Rotor In Hover Research Articles

Effects of the rotor tip gap on the aerodynamic and aeroacoustic performance of a ducted rotor in hover

Ducted rotors are configurations known to outperform their unducted reference baselines when aerodynamic performance is concerned. Aside from aerodynamic benefits in hover, a duct also affects acoustic emissions. One of the most contended design parameters of a duct-rotor assembly is the radial distance between the blade tip and the duct wall, referred to as the “tip gap”. The present study explains how the aerodynamic performance of a ducted-rotor system is affected by the tip-gap distance, taking into account the performance of the rotor and those of the duct's inlet lip and diffuser sections. Separate thrust measurements of the rotor and duct establish that the latter can generate up to half of the total thrust of the assembly. Static wall-pressure measurements along the inner wall of the duct reveal a low pressure suction zone over the duct's inlet lip area. This allows the assembly to generate more thrust than the rotor alone, even though the duct's diffuser section generates a drag component (negative thrust). From the velocity fields it is further shown that the performance-deterioration with an increasing tip gap distance is associated with a contraction of the rotor slipstream in the duct diffuser.

Design and evaluation of an acoustic liner applied to a UAV ducted rotor in hover

Unmanned aerial vehicles (UAVs) are currently being used for reconnaissance missions, tactical surveillance, and infrastructure inspection. These missions and operations could be performed near inhabited areas, possibly leading to noise complaints. For most UAVs, the rotor blades are the dominant source of noise. While some UAV concepts are equipped with rotor ducts for aerodynamic and safety reasons, few studies focus on the acoustic benefit of rotor ducts with integrated acoustic liners, similar to implementations on turbofan aircraft. The liner concept used in this study, called a Long Elastic Open Neck Acoustic Resonator (LEONAR), achieves good low frequency performance in a limited available space. The LEONAR concept extends the holes of the facesheet perforations into the resonator cavities with hollow tubes. As a result, the columns of air in the facesheet holes are prolongated without increasing facesheet thickness, thereby shifting the resonant frequencies of the liner lower. The objectives of this paper are to describe the simulation of noise radiated from a rotor in a duct with a LEONAR liner, the design of a liner integrated along the duct inner surface to mitigate radiated noise between 1000-5000 Hz, and impedance tube tests of liner samples to validate the predicted impedances.

Aerodynamic analysis of rotor-to-rotor interactions in different octocopter configurations

Rotor-to-rotor interaction among neighboring rotors of a multirotor has great significance for aerodynamically efficient multirotor design. Current research is conducted to analyze aerodynamic performance of different octocopter configurations amid hover and forward flight. Conventional and coaxial configurations are studied and a hybrid configuration is also proposed to rectify the disadvantages associated with the earlier two. Comparison is carried out for the aforementioned configurations along with comparison of coaxial and hybrid octocopters with bigger diameter rotors in the same confined space for high thrust requirement missions. Vertical spacing of coaxial configuration is also studied. Virtual Blade Method (VBM) is considered herein due to its great computational efficiency. The results show that there are 11.89% and 14.22% loss in thrust for coaxial octocopter compared to conventional and hybrid configurations with normal size rotors and 15.61% loss compared to hybrid configuration with bigger rotors in hover, whereas coaxial square configuration performs the worst in forward flight with a lift loss of 9.1%, 14.77% and 18.8% compared to coaxial diamond, conventional and hybrid configurations with normal size rotors and 9.96% and 17.82% loss compared to coaxial diamond and hybrid configurations with bigger rotors. Combined FM shows that hybrid configuration outperforms other octocopter configurations in overall aerodynamic performance.

Comparison of rotor hovering aerodynamic performance in free-surface and ground effects using numerical methods

The aerodynamic performance of the rotor hovering on the air–water free-surface, which is significant for cross-medium unmanned aerial vehicles, is merely studied. In this study, a compressible two-phase flow model is used to compare the aerodynamic performance in the free-surface effect (FSE) and the ground effect (GE) with various dimensionless distances, γ, between the rotor and the ground (or free-surface). According to the results, the vortex core in FSE moves further in both vertical and radial directions than in GE for the early stages. Additionally, the blade surface is separated into three parts. In zone I, the aerodynamic performance is mostly determined by proximity effects. For both FSE and GE, the downward induced velocity at the rotor disk rises with increasing γ, leading to a decrease in the sectional thrust coefficient CT,S. By the way, CT,S is larger in FSE. In zone III, the aerodynamic performance is mostly governed by the blade tip vortex. The trend of aerodynamic performance with γ is reversed compared with zone I. The above-mentioned two opposing tendencies result in a smaller rotor thrust in FSE than in GE within the range of 0.60≤γ≤3.00, but a higher rotor thrust in FSE within the range of γ≤0.60.

