Johannes Braukmann

German Aerospace Center (DLR) | DLR · Department of Helicopters (HE)

An experimental aerodynamic study was conducted, analyzing the wake structure of a four-bladed model rotor with a forward-backward swept tip geometry inspired by the ONERA-DLR “Etude d´un Rotor Aéroacoustique Technologiquement Optimisé”-design. Similar tip designs are used on some modern helicopter main rotors. The experiments were conducted at the...

This article describes recent tests and developments of imaging and evaluation techniques for particle image velocimetry (PIV) that exploit the forward scattering of tracer particles by placing the camera in-line with the illuminating light source, such as a laser or a light emitting diode. These techniques have been in use for some time in microsc...

The influence of interactional effects on quadrotor performance in forward flight was evaluated taking into account square and diamond configurations, forward and backward tilt angles, and a range of hub spacings including overlapping blades. The analysis was based on the wind-tunnel measurements and simulations from four midfidelity computational...

The present paper describes the results of an experimental wind tunnel test campaign aimed at investigating the aerodynamic performance and flow physics related to a wing section equipped with two propellers mounted on a boom. The configuration investigated is meant to be representative of a full-scale eVTOL aircraft in cruise flight condition. The...

Constant temperature anemometry and background-oriented schlieren (BOS) measurements on a CH-53G helicopter (take off mass n ≈ 14t) are used to investigate the rotor wake including the vortex breakdown to statistical turbulence. Hovering flights at a range of hub heights 0:50 < h < 0:83 were investigated. The position of the rotorcraft was opticall...

The study presents the aeroelastic behavior of a double-swept rotor blade, that was investigated under climb conditions on a four-bladed rotor in the rotor test facility at the German Aerospace Center in Göttingen. The results of two measurement campaigns are combined to obtain a detailed insight in the coupling of blade deformation, integral loads...

The influence of interactional effects on quadrotor performance in forward flight was evaluated taking into account square and diamond configurations, forward and backward tilt angles and a range of hub spacings including overlapping blades. The analysis was based on the wind-tunnel measurements and simulations from five computational methods with...

The present paper describes the results of an experimental wind tunnel test campaign aimed at investigating and characterizing the complicated aerodynamic flow patterns around a wing section equipped with propellers mounted on a boom. The investigated configuration is meant to be representative of a full-scale eVTOL aircraft in cruise flight condit...

Setup-related aspects of background-oriented schlieren (BOS) experiments are discussed focusing on a sensitivity parameter S , which represents the relation between light deflection and resulting BOS signal, and the geometric blur. An analytic expression for the geometric blur by means of the circle of confusion (CoC) was derived which shows a prop...

Experimental investigations of three-dimensional dynamic stall on a four-bladed Mach-scaled semielastic rotor with an innovative double-swept rotor blade planform are presented. The study focuses on the coupling between the aeroelastic behavior of the blade and the underlying aerodynamics. Blade bending moment and flap displacement measurements wer...

An experimental aerodynamic study was conducted, analyzing the wake structure of a four-bladed model rotor with a forward-backward swept tip geometry inspired by the ONERA-DLR "ERATO" design. Similar tip designs are used on some modern helicopter main rotors. The experiments were conducted at the rotor test stand Göttingen in hover-like conditions...

Experimental investigations of three-dimensional dynamic stall on a four-bladed Mach-scaled semi-elastic rotor with an innovative double-swept rotor blade planform are presented. The study focuses on the coupling between the aeroelastic behavior of the blade and the underlying aerodynamics. Blade bending moment and flap displacement measurements we...

Density-based schlieren methods are a well-known and well-established tool for studying flows. A representative of this category that has become increasingly important in recent years is the background-oriented schlieren method (BOS). Like other schlieren techniques, the BOS method is based on a changing refraction of light rays due to spatial dens...

The study presents an optimized pressure-sensitive paint (PSP) measurement system that was applied to investigate unsteady surface pressures on recently developed double-swept rotor blades in the rotor test facility at the German Aerospace Center (DLR) in Göttingen. The measurement system featured an improved version of a double-shutter camera that...

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–fil...

Experimental investigations of three-dimensional dynamic stall on a four-bladed Mach-scaled semielastic rotor with an innovative double-swept rotor blade planform are presented. The study focuses on the coupling between the aeroelastic behavior of the blade and the underlying aerodynamics. Blade bending moment and flap displacement measurements wer...

The wake of its main rotor plays a key role in the overall performance and operational safety of a helicopter. The flow is governed by complex and unsteady features, such as the blade shear layers and blade tip vortices. In recent years, experimental measurement techniques have been developed and refined for this application, to keep up with an inc...

Blade tip vortices are a dominant phenomenon in rotor wakes. The interaction of vortices with rotor blades can cause noise emissions, vibrations and even flow separation. Particle Image Velocimetry (PIV) has become a standard tool in vortex investigations. A disadvantage of this method is its dependence on the tracking accuracy of the particles use...

An investigation into blade tip vortices of a sub-scale rotor under cyclic pitch conditions is carried out. Background oriented schlieren (BOS), particle image velocimetry (PIV), and computational fluid dynamics (CFD) are applied to the same test cases. This approach allows to combine the velocity data from PIV in a measurement plane, the density r...

