This study quantifies the viscous interaction between propeller tip vortices and a turbulent boundary layer developing over a semi-elliptic leading-edge plate, located downstream. The experimental wind-tunnel set-up is designed to be representative of the tractor-propeller-wing configuration. Using stereoscopic particle image velocimetry and static wall-pressure measurements, the near-wall flow topology is resolved over the plate, semi-immersed in the propeller slipstream. The results show that the interaction exhibits high spatio-temporal coherence and is dominated by a coupling between primary and secondary vortical structures. Two distinct interaction regions are identified relative to the tip-vortex core: on the inboard side, towards the slipstream interior, the boundary-layer flow experiences strong velocity gradient transitions and amplified near-wall vorticity. The flow on the outboard side, moving out of the slipstream, exhibits wall-parallel velocity deficits and vorticity lift-up consistent with unsteady vortex-induced separation mechanisms. Spanwise velocity induced by the wall-normal component of the primary vortex connects these two regions, with the secondary vortex structure identified as enhancing boundary-layer lift-up on the outboard side. Although no local flow reversal occurs under the tested conditions, localised shear amplification and vorticity roll-up indicative of separation-like behaviour were observed. These findings advance the understanding of viscous slipstream-boundary-layer interaction and its implications for tractor-propeller-wing integration.

For the next generation of transport aircraft, boundary-layer ingestion (BLI) is being proposed as a promising technology to reduce energy consumption. However, the aerodynamic interaction between the propulsor and the fuselage boundary layer has received little attention. In this study, an experimental approach is used to study the effect of a fuselage aft-cone-mounted propeller on the flow around a fuselage aftbody. An idealized fuselage model with an integrated rear propeller is tested in a low-subsonic wind tunnel. The loads on the propeller were measured directly with the use of a rotating shaft balance. Integration of the aft-fuselage pressure field allowed for a complete force decomposition. Operation of the propeller was shown to significantly increase pressure and friction drag on the fuselage. Furthermore, hot-wire measurements show that the turbulence characteristics of the fuselage boundary layer upstream of the propeller were altered by the propeller. Compared to the propeller-off measurement, a clear deviation from the universal log law was observed. Phase-locked hot-wire and embedded microphone data reveal small in-phase fluctuations with the propeller blade passage. Despite their persistence throughout the boundary layer, the fluctuations are not believed to significantly impact the mean inflow to the propeller or affect its performance. Despite their insignificant impact on propeller performance, the fluctuations could still be relevant in terms of noise and vibrations.

This paper presents an improved approach for fast numerical modeling of the mutual aerodynamic interactions between a wing and tractor propellers for preliminary design purposes. Vortex methods are used to model the propeller and wing aerodynamic performance. The blade element momentum (BEM) method, which is used to model the propeller performance, is extended to allow for a nonuniform inflow field, such that the upstream effects of the wing can be included in the propeller performance modeling. The circulation distribution over the propeller blades is then used in the slipstream tube model (STM) to determine the time-averaged propeller slipstream velocities. Finally, an improved vortex lattice method (VLM) is used to model the wing’s spanwise lift distribution. The method includes an often overlooked correction for the finite slipstream dimensions experienced by the wing segments, in both vertical extent and spanwise extent. This physics-based correction, based on the image vortex technique, partially offsets the large discrepancies in the lift augmentation found in previous low-order numerical analyses of propeller–wing interactions, while keeping the analysis routine computationally very cheap. The BEM and STM are validated against experimental data, while the VLM and complete propeller–wing system are validated against high-fidelity numerical data, confirming the accuracy of the used models. Discrepancies are found in regions or operating conditions where viscosity becomes more relevant, such as separated flows. The numerical model derived in this paper can be used for a quick and accurate first-order estimation of the aerodynamic performance of new conceptual distributed propeller aircraft.

This paper explores the influence of the characteristics of the helical vortex system of a propeller slipstream on the resulting propeller–wing interaction, with a particular focus on how variations in helix angle impact slipstream deformation. Slipstream characteristics are changed by controlling the thrust and torque coefficient of the propeller through adjustments in blade pitch, advance ratio, and blade count. We conducted experimental measurements of a propeller–wing–flap model using seven-hole pressure probes, oil flow visualization, and infrared thermography in both cruise and high-lift configurations (with deployed slotted flap). The results presented in this paper demonstrate the dominance of the torque coefficient, and thereby longitudinal vorticity in the slipstream, on the slipstream deformation. The paper also underscores the role of the nacelle integration in the development of the slipstream, as well as the flow on the wing surface. The insights into the slipstream deformation provided in this work are essential for future closely coupled propeller–wing designs, particularly when it comes to high-lift configurations.

