Our current knowledge on helicopter rotor aerodynamics is largely based on quasi-steady hover- and wind-tunnel rotor tests. While these experiments give valuable insight into the rotor flow field for stationary rotor operation, they cannot capture the inherently unsteady aerodynamics and flight conditions of free-flying full-scale helicopters. Aerodynamic testing on a free-flying rotorcraft, on the other hand, has the advantage that no assumptions or simplifications are made for the flow field, flight controls and boundary conditions, and that unsteady flight conditions are naturally implemented, leading to a realistic representation of the rotor wake. In my research, I directly measure the flow fields of model- and full-scale helicopters in maneuvering flight using optical flow measurement techniques such as Particle Image Velocimetry (PIV) and Background-Oriented Schlieren (BOS). The main goal is to characterize the involved unsteady flow phenomena at realistic flow, trim, and flight conditions, while providing validation data for numerical methods.
This talk will give an overview of two of my previous projects on the characterization of unsteady rotor wakes on free-flying rotorcraft. The first part of this talk covers advanced measurement techniques that enable the study of full-scale helicopter blade tip vortices under realistic free-flight conditions. This is critical for studying aerodynamic phenomena such as vortex-structure interactions and vortex decay, and providing flight test data for comparison with numerical and sub-scale experiments. The focus of this study is on inflight visualization of vortices with the reference-free BOS method and on the application of a ten-camera BOS system for the visualization and three-dimensional reconstruction of the tip vortex system of a free-flying BO105 helicopter. These experiments demonstrate the feasibility of full-scale and non-invasive aerodynamic measurements on free-flying helicopters, which give unprecedented insights into the rotor wake.
The second project presented here focuses on the impact of unsteady maneuvers on the rotor wake of model- and full-scale helicopters operated in close proximity to the ground. Special focus is put on the interaction of the rotor wake with the ground plane, which plays an important role in particle uplift and corresponding deteriorated visual conditions ("brown out") during landing maneuvers. Time-resolved PIV and multi-camera BOS measurements are applied to track blade tip vortex convection in the rotor wake and study the wall jet velocity profile for different quasi-steady and unsteady flight maneuvers close to the ground. Flight regimes with peak recirculation or maximum vertical flow momentum flux in front of the rotor are identified for different forward flight speeds. Unsteady flight conditions are found to change the peak velocity and width of the wall jet profile, which influences the intensity of the brown out phenomenon. Through these examples, this talk illustrates the study of unsteady aerodynamic phenomena on free-flying rotorcraft with optical flow measurement techniques and gives future applications and potential next steps for this line of research.