# Experimental Study of the Aerodynamic Interaction between Side-by-Side Propellers in eVTOL Airplane Mode through Stereoscopic Particle Image Velocimetry

## Abstract

**:**

## 1. Introduction

## 2. Experimental Set-Up

#### 2.1. Propeller Model Design

#### 2.2. Stereo PIV Set-Up

## 3. Results and Discussion

#### 3.1. Test Matrix

#### 3.2. Flow Field Analysis

## 4. Conclusions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Abbreviations

CFD | Computational Fluid Dynamics |

D | propeller diameter (m) |

eVTOL | electrical Vertical Take Off and Landing aircraft |

J | advance ratio $={V}_{\infty}/\left(nD\right)$ |

${L}_{y}$ | lateral blade tips distance between side-by-side propellers (m) |

${M}_{t}$ | tip Mach number |

n | rotation rate (1/s) |

PIV | Particle Image Velocimetry |

R | propeller blade radius (m) |

$R{e}_{D}$ | Reynolds number based on propeller diameter |

u | freestream velocity component (m/s) |

v | vertical velocity component (m/s) |

w | out-of-plane velocity component (m/s) |

${U}_{t}$ | blade tip velocity (m/s) |

UAM | Urban Air Mobility |

VPM | Vortex Particle Method |

${V}_{\infty}$ | wind tunnel freestream velocity (m/s) |

$X-Y-Z$ | propeller reference system |

(${X}_{v},{Y}_{v}$) | in-plane coordinate of tip vortices core |

$\psi $ | blade azimuthal angle (deg) |

$\rho $ | air density (kg/m${}^{3}$) |

$\theta $ | blade pitch angle at 75% of the rotor radius (deg) |

$\left|\omega \right|$ | vorticity magnitude (1/s) |

## References

- Polaczyk, N.; Trombino, E.; Wei, P.; Mitici, M. A Review of Current Technology and Research in Urban On-Demand Air Mobility Applications. In Proceedings of the Vertical Flight Society’s 6th Annual Electric VTOL Symposium, Mesa, AZ, USA, 29–31 January 2019. [Google Scholar]
- Droandi, G.; Syal, M.; Bower, G. Tiltwing Multi-Rotor Aerodynamic Modeling in Hover, Transition and Cruise Flight Conditions. In Proceedings of the 74th Annual Forum, Phoenix, AZ, USA, 14–17 May 2018. [Google Scholar]
- Harris, F. Technical Note: Twin Rotor Hover Performance. J. Am. Helicopter Soc.
**1999**, 44, 34–37. [Google Scholar] [CrossRef] - Ramasamy, M. Hover Performance Measurements Toward Understanding Aerodynamic Interference in Coaxial, Tandem, and Tilt Rotors. J. Am. Helicopter Soc.
**2015**, 60, 1–17. [Google Scholar] [CrossRef] - Zhou, W.; Ning, Z.; Li, H.; Hu, H. An Experimental Investigation on Rotor-to-Rotor Interactions of Small UAV Propellers. In Proceedings of the 35th AIAA Applied Aerodynamics Conference, Denver, CO, USA, 5–9 June 2017. [Google Scholar]
- Shukla, D.; Komerath, N. Multirotor Drone Aerodynamic Interaction Investigation. Drones
**2018**, 2, 43. [Google Scholar] [CrossRef][Green Version] - Stokkermans, T.; Usai, D.; Sinnige, T.; Veldhuis, L.L. Aerodynamic Interaction Effects between Propellers in Typical eVTOL Vehicle Configurations. J. Aircr.
**2021**, 58, 1–19. [Google Scholar] [CrossRef] - Yin, J.; Ahmed, S. Helicopter Main-Rotor/Tail-Rotor Interaction. J. Am. Helicopter Soc.
**2000**, 4, 293–302. [Google Scholar] [CrossRef] - Tugnoli, M.; Montagnani, D.; Syal, M.; Droandi, G.; Zanotti, A. Mid-fidelity approach to aerodynamic simulations of unconventional VTOL aircraft configurations. Aerosp. Sci. Technol.