The goal of this study is to determine the aero-propulsive performance of an over-the-wing distributed propulsion (OTWDP) system, and to understand how it depends on various operating conditions. For this, a windtunnel test is performed with a simplified OTWDP geometry consisting of three unducted propellers placed side-by-side above a rectangular wing. A numerical model combining 2D panel methods, a slipstream vortex model, and a lower-order method for propeller performance in non-uniform inflow is then used to analyze additional operating conditions. A comparison to experimental data shows that the numerical method captures the changes in wing and propeller performance due to aerodynamic interaction in cruise conditions, though it is inaccurate if flow separation occurs on the wing surface beneath the propeller. For a setup with propellers of diameter-to-chord ratio 0.6 placed above the wing at 80% chord, the sectional lift-to-drag ratio of the wing is found to increase by 40% – 70% for typical cruise lift and thrust coefficients, while the propeller efficiency is decreased by 10% – 15%, compared to the two components in isolation. Parameter sweeps demonstrate that the combined aero-propulsive performance improves with a variable-pitch propeller and at higher lift coefficients, thrust settings, or Reynolds numbers.

Propeller–wing–flap systems are subject to complex aerodynamic interactions between each part of the system. Although the propeller–wing interaction in cruise conditions is well defined, the high-lift condition is relatively unexplored. Effective analysis of the complex aerodynamic relationship between propeller, wing, and flap is being impeded by a lack of understanding of the underlying mechanisms. In this paper, we therefore investigate the effects of a 2D jet impinging on a multisection airfoil. We quantify which factors that define a jet–wing–flap configuration dominate lift, drag, and moment responses. We further investigate interactions between these factors and discuss how they affect the flow. We find that the jet velocity ratio is by far the dominant factor in lift, drag, and moment responses, but it does not have strong interactions with other factors. The sensitivities of the multi-element airfoil do not change significantly when impinged upon by a jet, except when critical Mach numbers are exceeded. This strongly affects the aerodynamic response and dominant sensitivities. We furthermore conclude that the immersion of the flap is a key aspect when it comes to augmenting the lift by increasing the dynamic pressure in the flowfield. The conclusions from this paper can provide key insights for propeller–wing–flap flows.

Boundary-layer instability on a rotating cone induces coherent spiral vortices that are linked to the onset of laminar–turbulent transition. This type of transition is relevant to several aerospace systems with rotating components, e.g., aeroengine nose cones. Because a variety of options exist for the nose-cone shapes, it is important to know how their shape affects the boundary-layer transition phenomena. This study investigates the effect of varying cone angle on the boundary-layer instability on rotating cones facing axial inflow. It is found that increasing cone angle has a stabilizing effect on the boundary layer over rotating cones in axial inflow. The parameter space of Reynolds number Re _l and local rotational speed ratio S is experimentally explored to find the spiral vortex growth on rotating cones of half angle ψ 22.5°, 45°, and 50°. The previously addressed cases of ψ 15° and 30° are also revisited. Increasing half-cone angle is found to have a stabilizing effect on the boundary layer on the rotating cones with ψ ≲ 45°; i.e., the spiral vortex growth is delayed to higher Re _l and S. This effect diminishes when the half-cone angle increases from ψ 45° to 50°. The spiral vortex angle ϵ decreases with increasing rotational speed ratio S for all the investigated cones, irrespective of the half-cone angle. However, the instability on the broader cones is found to induce shorter azimuthal wavelengths.

