Axial Descent of Variable-Pitch Multirotor Configurations: An Experimental and Computational Study for Mars Deployment Applications

Veismann, Marcel; Conley, R. Scott; Delaune, Jeff; Burdick, Joel W.; Gharib, Morteza; Wei, Skylar X.; Young, Larry J.; Izraelevitz, Jacob

doi:10.4050/f-0077-2021-16697

Cited by 5 publications

(9 citation statements)

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“…rotor pair and it can be shown here that a greater geometric pitch, 𝜃 .75 (compare with table 1) reduces variation of the mean thrust with descent rate. This is consistent with previous studies of large-scale (Brown, Leishman, Newman, & Perry, 2002) and small-scale rotors (Langkamp & Crowther, 2010;Shetty & Selig, 2011;Veismann et al, 2021). Another noteworthy observation from figure 6 is that the critical descent rate ratio appears to remain largely unchanged at approximately −1.2 > v c /v h > −1.3 for all test runs and rotor sizes, confirming that v c /v h is the primary scaling for VRS aerodynamics.…”

Section: Discussion Of Resultssupporting

confidence: 93%

“…The obtained flow fields of this study can be compared with computational flow field results of closely arranged twin-rotor (Takii et al, 2020) and multirotor (McQuaid et al, 2020;Veismann et al, 2021) configurations in axial descent found in the literature. In the interest of brevity, this comparison will be qualitative in nature and readers are encouraged to consult the provided literature for their own, in-depth analysis.…”

Section: Flow Field Analysismentioning

confidence: 94%

“…To date, only few computational efforts have been dedicated to the investigation of non-overlapping, twin-rotors (Takii, Yamakawa, & Asao, 2020) or multirotors (McQuaid, Kolaei, Bramesfeld, & Walsh, 2020;Veismann et al, 2021) in descent, which similarly indicated severe aerodynamic losses and strong vehicle oscillations while providing a closer look into the flow field properties of these rotor systems in the VRS. It is worth noting that all aforementioned studies exclusively considered counter-rotating rotor arrangements, as this is typically used in practice for torque balancing.…”

Section: E7-3mentioning

confidence: 99%

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Effects of rotor separation on the axial descent performance of dual-rotor configurations

Veismann

Dougherty

Gharib

2023

Flow

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Rotorcraft can encounter highly unsteady flow when descending at a steep angle, leading to a flow condition called vortex ring state, which is associated with strong oscillatory airloads and substantial losses in mean rotor thrust. This study examines the aerodynamic coupling between closely arranged rotors in vertical flight and assesses the extent to which rotor–rotor interactions affect the rotor performance in this flight stage. Wind tunnel experiments were performed on a small-scale, dual-rotor set-up with adjustable rotor spacing, and the effect of rotor separation on thrust generation was quantified. Pairs of 4 in., 5 in. and 6 in. rotors ( $3.0 \times 10^4< Re<8.1 \times 10^4$ ) were investigated, with load cell measurements showing significant thrust losses and concomitantly increased thrust oscillations as descent rate increased. Peak losses and fluctuations were consistently recorded at descent rates of 1.2–1.3 times the hover induced velocity for all rotor sizes and separations. While tests showed that the mean aerodynamic performance of dual-rotor systems is generally similar to that of single rotors, appreciable changes to the descent characteristics could be observed at low rotor separations. Particle image velocimetry flow visualization suggests considerable changes to the flow field as rotor separation decreases, where individual vortex ring systems merge into a single vortex ring structure.

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Section: Discussion Of Resultssupporting

confidence: 93%

Section: Flow Field Analysismentioning

confidence: 94%

Section: E7-3mentioning

confidence: 99%

See 1 more Smart Citation

Effects of rotor separation on the axial descent performance of dual-rotor configurations

Veismann

Dougherty

Gharib

2023

Flow

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“…3 Additionally, a strong lowfrequency oscillatory behavior of the airloads is generally observed, 4,5 leading to severe vibrations that can ultimately manifest themselves in large vehicle attitude oscillations. 6 This performance degradation and the increased vibrational loads in axial descent can considerably limit the controllability and operational margins of multirotor platforms.…”