**2021**, 115, 106804. [Google Scholar] [CrossRef] - Wentrup, M.; Yin, J.; Kunze, P.; Streit, T.; Wendisch, J.; Schwarz, T.; Pinacho, J.; Kicker, K.; Fukari, R. An overview of DLR compound rotorcraft aerodynamics and aeroacoustics activities within the CleanSky2 NACOR Project. In Proceedings of the 74th AHS Annual Forum & Technology Display, Phoenix, AZ, USA, 14–17 May 2018. [Google Scholar]
- Tan, J.; Zhou, T.; Sun, J.; Barakos, G. Numerical investigation of the aerodynamic interaction between a tiltrotor and a tandem rotor during shipboard operations. Aerosp. Sci. Technol.
**2019**, 87, 62–72. [Google Scholar] [CrossRef][Green Version] - Cottet, G.H.; Koumoutsakos, P.D.; Petros, D. Vortex Methods: Theory and Practice; Cambridge University Press: Cambridge, UK, 2000. [Google Scholar]
- Winckelmans, G.S. Topics in Vortex Methods for the Computation of Three-and Two-Dimensional Incompressible Unsteady Flows. Ph.D. Thesis, California Institute of Technology, Pasadena, CA, USA, 1989. [Google Scholar]
- Alvarez, E.; Ning, A. Modeling Multirotor Aerodynamic Interactions Through the Vortex Particle Method. In Proceedings of the 54th AIAA Aviation Forum, Dallas, TX, USA, 17–21 June 2019. [Google Scholar]
- Alvarez, E.; Ning, A. High-Fidelity Modeling of Multirotor Aerodynamic Interactions for Aircraft Design. AIAA J.
**2020**, 58, 4385–4400. [Google Scholar] [CrossRef] - Raffel, M.; Willert, C.; Wereley, S.; Kompenhans, J. Particle Image Velocimetry—A Practical Guide; Springer: Berlin, Germany, 2007. [Google Scholar]
- Zanotti, A.; Ermacora, M.; Campanardi, G.; Gibertini, G. Stereo particle image velocimetry measurements of perpendicular blade—Vortex interaction over an oscillating airfoil. Exp. Fluids
**2014**, 55, 1–13. [Google Scholar] [CrossRef] - De Gregorio, F.; Pengel, K.; Kindler, K. A comprehensive PIV measurement campaign on a fully equipped helicopter model. Exp. Fluids
**2012**, 53, 37–49. [Google Scholar] [CrossRef] - Gibertini, G.; Mencarelli, A.; Zanotti, A. Oscillating Aerofoil and Perpendicular Vortex Interaction. Proc. Inst. Mech. Eng. Part G J. Aerosp. Eng.
**2014**, 228, 846–858. [Google Scholar] [CrossRef]

**Figure 1.**Examples of side-by-side propeller configurations in an eVTOL aircraft (pictures from https://evtol.news/aircraft, accessed on 28 August 2021).

**Figure 3.**(

**a**) Twist, dihedral angle, and (

**b**) chord distributions of the Varioprop 12C propeller blade along the spanwise radial coordinate.

**Figure 5.**Stereo PIV set-up for measurements of side-by-side propellers interaction at the S. De Ponte wind tunnel. The area of investigation is depicted by red the rectangular zone.

**Figure 6.**Sketch of the side-by-side propeller configuration including the reference system (

**left**) and the blade azimuthal angle definition (

**right**).

**Figure 7.**Comparison of the ensemble-averaged non-dimensional free-stream velocity component contours at $R{e}_{D}=1.96\times {10}^{6}$ and ${M}_{t}=0.325$.

**Figure 8.**Comparison of the phase-averaged non-dimensional vorticity magnitude $\omega D/{U}_{t}$ contours at blade azimuthal angle $\psi ={90}^{\circ}$, $R{e}_{D}=1.96\times {10}^{6}$, ${M}_{t}=0.325$.

