About

Eric Greenwood is a faculty member within Penn State's Department of Aerospace Engineering, which has a distinguished record of research excellence and scholarship. The department's research spans traditional disciplines associated with aeronautics and astronautics, with particular strengths in rotorcraft and aero-acoustics. The department is also expanding into new areas driven by increasing computational power for design, analysis, and on-board autonomy, as well as addressing industry challenges such as sustainable aviation and the growth of space systems. The faculty, including Professor Greenwood, are involved in leading research efforts, including the Vertical Lift and Rotorcraft Center of Excellence (VLRCOE), and participate in large, multidisciplinary projects supported by significant research funding. The department's faculty include experienced researchers and emerging scholars, many of whom have received prestigious awards and honors, such as NSF CAREER, DoD Young Investigator awards, and fellowships in professional societies. The department's research efforts are complemented by state-of-the-art facilities and collaborations across the university and with external research labs.

Research topics

• Computer Science
• Engineering
• Aerospace engineering
• Physics
• Acoustics
• Telecommunications
• Electrical engineering
• Mechanical engineering
• Structural engineering
• Aeronautics
• Geometry
• Computer vision
• Optics
• Mathematics
• Mechanics
• Simulation

Selected publications

• Broadband Noise Behavior in Coaxial Co-Rotating Rotors

  2025-05-20

  article

  This paper investigates the relationship between broadband noise behavior and helical wake structure in coaxial corotating rotors. Experimental measurements were conducted across variations in collective pitch (9.4°, 12.5°, and 15.0°) and rotor speeds (1500–4500 RPM). The inflow ratio (λ) was shown to govern the slope of broadband noise trends mapped in phase offset versus separation distance space, with experimental and theoretical λ values agreeing within 1%. Tip vortex core growth was estimated using the Ramasamy-Leishman model and normalized by the blade tip chord, reflecting the location of tip vortex formation. Across collective pitch variations, initial vortex core radii ranged between 7.5% and 9.1% and across rotor speeds, it ranged between 7.5% to 8.5% of the blade tip chord. When broadband noise trends became less coherent across phase offset angles, the corresponding vortex core radii were observed to approach or exceed 10% of the tip chord. At 4500 and 3500 RPM, vortex cores radii remained near or slightly above 10% at diffusion and depletion of broadband noise coherence. At 1500 RPM, initial vortex core radius is significantly beyond 10% of blade tip chord, leading to early diffusion, loss of organized wake structures, and broadband noise saturation across all phase offsets. If an inboard chord length were used instead for normalization, the specific normalized values would differ, but the overall trends in vortex diffusion and broadband noise coherence would remain similar. These results define a practical RPM threshold below which coherent wake structures and slope-based inflow prediction are no longer valid. Overall, the findings offer a framework for anticipating wake breakdown and broadband noise saturation, providing guidance for rotor design and operation to preserve low-noise performance.

  PublisherDOI

• Broadband noise modulation of multirotor aircraft

  The Journal of the Acoustical Society of America · 2025-02-01 · 8 citations

  articleSenior author

  Rotor broadband noise is typically analyzed over time scales encompassing multiple rotor periods. However, modulation of broadband noise levels with the blade passage frequency has been shown to be significant for human perception of wind turbine and helicopter noise. In contrast, broadband noise modulation has not been extensively studied for aircraft with many rotors, such as unmanned aerial vehicles (UAVs) or advanced air mobility aircraft. In this work, significant broadband noise modulation was measured in flight tests and anechoic chamber experiments of hexacopter UAVs. The amplitude of this modulation depended on the azimuthal phase offsets between rotors, demonstrating the potential for synchrophasing control to reduce broadband noise modulation, analogous to synchrophasing control of tonal noise. If rotors are not synchronized, as in typical flight, the azimuthal phase offsets between rotors vary with time. This variation was found to follow a uniform random distribution, resulting in modulation depth also varying randomly with time. The probability distribution of modulation depth was computed using offset copies of the modulation of a single rotor. These results contribute understanding to how the broadband noise modulation of rotors sum together, and showed that this modulation is likely to be significant in flight.

  PublisherDOI

• An Experimental Evaluation of an Electronic Rotor Phase Synchronization System for Multirotor Aircraft Noise Control

  Journal of the American Helicopter Society · 2025-09-12 · 2 citations

  articleOpen access

  Multirotor electric vertical take-off and landing vehicles and small unmanned aircraft systems commonly use distributed electric propulsion. One approach to reducing their noise involves synchronizing rotor phase relationships to control radiated noise directionally. This paper presents a practical electronic phase synchronization method for multirotor aircraft, experimentally evaluates its effectiveness using a hexacopter in an anechoic chamber, and investigates how vehicle configuration influences noise control capability. Harmonic noise reductions of 10 dB are demonstrated over a 20° azimuthal arc, with limited increases in other directions. Noise amplification of 6 dB is also achieved over the same region. Global reductions of 6 dB across nearly all observer angles are obtained using alternative rotor phase offsets. Statistical analysis of phase controller error indicates that maintaining phase accuracy within 5??? is necessary for 10 dB attenuation. Simulations across various rotor configurations show consistent noise control performance, with higher rotor counts enabling greater directional control of radiated noise.

