It is envisioned that the future generations of regional and short to medium-range aircraft employ a high level of propeller integration to achieve low-emission flight. The objectives of unconventional propeller installations include the enhancement of the airframe aerodynamic efficiency, increasing the propeller efficiency, improving cabin comfort, and improving the overall aircraft design by lower operating empty weight. Furthermore, by employing the aerodynamic interaction in specific phases of the flight, beneficial propulsion integration can also enable the use of alternative energy sources and increase the electrification level of the propulsion system. The closer proximity of the propeller and airframe requires a more dedicated integral design (approach) of both the airframe and propulsion unit. The objective of this dissertation is: to characterize the role of the aerodynamic interaction between the propeller and the airframe on the performance and static stability characteristics for selected aircraft configurations which aim for a beneficial propeller-airframe interaction. To this end, three different types of analyses are performed. First, fundamental phenomena are investigated which provide insight for related configurations and derivatives thereof. Second, a configuration study indicates the expected trends on various performance indicators. Finally, two detailed studies on aircraft level demonstrate the relative importance and the coupling between aerodynamic interactions. The first configuration features propellers that are mounted to the horizontal tailplane. This is an example where there is a strong interaction between the propeller and airframe that affects performance, stability, and control, and contains various interaction mechanisms that are of interest for other configurations as well. A second specific case is the a distributed propulsion configuration with propellers mounted to the inboard part of the wing (in front of the high lift devices), together with a propeller mounted to the tip of the wing. One of the focal points of the current study is extending the understanding of nonuniform inflow effects on propeller performance and its role in aircraft stability and trim. Compared to the conventional configuration, for various unconventional propeller installations, the nonuniform inflow to the propeller differs both in type and magnitude, and varies with flight condition. The slipstream shape and consequently its interaction with lifting surfaces are affected as well. These factors directly affect the gradients and offsets of the propeller force curves and therefore the aircraft stability and trim, respectively. By employing CFD results, a study has been performed on the {sensitivity} of the radial load distribution to a change in inflow condition that is expressed as a change in local advance ratio. The constructed distributions provide insight into what region of the disk is responsible for the largest changes of the propeller forces. This has been demonstrated to be the region of highest loading. It is also shown that for a given propeller design, nonuniform inflow can be represented by an `installation coefficient' kappa such that the efficiency curve of this uninstalled propeller is scaled along the advance ratio and efficiency directions by kappa to obtain the installed propeller efficiency. Using the data of the isolated propeller for an arbitrary blade angle, the advance ratio at which the installed case has highest efficiency, as well as the value of the maximum efficiency, can be quantified immediately. The computational intensive analyses to find the optimum blade angle for the installed cases are therefore redundant and the formulation of the installation coefficient is therefore highly valuable to the aircraft designer. The installation coefficient also gives insight in what regime of the efficiency-advance-ratio curve the largest changes occur due to nonuniform inflow...@en

This paper presents the identification of the aerodynamic model of the "Flying-V", a novel aircraft configuration. The aerodynamic model is estimated using flight test data from a 4.6\% sub-scale model. The dataset includes longitudinal and lateral-directional maneuvers performed by both the pilot and the autopilot to excite the aircraft dynamic modes. The so-called Two-Step Method is used to decouple and simplify the aerodynamic identification problem; the state estimation step is performed by an Iterated Extended Kalman Filter, and the parameter-estimation step using ordinary least squares. A stepwise regression technique and previous knowledge from wind-tunnel tests are combined to select the model structure, and the identified model is validated using a third of the gathered data. The estimated models are parsimonious and considered adequate in terms of model fit, with a maximum relative Root Mean Square Error of 10% for the worst validation case. For the considered location of the center of gravity and flight conditions, the estimated aerodynamic derivatives confirm that the aircraft is longitudinally stable, both statically and dynamically; and that it is also laterally and directionally statically stable. The analysis of the dynamic modes of the sub-scale model showed stable short period and roll subsidence modes, a lightly damped Dutch roll mode, and lightly damped/unstable phugoid and spiral modes.@en

Wingtip-mounted propellers are a promising solution for advanced propulsion integration on future (hybrid-)electric aircraft. Previous work has confirmed the favorable aerodynamic interactions between the propeller and the wing that occur for wingtip-mounted propellers in both tractor and pusher configuration. However, a direct comparison of the performance effects for the tractor and pusher configurations is unavailable in open literature. Moreover, the separate contributions of the propeller and wing forces to the overall system performance have not been sufficiently separated in previous studies. This paper presents the results of a wind-tunnel experiment performed at Delft University of Technology with a modular propeller-wing setup that addressed these knowledge gaps. A powered propeller model with a nacelle was installed at the tip of a cambered wing model. The nacelle could be reversed in order to change from tractor to pusher configuration. Measurements with an external balance quantified the system loading, while an internal balance provided a separate measurement of the propeller loading. The results highlight the differences between the interaction mechanisms for the tractor and pusher configurations. An assessment of the system performance showed that the pusher configuration required the lowest propeller shaft power to achieve a given system lift coefficient and net force coefficient in the flight direction. Power reductions of up to 9% were achieved compared to the tractor configuration for lift coefficients between 0.0 and 1.0 and net axial force coefficients between 0.00 (force balance in flight direction) and +0.08 (net positive force in flight direction).@en

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.@en

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.@en

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.@en

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.

