Breakthrough rotor model enables safe transitions for aerial-aquatic vehicles

Aerial-Aquatic Rotorcraft (AAR)—which combine the agility of drones with the versatility of underwater systems—hold transformative potential for reconnaissance, disaster response, and marine operations. However, transitioning between air and water poses critical aerodynamic challenges, especially when rotors operate close to or cross the air-water interface.

To address this, a research team led by Prof. Xiao Wang and Dr. Qi Zhan from the National Key Laboratory of Helicopter Aeromechanics at Nanjing University of Aeronautics and Astronautics developed an advanced adaptive aerodynamic model inspired by finite vortex theory. Their work, recently accepted by the Chinese Journal of Aeronautics, presents a high-precision method for predicting rotor performance in complex two-phase flow environments.

“Traditional rotor models often fail to capture the unique effects induced by proximity to a free water surface,” said Prof. Wang. “We designed a Finite Vortex Rotor Model (FVRM) that incorporates water surface deformation and accounts for the complex feedback between rotor downwash and liquid response.”

They identified three distinct flow regimes as rotors approached the water: (1) the depression stage, characterized by surface sinking under rotor downwash without droplet formation; (2) the fountain stage, where mild surface deformation and outward splashing droplets emerge; and (3) the boiling stage, marked by intense droplet ejection, atomization, and unstable thrust performance due to gas-liquid turbulence.

By integrating theoretical modeling with experimental validation, the researchers revealed that rotor thrust near the water surface undergoes a transition from a weak ground effect (fountain stage) to an inverse ground effect (boiling stage), driven by the rotor height-to-radius ratio (h/R) and Disc Loading (DL). This aerodynamic phenomenon significantly affects vehicle stability and control. To quantify this behavior, the team proposed a transition boundary that predicts the onset of thrust degradation and defines a safe operational envelope for aerial-aquatic vehicles operating near water.

“Our findings are not only theoretical but offer practical design guidance,” noted the team. “To prevent entering the unpredictable boiling stage, designers should consider increasing rotor diameter to reduce DL, limit rotor power near water, and enhance controller stability.”

The team envisions future studies using CFD and particle image velocimetry to further understand flow-field transitions at the gas-liquid interface.

Original Source

Qi ZHAN, Xiao WANG, Junhui HU, Xingzhi BAI, Pierangelo MASARATI. Aerodynamic modeling and analysis of aerial-aquatic rotorcraft performance near and crossing the air-water interface [J]. Chinese Journal of Aeronautics, 2025, https://doi.org/10.1016/j.cja.2025.103511.

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.