  PublisherDOI

• Passive Blade–Vortex Interaction Noise Reduction Using Embedded In-Blade Resonator Cavities

  AIAA Journal · 2025-08-06

  articleSenior author

  A novel passive blade–vortex interaction noise reduction concept is proposed that integrates open-closed resonator cavities within helicopter rotor blades. These cavities attenuate blade–vortex interaction noise through two physical mechanisms. First, they weaken the unsteady surface pressure fluctuations caused by the blade–vortex interactions. Secondly, they disrupt the acoustic focusing process associated with blade–vortex interaction noise. A midfidelity computational framework is used to design and evaluate the effectiveness of the treatment. Two variations of the treatment are considered: one consisting of uniform-depth cavities and the other of variable-depth cavities. The effects of varying the spanwise distribution of both treatment variants are evaluated. The most effective treatment design consists of uniformly distributed, variable-depth resonators, and it is predicted to attenuate noise levels below the rotor by over 8 dB. When both treatments are applied uniformly along the blade span, the primary mechanism of noise reduction is through weakening the unsteady surface pressure fluctuations. However, when the treatments are staggered along specific portions of the blade span, noise is attenuated through a reduction in the focusing of blade–vortex interaction noise. The staggered uniform- and variable-depth resonators yield comparable noise benefits because they disrupt the focusing of blade–vortex interaction noise in a similar manner.

  PublisherDOI

• Noncompactness corrections to the Brooks, Pope, and Marcolini self-noise model for small rotor noise prediction

  The Journal of the Acoustical Society of America · 2025-12-01 · 1 citations

  articleSenior author

  Accurate prediction of high-frequency broadband noise is essential for assessing the environmental impact of small uncrewed aerial systems (sUAS). The widely adopted Brooks, Pope, and Marcolini (BPM) airfoil self-noise model has been validated for rotorcraft and wind turbine applications but shows limitations for sUAS-scale rotors, partly due to its assumption of acoustic compactness (chord smaller than the wavelength of sound). This paper introduces a modification to the BPM model to account for noncompactness by incorporating an alternative directivity function and an exact numerical formulation based on Amiet's aeroacoustic transfer function (non-dimensional radiation integral). The modified models were applied to predict the noise from a hexacopter hovering at 20 and 40 feet and compared with outdoor measurements from a ground microphone grid. For all ground microphones, the median A-weighted SPL prediction error with the original BPM model was reduced from 8.7 dBA to 1.0 dBA at 20 feet and from 2.7 dBA to 1.8 dBA at 40 feet using the modified formulation. The modified models also showed substantially improved one-third octave spectral agreement, demonstrating the importance of noncompactness corrections for accurate sUAS trailing edge broadband noise prediction using the BPM model.

  PublisherDOI

• Small-scale testing of boundary conditions of a floor– assembly

  The Journal of the Acoustical Society of America · 2025-10-01

  article

  In laboratory measurements for floor– assemblies, the primary transmission path for sound should be through the panel. Section 6 of ASTM E492-22 suggests a best practice to use resilient material around the specimen and test frame to reduce airborne and structural transmission into the test rooms. The standard does not specify any requirements regarding the resilient material. Lab tests have shown the boundary conditions of the panel affect the test results. A small-scale experiment was designed to investigate if the resilient material shifts the natural frequency of a test system. From this experiment, we hope to learn how the natural frequency is affected by resiliently supporting the test specimen and what it might indicate for full-scale testing.

  PublisherDOI

• On the Design and Testing of a Full-Scale Quiet and Efficient eVTOL Propeller

  2025-05-20

  article

  This paper describes the design, development, and testing of a full-scale eVTOL propulsor optimized for quiet and efficient operation. To design the propulsor, a design tool was developed for predicting the aerodynamic and acoustic performance of eVTOL propellers and rotors. The design tool consists of an aerodynamic prediction code, AMP (Aerodynamic Modeling of Propulsor), and an acoustics prediction code, OpenCOPTER, coupled with an acoustics propogator, PSU-WOPWOP, which can receive inputs from either an acoustic solver or high-fidelity CFD. The tool was used to design a coaxial eVTOL propulsor, and both subscale and full-scale blades were manufactured. The aerodynamic and acoustic performance of the subscale propulsor was tested in hover and edgewise flight in an anechoic wind tunnel. A custom test stand was developed and used to measure the aerodynamic and acoustic performance of the 8-ft diameter full-scale propeller in hover. The experimental results were used to validate the design tool.

