The Quiet Electric Engines are being developed for the 25% scale model of the Subsonic Aft Engine (SUSAN) Flight Research Vehicle and are intended to explore the potential of distributed electric propulsion for large regional single-aisle aircraft. Lessons learned will be used to guide development of future QUEEN prototypes. Results of a thermal test of the Electronic Speed Controller measured during this test are presented in a separate report.

L Danielle Koch↗

NASA NTRSDate not supplied

NASA’s Efficient Quiet Integrated Propulsors (Equip) Technical Challenge

Advances in aircraft propulsor technology - ducted and unducted - are key elements to developing a sustainable aviation future. Increasing propulsive efficiency comes with significant challenges. The long-term trend in aircraft engine design has been towards higher bypass ratios to increase efficiency and decrease noise. Current designs are pushing against limits on engine size; the engine must fit under the wing and overcome the nacelle weight, aerodynamic drag, and airframe integration penalties associated with an ultra-high bypass ratio. The next-generation ducted engines will have shorter inlets and smaller rotor-stator spacing to minimize nacelle area. This increases the potential for non-uniform flow at the fan face and less area for acoustic liners. Unducted engines eliminate the nacelle penalty while presenting challenges to noise and operability in distorted flow environments. The overall diameter in both cases raises questions about engine-airframe integration to maximize efficiency gains and minimize installation penalties. The National Aeronautics and Space Administration (NASA) Advanced Air Vehicles Program (AAVP) approved the Efficient Quiet Integrated Propulsor (EQuIP) Technical Challenge (TC) under the Advanced Air Transport Technology (AATT) Project to work on next-generation propulsor technologies in collaboration with NASA’s government and industry partners in the Sustainable Flight National Partnership. The EQuIP TC leverages NASA resources with Federal Aviation Administration (FAA) and industry investments under the FAA’s Continuous Lower Energy Emissions and Noise (CLEEN) program to address technical challenges on the propulsor subject to the flow field imposed by the engine-airframe-flight environment. The EQuIP TC complements NASA Aeronautics Research Mission Directorate’s (ARMD) existing investments in sustainable aviation for the next-generation of commercial aircraft and contributes to meeting the noise and efficiency goals set by the Sustainable Flight National Partnership and U.S. Aviation Climate Action Plan. This paper introduces the EQuIP TC, describes key parts of its development, and presents the background research used to scope its impact.

Propulsion↗

NASA NTRS2019

Design and Acoustic Efficacy of a Broadband Liner for the Inlet of the DGEN Aero-Propulsion Research Turbofan

Advanced fan designs (including higher bypass ratios) and shorter engine nacelles have highlighted a need for increased fan noise reduction over a broader frequency range. With these observations in mind, an acoustic liner optimization process has been developed to achieve improved broadband liner designs. A series of advanced liner design studies at increasing technology readiness levels have been conducted as the overall optimization methodology has been enhanced. As part of the analysis, the overall design and evaluation capability was extended such that external observer locations may be included in the optimization process. This enhancement will provide a much wider design space in designing advanced broadband liners. In this work, further validation was pursued through the fabrication and testing of two liner designs for the NASA Glenn DGEN Aero-propulsion Research Turbofan (DART). The DART was used to document the efficacy of acoustic liners installed in the inlet of the DGEN380. An advanced Multi-Degree of Freedom Liner was designed, fabricated, and tested, along with a traditional Single-Degree-of-Freedom Liner, and those results compared to a hardwall baseline inlet. Farfield acoustic data were acquired from and external array, evaluated and reported herein terms of overall, broadband, and tonal components of the insertion loss. The predicted attenuation results generally matched expected trends of the measured data for the conditions considered.

Liners↗

NASA NTRS1977

Analytical and experimental studies of acoustic performance of segmented liners in a compressor inlet

The performance of axially segmented (phased) acoustic treatment liners in the inlet of a compressor was investigated. Topics discussed include: (1) the validation of a theoretical procedure to predict propagation and suppression characteristics of duct liners; (2) the in-duct measurement of spinning modes; (3) investigation of phased treatment designs; (4) high Mach inlet acoustic tests; and (5) an experimental investigation of inlet turbulence. The analytical prediction for the multi-segmented treatment was found to provide the correct order of magnitude of suppression and was generally within 50% of that determined experimentally. Refinements required to improve the correlation are identified. Suppression due to high subsonic Mach number flow effects was found to become significant above an average throat Mach number of 0.65 to 0.7 and 20 PNdB was achieved with an average throat Mach number in the range of 0.80 to 0.85. For the measured turbulence in the inlet, including the axial and circumferential turbulence intensities and the axial integral length scale, data are presented with and without an inlet screen showing that the screen reduced the turbulence intensities and that the BPF noise was reduced as a consequence.