Experimental and computational investigations of flexible membrane nano rotors in hover

With reduced size, the flight Reynolds number of the nano rotor decreases, leading to a sharp drop in the aerodynamic efficiency of the nano rotor. Therefore, improving the aerodynamic performance of the nano rotor at low Reynolds numbers through flow control methods becomes imperative. In this study, from the perspective of bionics, flexible materials are employed in nano rotor design. Seven different layouts of flexible membrane rotor blades are designed and fabricated. The influence of leading and trailing edge flexibility, as well as membrane occupancy ratio on rotor blade propulsion characteristics, is explored through propulsion performance tests in hover.Results show that among several layouts proposed in this study, the layout with both reinforced leading and trailing edges of the flexible membrane nano rotor blade exhibits excellent propulsive performance. However, the propulsion performance of the flexible membrane rotors doesn't vary linearly with the ratio of membrane area to the whole rotor area. The appropriate ratio or flexibility can increase the propulsive performance of the rotor, especially at medium and high collective angles. At 7000 RPM and a 20° collective angle, the Figure of Merit of the flexible membrane nano rotor increases up to 4.2% when comparing with the nano rotor without membrane. This improvement becomes more significant with higher collective angles. The structural natural vibration characteristics of each flexible membrane with different layouts are analyzed through modal tests, ensuring the accuracy of the finite element model for flexible membrane rotors. Numerical analysis of the fluid-structure coupling of a flexible membrane rotor with different layouts indicates that rotor structural vibrations is highly consistent with fluctuation of aerodynamic parameters. The deformation of the flexible membrane under aerodynamic forces enhances local blade camber, but reduces angles of attack. This, subsequently, minimizes the size of laminar separation bubbles and the intensity of the blade tip vortices at high collective angles. Consequently, rotor power coefficients decrease, and overall aerodynamic performance improves.

Higher-Order Statistical Metrics for Characterizing Multirotor Acoustics

Several higher-order statistical metrics are used to evaluate the significance of waveform nonlinearities in the sound field of a laboratory-scale coaxial, corotating rotor in hover. These comprise the magnitude-squared coherence, skewness, and kurtosis of the pressure waveform and its time derivative, number of zero crossings per rotation, a wave steepening factor, and the quadrature spectral density and its integral. A unique feature of this rotor setup is the constructive and destructive interference of sound waves produced by neighboring blades, which are incubators for signal distortion effects. Waveform distortions are evaluated for changes to rotor index angle, the separation distance between the upper and lower rotors, as well as changes to rotor speed for different observer positions. Significant sensitivities in the kurtosis of the pressure waveform and its time derivative, the number of zero crossings, and the integral of the quadrature spectral density are shown for changes in rotor index angle, observer position, and rotor speed; stacking distance appears less important at affecting changes to these metrics. The trade space between these metrics and rotor figure of merit demonstrates how changes to the rotor index angle can invoke relatively small changes in rotor performance while generating large changes in acoustic waveform nonlinearities.

Analytic Prediction of Rotor Broadband Noise with Serrated Trailing Edges with Applications to Urban Air Mobility Aircraft

Theoretical predictions for broadband noise generated by rotors with serrated trailing edges are presented. This study extends Lyu and Ayton’s model for serrated trailing-edge noise to account for factors such as finite span length, the accurate acoustic attenuation rate of trailing-edge noise, modified spanwise wavenumber, and the proper scattering factor. This new model is implemented in the rotorcraft broadband noise tool, UCD‐QuietFly. The validation results demonstrate a satisfactory level of agreement with the experimental data for serrated trailing‐edge noise of a wing section, a hovering rotor, and a forward‐flight rotor. Next, the effects of serration parameters, such as the number of serrations, serrations starting radial location, serration geometry, and serration height, on rotor broadband noise are investigated. For example, it is observed that sine-wave, chopped peak, or sawtooth serration shapes provide the most significant noise reduction among the various shapes considered. Finally, broadband noise reduction through serrations is applied to two urban air mobility aircraft scenarios: a six‐passenger quadrotor and a quiet helicopter. For the quadrotor, serrated trailing edges show a noise reduction potential of over 9 dB, with higher reductions observed at higher tip speeds and fewer blades. A quiet helicopter equipped with serrated blades in forward flight achieves a maximum noise reduction of over 10 dB in the forward direction.

