Experimental Studies of Open Rotor Installation Effects

Open rotor propulsion technologies offer an opportunity for reducing fuel burn due to the very high effective bypass ratio that results in increased propulsive efficiency. Open rotor effective bypass ratio can be 25 or higher and represents a potential advantage over even advanced ultra high bypass ratio turbofan engines. At the same time, great challenges arise from this radically different engine architecture in terms of aircraft system integration. The propulsion airframe aeroacoustic (PAA) effects of integration are one of those key challenges. Total installed noise, open rotor noise including integration effects, can be impacted by angle of attack, spacing between rotors and airframe elements, flow effects from wake ingestion or distortion from the airframe elements and several other parameters that generally depend on the aircraft configuration. In general, these effects increase noise compared to that of an isolated open rotor. This inter-relationship of the aerodynamic and aeroacoustic system integration effects is particularly important to enable future application. Furthermore, innovative integration and advanced technology may also offer the possibility of mitigating these usually negative aeroacoustic effects for a total aircraft system noise reduction. Understanding of these installation effects is essential to be able to assess the aircraft system benefits and to develop technology and approaches to achieve the best aircraft system benefits possible. An extensive model scale test campaign was conducted to investigate a broad range of these open rotor installation effects for both a conventional and an unconventional airframe. The conventional airframe was patterned after a modern twinengine aircraft configuration. The unconventional airframe was a hybrid wing body aircraft concept. The contra-rotating, eight by eight, open rotor used in this experiment was legacy technology from the 1980s flight test project. The experimental campaign was conducted in the Boeing Low Speed Aeroacoustic Facility (LSAF), shown in Figure 1. A 9 by 12 ft open jet is used to produce the forward flight simulation with a maximum Mach number of 0.25 for this experimental setup. Figure 1 shows the basic setup for this campaign with the airframe attached from the overhead structure and the open rotor rig attached on a strut from below the open jet. LSAF installed specially designed modifications for efficient positioning of the airframe relative to the open rotor. The airframe was traversed remotely relative to the fixed open rotor rig providing for the investigation of a large number of installation positions. Eight positions around the main wing of the conventional airframe and eleven positions above the hybrid wing body airframe were documented. Figure 2 shows a typical spectrum of the open rotor. In this case, the forward and aft blade rows were run intentionally at slightly different speeds. This allows the engine 3rd AIAA Atmospheric Space Environments Conference 27 30 June 2011, Honolulu, Hawaii AIAA 2011-4047