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Advances in aerodynamic and propulsive efficiency of future aircraft can be achieved by strategic installation of propellers near the airframe. This paper presents a robust and computationally efficient engineering method to estimate the load distribution of a propeller operating in arbitrary nonuniform flow that is induced by the airframe and by different flight conditions. The time-resolved loading distribution is computed by determining the local blade section advance ratio and using the sensitivity distribution along the blade, which is a property of the propeller in isolated conditions. The method is applied to four representative validation cases by comparing to full-blade computational fluid dynamics (CFD) simulations and experimental data. For the evaluated cases, it is shown that the changes in the propeller loads due to the nonuniform inflow are predicted with errors ranging from 0.5 up to 12% compared to the validation data. By extending the quasi-steady approach with a correction to account for unsteady effects, the time-resolved blade loading is also well approximated, without adding computational cost. The proposed method provided a time-resolved solution within several central processing unit seconds, which is seven orders of magnitude faster compared to full-blade CFD computations.

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This paper presents an experimental and numerical study of the aerodynamic in-plane and out-of-plane loads of a propeller which are induced by the wake of an upstream wing impinging on the lower half of the propeller disk. A propeller was installed behind a wing model in a low-speed wind-tunnel and measurements were taken with an external balance and a rotating shaft balance to determine the aerodynamic characteristics of the wing and propeller. The installation of the wing shows negligible changes in propeller thrust coefficient at low advance ratios, while at medium thrust conditions (C _T ≈ 0.3), the wing shows a small increase in propeller thrust in the order of 1%. The installation of the propeller aft of the wing shows a change on propeller efficiency ranging from ∆η _p =–0.01 to +0.04. The location of the wake impingement at the propeller plane is shown to play an important factor for the time averaged and unsteady propeller loads. The radial location where the largest change in load occurs due to wake impingement, coincides with the location of highest propeller loading. A simplified and computationally efficient method is presented for estimation of these unsteady propeller loads in non-uniform inflow. The method shows good agreement for the integral unsteady blade thrust and integral propeller for different wake impingement locations.

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Wingtip-mounted tractor propellers enhance aerodynamic performance by attenuating the wingtip vortex with the propeller slipstream and inducing a favorable upwash on the wing. However, the close coupling between propeller and wing means that wing performance may be degraded when the propeller produces negative thrust. This paper analyzes the aerodynamic interaction effects due to the wingtip-mounted propeller under such conditions, that occur when the propeller is windmilling or used for energy harvesting. Experiments in a low-speed wind tunnel and simulations with a RANS solver highlighted the drop in wing performance at negative thrust for the case with inboard-up rotation. The interaction phenomena are reversed compared to the beneficial propulsive case, since the inflow velocity and angle of attack are now reduced on the part of the wing washed by the slipstream. Because of the reversal of the swirl in the slipstream at negative thrust, the interaction is then favorable with outboard-up rotation. For the considered propeller, that was not optimized for operation at negative thrust, the energy-harvesting efficiency was about 10%. This can be improved for future designs by optimizing the blade geometry and pitch setting of the propeller.@en

This paper presents an experimental and numerical study of the aerodynamic interaction between horizontal tail-mounted propellers and the airframe. A representative aircraft model was installed in a low-speed wind-tunnel and measurements were taken with an external balance to determine the effect of propeller installation on integral forces and moments. Total pressure measurements were performed downstream of the model for qualitative analysis of the propeller–airframe interaction. The experimental data were complemented by full blade CFD analyses, which correlate excellently to the experimental data. Balance measurements indicate that the propeller installation results in an offset and a change in the slope of the pitching moment curve over the complete range of angles of attack. The extent to which the propellers contribute to the longitudinal control and stability was shown to be dependent on the angle of attack of the aircraft and the rotation direction of the propellers. The flowfield and computed propeller loads show that an inboard-up rotating propeller results in a neutral contribution to longitudinal stability towards higher angles of attack, while an outboard-up rotation enhances the stability for all positive angles of attack. The non-uniform inflow to the propeller induced by the airframe leads to a lateral shift of the thrust which influences the trim condition.

