A key challenge in open rotor design is getting the optimum aerodynamics at both the cruise and take-off conditions. This is particularly difficult because the operation and the requirements of an open rotor are very different at cruise compared to takeoff. This paper uses CFD results to explore the impact of various design changes on the cruise and take-off flow-fields. The paper then considers how a given open rotor design is best operated at take-off to minimise noise whilst maintaining high thrust. The main findings are that various design modifications can be applied to control the flow features that lead to lost efficiency at cruise and increased noise emission at take-off. A breakdown of the lost power terms from CFD solutions demonstrates how developments in open rotor design have led to reduced aerodynamic losses. At take-off, the operating point of the open rotor should be set such that the non-dimensional lift is as high as possible, without causing significant flow separation. This can be achieved through suitable amounts of re-pitch and speed up applied to a design. Comparisons with fully three-dimensional CFD show that the amount of re-pitch required can be determined using simplified methods such as two-dimensional CFD and a Blade Element Method.

The purpose of this paper is to describe the current status of open rotor noise prediction methods and to highlight future challenges in this area. A number of analytic and numerical methods are described which can be used for predicting ‘isolated’ and ‘installed’ open rotor tonal noise. Broadband noise prediction methods are also described and it is noted that further development and validation of the current models is required. The paper concludes with a discussion of the analytical methods which are used to assess the acoustic data collected during the high-speed wind-tunnel testing of a model scale advanced open rotor rig.

Contra-Rotating Open Rotor (CROR) propulsion systems have seen renewed interest as an economic and environmentally friendly powerplant for future transport aircraft. Installation effects, i.e. the mutual interactions between airframe components and the rotors, have a pronounced impact on the aerodynamic and aeroacoustic performance for this type of engine. In the past five years, DLR’s Institute of Aerodynamics and Flow Technology has performed a number of numerical studies investigating important aspects relating to engine-airframe integration of CROR engines. In this article an overview of coupled aerodynamic and aeroacoustic simulations investigating representative pusher-configuration CROR engines will be given, focusing on the impact on aerodynamic performance and aeroacoustics caused by the presence of a pylon, the potential for noise reduction by employing trailing-edge blowing at the pylon trailing edge as well as the performance and noise variations caused by different senses of rotation of the rotors. It is shown that the interaction with the pylon strongly impacts blade performance and front rotor noise emissions but that the use of active flow control in the form of pylon trailing-edge blowing can alleviate these adverse installation effects to a notable extent.

At DLR’s Institute of Propulsion Technology, the prediction tools and multi-disciplinary optimisation strategies developed for turbofan engines have been extended to contra-rotating open rotors (CROR). Thereby the objective has been to appraise and improve the performance of CROR engines and thus to reduce their environmental impact. The present paper reviews the intermediate progress achieved in this scope. The prediction is based on analytical and CFD methods and covers the fields of engine performance analysis, aerodynamics and acoustics. The aerodynamic and acoustic results could be partly validated through the comparison to experimental data obtained from wind-tunnel tests. In a multi-disciplinary approach the aforementioned aspects are optimised together. First results of an aero-acoustic optimisation are presented. Furthermore this paper undertakes some comparison between high-bypass ratio turbofan engines and open-rotor concepts.

Model scale tests of modern ‘open rotor’ propulsor concepts that have potential for significant fuel burn reduction for aircraft applications were completed at NASA Glenn Research Center. The recent test campaign was a collaboration between NASA, FAA, and General Electric (GE). GE was the primary industrial partner, but other organisations were involved such as Boeing and Airbus who provided additional hardware for fuselage simulations. The open rotor is a modern version of the UnDucted Fan (UDF®) that was flight tested in the late 1980s through a partnership between NASA and GE. The UDF® was memorable for its scimitar shaped propeller blades and its unique noise signature. Design methods of the time were not able to optimise for both high aerodynamic efficiency and low noise simultaneously. Contemporary CFD/CAA based design methods can produce open rotor blade designs that maintain efficiency with acceptable acoustic signatures. Tests of two generations of new open rotor designs were conducted in the 9’ × 15’ Low Speed Wind Tunnel and the 8’ × 6’ Supersonic Wind Tunnel starting in late 2009 and completed in early 2012. Aerodynamic performance and acoustic data were obtained for take-off, approach and cruise conditions in isolated and semi-installed configurations. Additional detailed flow diagnostic measurements and acoustic measurements, including canonical shielding configurations, were obtained by NASA. NASA and GE conducted joint systems analysis to evaluate the performance of the new blade designs on a Boeing 737 class aircraft. The program demonstrated a 2-3% improvement in overall net efficiency relative to the best efficiency designs of the 1980s while nominally achieving 15-17 EPNdB noise margin to Chapter 4 (at a Technology Readiness Level of 5) for a notional aircraft system defined by NASA.