Internal Acoustics Measurements of a Full Scale Advanced Ducted Propulsor Demonstrator

Acoustics measurements of a Pratt & Whitney full-scale ADP (Advanced Ducted Propulsor), an ultrahigh by-pass ratio engine, were conducted in the NASA Ames 40by 80-Foot Wind Tunnel. This paper presents data from measurements taken from sensors on a fan exit guide vane in the ADP. Data from two sensors, one at mid-span and the other at the tip of the fan exit guide vane, are presented. At the blade passage frequency (BPF), the levels observed at the various engine and wind speeds were higher at the mid-span sensor than the tip sensor. The coherence between these internal sensors and external microphones were calculated and plotted as a function of angle (angles ranged from 5° to 160°) relative to the ADP longitudinal axis. At the highest engine and wind speeds, the coherence between the tip sensor and the external microphones was observed to decrease at higher multiples of the BPF. These results suggest that the rotor-stator interaction tones are stronger in the mid-span region than at the tip. Introduction The requirement for increased fuel efficiency and reduced noise in aircraft operations has lead to the investigation of an ultra-high by-pass ratio engine for commercial aircraft of the 21st century. One engine configuration under consideration is an ADP (Advanced Ducted Propulsor), characterized by ultrahigh by-pass ratio, very low fan pressure ratio, large diameter, and very low fan rotational speeds. The low fan speeds are achieved through the use of a gear box, allowing the engine’s low rotor and fan to run at different and optimum speeds. ADP’s will have by-pass ratios of 10 to 20 and, depending on the airframe application, fan diameters as large as 160 inches. Noise concerns resulting from such large fans and annular ducts are offset by the characteristically low fan speeds. However, fan noise is still expected to be the major noise contributor to the total engine noise. Therefore, it is important to