Different configurations, same performance: A new equivalent criterion for distributed ducted propellers

For years, engineers have recognized the potential of ducted propellers – propellers housed within a cylindrical duct – for eVTOLs due to their enhanced propulsion efficiency, reduced noise, and improved safety compared to open rotors. However, optimizing these systems, primarily when multiple propellers are distributed across an aircraft, has been a complex challenge. A key hurdle has been the lack of a standardized method to compare the aerodynamic performance of different designs.

Professor Wu's team tackled this issue by developing "analytical criteria for equivalent ducted propellers." This innovative approach, derived from fundamental momentum and energy conservation principles, effectively creates a universal yardstick. It allows designers to accurately assess and compare configurations – such as single ducts with one or two rotors or dual-duct setups – on a level playing field. The study demonstrated that different multi-rotor configurations could produce similar total thrust for the same power input, validating the new criteria.

One of the most exciting outcomes highlighted in the paper is the potential for significant size and weight reduction. Numerical simulations showed that multi-rotor ducted propeller systems can be considerably more compact. For instance, a single-duct system with two rotors was found to be approximately 32% smaller in volume and 35% lighter in mass compared to a traditional single-duct, single-rotor design, all while maintaining comparable thrust. Such reductions are paramount for eVTOLs, where every kilogram saved translates to increased payload capacity or longer flight times.

Furthermore, the research delved into the critical role of "rotational consistency" in single-duct systems housing multiple propellers. The study revealed that how these propellers spin about each other dramatically impacts performance, especially at lower speeds. When propellers rotate non-uniformly, it can create detrimental airflow patterns, leading to a significant drop in the duct's thrust and causing system instability, even if the propellers generate slightly more thrust. Conversely, when propellers spin consistently, it smooths out the airflow, improves the interaction between the duct and propellers, reduces energy loss, and boosts overall thrust and stability. This finding provides a clear directive for optimizing the control strategies of these advanced propulsion units.

The implications of this study are far-reaching. The validated analytical criteria provide a much-needed standardized evaluation tool for the industry. The findings on rotational speed and consistency offer direct insights for engineers to enhance thrust efficiency and ensure the stability of these complex systems.

Looking ahead, Professor Wu's team plans to investigate the influence of different multi-rotor layouts further, refine rotational strategies, and develop advanced optimization methods. Their ultimate goal is to provide robust theoretical and practical support for developing highly efficient, exceptionally safe, and environmentally considerate urban air mobility vehicles, bringing the vision of accessible city-wide air travel closer to reality. This research marks a vital step in making advanced air mobility not just a futuristic dream but an impending transformation in transportation.

Original Source

Zeyu LI, Jianghao WU, Bairui PEI, Long CHEN, Zhengping ZOU, Haiying LIN. Numerical investigation on aerodynamic characteristics of equivalent distributed ducted propellers[J]. Chinese Journal of Aeronautics, 2025, 38(6): 103487, https://doi.org/10.1016/j.cja.2025.103487.

About Chinese Journal of Aeronautics 

Chinese Journal of Aeronautics (CJA) is an open access, peer-reviewed international journal covering all aspects of aerospace engineering, monthly published by Elsevier. The Journal reports the scientific and technological achievements and frontiers in aeronautic engineering and astronautic engineering, in both theory and practice. CJA is indexed in SCI (IF = 5.3, top 4/52, Q1), EI, IAA, AJ, CSA, Scopus.