Helicopter vibration control employing superelastic pitch link with shape memory alloy

One of the main applications of shape memory alloy (SMA) concerns vibration control, especially superelastic SMA (SMA-SE), which presents a significant stress hysteresis and can work as a damper while adding minimal weight to the structure. In helicopters, vibration is a phenomenon inherent to their operation, and although cannot be eliminated, must be minimized to acceptable levels of safety and comfort. In this context, this paper aims to present the design, testing, and dynamic behavior of a new device entitled smart pitch link, a device with superelastic material for passive vibration control. The paper depicts the design process, assembly, and device testing using a whirl tower that simulates a helicopter rotor in hover. The approach adopts a comparative analysis of the dynamic response on the prototype in reference condition, with a stiff pitch link, with the modified prototype, and with the superelastic pitch link installation. In addition, beyond hover tests, cyclic commands and an asymmetrical lateral gust were employed to observe the output response of the device under transient loadings. Contrary to the literature data about the low time response of SMA, the device presents satisfactory time response and attenuation in frequencies close to main rotor frequencies, with signal attenuations between 2 to 4 dB without any external energy consumption, which, in a comparative analysis, may exceed more than 50% of vibrational amplitude attenuation.

A high-resolution numerical investigation of unsteady wake vortices for coaxial rotors in hover

High-resolution numerical simulations for wake vortical flows have long been a challenge in rotor aerodynamics. A novel spectrum-optimized sixth-order Weighted Essentially Non-Oscillatory (WENO) scheme is proposed to discretize inviscid fluxes on moving overset grids, and the Improved Delayed Detached Eddy Simulation (IDDES) is employed to resolve turbulent vortices. The integration of these methods facilitates a comprehensive numerical investigation into the unsteady vortical flows over coaxial rotors in hover. The results highlight the substantial improvement in numerical resolution, in terms of both spatial structure and temporal evolution of unsteady multiscale wake vortices. Coaxial rotors in hover manifest three primary scales of wake vortex structures: (A) the helical evolution of primary blade tip vortices and the periodic occurrence of strong Blade-Vortex Interactions (BVI); (B) the continuous shedding of small-scale horseshoe-shaped vortices from the trailing edges of rotor blades, forming the vortex sheets; (C) the emergence of small-scale secondary vortex braids induced by interactions between rotor tip vortices and the vortex sheets. These vortex structures and their interactions cause high-frequency oscillations in rotor disk loads and induce unsteady perturbations in the local flow field. Interactions among these primary vortices, coupled with the generation of secondary vortices, result in the dissipation, distortion, and breakup of the rotor tip vortices, ultimately forming a vortex soup. Notably, a substantial quantity of seemingly weak small-scale secondary vortex braids significantly contribute to energy dissipation during the evolution of wake vortices for coaxial rotors in hover.

High-fidelity aero-acoustic evaluations of a heavy-lift eVTOL in hover

Electric Vertical Take-off Landing (eVTOL) vehicles can transform aviation, but the unconventional aircraft development currently requires an extensive knowledge base. This paper presents a high-fidelity aerodynamic and acoustic evaluation of a heavy-lift multi-rotor eVTOL design in hover. The design, known as Skybus, has an MTOW of 14,000 kg, and six tiltable rotors attached at the tip of three sets of tandem wings with flaps. High-fidelity scale-adaptive CFD simulations of the full-scale Skybus vehicle in hover, with three design variations, were conducted using the HMB3 framework. The aerodynamic performance and interactions were analysed in detail. The baseline airframe caused a blockage force of about 13% of the MTOW, and the compact hovering rotors induced a sinusoidal single-blade loading variation, blended with small local interactions. The near-field acoustics were then directly extracted from the high-fidelity CFD results, and far-field acoustic propagation was computed using the acoustic analogy, following reference acoustic measurements for eVTOLs proposed by EASA. Different aerodynamic interactions and propeller acoustic characteristics showed strong impacts on the vehicle acoustics. The EPNL metric was also computed and analysed assuming various hover duration, and restrictions on maximum EPNL and hover duration are recommended for future rotorcraft hover noise certification. These novel results provide valuable guidance for eVTOL acoustic design, infrastructure planning, and policy-making.