The present work discusses a methodology to reconstruct three-dimensional positions of rotor blade tip vortices based on stereoscopic background-oriented schlieren data. The reconstruction procedure is first applied on a rotor test stand where a comparison with previous PIV measurements is performed to validate the reconstruction scheme. Subsequent...

An investigation on blade tip vortices of a sub-scale rotor under cyclic pitch conditions is carried out. Background oriented schlieren (BOS), Particle image velocimetry (PIV), and computational fluid dynamics (CFD) are applied to the same test cases. This approach enables to combine the velocity data from PIV in a measurement plane, the density re...

The main rotor wake of free–flying helicopters in ground effect was investigated during a measurement campaign with the DLR test rotorcraft Airbus Bo105 and EC135. An advanced measurement system combined high–speed schlieren imaging with point–wise constant temperature anemometry for a comprehensive analysis of the flow. In hover conditions, the wa...

The main rotor wakes of the free-flying DLR test helicopters Airbus Bo105 and EC135 were investigated in ground effect during hover, vertical takeoff, 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...

The present work discusses a methodology to reconstruct three-dimensional positions of helicopter blade tip vortices based on stereoscopic background-oriented schlieren data. The reconstruction methodology is first applied on a rotor test stand where a comparison with previous PIV measurements is performed to validate the reconstruction scheme. Sub...

The vortex system of four rotating and pitching DSA-9A blades was examined numerically and experimentally. Numerical computations were performed using German Aerospace Center (DLR)’s finite-volume solver TAU and were validated against experimental data gathered using particle image velocimetry carried out at the rotor test facility (RTG) in Götting...

In the published version, graphical abstract was missing. It is given below.

To characterize the vortex system of a helicopter rotor with cyclic pitch, an experimental investigation is carried out at the rotor test facility of the DLR, German Aerospace Center in Göttingen. A test case with attached flow conditions is chosen to study the blade tip vortex aerodynamics on a four-bladed rotor. The resulting azimuth-dependent vo...

The presented work tackles the lack of experimental investigations of unsteady laminar-turbulent boundary-layer transition on rotor blades at cyclic pitch actuation, which are important for accurate performance predictions of helicopters in forward flight. Unsteady transition positions were measured on the blade suction side of a four-bladed subsca...

A method of measuring density second derivative (shadowgraph) as a variant of background oriented schlieren (BOS) is presented. Pairs of images of a moving background with a small time difference are analysed using moving-window cross-correlation, giving the second derivative of density in the axis of background travel. The advantage over standard...

The presented work tackles the lack of experimental investigations of unsteady laminar-turbulent boundary-layer transition on rotor blades at cyclic pitch actuation, which are important for accurate performance predictions of helicopters in forward flight. Unsteady transition positions were measured on the blade suction side of a four-bladed subsca...

The presented work tackles the lack of experimental investigations of unsteady boundary-layer transition on rotor blades at cyclic pitch actuation, which are important for accurate performance predictions, for instance for helicopters in forward flight. Unsteady boundary-layer transition positions were measured on the blade suction side of a four-b...

The vortex system of four rotating and pitching DSA-9A blades was examined numerically and experimentally. Numerical computations were performed using DLR's finite-volume solver TAU and were validated against experimental data gathered using particle image velocimetry (PIV) carried out at the rotor test facility in Gottingen (RTG). Algorithms deriv...

The tip vortex system downstream of a four-bladed instrumented rotor was investigated experimentally through the application of stereoscopic particle image velocimetry (PIV). A dynamic stall test case was facilitated by a high cyclic pitch setting of the swashplate, with additional attached-flow and constant-pitch test cases for comparison reasons....

To characterize the vortex system of a rotor with collective and cyclic pitch settings, an experimental investigation was carried out at the rotor test facility (RTG) of the German Aerospace Center (DLR) in Gottingen. In addition to former studies based on velocity fields from particle image velocimetry (PIV) in the current measurement also a backg...

The blade-tip vortex of a rotating and pitching DSA-9A blade was investigated numerically and experimentally. Numerical computations were performed using DLR, German Aerospace Center's finite volume solver TAU, and experimental data were gathered by using particle image velocimetry carried out in a stereoscopic setup at the rotor test facility in G...

The tip vortex–system downstream of a four–bladed instrumented rotor was investigated experimentally through the application of stereoscopic particle image velocimetry (PIV). A dynamic stall test case was facilitated by a high cyclic pitch setting of the swashplate, with additional attached–flow and constant–pitch test cases for comparison reasons....

A new method has been applied for the detection of dynamic stall on rotating blades with cyclic pitch variation. The method works robustly for carbon-fiber composite blades and does not require contact with the surface or any kind of surface treatment or on-blade instrumentation. It can produce maps of the rotor plane where areas of stalled and att...

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...

Flow separation plays an important role in the aerodynamics of propellers and helicopter rotors. Although the ideal is to retain attached flow under all flight conditions, the main rotor of a helicopter sees partial separation under highly loaded forward flight or for flight conditions which include the blades intersecting their own wakes. The pres...