Andre Bauknecht

Blade-Tip Vortex Detection in Maneuvering Flight Using the Background-Oriented Schlieren Technique

The background-oriented schlieren technique was used to visualize the blade-tip vortices of a Eurocopter AS532UL Cougar helicopter in maneuvering flight. The test program covered a large part of the flight envelope, including maneuvers such as hover flight, fast forward flight, flare maneuvers, and high-speed turns. For selected flight conditions, the aerodynamic results are presented here. It is shown that, with the reference-free background-oriented schlieren method, the detection of vortex filaments up to vortex ages of ψv=540  deg is possible. The visualization of the vortex system is used to detect aerodynamic phenomena such as blade–vortex interactions, vortex–airframe interactions, and the occurrence of smooth sinuous disturbances. A detailed description of the applied reference-free background-oriented schlieren setup is given, and the suitability of different natural backgrounds for the background-oriented schlieren method is analyzed.

Contributions of Particle Image Velocimetry to Helicopter Aerodynamics

The advancement of flow measurement techniques continues to extend experimental boundaries and thus significantly contributes to improving our understanding of both basic and applied aerodynamics. This is particularly apparent in the case of particle image velocimetry (PIV), where its application has furthered the existing knowledge in several areas of helicopter rotor aerodynamics. The complex nature of helicopter rotor flows presents unique challenges to experimentalists, including transonic flow, concentrated vortices and dynamic stall. To illustrate the impact of the technological advancements on the way helicopter aerodynamics is studied today, the development of PIV since the early nineties of the last century is reviewed and some recent PIV applications are described. Using examples of main rotor wakes, dynamic stall and flow control investigations, the capabilities of large–scale, time–resolved and volumetric PIV are summarized.

Towards Density Reconstruction of Helicopter Blade Tip Vortices from High-Speed Background-Oriented Schlieren Data

A method for the reconstruction of 3D density distributions of blade tip vortices is presented. The reconstruction is performed using a filtered back-projection on 2D density gradient data from a high-speed LS-BOS (laser-speckle-illuminated background-oriented Schlieren) setup, captured during the STAR (smart-twisting active rotor) project. Density profiles of blade tip vortices are presented, as well as comparisons of the size and development of the vortices. In addition the comparison to a PIV (particle image velocimetry) measurement is shown.

Airborne Acquisition of Blade Tip Displacements and Vortices on a Coaxial Ultralight Helicopter

A series of comprehensive flight tests investigated the flight dynamics and explored the flight envelope of a newly developed ultralight, coaxial, counter-rotating helicopter from the company edm aerotec. The present paper focuses on optical measurement techniques that were applied to study the minimum blade tip clearance and convection of blade tip vortices around the dual-rotor system. A calibrated single-camera system was used to quantify the movement and distances of the blade tips at a critical rotor azimuth for multiple steady and unsteady flight maneuvers. The measurements allowed critical flight conditions to be identified and provided flight data for the validation of comprehensive rotor codes. In an additional flight test, an airborne background-oriented schlieren (BOS) system was operated aboard a second helicopter to visualize the blade tip vortices of the ultralight helicopter in forward flight. The BOS measurements resulted in the first visualization of blade-vortex and vortex-structure interaction effects on a full-scale coaxial helicopter. The application of optical measurement systems in the present flight tests gives valuable insight into the aerodynamics and blade dynamics of a free-flying counter-rotating helicopter and demonstrates how these methods can effectively support the development process of new rotorcraft.