Experimental and Computational Investigation of Rotor Noise in Hover

The noise magnitude and directivity of a four-bladed 1.108-m-radius rotor in hover were measured at a tip Mach number of 0.42, representative of an electric vertical takeoff and landing vehicle rotor. The tonal noise was filtered from the acoustic spectra, allowing for analysis of the separate contributions of tonal and broadband noise. Accompanying computations of the same rotor and operating conditions were performed using the rotorcraft comprehensive analysis system (RCAS) with a viscous vortex particle method (VVPM) for aerodynamic calculations and PSU-WOPWOP with the semi-empirical Brooks, Pope, and Marcolini (BPM) broadband model for predicting acoustics. Measured broadband and tonal noise contributed similar magnitudes to the overall sound pressure level, with broadband always several decibels greater. This is in contrast to conventional helicopter rotors, which are dominated by tonal noise. Simulated trends of tonal noise matched the measurements, though the higher harmonic components of the acoustic spectra were underpredicted by up to 30 dB. The broadband noise was underpredicted by 10 dB at moderate rotor collectives, as the measured noise contained noise from laminar and turbulent boundary layers, while the model predicted predominantly laminar boundary layer–vortex shedding noise. Comparing the time histories of the measured and simulated rotor thrust revealed that the RCAS-VVPM method does not capture the amplitude of the 4/rev component of the thrust and lacks the higher frequency content present in the measured thrust, indicating a possible source of discrepancy in the tonal acoustic predictions. The results of this study show that methods to predict rotor tonal and broadband noise without the use of lengthy computational fluid dynamics computations must be further developed.

Numerical study on aerodynamic performance of a ducted tail rotor in hover and sideward flight

The present paper examines the aerodynamic performance of a ducted tail rotor in hover and sideward flight conditions using high-fidelity CFD methods. The wake features, flow separation, and aerodynamic force pulsation of a full-scale SA365N1 ducted tail rotor model under different flight conditions are compared. The coupling influence mechanism of the sideward inflow, wake flow, and shroud shielding on the aerodynamic loads of the ducted tail rotor are revealed through numerical evaluation of variations in collective pitch and sideward flight speed. Special attention is dedicated to studying the wake recirculation phenomenon and thrust breakdown of the ducted tail rotor during right sideward flight, in order to analyze the flight speed boundary that notably affect the aerodynamic performance. It is found that the duct effectively shields the tail rotor from intricate vortex interference during right sideward flight, thus averting tail rotor failure. Additionally, when the ratio of right sideward flight speed to hover equivalent induced velocity is below 0.2, it results in favorable recirculation and enhanced shroud thrust. However, increasing the speed ratio results in a reduction in duct thrust. If the right sideward flight speed becomes excessive, it induces a vortex ring state, resulting in failure of the shroud aerodynamics and a significant decline in the overall aerodynamic performance of ducted tail rotor.

Parametrization Effects of the Non-Linear Unsteady Vortex Method with Vortex Particle Method for Small Rotor Aerodynamics