With the rise of distributed propeller concepts, there is an increased interest in quantifying the interaction between propeller slipstream and wing. It is known from literature that the local upwash induced by the propeller swirl can lead to a reduction of the wing-induced drag, a phenomenon often referred to as swirl recovery. However, at the same time, the distortion of the lift distribution due to the slipstream interaction cancels part of the swirl recovery benefit. These two separate mechanisms are often grouped together, but their relative contribution to the change in induced drag of a propeller-wing system is unknown. The goal of this paper is to separate the two mechanisms and to quantify their relative importance in terms of the induced drag of a wing immersed in a propeller slipstream. To this end, an improved approach for fast low-order modeling of the interaction between propeller and wing was implemented. The propeller performance is calculated using a BEM model, after which the induced velocities in the slipstream are modeled using a slipstream tube model. The propeller-induced velocities then have been implemented into a vortex-lattice analysis of the wing, including an often overlooked correction for the finite slipstream height experienced by the wing sections immersed in the propeller slipstream. It was found that the tip-mounted configuration with an inboard-up rotating propeller showed the largest reduction in total induced drag, even though the spanwise lift distribution was disturbed the most compared to other spanwise propeller positions. The swirl-induced drag mechanism outweighs the trailing vortex-induced drag mechanism. This is also true when the propeller is rotating in the other direction, when the largest performance degradation is obtained for the tip-mounted configuration.

This work shows the behaviour of an unstable boundary-layer on rotating cones in high-speed flow conditions: high Reynolds number Rel > 106, low rotational speed ratio S < 1–1.5, and inflow Mach number M = 0.5. These conditions are most-commonly encountered on rotating aero-engine-nose-cones of transonic cruise aircraft. Although it has been addressed in several past studies, the boundary-layer instability on rotating cones remained to be explored in high-speed inflow regime. This work uses infrared-thermography with POD approach to detect instability-induced flow structures by measuring their thermal footprints on rotating cones in high-speed inflow. Observed surface temperature patterns show that the boundary-layer instability induces spiral modes on rotating cones, which closely resemble the thermal footprints of the spiral vortices observed in the past studies at low-speed flow conditions: Rel < 105, S > 1, and M ≈ 0. Three cones with half-cone angles y = 15◦, 30◦, and 40◦ are tested. For a given cone, the Reynolds number relating to the maximum amplification of the spiral vortices is found to follow an exponential relation with the rotational speed ratio S, extending from low- to high-speed regime. At a given rotational speed ratio S, the spiral vortex angle appears to be as expected from the low-speed studies, irrespective of the half-cone angle.

Boundary-layer ingestion (BLI) is a propulsor–airframe integration technology that promises substantial fuel consumption benefits for future civil aircraft. This paper discusses an experimental study, conducted within the European Union–funded Horizon 2020 CENTRELINE project, on the aerodynamic performance of an aircraft with a BLI propulsor integrated at the aft-fuselage section (known as the Propulsive Fuselage Concept). The low-speed wind-tunnel experiments were carried out at Reynolds and Mach numbers of 460,000 and 0.12, whereas the Reynolds and Mach numbers are 40,000,000 and 0.82 at full-flight scale. Aerodynamic loads measurements show that the BLI propulsor affects the longitudinal and lateral-directional equilibrium of the aircraft in off-cruise conditions. Moreover, velocity and total pressure measurements characterize the flowfield around the BLI propulsor in cruise and off-cruise conditions. The analysis of the momentum and power fluxes in the flowfield shows that, while around 20% of the total aircraft drag is due to the fuselage body, only less than 5% of the total aircraft drag power is dissipated in the fuselage wake. Furthermore, the BLI propulsor recovers around 50% the axial kinetic energy flux in the fuselage boundary layer (the so-called wake-filling effect), suggesting an increased propulsive efficiency.

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Growing interest in unconventional aircraft designs coupled with miniaturization of electronics and advancements in manufacturing techniques have revived the interest in the use of Sub-scale Flight Testing (SFT) to study the flight behaviour of full-scale aircraft in the early stages of design process by means of free-flying sub-scale models. SFT is particularly useful in the study of unconventional aircraft configurations as their behaviour cannot be reliably predicted based on legacy aircraft designs. In this paper, we survey the evolution of various design approaches (from 1848 to 2021) used to ensure similitude between a sub-scale model and its full-scale counterpart, which is an essential requirement to effectively perform SFT. Next, we present an exhaustive list of existing sub-scale models used in SFT and analyse the key trends in their design approaches, test-objectives, and applications. From this review, we conclude that the state-of-the-art sub-scale model design methods available in literature have not been used extensively in practice. Furthermore, we argue that one sub-scale model is not sufficient to predict the complete flight behaviour of a full-scale aircraft, but a catalog of tailored sub-scale models is needed to predict full-scale behaviour. An introduction to the development of such a catalog is presented in this paper, but the development of a formal methodology remains an open challenge. Establishing an approach to develop and use a SFT catalog of models to predict full-scale aircraft behaviour will help engineers enhance confidence on their designs and make SFT a viable and attractive testing method in the early stages of design.