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Wingtip-mounted propellers installed in a tractor configuration can decrease the wing induced drag by attenuating the wingtip vortex by the propeller slipstream. This paper presents an aerodynamic analysis of the propeller-wing interaction effects for the wingtip-mounted propeller configuration, including a comparison with a conventional configuration with the propeller mounted on the inboard part of the wing. Measurements were taken in a low-speed wind tunnel at Delft University of Technology, with two wing models and a low-speed propeller. Particle-image-velocimetry measurements downstream of a symmetric wing with integrated flap highlighted the swirl reductions characteristic of the wingtip-mounted propeller due to wingtip-vortex attenuation and swirl recovery. External-balance and surface-pressure measurements confirmed that this led to an induced-drag reduction with inboard-up propeller rotation. In a direct comparison with a conventional propeller-wing layout, the wingtip-mounted configuration showed a drag reduction of around 15% at a lift coefficient of 0.5 and a thrust coefficient of 0.12. This aerodynamic benefit increased upon increasing the wing lift coefficient and propeller thrust setting. An analysis of the wing performance showed that the aerodynamic benefit of the wingtip-mounted propeller was due to an increase of the wing's effective span-efficiency parameter.

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This experimental study focuses on the aerodynamic interaction effects that occur between an over-the-wing (OTW) propeller and a wing boundary-layer. An OTW propeller is positioned above the hinge-line of a wing featuring a plain flap. The measurements are carried out with and without axial pressure gradients by deflecting the flap and by extending the wing in streamwise direction to simulate a flat-plate configuration, respectively. Wing pressure taps and phase-free particle-image-velocimetry (PIV) are used to quantify the time-averaged interaction effects, while embedded microphones and phase-locked PIV are used to analyze unsteady interaction effects. Results show that the propeller generates an adverse pressure gradient on the wing surface which increases linearly with thrust and decreases as the blade tipclearance is increased. The pressure gradient is partially caused by the slipstream contraction, which creates a streamwise velocity deficit near the wall immediately behind the propeller disk. Moreover, the rotation of the propeller blades generates pressure fluctuations on the surface, the amplitude of which exceeds both the pressure fluctuations produced by the tip-vortices and the time-averaged pressure effect of the slipstream. Consequently, the propeller triggers flow separation 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 induces a Coanda effect which allows the flow to remain attached. @en

The potentially favorable interaction of a propeller slipstream with a wingtip is a complex problem, because of the importance of vortex interaction and viscous effects. This paper examines the capability of different propeller modeling methods in a Reynolds-averaged Navier–Stokes (RANS) solver for the simulation of wingtip-mounted propellers. The applicability of actuator-disk and actuator-line models to reduce the cost of propeller modeling is investigated in its most accurate form, by applying propeller-blade-loading results extracted from simulations, in which the blades are fully resolved. The numerical results are validated by comparison with measurement data from in-house wind-tunnel experiments. It is concluded that the aerodynamic interactions for the wingtip-mounted propeller in tractor configuration can be predicted by RANS simulations with a simple one-equation turbulence model (Spalart–Allmaras), provided that the uncertainty due to numerical diffusion is accounted for by a griddependency study, or reduced by local grid refinement. The actuator-line model reduced the computational time by 17% without introducing errors into the time-accurate and time-averaged wing loading. The actuator-disk model reduces the computational cost by 85% by removing time dependency, with a small penalty in the accuracy of the time-averaged flowfield and a 3.9% overprediction of the wing lift. @en

This paper examines the capability of a commercial RANS solver for the simulation of wingtip-mounted propellers. The applicability of actuator-disk and actuator-line models to reduce the cost of propeller modeling is investigated in its most accurate form, by extracting and applying propeller blade loading from full-blade simulations. The results obtained from all numerical simulations are validated based on measurement data from an in-house wind-tunnel experiment. An extensive grid dependency study is presented for the isolated propeller and the wing to distinguish discretization errors from model errors. It is concluded that RANS CFD with a simple one-equation turbulence model (Spalart–Allmaras) is capable of modeling the aerodynamic interactions for the wingtip-mounted propeller in tractor configuration, provided that numerical diffusion is accounted for by a grid dependency study or prevented by local grid refinement. The actuator-line model is fully able to replace propeller blade modeling in the simulation, and agreement with the full-blade simulations is found in time-accurate and time-average wing loading. The actuator-disk model further reduces the cost of the simulation by removing time dependency, at the cost of a small penalty in the accuracy of the time-averaged flowfield and lift distribution on the wing.@en

This paper addresses the effects of propeller installation on the aerodynamic performance of a tailplane featuring tip-mounted propellers. A model of a low aspect ratio tailplane equipped with an elevator and a tip-mounted propeller was installed in a low-speed wind-tunnel. Measurements were taken with an external balance and surface pressure taps to determine the aerodynamic characteristics of the tailplane, while the flowfield in the wake of the model was investigated using particle-image velocimetry. The experimental data are supported by CFD analyses, involving both transient simulations of the full-blade configuration and steady-state simulations the propeller replaced by an actuator-disk model. The upstream effects on the propeller time-average and time-accurate thrust and normal-forces are found to be limited for different tailplane operating conditions. It is shown that for a given propeller rotation direction, the load distribution on the tailplane is highly dependent on the direction of elevator deflection. The rotation direction of the tailplane tip-vortex relative to the propeller swirl therefore significantly affects the integral loads on the tailplane, resulting in differences in the normal-force gradient and elevator effectiveness.@en