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Wake Unsteadiness and Tip Vortex System of Full-Scale Helicopters in Ground Effect

Wolf, Christian and Weiss, Armin and Schwarz, Clemens and Braukmann, Johannes N. and Koch, Stefan and Raffel, Markus (2022) Wake Unsteadiness and Tip Vortex System of Full-Scale Helicopters in Ground Effect. Journal of the American Helicopter Society, 67 (1), pp. 1-17. American Helicopter Society. doi: 10.4050/JAHS.67.012010. ISSN 0002-8711.

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Official URL: https://doi.org/10.4050/JAHS.67.012010


The main rotor wakes of the free-flying DLR test helicopters Airbus Bo105 and EC135 were investigated in ground effect during hover, vertical take-off, and forward flight. A high–speed schlieren system tracked the blade tip vortices at about 60 images per revolution. In addition, a constant temperature anemometry-system utilized arrays of fiber–film sensors, providing velocity statistics and spectra in the rotor flow. The overall wake structure agreed to preceding studies, but the velocity profiles and tip vortex trajectories were sensitive towards the environmental wind conditions. The tip vortices were observed in the schlieren images up to an age corresponding to about two revolutions below the rotor plane, before developing instabilities and falling below the detection limit. Systematic vortex pairing was found for the Bo105 but not for the EC135. The remnants of the tip vortices were identified further downstream in the wake by means of rotor-harmonic velocity signals, but they play a minor role in comparison to broad banded turbulent fluctuations with a Kolmogorov- like spectrum. For vertical take-off cases, the rotor wake had a hover–like structure until breaking down into low–frequency oscillations when exceeding a hub height of approximately 1.4 rotor radii. In forward flight, different types of wake velocity footprints were categorized on the basis of the normalized advance ratio. Blade vortex interactions were found in the frontal area of the main rotor planes, and between the main rotor tip vortices and the Bo105's tail rotor. The interactions prevent a further evolution of the tip vortices.

Item URL in elib:https://elib.dlr.de/143786/
Document Type:Article
Title:Wake Unsteadiness and Tip Vortex System of Full-Scale Helicopters in Ground Effect
AuthorsInstitution or Email of AuthorsAuthor's ORCID iDORCID Put Code
Wolf, ChristianUNSPECIFIEDhttps://orcid.org/0000-0002-9052-7548UNSPECIFIED
Weiss, ArminUNSPECIFIEDhttps://orcid.org/0000-0002-7532-2974UNSPECIFIED
Braukmann, Johannes N.UNSPECIFIEDhttps://orcid.org/0000-0001-8046-9623UNSPECIFIED
Raffel, MarkusUNSPECIFIEDhttps://orcid.org/0000-0002-3340-9115133716116
Date:January 2022
Journal or Publication Title:Journal of the American Helicopter Society
Refereed publication:Yes
Open Access:Yes
Gold Open Access:No
In ISI Web of Science:Yes
Page Range:pp. 1-17
EditorsEmailEditor's ORCID iDORCID Put Code
Publisher:American Helicopter Society
Keywords:Helicopter, Rotor Wake, Background Oriented Schlieren, Constant Temperature Anemometry, Full-Scale Testing, Blade Tip Vortex
HGF - Research field:Aeronautics, Space and Transport
HGF - Program:Aeronautics
HGF - Program Themes:Efficient Vehicle
DLR - Research area:Aeronautics
DLR - Program:L EV - Efficient Vehicle
DLR - Research theme (Project):L - Virtual Rotorcraft and Validation
Location: Göttingen
Institutes and Institutions:Institute for Aerodynamics and Flow Technology > Helicopter, GO
Institute for Aerodynamics and Flow Technology > High Speed Configurations, GO
Deposited By: Carter, Beatrice
Deposited On:21 Jan 2022 16:51
Last Modified:25 Apr 2023 10:05

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