An experimental analysis was performed of the unsteady aerodynamic loading caused by the impingement of a propeller slipstream on a downstream lifting surface. When installed on an aircraft, this unsteady loading results in vibrations that are transmitted to the fuselage and are perceived inside the cabin as structure-borne noise. A pylonmounted tractor-propeller configuration was installed in a low-speed wind tunnel at Delft University of Technology. Surface-microphone and particle-image-velocimetry measurements were taken to quantify the pressure fluctuations on the pylon and visualize the impingement phenomena. It was confirmed that the propeller tip vortex is the dominant source of pressure fluctuations on the pylon. Along the path of the tip vortex on the pylon, a periodic pressure response occurs with strong harmonics. The amplitude of the pressure fluctuations increases with increasing thrust setting, whereas the unsteady lift coefficient displays a nonmonotonic dependency on the propeller thrust. The lowest integral unsteady loads were obtained for cases with approximately integer ratios between the pylon chord and the wavelength of the perturbation associated with the propeller tip vortices. This implies that structure-borne-noise reductions might be obtained by matching the pylon chord with an integer multiple of the axial separation between the propeller tip vortices.

Key results from the EU H2020 project CENTRELINE are presented. The research activities undertaken to demonstrate the proof of concept (technology readiness level—TRL 3) for the so-called propulsive fuselage concept (PFC) for fuselage wake-filling propulsion integration are discussed. The technology application case in the wide-body market segment is motivated. The developed performance bookkeeping scheme for fuselage boundary layer ingestion (BLI) propulsion integration is reviewed. The results of the 2D aerodynamic shape optimization for the bare PFC configuration are presented. Key findings from the high-fidelity aero-numerical simulation and aerodynamic validation testing, i.e., the overall aircraft wind tunnel and the BLI fan rig test campaigns, are discussed. The design results for the architectural concept, systems integration and electric machinery pre-design for the fuselage fan turbo-electric power train are summarized. The design and performance implications on the main power plants are analyzed. Conceptual design solutions for the mechanical and aero-structural integration of the BLI propulsive device are introduced. Key heuristics deduced for PFC conceptual aircraft design are presented. Assessments of fuel burn, NOx emissions, and noise are presented for the PFC aircraft and benchmarked against advanced conventional technology for an entry-into-service in 2035. The PFC design mission fuel benefit based on 2D optimized PFC aero-shaping is 4.7%.

The impingement of the propeller slipstream on a downstream surface acts as a source of structure-borne noise. The experimental study presented in this paper aims at localizing and quantifying the main sources of unsteady loading for a pylon-mounted tractorpropeller configuration. Balance measurements showed that the installation of the pylon had no significant influence on the steady-state propeller performance. Measurements of the surface-pressure fluctuations on the pylon using microphones indicated harmonic unsteady loading at integer multiples of the blade-passage frequency, caused by the periodic interaction with the propeller blade wakes and tip vortices. The average amplitude of the pressure fluctuations on the pylon surface decreased by 85% between high thrust conditions (J = 0.6) and the zero thrust condition (J ≈ 1.0). The main source of unsteady loading was the impingement of the blade tip vortices, which modified the pylon loading along the entire chord. Only at low thrust settings the impingement of the blade wakes on the leading edge of the pylon became dominant. The amplitude of the integral unsteady pylon loading showed a more complicated dependence on advance ratio, mainly due to relative phase differences between the excitations at different locations on the surface of the pylon. Increasing the propeller-pylon spacing decreased the amplitude of the unsteady pylon loading, without appreciably modifying the distribution of the pressure fluctuations over the pylon. Results show that a reduction of unsteady pylon loading can most effectively be achieved by surface treatments at the tip-vortex impingement region or with a pylon design optimized for specific operating conditions of the propeller.

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Structure-borne noise can be a relevant source of cabin noise in advanced aircraft configurations with pylon-mounted tractor propellers. The periodic impingement of the propeller slipstream on the pylon causes unsteady loading that can be mitigated by applying passive porosity at the leading edge of the pylon. Pressure reconstruction from particle-image velocimetry was used to extract the unsteady pressure field around the pylon and the associated aerodynamic loads. Experimental results showed that porosity locally modifies the pylon near-wall pressure distribution because of the crossflow through the porous surface. Application of a porous leading edge reduced the intensity of the unsteady pressure fluctuations in the tip-vortex impingement region by approximately 5% and 30% on the suction and pressure sides of the pylon, respectively. Consequently, the unsteady pylon loading induced by the propeller tip vortices was reduced by 25%, thus resulting in a less intense source of vibrations. On the other hand, pylon drag was locally increased. Based on the experimental data, projections of the drag penalty were made for a generic pylon design. The resulting pylon drag penalty equaled 15% up to 35% for angles of attack of 0• up to 9• . This drag penalty was attributed to the early onset of transition caused by the increased surface roughness of the porous insert, and viscous dissipation of the crossflow through the cavity of the porous insert.

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Junction flow caused by the aerodynamic interaction between a wind-tunnel model and the support structure can largely influence the flowfield and hence the experimental results. This paper discusses a combined numerical and experimental study which was carried out to mitigate the model–support interference in a wind-tunnel test setup for the study of fuselage boundary-layer ingestion. The setup featured an axisymmetric fuselage mounted through a support beam, covered by a wing-shaped fairing. The junction flow appearing at the fuselage–fairing connection produced undesired flow distortions at the fuselage aft section, due to the formation of an horseshoe vortex structure at the fairing leading edge. Numerical and experimental analysis were performed with the aim of reducing the distortion intensity by improving the fairing design. Results show that modifying the leading-edge shape of the fairing effectively decreased the flowfield distortions. Moreover, the addition of a dummy fairing diametrically opposed to the first one was found to be beneficial due to the enhancement of the configuration symmetry.

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.