The aim of this article is to investigate the parameter sensitivity of the (Non-Linear) Unsteady Vortex Lattice Method-Vortex Particle Method [(NL-)UVLM-VPM] with Particle Strength Exchange-Large Eddy Simulations (PSE-LES) method on a lower Reynolds number rotor. The previous work detailed the method, but introduced parameters whose influence were not investigated. Most importantly, the Vreman model coefficient was chosen arbitrarily and was not suitable to ensure stability for this lower Reynolds number rotor simulation. In addition, the previous work presented a consistency study where geometry and time discretization were refined simultaneously. The present article starts with a comparative literature review of potential methods used to solve the aerodynamics of an isolated hovering rotor. This review highlights the differences in modeling, discretizations, sensitivity analysis, validation cases, and the results chosen by the different studies. Then, a transparent and thorough parametric study of the method is presented alongside discussions of the observed results and their physical interpretation regarding the flow. The sensitivity analysis is performed for the three free parameters of UVLM, namely Vatistas core size, the geometry and the temporal discretizations, and then for the three additional parameters introduced by UVLM-VPM, which are the Vreman model coefficient, the particle spacing, and the conversion time. The effect of different databases in the non-linear coupling is also shown. The method is shown to be consistent with both geometry and temporal refinements. It is also consistent with the expected behavior of the different parameters change, including the numerical stability that depends on the strength of the LES diffusion controlled by the Vreman model coefficient. The effect of discretization refinement presented here not only shows the integrated coefficients where different errors can cancel each other, but also looks at their convergence and where relevant, the distributed loads and tip singularity position. Finally, the aerodynamics results of the method are compared for different databases and with higher fidelity Unsteady Reynolds Averaged Navier–Stokes (URANS) 3D results on a lower Reynolds number rotor.

An Experimental Study of Stacked Rotor Tip Vortex Interaction in Hover

Measurements were made using flow visualization to track the blade tip vortices of a 1.108-m radius stacked rotor in hover. Vortex measurements of an isolated, two-bladed rotor were also made and correlated with the Landgrebe wake model. When a second rotor was introduced to the system, the interaction between the upper and lower rotor vortices resulted in deviations from the Landgrebe wake model. Changing the azimuthal spacing revealed large changes in the trajectory of the upper and lower rotor vortices, particularly at small, negative azimuthal spacings. The miss distance, or distance between a rotor blade and vortex, was measured. Positive miss distances where the lower rotor blade passed below the upper rotor tip vortex were associated with greater noise between 1 and 5 kHz, possibly from turbulent boundary layer noise or blade–vortex interaction noise. The greatest performance and lowest broadband noise corresponded with large miss distance and minimal vortex–vortex interaction, which occurred at an azimuthal spacing of −45°. The tip vortex trajectory, and thus miss distance, could be modified by utilizing differential collective. This resulted in a 4.5-dB reduction in broadband noise at negative differential collectives by allowing the lower rotor blade to pass above the upper rotor vortex and achieving negative miss distances. At azimuthal spacings where the miss distance was always negative, the airfoil self-noise dictated the broadband noise level.

Performance and Acoustics of a Stacked Rotor with Differential Collective Pitch

Performance and acoustics of a 1.108-m radius stacked rotor in hover with an axial spacing between the upper and lower rotors of 0.73 chords were measured. The differential collective pitch of the rotor system, de ned as half the difference between the upper rotor pitch and lower rotor pitch was varied along with the azimuthal spacing between the upper and lower blade sets. Coupled comprehensive analysis and acoustics simulations were conducted using the Rotorcraft Comprehensive Analysis System, the acoustic solver PSU-WOPWOP, and the Brooks, Pope, and Marcolini method implemented in ANOPP2. Increased sensitivity to azimuthal spacing was measured for large, negative differential collectives, resulting in a 76.5% change in total rotor thrust and a 46% change in total rotor power over 11.25 ° change in azimuthal spacing. The trends in individual and total system thrust were predicted with good accuracy, but the total power was overpredicted. Overall sound pressure level decreased by 3.5 dB at – 2° differential collective and 90° azimuthal spacing due to reduced broadband levels, indicating a possible quiet mode of operation for this type of rotor. By assessing the highest performance and lowest noise, an optimum rotor con guration was observed at an azimuthal spacing of – 45° and differential collective of 2°. Acoustic predictions captured tonal and broadband noise trends in most conditions. Conditions where broadband noise was underpredicted could be a result of vortex interactions not modeled by the broadband simulations.