Experiments were performed using a wall-to-wall unswept and untapered wing with a single slotted flap and a propeller, to obtain a validation dataset and gain insight into primary flow phenomena in propeller-wing-flap interactions. Measurements were taken using pressure taps, a wake rake and oil flow visualization, for several flap deflections (0, 15 and 30 degrees) and thrust settings (unpowered, J = 0.8 / T _c = 1.05 and J = 1.0 / T _c = 0.45). Similarity of the measured data to similar experiments was poor, which was believed to be due to the low Reynolds number of Re = 6e5 and sensitivity of local measurements due to occurrence of stall cells. Oil flow visualizations showed significant induction of flow separation from nacelle-wing interactions in unpowered conditions, traced to boundary layer growth. For powered cases it was shown that both sides of the deployed flap are immersed in the part of the slipstream that passes the pressure side of the main element. This part of the slipstream deforms significantly before it reaches the flap and thus results in complex spanwise variations for the flap flow. This stresses the need to investigate slipstream development in propeller-wing-flap systems and the effects on flap flow specifically to gain in-depth understanding of the interactions. The results presented in this paper expose the inherent complexity of investigating propeller-wing-flap systems and gaining viable validation data, and might serve to guide for future investigations of propeller-wing-flap systems.

Accurately measuring small changes in aerodynamic drag over a flat surface stands at the core of the development of technologies capable of reducing turbulent friction drag. A wind tunnel drag measurement system was developed which improves significantly on the state of the art. Experimental tests demonstrated that an uncertainty of less than 0.5% of C D at a 95% confidence level was typically achieved, already at drag values below 1 N. This was replicated in two different wind tunnels. A match with literature on riblet performance within 1% of C D was obtained. A crucial aspect of the design is the implementation of a correction for the pressure forces on the streamwise-facing surfaces of the test plate assembly. The flexible architecture of the system in the present realisation makes it suitable for most wind tunnels having a test section width of 400 mm or larger, which allows for accelerated development of turbulent drag reduction concepts from moderate-size low-cost facilities towards flow conditions relevant to the intended industrial application.

This experimental study focuses on the aerodynamic interaction between an over-the-wing (OTW) propeller and a wing boundary layer. An OTW propeller is positioned above the hinge line of a wing with a trailing-edge flap. Measurements are carried out with and without axial pressure gradients by deflecting the flap and by extending the flat upper surface of the wing in the streamwise direction, respectively. Surface-pressure taps, microphones, and particle image velocimetry are combined to quantify both the time-averaged and unsteady interaction effects. Results show that the propeller generates an adverse pressure gradient on the wing surface that scales linearly with thrust and decreases with increasing blade-tip clearance. The pressure gradient is partially caused by slipstream contraction, which decelerates the flow near the wall. Additionally, the surface-pressure fluctuations generated beneath the propeller blades and slipstream are stronger than the time-averaged pressure increase due to flow deceleration. Consequently, the propeller triggers flow separation over the hinge line when the flap is deflected. A parametric study of different propeller locations indicates that increasing the tip clearance is not an effective way to mitigate flow separation. However, displacing the propeller half a radius upstream of the hinge line creates a Coandă effect, which allows the flow to remain attached.

An experimental dataset is presented of propeller performance in static condition and at low subsonic airspeeds for various angles of attack up to 90 deg. Numerical investigation through a Reynolds-averaged Navier–Stokes computational fluid dynamics model revealed the mechanisms behind performance changes with advance ratio, angle of attack, and configuration changes. The experimental dataset was found to be free of major errors and is very suitable for validation of propeller models. Furthermore, aerodynamic interaction with an upstream wing was tested with the propeller and wing normal to the flow, to represent the interaction occurring with a time-averaged main rotor slipstream on a compound helicopter. From numerical investigation it was concluded that the results are qualitatively representative of this interaction. The experimental data showed that addition of the wing results in a net reduction of all propeller performance quantities, with thrust reducing up to 20%. Thrust decreasing and thrust increasing mechanisms were found numerically. For most tested operating conditions, the wing resulted in a small decrease of propeller thrust-over-power ratio. Decreasing propeller advance ratio, increasing wing distance, and increasing flap deflection generally decreased the effect of the wing on thrust and power; however, the influence of flap deflection was found to be small.

