Tractor Propeller-Pylon Interaction, Part II: Mitigation of Unsteady Pylon Loading by Application of Leading-Edge Porosity

Abstract

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.

Impingement of a propeller-slipstream on a leading edge with a flow-permeable insert: A computational aeroacoustic study

“…A possible way to reduce the amplitude of the unsteady loading is to use a pylon with a flowpermeable leading edge, also referred to as passive porosity. [13][14][15][16] Tinetti et al 13,14 applied passive porosity to mitigate the unsteady loading for rotor-stator interaction in turbomachinery. They showed a reduction of the unsteady loading up to 21%.…”

“…Lee 15 applied a flow-permeable leading edge for blade-vortex interaction noise for helicopter applications, showing a reduction of the amplitude of the interaction noise up to 30%. Della Corte et al 16 investigated experimentally the potential of flow-permeable leading edges to alleviate the unsteady loading caused by the impingement of the propeller slipstream on a downstream surface, for which wake-impingement and vortex-impingement phenomena occur simultaneously. The spatial distribution of the cavities was based on the optimal configuration identified by Tinetti et al 13,14 They investigated the effect of both the cavity size and of the hole diameter with particle image velocimetry (PIV).…”

“…The flow is computed by solving the explicit, transient, compressible Lattice-Boltzmann (LB) equation, while the acoustic field is obtained by means of the Ffowcs Williams and Hawkings (FWH) acoustic analogy. 19 The configuration replicates the experiments performed at Delft University of Technology, 9,10,16 which are used as benchmark.…”

Analysis and Design of a Wing Trailing Edge Mounted Over-The-Wing Distributed Propeller Propulsion system

“…Designs in which use is made of leading edge mounted tractor propeller design may have interesting performance characteristics although engine failure may have serious detrimental effects on the stall behaviour. Moreover, a non-negligible noise penalty may be associated which such designs [4]. To overcome these problems a wing trailing edge (TE) mounted distributed propulsion (DP) system is being investigated at Delft University as part of an EU-funded framework program.…”

Tractor Propeller-Pylon Interaction, Part I: Characterization of Unsteady Pylon Loading

“…This corresponds to a Reynolds number based on the pylon chord of Re c = 1.37 · 10 5 . In addition, PIV measurements were taken at 40 m/s, since in the companion study 23 higher freestream velocities were required. Three parameters were varied during the experiments: advance ratio, angle of attack, and propeller-pylon spacing.…”