  PublisherDOI

• Multirotor broadband noise modulation

  The Journal of the Acoustical Society of America · 2024-03-01 · 1 citations

  articleSenior author

  Rotor broadband noise spectra are typically analyzed over time scales on the order of one or more rotor periods. However, modulation of the broadband noise spectrum with the blade passage frequency (BPF) has been shown to be significant for noise levels and perception of wind turbines and helicopters. In contrast, time-varying broadband noise has not been extensively studied for aircraft with many rotors, such as unmanned aerial vehicles (UAVs) or advanced air mobility aircraft. In this work, significant broadband noise modulation was measured in flight and anechoic chamber tests of hexacopter UAVs at various observer angles. This modulation is aperiodic with the BPF such that the modulation amplitude varies substantially between blade passages, even when the BPFs are controlled to be nearly constant between all rotors at all times. Furthermore, the azimuthal phasing between rotors greatly affects the measured modulation, such that the modulation of multiple rotors may be less than or greater than for a single rotor, depending on the phase offsets. The effects of phase variations on acoustic interactions between rotors is studied by comparing the sum of the modulation of individual rotors to the modulation of those rotors operating simultaneously. This is done not only using measurements, but also noise predictions made using PSU-WOPWOP. These results contribute understanding to how the noise modulation of rotors sum together, including the resulting directivity and aperiodicity.

  PublisherDOI

• Time-varying broadband noise of multirotor aircraft

  Proceedings of meetings on acoustics · 2024-01-01 · 3 citations

  articleOpen accessSenior author

  Rotor broadband noise is typically analyzed over time scales encompassing multiple rotor periods. However, modulation of broadband noise levels with the blade passage frequency has been shown to be significant for human perception of wind turbine and helicopter noise. Time-varying broadband noise has not been extensively studied for aircraft with many rotors, such as unmanned aerial vehicles (UAVs) or advanced air mobility aircraft. In this work, significant broadband noise modulation was measured in flight and anechoic chamber tests of hexacopter UAVs. Envelope analysis showed that the modulation depth depends on the azimuthal phasing between rotors, demonstrating the potential for synchrophasing control to reduce broadband noise modulation. If rotors are not synchronized, as in typical flight, the phasing between rotors varies with time. This phase variation followed a uniform random distribution, resulting in modulation depth also varying randomly with time. The probability distribution of modulation depth was computed using offset copies of the modulation of a single rotor. These results contribute understanding to how the noise modulation of rotors sum together, demonstrating that broadband noise modulation is likely to be significant in flight.

  PublisherOA PDFDOI

• Optimization of Variable Depth Acoustic Liners with Grazing Flow

  2024-01-04

  articleSenior author

  Acoustic liners, typically used as a noise control treatment in the engine nacelles of conventional aircraft, are being considered for noise treatment in the proprotor ducts of a vertical takeoff and landing aircraft. This work considers a new optimization method to design an acoustic liner with variable depth cavities for broadband and low-frequency attenuation. This method, termed the direct optimization method, minimizes the radiated sound from a duct. In this paper, the new method is compared with an existing indirect method to design multiple variable depth acoustic liners. Acoustic impedances of liners designed using both methods are predicted using a semianalytical impedance model and impedance predictions for two designs are compared to experimental results acquired from grazing flow impedance testing. For the work presented here, liners designed using the indirect approach provide improved attenuation spectra over those designed using the direct approach but potential improvements to the performance of the direction optimization method are discussed.

  PublisherDOI

Frequent coauthors

• James H. Stephenson

  University of Glasgow

  13 shared

• Kenneth S. Brentner

  Langley Research Center

  12 shared

• Fredric H. Schmitz

  University of Maryland, College Park

  10 shared

• Michael E. Watts

  Analytical Mechanics Associates (United States)

  10 shared

• Ben Wel-C. Sim

  United States Department of the Army

  8 shared

• Vítor Valente

  8 shared

• Ze Feng Gan

  6 shared

• Charles D. Smith

  United States Navy

  5 shared

Labs

• Aerospace Engineering Research at Penn StatePI

Education

• Doctor of Philosophy, Aerospace Engineering

  University of Maryland

  2011

• Master of Science, Aerospace Engineering

  University of Maryland

  2008

• Bachelor of Science, Mechanical Engineering

  Rochester Institute of Technology

  2005

Awards & honors

• AIAA Aero Acoustics Award (3)
• AIAA Sperry Award (2)
• AIAA Applied Aerodynamics Award
• Am Astronautical Society Brouwer Award
• AIAA de Florez Award in Flight Simulation