Motsinger, R. E.↗

NASA NTRS1982

A method to determine the acoustical properties of locally and nonlocally reacting duct liners in grazing flow

The acoustical properties of locally and nonlocally reacting acoustical liners in grazing flow are described. The effect of mean flow and shear flow are considered as well as the application to rigid and limp bulk reacting materials. The axial wavenumber of the least attenuated mode in a flow duct is measured. The acoustical properties of duct liners is then deduced from the measured axial wavenumber and known flow profile and boundary conditions. This method is a natural extension of impedance-like measurements.

Succi, G.↗

NASA NTRS1981

Noise suppression characteristics of peripherally segmented duct liners

The acoustic fields and transmission losses produced in semi-infinite circular ducts with peripherally segmented liners are analyzed using a series expansion of hard-wall duct modes. The coefficients of the series are computed using Galerkin's method. Unlike finite element approaches, this analysis includes the effects of realistic sources and the number of peripheral strips need not be small. It is shown that peripherally segmented liners redistribute the acoustic energy in waves composed of only a single circumferential mode at the source into other waves which contain a multitude of circumferential modes in the lined section. The accuracy of eigenfunctions computed from the analysis was observed to increase as either the frequency or radial mode order increased. The transmission losses were found to be accurate at frequencies above the cut-on value of the first-order radial mode in a hard-wall duct. The results show that for plane wave sources, peripherally segmented liners may attenuate as much sound as an optimized uniform liner at the optimal point while giving more noise suppression at most other frequencies.

Watson, W. R.↗

NASA NTRS1976

Prediction of the acoustic impedance of duct liners

Recent research which contributes to the prediction of the acoustic impedance of duct liners is reviewed. This review includes the linear and nonlinear properties of sheet and bulk type materials and methods for the measurement of these properties. It also includes the effect of grazing flow on the acoustic properties of materials. Methods for predicting the properties of single or multilayered, point reacting or extended reaction, and flat or curved liners are discussed. Based on this review, methods for predicting the properties of the duct liners which are typically used in aircraft engines are recommended. Some areas of needed research are discussed briefly.

Zorumski, W. E.↗

NASA NTRSDate not supplied

Assessment of Acoustic Behavior for Perforate-Over-Large-Cell Liners

This study explores the effects of increasing the cell size for large-cell acoustic liners. Tests are conducted in the NASA Langley Grazing Flow Impedance Tube to evaluate liners with increasingly larger cell dimensions (up to 2” x 4” cross-section). Conventional impedance eduction confirms that liners with 2” x 3” cells (or larger) must be evaluated using nonlocally reacting assumptions. In addition, due to the sound propagation within cavities, the liners must be modeled using higher fidelity techniques. Thus, grazing flow duct and acoustic liner are modeled simultaneously using finite element methods. The facesheet is modeled using a transfer impedance, while the rest of the domain is modeled using the convected Helmholtz equation. The acoustic pressures predicted are shown to compare favorably with those measured in the Grazing Flow Impedance Tube.

large diameter↗

NASA NTRS2019

Acoustic Phased Array Quantification of Quiet Technology Demonstrator 3 Advanced Inlet Liner Noise Component

