Investigation of the Performance Characteristics of Unequal Co-Axial Rotors

Abstract

The behaviour of co-axial rotors is well understood, and they are especially practical for large UAVs due to their increased thrust without changing the vehicle footprint. However, for co-axial systems with varying propeller diameters between the two disks, research is more limited. The goal of this paper was to determine an optimal configuration for several different unequal co-axial setups using numerous different propeller combinations and separation ratios. Propellers with diameters of 26 and 29 inches are tested at separation ratios of 0.05 to 0.35. Thrust and power were collected using an off-the-shelf FS15-TYTO thrust stand, with the upstream and downstream propellers running at equal throttles. From this, performance was assessed through efficiency, thrust, and power consumption, and comparisons were made to an ideal combination without losses. The results show that for unequal combinations, the user should place the smaller propeller upstream for greater efficiency, but for maximum thrust capacity, two equal propellers are preferred. When compared to two independent rotors of the same size, a 26″ upstream rotor and a 29″ downstream rotor minimised thrust loss to 16%, compared to 23% for the opposite arrangement. It was also found that the optimal separation ratio is always approximately 0.2.

Keywords

Unequal / Propellers / Co-axial / Aerodynamics / Efficiency / z/D ratio / Optimisation

Acknowledgments

We hereby acknowledge the administrative and technical support from within the University of Southampton, which enabled this work to be undertaken.

Author Contributions

This research article was written by both authors, the individual contributions are as follows: Conceptualization, S.D.P. and M.M.; methodology, S.D.P. and M.M.; software, M.M.; validation, S.D.P. and M.M.; formal analysis, M.M.; investigation, S.D.P. and M.M.; resources, S.D.P.; data curation, S.D.P. and M.M.; writing—original draft preparation, M.M. and S.D.P.; writing—review and editing, S.D.P. and M.M.; visualization, M.M. and S.D.P.; supervision, S.D.P.; project administration, S.D.P.; funding acquisition, S.D.P. All authors have read and agreed to the published version of the manuscript.

Ethics Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availibility Statement

A range of propeller test data is available online via the UIUC Propeller Data Site, Department of Aerospace Engineering, Vol. 1-4. This can be accessed for free from: https://m-selig.ae.illinois.edu/props/propDB.html (accessed on 11 November 2024).

Funding

This research was funded by the Engineering and Physical Sciences Research Council (EPSRC), grant number (EP/V05614X/1), entitled “Aerodynamics and aeroacoustics of closely coupled rotors”. This research funding also paid for the APC charges.

Declaration of Competing Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

References

[1] Chaitanya PJP, Prior SD, Parry AB. On the optimum separation distance for minimum noise of contra-rotating rotors. J. Sound Vib. 2022, 535, 117032.

[2] Xu ZY. Numerical Simulation of Unsteady Flow Around Forward Flight Helicopter with Coaxial Rotors. Chin. J. Aeronaut. 2011, 24, 1-10.

[3] Shukla NKD. Drone Scale Coaxial Rotor Aerodynamic Interactions Investigation. J. Fluids Eng. 2019, 141, 071201.

[4] Bell MB, Prior SD, Erbil MA, Karamanoglu M. Development of a Test-Rig for Exploring Optimal Conditions of Small Unmanned Aerial Vehicle Co-Axial Rotor Systems. In Proceedings of the International Conference on Manufacturing Engineering Systems, Taiwan, Republic of China (ROC), 16-20 December 2010.

[5] Zhu SN, He YL, Chen CX, Wang SY. Research on Hover Characteristics of Ducted Fan with Coaxial Rotors. Appl. Mech. Mater. 2013, 427, 216-221.

[6] Lim KWMJ, Johnson W. Hover Performance Correlation for Full-Scale and Model-Scale Coaxial Rotors. J. Am. Helicopter Soc. 2009, 54, 032005.

[7] Ostar-Exel L. The Effects of Varying Diameter on Coaxial Propellers for the Propulsion of Multirotor Systems. PhD Thesis, School of Graduate Studies, The State University of New Jersey, Brunswick, NJ, USA, 2019.

[8] Simões M. Optimizing a Coaxial Propulsion System to a Quadcopter Cédric. Master’s Thesis, Department of Engenharia Mecânica, Instituto Superior Técnico, Av. Rovisco Pais, Lisboa, Portugal, 2014.

[9] Palleja-Cabre SCP, Joseph P. Acoustic and Aerodynamic Performance of Co-Axial Contra-Rotating Rotors with Unequal Diameters. In Proceedings of the 30th AIAA/CEAS Aeroacoustics Conference, Rome, Italy, 4-7 June 2024.

[10] Cai GM, Li ZX. Aerodynamic Characteristics of a Ducted Fan System Based on Momentum Source Method. In Proceedings of the 3rd International Conference on Fluid Mechanics and Industrial Applications, Taiyun, China, 29-30 June 2019.

[11] Sunada KTS, Kawashima K. Maximization of Thrust-Torque Ratio of a Coaxial Rotor. J. Aircraft 2005, 42, 300-307.

[12] Che Muhammad Ikram CF, Che Umar M. Distance and Rotational Speed Analysis of Coaxial Rotors for UTHM CDrone. Prog. Aerosp. Aviat. Technol. 2021, 1, 45-55.

[13] Deng SW, Zhang Z. Aerodynamic Performance Assessment of a Ducted Fan UAV for VTOL Applications. Aerosp. Sci. Technol. 2020, 103, 105895.

[14] Fahlstrom TJG. Introduction to UAV Systems, 4th ed.; Wiley: Hoboken, NJ, USA, 2012; p. 76.

[15] Leishman GSJ. Figure of Merit Definition for Coaxial Rotors. J. Am. Helicopter Soc. 2008, 53, 290-300.

[16] Coleman CP.A Survey of Theoretical and Experimental Coaxial Rotor Aerodynamic Research. Available online: https://ntrs.nasa.gov/citations/19970015550 (Accessed on 1 November 2024).

[17] Leishman JG. Aerodynamic Optimization of a Coaxial Proprotor. In Proceedings of the American Helicopter Society 62nd Annual Forum, Phoenix, USA, 9-11 May 2006.

[18] Prior SD. Reviewing and Investigating the Use of Co-Axial Rotor Systems in Small UAVs. Int. J. Micro Air Veh. 2010, 2, 1-12.

[19] Fernandes SD. Performance Analysis of a Coaxial Helicopter in Hover and Forward Flight. Master’s Thesis, Embry-Riddle Aeronautical University, Prescott, AZ, USA, 2017.

[20] Holzsager JE. The Effects of Coaxial Propellers for the Propulsion of Multirotor Systems. PhD Thesis, School of Graduate Studies, Rutgers University, New Brunswick, NJ, USA, 2017.