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

Abstract

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.

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

“…The propeller has four blades and a diameter D equal to 0.237 m. The blade angle relative to the chord at 75% of the propeller radius (r=R ¼ 0:75, where r is the local radial location and R is the propeller radius) is 23.9 . Additional details of the propeller geometry are reported by de Vries et al 9 The pylon features a straight wing with an NACA 0012 aerofoil cross-section. The pylon chord length c is 0.200 m and the span b is 0.592 m. The leading edge of the pylon is located at 0.100 m from the propeller center, corresponding to an axial spacing of 0.42 D. Two leading-edge geometries are investigated: the baseline clean leading edge (named also solid leading edge) and a flow-permeable one.…”

“…6 The periodic impingement of the rotor blade wakes and tip vortices is experienced by the pylon as an unsteady inflow condition, resulting in an unsteady loading with span-wise and chord-wise gradients. [7][8][9][10] The unsteady pylon loads cause vibrations, which are transmitted into the structure of the aircraft and can be perceived by the passengers as structure-borne noise. 11 This noise source can be mitigated by modifying the transmission path of the vibrations, 12 or by decreasing the amplitude of the unsteady aerodynamic loading.…”

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

“…The intensity of the pressure fluctuations reduced in chordwise direction, as also reported in the companion study. 19 Application of porosity reduced the intensity of the pressure fluctuations by approximately 5% on the suction side and 30% on the pressure side with respect to the solid leading edge.…”

“…A spanwise extent of the porous insert of 10% of the propeller diameter was selected, such that it only covered the region closest to the tip-vortex impingement position where the pressure fluctuations are the largest. 19 It was assumed that the effect of the porous material was limited to its spanwise extent on the pylon. Moreover, the pylon span was assumed equal to the propeller diameter to provide sufficient propeller tip-fuselage clearance.…”

“…The propeller operating point was selected after measurements of the isolated propeller performance. 19 The thrust setting was chosen such that the propeller operated in the linear part of the thrust curve, thereby avoiding separation on the blades. This corresponded to an advance ratio of J = U ∞ /nD = 0.8 and a thrust coefficient…”