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

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.