Study on Aerodynamic Interaction of Rotor and Fuselage in Hover and Its Application to Flap Width Determination of Winged Helicopters

Study on Aerodynamic Interaction of Rotor and Fuselage in Hover and Its Application to Flap Width Determination of Winged Helicopters

Stable Vortex Particle Method Formulation for Meshless Large-Eddy Simulation

A novel formulation of the vortex particle method (VPM) is developed for large-eddy simulation (LES) in a meshless scheme that is numerically stable. A new set of VPM governing equations are derived from the LES-filtered Navier–Stokes equations. The new equations reinforce the conservation of angular momentum by resizing vortex elements subject to vortex stretching. In addition to the VPM reformulation, a new anisotropic dynamic model of subfilter-scale (SFS) vortex stretching is developed. This SFS model is well suited for turbulent flows with coherent vortical structures, where the predominant cascade mechanism is vortex stretching. The mean and fluctuating components of turbulent flow and Reynolds stresses are validated through the simulation of a turbulent round jet. The computational efficiency of the scheme is showcased in the simulation of an aircraft rotor in hover, showing our meshless LES to be 100 times faster than a mesh-based LES with similar fidelity. The implementation of our meshless LES scheme is released as open-source software, called FLOWVPM.

Investigation of the Flow Fields of Coaxial Co-Rotating and Counter-Rotating Rotors in Hover Using Measurements and Simulations

The flow fields of a 2-m diameter two-bladed single rotor, a 2× 2-bladed coaxial corotating (stacked) rotor, and a 2× 2-bladed coaxial counterrotating (CCR) rotor in hover were measured using particle image velocimetry and computed using a finitevolume unsteady Reynolds-averaged Navier–Stokes (URANS) CFD model. Phase-resolved measurements were performed on the stacked rotor at nine azimuthal locations, and time-resolved measurements were performed on the CCR rotor at 64/rev with at least 500 flow realizations per azimuth for each operating condition. The goal of this study was to compare the flow features of these rotor configurations and explore the interactions between the rotors. Overall, there was good correlation between the measurements and simulations. In particular, the effect of index angle on the upper and lower rotor thrust sharing for the stacked rotor was predicted well by the simulation. The slipstream boundary for the stacked rotor was found to vary with the index angle. The slipstream boundary and vortex trajectories for the CCR rotor were found to vary with azimuthal location, indicating the effect of blade passage on the wake geometry. Simulations indicated a stronger dependence of the tip vortex trajectory on the index angle and thrust for the stacked rotor compared to the CCR rotor. The radial thrust distribution along the upper blades was found to depend on the index angle for the stacked rotor and showed small variation due to blade passage for the CCR rotor. A larger azimuthal dependence was seen for the radial thrust distribution on the lower rotor blades, primarily due to the proximity of the upper rotor tip vortices. The lower rotor radial thrust distribution was biased towards the blade tip, outside the upper rotor slipstream.

A Quantitative Approach for the Accurate CFD Simulation of Hover in Turbulent Flow

Time-dependent Navier–Stokes simulations have been carried out for a V22 rotor in hover using an improved, lowdissipation, HLLE ++ upwind algorithm in the OVERFLOW code. Emphasis is placed on lessons learned over the past decade regarding the effects of high-order spatial accuracy, grid resolution, and the use of detached eddy simulation in predicting the figure-of-merit, the rotor's chief performance parameter. A general quick-start procedure is described together with a statistical measure of FM convergence that reduces hover computations by fourfold, similar to computational work for forward flight. Cartesian adaptive mesh refinement is used to resolve the tip–vortex to its correct physical size. This adaptive mesh refinement in the rotor wake also revealed a complex turbulent flow with worm-like structures of various scales. These structures were numerically found a decade ago and recently observed in experiment.

Influence of Propeller Parameters on the Aerodynamic Performance of Shrouded Coaxial Dual Rotors in Hover

A numerical method is used to evaluate the influence of propeller parameters on the aerodynamic performance of shrouded coaxial dual rotors in hover. Compared with the open-rotor configuration, the shrouded rotors reduce the tip vortex, resulting in a higher thrust and figure of merit (FoM). The varying inflow distribution over the inlet lip of the shroud introduces a different working condition for the propellers, thereby providing an opportunity for propeller parameter optimization. The pitch length, chord length, and tip clearance are chosen as the design parameters to improve the aerodynamic performance of the rotors. When the pitch lengths of the upper and lower propellers increase, the shrouded rotors outperform the original ones in both the coefficient of thrust (Ct) and in terms of the (FoM). The results show that increasing the chord length improves the Ct but that the (FoM) is almost unaffected. Moreover, smaller tip clearance can significantly improve the aerodynamic performance of the shrouded rotors. Results presented in this work provide insights into the parametric design and optimization of shrouded rotors.