Acoustic phased array flyover noise measurements were acquired as part of the Boeing 737 MAX-7 NASA Advanced Inlet Liner segment of the Quiet Technology Demonstrator 3 (QTD3) flight test program. This paper reports on the processes used for separating and quantifying the engine inlet, exhaust and airframe noise source components and provides sample phased array-based comparisons of the component noise source levels associated with the inlet liner treatment configurations. Full scale flyover noise testing of NASA advanced inlet liners was conducted as part of the Quiet Technology Demonstrator 3 flight test program in July and August of 2018. Details on the inlet designs and testing are provided in the companion paper of Reference 1. The present paper provides supplemental details relating to the acoustic phased array portion of the analyses provided in Ref. 1. In brief, the test article was a Boeing 737MAX-7 aircraft with a modified right hand (starboard side) engine inlet, which consisted of either a production inlet liner, a NASA designed inlet liner or a simulated hard wall configuration (accomplished by applying speed tape over the inlet acoustic treatment areas). In all three configurations, the engine forward fan case acoustic panel was replaced with a unperforated (hardwall) panel. No other modifications to any other acoustic treatment areas were made. The left hand (port side) engine was a production engine and was flown at idle thrust for all measurements in order to isolate the effects of the inlet liners to the right hand engine. As described in Ref. 1, the NASA inlet treatment consists of laterally cut slots (cut perpendicular to the flow direction) which are designed to reduce excrescence drag while maintaining or exceeding the liner acoustic noise reduction capabilities. The NASA inlet liner consists of a Multi-Degree of Freedom (MDOF) design with two breathable septum layers inserted into each honeycomb cell [1]. The aircraft noise measurements were acquired for both takeoff (flaps 1 setting, gear up) and approach (flaps 30 gear up and gear down) configurations. The inlet and flight test configurations are summarized in Table 1. Table 1: Inlet Treatment and Flight Configurations Inlet Forward Fan Case Aircraft Production Hardwall Flaps 1, gear up; flaps 30 gear up; flaps 30 gear down NASA Hardwall Flaps 1, gear up; flaps 30 gear up; flaps 30 gear down Hardwall Hardwall Flaps 1, gear up; flaps 30 gear up; flaps 30 gear down III.Test Description and Hardware The flight testing was conducted at the Grant County airport in Moses Lake, WA, between 27 July and 6 August 2018. The noise measurement instrumentation included 8 flush dish microphones arranged in a noise certification configuration as well as an 840 microphone phased array. The flush dish microphones were used to quantify the levels and differences in levels between the various inlet treatments. The phased array was used to separate and quantify the narrowband (tonal) and broadband noise component levels from the engine inlet/exhaust and from the airframe. Phased array extraction of the broadband component was critical to this study because it allowed for the separation of the inlet component from the total airplane level noise even when it was significantly below the total level. Figure 1 provides an overview of the phased array microphone layout as well as a detailed image of an individual phased array microphone mounted in a plate holder (the microphone sensor is the dot in the center of the plate). The ground plane ensemble array microphones (referred to as “ensemble array” in this paper) were mounted in plates with “flower petal” edges designed to minimize edge scattering effects. Fig. 1 Flyover test microphone layout. The phased array configuration was the result of a progressive development of concepts originally implemented in Ref. 2 and refined over the following years, consisting namely of multiple multi-arm logarithmic spiral subarrays designed to cover overlapping frequency ranges and optimized for various aircraft emission angles. For the present case, the signals from all 840 microphones were acquired on a single system. The 840 microphones were parsed into 11 primary subarray sets spanning from smallest to largest aperture size and labeled accordingly as “a, b, …, k”, where “a” corresponds to the smallest fielded subarray and “k” corresponds to the largest aperture subarray. The apertures ranged from approximately 10 ft to 427 ft in size (in the flight direction) with the subarrays consisting of between 215 and 312 microphones. Figure 2 shows three such subarrays, k, h and a. As done in Ref. 2, microphones were shared between subarrays in order to reduce total channel count. Fig. 2 Sample subarray sizes (20° from overhead – refer to Figure 3a discussion). In addition to the above, each of the 11 primary subarray sets consisted of four subarrays optimized to provide near equivalent array spatial resolution in both the flight and lateral directions within 30 degrees of overhead (i.e., airplane directly above the center of the array), namely, at angles  of 0, ±10, ±20 and ±30 degrees relative to overhead where angle  is defined as shown in Figure 3a. This allowed for optimized aircraft noise measurements from 60 to 120 degree emission angle.6 An example of this pletharray design is shown in Figure 3b for the k subarray. When the aircraft is at overhead, the microphones indicated by the blue markers are used for beamforming. When the aircraft is at angles ±10 degrees from overhead, both the blue and red colored microphones are used, and so on for the ±20 and ±30 degree aircraft locations. See Ref. 3 for extensive details on pletharray design for aeroacoustic phased array testing. 6 In the discussions that follow, emission angle values are used. These are the angles at the time sound is emitted relative to the engine axis and are calculated based on flight path angle, body aircraft body angle with respect to the relative wind direction, and engine axis angle relative to aircraft body angle.

Brusniak, Leon↗