**Figure 9.**Comparison of the tip vortex core displacements at advance ratios J = 0.4 and J = 0.8 between a single propeller and an upper side-by-side propeller at ${L}_{y}/D$ = 0.05, $R{e}_{D}=1.96\times {10}^{6}$, and ${M}_{t}=0.325$.

**Figure 10.**Comparison of the phase-averaged non-dimensional velocity component contours at blade azimuthal angle $\psi ={90}^{\circ}$ for advance ratio J = 0.4, $R{e}_{D}=1.96\times {10}^{6}$, and ${M}_{t}=0.325$.

**Figure 11.**Comparison of the phase-averaged non-dimensional velocity component contours at blade azimuthal angle $\psi ={90}^{\circ}$ for advance ratio J = 0.8, $R{e}_{D}=1.96\times {10}^{6}$, and ${M}_{t}=0.325$.

**Figure 12.**Comparison of the phase-averaged non-dimensional freestream velocity component profiles u extracted in correspondence with the tip vortex core locations (${X}_{v}$, ${Y}_{v}$) for blade azimuthal angle $\psi ={90}^{\circ}$ and advance ratio J = 0.4, $R{e}_{D}=1.96\times {10}^{6}$, and ${M}_{t}=0.325$.

**Figure 13.**Comparison of the phase-averaged non-dimensional freestream velocity component profiles u extracted in correspondence with the tip vortices core locations (${X}_{v}$, ${Y}_{v}$) for blade azimuthal angle $\psi ={90}^{\circ}$ and advance ratio J = 0.8, $R{e}_{D}=1.96\times {10}^{6}$, nad ${M}_{t}=0.325$.

**Figure 14.**Comparison of the phase-averaged non-dimensional out-of-plane velocity component profiles w extracted in correspondence with the tip vortices core locations (${X}_{v}$, ${Y}_{v}$) for blade azimuthal angle $\psi ={90}^{\circ}$ for advance ratio J = 0.4, $R{e}_{D}=1.96\times {10}^{6}$, and ${M}_{t}=0.325$.

**Figure 15.**Comparison of the phase-averaged non-dimensional out-of-plane velocity component profiles w extracted in correspondence with the tip vortices core locations (${X}_{v}$, ${Y}_{v}$) for blade azimuthal angle $\psi ={90}^{\circ}$ for advance ratio J = 0.8, $R{e}_{D}=1.96\times {10}^{6}$, and ${M}_{t}=0.325$.

$RPM$ | J | ${\mathit{L}}_{\mathit{y}}$/D | $\mathit{\psi}$ [deg] | |
---|---|---|---|---|

Single Propeller | 7050 | 0.4, 0.8 | - | ${0}^{\circ}$, ${30}^{\circ}$, ${60}^{\circ}$, ${90}^{\circ}$ |

Side-by-side Propellers | 7050 | 0.4, 0.8 | 0.1 | ${0}^{\circ}$, ${30}^{\circ}$, ${60}^{\circ}$, ${90}^{\circ}$ |

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**MDPI and ACS Style**

Zanotti, A.
Experimental Study of the Aerodynamic Interaction between Side-by-Side Propellers in eVTOL Airplane Mode through Stereoscopic Particle Image Velocimetry. *Aerospace* **2021**, *8*, 239.
https://doi.org/10.3390/aerospace8090239

**AMA Style**

Zanotti A.
Experimental Study of the Aerodynamic Interaction between Side-by-Side Propellers in eVTOL Airplane Mode through Stereoscopic Particle Image Velocimetry. *Aerospace*. 2021; 8(9):239.
https://doi.org/10.3390/aerospace8090239

**Chicago/Turabian Style**

Zanotti, Alex.
2021. "Experimental Study of the Aerodynamic Interaction between Side-by-Side Propellers in eVTOL Airplane Mode through Stereoscopic Particle Image Velocimetry" *Aerospace* 8, no. 9: 239.
https://doi.org/10.3390/aerospace8090239