Progress in Open Rotor Research: A U.S. Perspective

Abstract

In response to the 1970s oil crisis, NASA created the Advanced Turboprop Project (ATP) to mature technologies for high-speed propellers to enable large reductions in fuel burn relative to turbofan engines of that era. Both single rotation and contra-rotation concepts were designed and tested in ground based facilities as well as flight. Some novel concepts/configurations were proposed as part of the effort. The high-speed propeller concepts did provide fuel burn savings, albeit with some acoustics and structural challenges to overcome. When fuel prices fell, the business case for radical new engine configurations collapsed and the research emphasis returned to high bypass ducted configurations. With rising oil prices and increased environmental concerns there is renewed interest in high-speed propeller based engine architectures. Contemporary analysis tools for aerodynamics and aeroacoustics have enabled a new era of blade designs that have both high efficiency and lower noise characteristics. A recent series of tests in the U.S. have characterized the aerodynamic performance and noise from these modern contra-rotating propeller designs. Additionally the installation and noise shielding aspects for conventional airframes and blended wing bodies have been studied. Historical estimates of ‘propfan’ performance have relied on legacy propeller performance and acoustics data. Current system studies make use of the modern propeller data and higher fidelity installation effects data to estimate the performance of a contemporary aircraft system. Contemporary designs have demonstrated high net efficiency, ∼86%, at 0.78 Mach, and low noise, >15 EPNdB cumulative margin to Chapter 4 when analyzed on a NASA derived aircraft/mission. This paper presents the current state of high-speed propeller/open rotor research within the U.S. from an overall viewpoint of the various efforts ongoing. The remaining technical challenges to a production engine include propulsion airframe integration, acoustic sensitivity to aircraft weight and certification issues.

Acoustic Analysis of Counter-Rotating Open Rotors with a Locked Blade Row

“…The CROR can offer significant improvements in efficiency due to the counter-rotation of the aft rotor removing residual swirl from the fore row. Furthermore, without the drag of a duct and their high bypass ratios, CROR can significantly reduce aircraft emissions when compared to conventional aircraft propulsion systems [1].…”

Probability-damage approach for fail-safe design optimization (PDFSO)

“…Commercial aviation companies have investigated novel concepts of aircraft propulsion aimed at reducing fuel burn and improving overall aircraft performance. These new designs contemplate the possibility of using open rotor engines (also called propfan or unducted fan), with less noise and emission levels (Schimming 2003;Negulescu 2013;Van Zante et al 1968;Van Zante 2015;Brouckaert et al 2018;Langston 2018). In this aircraft concept, the engine system is generally located in the rear part of the aircraft, with the required redesign and sizing of the fuselage.…”

Open Rotor Engines—Still an Open Question?

“…Cruise Mach numbers, M, of airliners range from 0.7 to 0.9. For the higher end of this range, propeller losses increase markedly (shocks forming in tip regions) so open rotor engines are typically being designed to run at a maximum of about M=0.78, taking full advantage of swept scimitar shaped blades to mitigate compressibility effects [2].…”

“…propeller/fan blades, it had a direct turbine drive. The GE36 demonstrated a 15% fuel burn reduction, compared to contemporary turbofans [2], based on some 281 hours of flight testing. Faced with a market where fuel prices were dropping, the program was ended in 1989.…”