This paper discusses the early-stage development of a fast propeller design tool using low-fidelity methods. Aerodynamics, aeroacoustics, and structural behavior of the propeller have been incorporated into an optimization framework to generate more efficient and quieter propeller designs. A first optimization process has successfully provided a set of more efficient and/or quieter designs among which one specific geometry has been manufactured. CFD validation has confirmed its aerodynamic performances and reasonable agreements have been observed with experimental results, with some discrepancies, however. Additional parametric studies are also discussed. @en

A wind-tunnel experiment was performed at the DNW Low-Speed Tunnel with a powered propeller-wing model to prove the concept of energy-harvesting with propellers and assess its impact on the wing performance. By separating the contributions of the propeller and wing to the overall system forces, both for positive and negative thrust settings improved understanding was obtained of the propeller-wing interaction. A tip-mounted propeller configuration was simulated. At positive thrust settings, the operation of the propeller increased the lift gradient and improved the aerodynamic efficiency of the wing (L/D) by 10-35% compared to the propeller-off configuration. At CL = 0.5 and net zero force in streamwise direction the benefit was 12%, while at CL = 1.0 and a net force in streamwise direction of approximately three times the wing drag the benefit was 32%. At negative thrust, the propeller operation decreased the lift gradient, but the wing aerodynamic efficiency was still higher than that of the propeller-off configuration. This was an unexpected result, which was explained by the reduction in friction drag on the wing immersed in the propeller slipstream due to the lower dynamic pressure, and a possible reduction in wing induced drag due to downwash on the outboard part of the wing. The aileron effectiveness was decreased by about 10% when switching from positive to negative thrust operation. However, for angles of attack up to approximately 14 degrees even at negative thrust, the aileron effectiveness was still higher than for the clean wing.

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Fuselage Boundary-Layer Ingestion (BLI) is a promising example of synergistic design and propulsion-airframe integration to reduce fuel burn. For a BLI configuration, the aero propulsive performance of the aircraft is a result of the complex aerodynamic interaction between the fuselage airframe and the BLI propulsor. This paper presents a design method for the aft fuselage including the propulsor shrouding to minimize the required shaft power of an aft-mounted propulsor in the conceptual design phase. First, a global aerodynamic design space exploration is carried out using Computational Fluid Dynamics (CFD) to identify the key design parameters and their influence to the aerodynamic performance of the propulsive fuselage. An optimization study is subsequently carried out to improve the aerodynamic performance of a baseline design. The optimization was performed for a turbo-electric BLI configuration and within representative design constraints. The optimization achieved a decrease of approximately 10% of the isentropic shaft power required for the aft-mounted propulsor for a constant net force acting on the propulsive fuselage. The presented methodology and the resulting design practices can be effectively applied to other advanced aircraft configurations.@en

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.@en

Boundary-layer ingestion (BLI) has been proposed as one of the novel airframe–engine integration technologies to reduce aircraft fuel consumption. The current numerical analysis involves the evaluation of the effect of fuselage design on the power consumption of a boundary-layer ingesting propulsor modeled as an actuator volume without nacelle. An axisymmetric fuselage model is used as a canonical case to study BLI in transonic flight conditions. The flowfield is investigated through the power balance and the exergy analysis methods. Results show that the fuselage geometry and flight conditions only have a minor effect on the BLI power saving benefit when compared to the effect on the drag power of the fuselage. This indicates that, for the range of fuselage geometries and flight conditions studied, the isolated fuselage drag can be used for a qualitative performance assessment of different fuselage designs even for BLI configurations. Also, the power saving results obtained based on the power balance and the exergy analysis methods show similar qualitative trends for the fuselage geometries and flight conditions considered. Furthermore, the BLI propulsor has a negligible effect on the upstream anergy generation rate. Turbulence and temperature gradients within the flow are the important reasons for the deterioration of the BLI propulsor performance as expected.

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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 aerostructural 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%.

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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.@en

Boundary Layer Ingestion (BLI) is a promising propulsion integration technology capable of enhancing aircraft propulsive efficiency. The Propulsive Fuselage Concept (PFC), a tube-and-wing configuration with an aft-fuselage-mounted BLI propulsor, is particularly suited for BLI. Although extensively studied on a system level, the aerodynamic performance of the PFC, resulting from the complex interaction between the airframe and the propulsor, is still largely uncharted. In this paper, the results of wind-tunnel tests on a simplified PFC model are presented. The model featured an axisymmetric fuselage body with an integrated BLI shrouded fan. Flowfield measurements were performed through particle image velocimetry to analyze the key aerodynamic phenomena and to assess the distribution of momentum and mechanical energy around the aft-fuselage propulsor. Results show that the BLI fan alters the surrounding flowfield by increasing the mass flow in the inner part of the fuselage boundary layer and by reducing the boundary-layer thickness. Moreover, the power analysis indicates that the potential benefit of BLI is strongly dependent on the fan setting. Increasing the fan shaft power leads to a higher amount of power dissipated in the near wake. However, an increasing share of the energy flux is associated with the momentum excess contained in the wake.

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The impingement of a propeller slipstream on a downstream surface causes unsteady loading, which may lead to vibrations responsible for structure-borne noise. A low-speed wind-tunnel experiment was performed to quantify the potential of a flow-permeable leading edge to alleviate the slipstream-induced unsteady loading. For this purpose, a tractor propeller was installed at the tip of a pylon featuring a replaceable leading-edge insert in the region of slipstream impingement. Tests were carried out with four flow-permeable inserts, with different hole diameters and cavity depths, and a baseline solid insert. Particle-image-velocimetry measurements showed that the flow through the permeable surface caused an increase in boundary-layer thickness on the pylon's suction side. This led to a local drag increase and reduced lift, especially for angles of attack above 6 deg. Furthermore, it amplified the viscous interaction with the propeller tip-vortex cores, reducing the velocity fluctuations near the pylon surface by up to 35%. Consequently, lower tonal noise emissions from the pylon were measured in the far field. This suggests that the desired reduction in surface pressure fluctuations was achieved by application of the flow-permeable leading edge.

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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 pylon-mounted 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.

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