The Effects of Pylon Blowing on Pusher Propeller Performance and Noise Emissions

Abstract

Growing concerns about the environmental impact of aircraft operations and increasing fuel prices have led to the demand for more fuel-efficient aircraft. One of the technologies with the potential to offer a significant reduction in fuel burn is the open rotor engine. However, the noise emissions of such engines are higher than for turbofans, partially due to the typical installation of the open rotor in a pusher configuration. The wake shed by the upstream pylon results in a velocity deficit at the propeller disk, leading to unsteady blade loads and additional noise emissions. This thesis investigated the potential of pylon trailing edge blowing to reduce the adverse effects of airframe installation on the performance and noise emissions of pusher propellers. Experimental and numerical analyses were performed, focusing on the pylon wake profiles, the propeller performance, and the propeller noise emissions. The experiments were executed in Delft University of Technology's Open Jet Facility using a scale model pylon and a single-rotating powered propeller model. The numerical analysis combined an existing propeller lifting line code with analytic methods suited to predict the effects of installation on the propeller performance and noise emissions. Pylon wake measurements showed that application of the pylon trailing edge blowing system resulted in reductions in the integral wake velocity deficit of up to 60% when compared to the unblown configuration. However, no full mixing of the external flow and the flow blown into the pylon wake was obtained. As a result, the application of blowing did not completely eliminate the pylon wake, but instead led to a velocity overshoot in the wake center and two local minima left and right of the wake centerline. From the studies of the propeller performance it was concluded that the effects of installation on the time-averaged performance are small, with maximum differences between the isolated (without pylon) and installed (with pylon) thrust and torque of less than 2% for advance ratios below 1.4. For the same advance ratio range the peak-to-peak variations in the installed thrust and torque were computed to be smaller than 4% of the mean value. Excellent agreement was obtained between the experimental and numerical time-averaged isolated propeller performance for advance ratios above 0.7, with a maximum difference of 1% between the computed and measured data. The measurements of the propeller noise emissions showed that the sound pressure level (SPL) of the propeller tones is strongly increased due to the installation effects, with noise penalties of up to 10 to 25 dB for the first six tones occurring at integer multiples of the blade passage frequency (BPF). Broadband levels on the other hand were unaffected by the presence of the upstream pylon. The application of blowing resulted in significant noise reductions when compared to the unblown installed case. Depending on the operating conditions, at the highest blowing rate considered SPL reductions were obtained of up to 4 dB for the 1BPF tone, 8 dB for the 2BPF tone, and 12 dB for the 3BPF tone. Furthermore, the higher BPF tones (4BPF and above) were practically eliminated. The evolution of the noise reduction due to blowing as a function of the advance ratio followed the trend in the noise penalty due to installation, thereby confirming that the application of blowing indeed successfully opposes the installation effects.