TY - JOUR

T1 - Airborne visualization of helicopter blade tip vortices

AU - Bauknecht, André

AU - Merz, Christoph B.

AU - Raffel, Markus

PY - 2016/9/2

Y1 - 2016/9/2

N2 - Abstract: Blade tip vortices and their interaction with the helicopter blades play an important role in the generation of noise on rotorcraft. Full-scale vortex visualization is essential for the understanding of these effects and the validation of sub-scale experiments and numerical simulations. In the present work, the reference-free background-oriented schlieren (BOS) method was used to visualize blade tip vortices of a full-scale BO 105 helicopter in hover and under free flight conditions. A ground-based dual-camera BOS system with an artificial background was applied for the visualization of the helicopter’s main and tail rotor tip vortices during take-off. In a second flight test, a similar dual-camera BOS system was used to visualize the main rotor blade tip vortices of the BO 105 during high speed forward, curve, and accelerating forward flight. The camera system was deployed aboard a microlight airplane flying above the helicopter, with vegetation on the ground serving as natural backgrounds. Different natural and artificial backgrounds were analyzed and compared to assess their suitability for the BOS evaluation. The airborne setup enabled the visualization of vortices up to a maximum vortex age of ψv= 630 ∘ at distances of up to one rotor diameter behind the rotor plane. Blade-vortex interaction effects and deformations of the vortex system were observed. The visualization results obtained during the inflight measurements show that the reference-free BOS method is highly suitable for the detection of helicopter blade tip vortices during full speed forward and maneuvering flight, therefore removing the restrictions imposed by ground-based measurements. Graphical abstract: [Figure not available: see fulltext.]

AB - Abstract: Blade tip vortices and their interaction with the helicopter blades play an important role in the generation of noise on rotorcraft. Full-scale vortex visualization is essential for the understanding of these effects and the validation of sub-scale experiments and numerical simulations. In the present work, the reference-free background-oriented schlieren (BOS) method was used to visualize blade tip vortices of a full-scale BO 105 helicopter in hover and under free flight conditions. A ground-based dual-camera BOS system with an artificial background was applied for the visualization of the helicopter’s main and tail rotor tip vortices during take-off. In a second flight test, a similar dual-camera BOS system was used to visualize the main rotor blade tip vortices of the BO 105 during high speed forward, curve, and accelerating forward flight. The camera system was deployed aboard a microlight airplane flying above the helicopter, with vegetation on the ground serving as natural backgrounds. Different natural and artificial backgrounds were analyzed and compared to assess their suitability for the BOS evaluation. The airborne setup enabled the visualization of vortices up to a maximum vortex age of ψv= 630 ∘ at distances of up to one rotor diameter behind the rotor plane. Blade-vortex interaction effects and deformations of the vortex system were observed. The visualization results obtained during the inflight measurements show that the reference-free BOS method is highly suitable for the detection of helicopter blade tip vortices during full speed forward and maneuvering flight, therefore removing the restrictions imposed by ground-based measurements. Graphical abstract: [Figure not available: see fulltext.]

KW - Background-oriented schlieren

KW - Blade tip vortex

KW - Flight test

KW - Helicopter

UR - http://www.scopus.com/inward/record.url?scp=84984910961&partnerID=8YFLogxK

U2 - 10.1007/s12650-016-0389-z

DO - 10.1007/s12650-016-0389-z

M3 - Article

AN - SCOPUS:84984910961

VL - 20

SP - 139

EP - 150

JO - Journal of visualization

JF - Journal of visualization

SN - 1343-8875

IS - 1

ER -