Assessment of Propfan Propulsion Systems for Reduced Environmental Impact

Andreas Peters
Advisor: Prof. Spakovszky

Baseline CRP blade-tip vortex system: Front rotor tip-vortices and viscous wakes interacting with rear rotor contribute to interaction tone noise.

Current aircraft engine design studies tend towards higher bypass ratio, low-speed fan configurations in order to attain reductions in fuel consumption, emissions, and noise. Propfan (advanced turboprop) engine concepts investigated in the past by American, European, and Russian aircraft manufacturers have demonstrated significant benefits in these areas. However, considerable concern remains about the potential noise generated by propfan engines, including both inflight cabin noise and community noise during takeoff and approach. The overall goal of this project is to define an advanced CRP configuration with improved noise characteristics while maintaining the required aerodynamic performance for a given aircraft mission.
An aircraft performance, weight and balance, and mission analysis is conducted on a candidate CRP-powered aircraft configuration and a detailed aerodynamic design of a pusher CRP is carried out. Full wheel unsteady 3-D RANS simulations are then used to determine the time-varying blade surface pressures and unsteady flow features necessary to define the acoustic source terms.

Polar directivity at first interaction tone frequency: Implementing advanced source mitigation concepts in re-designed CRP significantly reduces interaction tone noise compared to baseline CRP design.

A frequency domain approach based on Goldstein’s formulation of the acoustic analogy for moving media and an existing single rotor noise method is extended to counter-rotating configurations. Using the developed CRP noise estimation method, the underlying noise mechanisms front-rotor wake interaction, aft-rotor upstream influence, hub-endwall secondary flows, and front-rotor tip-vortices to interaction tone noise are dissected and quantified. Based on this investigation, the CRP is re-designed for reduced noise incorporating a clipped rear-rotor and increased rotor-rotor spacing to reduce upstream influence, tip-vortex, and wake interaction effects. Maintaining the thrust and propulsive efficiency at takeoff, the noise is calculated for both designs. On the engine/aircraft system level, the re-designed CRP demonstrates significant noise reductions and the results suggest that advanced open rotor designs can possibly meet Stage 4 noise requirements.

Re-designed CRP for low noise: Clipping rear rotor reduces interaction of front rotor tip-vortices with rear rotor, thereby decreasing interaction tone noise.