Otto Lilienthal developed a propulsion system for his gliders that used flapping feathered wingtips, which were actuated by a piston engine. This article presents the known historical and technical facts about this propulsion system and its application in Lilienthal's ornithopters. Although the motor capacity was insufficient for steady level flight, the configuration of the propulsion system is interesting. Wind tunnel experiments with scaled-down wingtips and their results will be presented. Simulations using the unsteady vortex lattice method were performed for comparison. The sensitivity of efficiency to different parameters, as well as the flight capacities of the configuration, will be discussed.

This study investigates Reynolds number effects on rotor wake vortex development using a hyperbaric rotor facility capable of pressurizing air up to 100 bar. Background-oriented schlieren (BOS) and hot-wire anemometry (HWA) were applied to characterize vortex trajectories, core growth, and circumferential velocity distribution. BOS measurements revealed consistent blade-to-blade trajectory deviations and vortex pairing across all operating conditions, despite that the investigated three-bladed rotor was milled from a single piece of aluminum, ensuring precise manufacturing and a highly symmetric geometry. A statistical scheme was developed to analyze the radial structure of fluctuating tip vortices, which traverse the pointwise fiber-film sensor in a fixed position. With increasing vortex Reynolds number, the tip vortices are more compact with a reduction in core growth. The circulation in the vortices grows with the vortex radial coordinate, and converges at a radial position basically independent of the vortex core size. Observed asymmetries in young vortices at low Reynolds numbers indicate enhanced roll-up dynamics. The results demonstrate the facility’s ability to isolate Reynolds number effects in rotor wake dynamics.

The blade tip vortices in the wake of a rotor are essential for the aerodynamic behavior of a helicopter. Due to the high complexity, a detailed simulation of a rotor wake is challenging and in many cases requires experimental validation. In particular, the study of secondary vortices that occur between the helically arranged blade tip vortices is part of the current research. Therefore, this paper presents an experimental study of the influence of different configurational parameters on the development of secondary vortices and their contribution to the vortex breakdown of a rotor in hover. The aim is to gain insight into which configurational parameters have an influence on the occurrence of secondary structures. An extensive database of more than 30 different configurations was created to validate the numerical simulations and methods. It was found that the blade passing frequency is the main contributor to the occurrence of secondary structures. A linear dependence between the number of secondary vortices detected and the blade passing frequency was found.

Since its introduction in the year 2000, background-oriented schlieren (BOS) has become a cornerstone technique for visualizing variable-density flows. In this review, we provide a rigorous examination of the optical principles underpinning BOS and related refractive-index-based techniques, complemented by an appendix linking schlieren imaging to Maxwell's equations. The core sections delve into the practical aspects of BOS, with detailed discussions on image processing algorithms and critical considerations for experimental setups. We then explore recent advancements and innovations, including extensions of BOS with tomography, data assimilation, and event-based imaging. Finally, we present notable applications of BOS in challenging and unconventional environments, showcasing the method's versatility, and offer inspiration for future research directions.

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 microscopy and in the design of optical instruments in astronomy. However, they have not yet been used for macroscopic PIV flow measurements. This study highlights the most promising approaches of the various recording configurations and evaluation techniques.

Measurements of the flow field around a free-flying model helicopter in ground effect for both quasi-steady and unsteady maneuvering flights were performed using stereoscopic particle image velocimetry (PIV). The wake features for hover and forward flight at low advance ratios were characterized and changing flow patterns like recirculation and ground vortex flow were observed to be in good agreement with existing wind tunnel data. Parameters describing both general flow patterns and single blade tip vortices were extracted and showed significant dependence on the forward flight velocity. Landing approaches were performed and large vortical structures were observed close to the rotor disk, which contained high amounts of vorticity due to entrained blade tip vortices. The vortex structures developing for unsteady landing approaches contained distinctly higher velocities and momentum fluxes than expected from quasi-steady conditions at the same advance ratios. For a vertical takeoff maneuver with a rapid increase of collective pitch, the bundling of blade tip vortices into a "starting vortex" with circulation values up to 6 times higher than for a single blade tip vortex was observed. These results show the significance of experimental data on free-flying helicopters in unsteady maneuvering flight because the resulting flow fields cannot be predicted using steady-state experiments or simulations.