Section: Introductionmentioning

confidence: 99%

Parametric study of small-scale rotors in axial descent

2022

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Despite extensive research in multirotor aerodynamics in the recent past, axial descent, specifically the vortex ring state, still poses great challenges for multirotor configurations as this flight stage is typically accompanied by severe losses in rotor thrust and strong thrust fluctuations. This paper presents a parametric study to investigate the influence of relevant geometric parameters of a small-scale rotor blade on the rotor performance in axial descent. Design variables subject to variation were the collective pitch, chord length, taper ratio, number of blades, as well as the tip geometry. Custom rotors for each parameter modification were manufactured and experimentally evaluated in wind tunnel tests with mean thrust recordings and measurements of the thrust fluctuations serving as performance metrics. Results indicated that rotor blades with larger aspect ratio and higher blade loading coefficient are less affected by the adverse aerodynamics in the vortex ring state, experiencing lower thrust losses and vibrational loads. Particle image velocimetry flow visualization confirmed that the aerodynamic losses in the vortex ring state can be attributed to blade vortex interactions. Comparison of the rotor flow structure in hover of all investigated rotor designs suggested that improvements in the descent performance of a rotor stem from a combination of reduced tip vortex strength and increased axial tip vortex convection rate. Using the experimental findings of this study, a predictive model for approximating the maximum extent of mean thrust losses in axial descent for a given blade geometry and hover thrust coefficient could be established.

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“…New studies delving further into the analysis of these vehicles range topics covering comprehensive and CFD analyses, handling qualities, system design, hazard analysis, and wind-tunnel testing [36][37][38][39][40][41][42][43][44][45][46][47]. Of particular interest to this work are recent studies of mid-air deployment and high rates of descent with these vehicles [48][49][50][51][52]. Some works have also looked at improving the design approach specifically for coaxial and octocopter configurations [53,54].…”

Section: Introductionmentioning

confidence: 99%

Rotor Performance Predictions for Urban Air Mobility: Single vs. Coaxial Rigid Rotors

Cornelius,

Schmitz,

Palacios

et al. 2024

Aerospace

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This work details the development and validation of a methodology for high-resolution rotor models used in hybrid Blade Element Momentum Theory Unsteady Reynolds Averaged Navier–Stokes (BEMT-URANS) CFD. The methodology is shown to accurately predict single and coaxial rotor performance in a fraction of the time required by conventional CFD methods. The methodology has three key features: (1) a high-resolution BEMT rotor model enabling large reductions in grid size, (2) a discretized set of momentum sources to interface between the BEMT rotor model and the structured URANS flow solver, and (3) leveraging of the first two features to enable highly parallelized GPU-accelerated multirotor CFD simulations. The hybrid approach retains high-fidelity rotor inflow, wake propagation, and rotor–rotor interactional effects at a several orders of magnitude lower computational cost compared to conventional blade-resolved CFD while retaining high accuracy on steady rotor performance metrics. Rotor performance predictions of thrust and torque for both single and coaxial rotor configurations are compared to test the data that the authors obtained at the NASA Langley 14- by 22-ft. Subsonic Tunnel Facility. Simulations were run with both fully turbulent and free-transition airfoil performance tables to quantify the associated uncertainty. Single rotor thrust and torque were predicted on average within 4%. Coaxial thrust and power were predicted within an average of 5%. A vortex ring state (VRS) shielding phenomenon for coaxial rotor systems is also presented and discussed. The results support that this hybrid BEMT-URANS CFD methodology can be highly parallelized on GPU machines to obtain accurate rotor performance predictions across the full spectrum of possible UAM flight conditions in a fraction of the time required by conventional higher-fidelity methods. This strategy can be used to rapidly create look-up tables with hundreds to thousands of flight conditions using a three-dimensional multirotor CFD for UAM.

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Axial Descent of Variable-Pitch Multirotor Configurations: An Experimental and Computational Study for Mars Deployment Applications

Cited by 5 publications

References 0 publications

Effects of rotor separation on the axial descent performance of dual-rotor configurations

Effects of rotor separation on the axial descent performance of dual-rotor configurations

Parametric study of small-scale rotors in axial descent

Rotor Performance Predictions for Urban Air Mobility: Single vs. Coaxial Rigid Rotors

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