At the 1998 Nagano Winter Olympic Games, zigzag tape was introduced on the race suit lower legs and cap of speed skaters. Application of these zigzag devices on live skaters and cylinders in the wind tunnel showed large improvements in the aerodynamic drag. These wind-tunnel results were unfortunately not widely published, and the impact of the zigzag strips in a real skating environment was never established. This paper aims to show the background of the application of the zigzag tape and to establish the impact it may have had on speed-skating performance. From comparisons of 5000 m races just before, during and just after the Nagano Olympics and an analysis of historic world record data of the 1500 m men’s speed skating, the impact of the zigzag tape turbulators on average lap times on 1500 and 5000 m races is calculated to be about 0.5 s.

Centrifugal instability of the boundary layer is known to induce spiral vortices over a rotating slender cone that is facing an axial inflow. This paper shows how a deviation from the symmetry of such axial inflow affects the boundary layer instability over a rotating slender cone with half-angle. The spiral vortices are experimentally detected using their thermal footprint on the cone surface for both axial and non-axial inflow conditions. In axial inflow, the onset and growth of the spiral vortices are governed by the local rotational speed ratio and Reynolds number in agreement with the literature. During their growth, the spiral vortices significantly affect the mean velocity field as they entrain and bring high-momentum flow closer to the wall. It is found that the centrifugal instability induces these spiral vortices in non-axial inflow as well; however, the asymmetry of the non-axial inflow inhibits the initial growth of the spiral vortices, and they appear at higher local rotational speed ratio and Reynolds number, where the azimuthal variations in the instability characteristics (azimuthal number and vortex angle) are low.

In this combined experimental and numerical study, the propeller–airframe aerodynamic interaction is characterized for an aircraft configuration with propellers mounted to the horizontal tailplane. The contributions of the propeller and airframe to the overall loading are distinguished in the experimental analyses by using a combination of external balance and internal load cell data. Validated computational fluid dynamics simulations are then employed to quantify the interaction at a component level. The results show that the propeller installation shifts the neutral point aft with increasing propeller thrust. For the configuration considered herein, the yawing moment due to sideslip is increased by approximately 10%, independent of the propeller thrust coefficient. The changes in propeller loading due to the airframe-induced flowfield are the dominant factor to change the airframe stability and performance. The prominent installation effects occur at high angle of attack, because in that condition the propeller experiences a significant nonuniform inflow that affects the propeller and tailplane. The relatively large propeller diameter compared with tailplane span leads to a change of the tailplane root vortex that causes the tailplane effectiveness to reduce with an inboard-up rotating propeller.

View Video Presentation: https://doi-org.tudelft.idm.oclc.org/10.2514/6.2021-2467.vid

Boundary Layer Ingestion (BLI) is a technology that promises fuel consumption benefits for future civil aircraft. However, it introduces detrimental aerodynamic interactions between the propulsor and the airframe. In particular, the inflow to the BLI propulsor is affected by the flow around the airframe elements. The non-uniform inflow can influence the fan aerodynamic, aeroacoustic and aeroelastic performance. As a consequence, the fan design needs to tolerate the inlet distortions in all the flight phases. This paper discusses an experimental study of the aerodynamic performance of an aircraft with a BLI propulsor integrated at the aft-fuselage section, representative of a Propulsive Fuselage Concept (PFC) aircraft. Aerodynamic load measurements show that the BLI propulsor affects the longitudinal and lateral-directional equilibrium of the aircraft in off-cruise conditions. Flow measurements at the BLI propulsor inlet indicate that the fuselage boundary layer induces the strongest total pressure distortion. However, particularly at a non-zero sideslip angle, the vertical tail plane strongly affects the inflow to the BLI propulsor, introducing non-symmetric total pressure and velocity distortions. The analysis of the momentum and power fluxes in the flowfield show that around 20% of the total aircraft drag is produced in the fuselage boundary layer, while around 5% of the total aircraft drag power is dissipated in the fuselage wake. Furthermore, the BLI propulsor substantially reduces the axial kinetic energy flux in the fuselage boundary layer (the so-called ``wake-filling'' effect), suggesting an increased propulsive efficiency.