The blade tip vortices in the wake of a rotor are essential for the aerodynamic behavior of a helicopter. Due to the high complexity, a detailed and high resolution simulation of a rotor wake is challenging and in many cases requires experimental validation. In particular, the study of secondary vortices that occur between the helically arranged blade tip vortices is part of the current research. Therefore, this paper presents an experimental study of the influence of different configurational parameters on the development of secondary vortices and their contribution to the vortex breakdown of a rotor in hover. The aim is to gain insight into which configurational parameters have an influence on the occurrence of secondary structures. An extensive database of more than 30 different configurations was created to validate the numerical simulations and methods. It was found that the blade passing frequency is the main contributor to the occurrence of secondary structures. Taking into account all the cases studied, a linear dependence between the number of secondary vortices detected and the blade passing frequency was found.

A background-oriented schlieren technique (BOS) using projected background patterns is presented. The projected backgrounds can be scaled to the measurement requirements, providing greater flexibility than printed backgrounds. The evaluation of the BOS image pairs using cross-correlation or optical flow analysis was demonstrated to produce equivalent results. The background pattern can dynamically be shifted by a rotating mirror, allowing evaluation using the reference-free shadowgraphy or reference-free BOS methods. The forward BOS technique is demonstrated, in which a projected speckle pattern is illuminated through a density object and recorded by a camera focused on the screen. In contrast to standard shadowgraphy, where the local image intensity is the parameter proportional the second derivative of density, this technique is robust with respect to varying lighting of the field as long as the displacement of small-scale image structures can be determined.

"The article describes recent tests and developments of PIV recording and evaluation techniques that take advantage of strong forward scattering of tracer particles by in-line recording set ups. By collimating the light of a low-power light source between a pair of schlieren mirrors the observation area is increased to more than 350 mm. The same setup allows for schlieren recording of density gradients. The in-line imaging arrangements have a relatively high background intensity level on which brighter particle images can be detected. Ideally this background is constant in time such that it can be removed through subtraction. The use of forward scattering has a significant influence on the required pulse energy of the light source used. Typically, sufficient illumination is achieved with about 1% of the pulse energy that would normally be used for a conventional orthogonal PIV arrangement employing a laser light sheet. "

The article describes the historical facts known of Otto Lilienthal’s Ornithopters from 1893 and 1896 in some detail. Example results of first numerical simulations of the flapping wing feathers will be discussed at the end of this article.

The article describes recent tests and developments of PIV recording and evaluation techniques that take advantage of strong forward scattering of tracer particles by in-line recording set ups. By collimating the light of a low-power laser between a pair of schlieren mirrors the observation area is increased to 200 mm. Another setup blocks the expanding laser light close to the source and is applied for the measurement of cylinder wake in a wind tunnel with a similar field of view.

The article describes recent tests and developments of PIV recording and evaluation techniques that take advantage of strong forward scattering of tracer particles by in-line recording set ups. By collimating the light of a low-power laser between a pair of schlieren mirrors the observation area is increased to 200 mm. Another setup blocks the expanding laser light close to the source and is applied for the measurement of cylinder wake in a wind tunnel with a similar field of view.

Mid-Air Helicopter Delivery (MAHD) is a new Entry, Descent and Landing (EDL) architecture for enabling future Martian helicopter-only missions (e.g., Mars Science Helicopter (MSH)), that offer much greater in situ mobility compared to traditional rover missions at lower cost. This EDL concept utilizes a delivery jetpack to slow down the rotorcraft free fall after separation from the parachuting backshell, thus avoiding unfavorable rotorcraft descent aerodynamics, and provides suitable aerodynamic conditions for helicopter take-off in mid air. While Martian rotorcraft operation has been successfully demonstrated by the Ingenuity system, the mid-air helicopter take-off from a self-propelled jetpack platform has been identified as one of the critical aspects of this EDL strategy. This paper presents the development of an experimental sub-scale test-bench to assess the aerodynamic interactions between the MSH, a jetpack analogue system, and the wind to evaluate the technical feasibility of MAHD. Aerodynamic measurements and various qualitative and quantitative flow visualizations were performed in a (1 atm / 1 g) environment and compared to computational fluid dynamics (CFD) simulation for validation. We also demonstrate in-flight capabilities of wind sensing as well as active trimming of the rotorcraft under relative crosswinds using an integrated force-torque sensor to be placed between rotorcraft and jetpack.