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 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.

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.

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.

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.

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.

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.

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.

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.

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.

The Faculty of Aerospace Engineering is one of eight faculties at Delft University of Technology. It is one of the most comprehensive academic and innovation communities worldwide focusing on aerospace engineering. Its 120 professors and 70 researchers are mentoring and teaching around 2,800 BSc/MSc students and more than 350 PhD candidates while working in all aerospace disciplines. It’s a powerhouse in aerospace education, research, and innovation, within the top 10 in the world. Our priority themes? Sustainable aerospace, digital transformation, including Artificial Intelligence, bio-inspired engineering and smart instruments and systems. Here’s our story.

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.

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.

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.

Many electric vertical takeoff and landing concepts are characterized by nontraditional vehicle layouts with distributed propellers. Two propeller interaction types were distinguished in this Paper, which investigates how propeller interaction in side-by-side and one-after-another configuration affects performance, in terms of thrust, power, in-plane forces, and out-of-plane moments, and how those performance effects depend on axial and lateral propeller spacing. A wind-tunnel experiment was performed with two propeller units, one instrumented with a force/torque sensor and the other introducing the aerodynamic interaction. Total pressure and planar particle-image velocimetry measurements were taken to investigate slipstream characteristics. A strong dependency of interaction effects on the geometric layout was found. For side-by-side interaction characteristic of vertical takeoff and transition, interaction effects varied from weak at small angle of attack to strong at larger angles. A drop in rear propeller thrust of up to 30% was found at constant advance ratio. Keeping thrust constant resulted in power penalties up to 13% for the two propellers combined. For one-after-another interaction, characteristic of cruise, a maximum reduction of thrust of up to 80% was observed. Thrust compensation led to power penalties up to 30% for the rear propeller alone. An extended blade element momentum model captured most interaction effects with sufficient accuracy.

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.

In this study, unsteady RANS simulations are performed to investigate the effect of over-the-wing (OTW) propeller inclination on the aerodynamic interaction with a wing featuring a plain flap. A comparison to experimental data shows that the numerical approach is capable of modeling the wing and propeller separately and can capture the effect of the wing on the propeller in the OTW configuration, but under-predicts propeller-induced flow separation over the flap. The results show that, if the propeller is installed over the flap hinge and aligned with the freestream velocity (baseline configuration), the slipstream and blade tip-vortices generate additional adverse pressure gradients on the wing surface, leading to a local increase in flow separation downstream. However, if the propeller is tilted and aligned with the flap surface (inclined configuration), the slipstream increases the momentum in the boundary layer and the flow remains attached. The propeller alters the pressure distribution of the wing such that higher lift is generated in the baseline case, while a larger drag reduction is achieved in the inclined case. However, combined with the thrust vector of the propeller, the baseline configuration is found to have the largest combined axial force in thrust direction, while the inclined configuration presents the highest effective lift. These results indicate that inclining the propeller can enhance the low-speed performance of OTW distributed-propulsion systems.

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.

This article describes an experimental investigation of the aerodynamic interaction that occurs between distributed propellers in forward flight. To this end, three propellers were installed in close proximity in a wind tunnel, and the changes in their performance, flow-field characteristics, and noise production were quantified using internal force sensors, total-pressure probes, particle-image velocimetry (PIV), and microphones recessed in the wind-tunnel wall. At the thrust setting corresponding to maximum efficiency, the efficiency of the middle propeller is found to drop by 1.5% due to the interaction with the adjacent propellers, for a tip clearance equal to 4% of the propeller radius. For a given blade-pitch angle, this performance penalty increases with angle of attack, decreasing thrust setting, or a more upstream propeller position, while being insensitive to the rotation direction and relative blade phase angle. Furthermore, the velocities induced by the adjacent propeller slipstreams lead to local loading variations on the propeller disk of 5% – 10% of the average disk loading. Exploratory noise measurements show that the interaction leads to different tonal noise waveforms of the system when compared to the superposition of isolated propellers. Moreover, the results confirm that an active control of the relative blade phase angles between propellers can effectively modify the directivity pattern of the system.