Experimental investigation of vortex ring state for multicopters

The vortex ring state (VRS) regime for multicopters has been experimentally investigated within ”GARTEUR AG28: Vortex Ring State” via thrust and power measurements as well as stereo particle image velocimetry (S-PIV), at descent angles between 40 ∘ and 90 ∘ , and normalised descent rates between −0.4 and −1.7, covering the whole area of flight susceptible to VRS, with the aim of investigating the interaction effects on multirotor configurations in VRS and the deviation of their performance from existing experimental data for conventional helicopters. Four different configurations have been investigated: the single rotor (SR), used as a reference test case, the dual rotors in side-by-side (DRSbS) and tandem (DRT) arrangement and the quadrotor (QR) configuration. Thrust data and S-PIV velocity fields show that while multirotor configurations exhibit qualitative similarities to helicopters in their VRS behaviour, they proved more susceptible to VRS: based on mean thrust reduction, the multirotor configurations show broader regions of the flight envelope affected by VRS compared to the SR configuration. However, similar mean thrust reduction levels are obtained for all configurations, with the highest reduction measured at roughly 25-30%. Significant configurational effects are also found regarding rotor placement, with the DRT and QR configurations showing reduced thrust levels for the rear rotor at shallow descent angles. The QR configuration also showed the highest thrust fluctuations, growing by a factor of 2 compared to the SR configuration. The velocimetry measurements proved that, for the QR configuration, a single recirculation region wraps around the device when in VRS, forming a unified vortex ring. Additionally, the minimum thrust experienced by the configuration is found to correlate with the centre of the recirculation regions being approximately positioned at 1/2 of the device characteristic length from the rotor plane.