Early Onset Prediction for Rotors in Vortex Ring State

AbstractThe OpenFOAM version 5.0 CFD package was used to investigate detection methods for a small quadrotor drone entering the vortex-ring state. The vortex-ring state causes increased thrust peak-to-mean fluctuations that can be partially attributed to a breakdown of the stable helical wake vortex into a single vortex-ring trapped around the rotor tip. These thrust fluctuations cause significant flight control issues for rotorcraft. Detection of a rotorcraft approaching this flight condition becomes important to avoid any loss of vehicle control. A method for detecting the onset of the vortex-ring state is proposed using various differential pressure measurements on the quadrotor. It has been shown that, by monitoring the pressure difference between the top of the quadrotor body and the bottom of one of the quadrotor legs, a pressure drop can be observed just prior to the onset of the vortex-ring state. This pressure drop was shown to occur during descending flight regimes and may prove to be an effective pre-vortex-ring state warning system.