Drag Reduction Techniques for eVTOL Configuration with Shrouded Rotors

This study serves as a proof of concept of using flow directing mechanisms such as asemi circular obstacle instead of rotor doors to reduce drag of eVTOL configurations with shrouded lifting fans. Two drag reduction techniques, namely rotor doors and step, were integrated on current design of Rapper eVTOL. A step added to the pressure surface before the rotor opening will divert flow and prevents flow recirculation into the rotor duct, thus achieving drag reduction with lower integration complexity and weight penalties compared to rotor doors. Adding a step in front of the first rotor at pressure surface and covering the first duct opening at suction surface results in identical aerodynamic performances with respect to adding rotor doors to both openings of the first duct. Applying the same drag reduction technique to the first and second rotor ducts from the leading edge results in, on average, 76.2%of the performance ceiling of this configuration, compared to 48.7% of the performance ceiling with no drag reduction elements applied. Increasing height of the step will result in turbulent flow downstream, and increased drag introduced by flow stagnation at the step, thus observing negative impact on drag reduction due to separated flow. The drag reduction is shown to be three dimensional effects as similar drag reduction was not observed on two dimensional simulations over the same geometry and circular step coupled with circular cavity shows higher efficiency on drag reduction over straight step coupled with rectangular duct.