Far-field noise and internal modes from a ducted propeller at simulated aircraft takeoff conditions

The ducted propeller offers structural and acoustic benefits typical of conventional turbofan engines while retaining much of the aero-acoustic benefits of the unducted propeller. A model Advanced Ducted Propeller (ADP) was tested in the NASA Lewis LowSpeed Anechoic Wind Tunnel at a simulated takeoff velocity of Mach 0.2. The ADP model was designed and manufactured by the Pratt & Whitney Division of United Technologies. The 16-blade rotor ADP was tested with 22and 40-vane stators to achieve cut-on and cut-off criterion with respect to propagation of the fundamental rotor-stator interaction tone. Additional test parameters included three inlet lengths, three nozzle sizes, two spinner configurations, and two rotor rub strip configurations. The model was tested over a range of rotor blade setting angles and propeller axis angles-of-attack. Acoustic data were taken with a sideline translating microphone probe and with a unique inlet microphone probe which identified inlet rotating acoustic modes. The beneficial acoustic effects of cut-off were clearly demonstrated. A 5 dB fundamental tone reduction was associated with the long inlet and 40-vane stator, which may relate to inlet duct geometry. The fundamental tone level was essentially unaffected by propeller axis angle-of-attack at rotor speeds of at least 96 percent design. In f.rnA 11 rf'.l nn The advanced propeller program"' had successfully demonstrated significant performance improvements for single and counterrotating propellers relative to that of current turbofan engines at typical cruise conditions of Mach 0.8 and 10 688 m (35 000 ft) altitude. However, uncertainties over new propeller technologies and inherent structural and acoustic benefits associated with propeller shrouds have directed current research toward the advanced ducted propeller, which is a marriage of the turbofan and propeller technologies. The advanced ducted propeller will typically feature a low number of rotor blades and a stator designed to satisfy the acoustic cut-off criterion. 3 Bypass ratios of 15 or greater will be characteristic of these powerplants. The objective of the present study was to quantify the aero-acoustic effects of flow parameters such as inlet length and stator vane number for a model ducted propeller. This paper will present acoustic results for a model Advanced Ducted Propeller (ADP) designed and built by Pratt & Whitney Division of United Technologies, which was tested in the NASA Lewis 9by 15-Foot Anechoic Wind Tunnel (Fig. 1(a)). The rotor diameter was 43.8 cm (17.25 in.). The model was tested with two different stator vane numbers (for cut-on and cut-off conditions), three inlets, two spinners, three nozzles, and with grooved and smooth rotor rub strips. A range of rotor blade setting angles was investigated. Limited diagnostic data are included from a unique rotating inlet microphone probe which allowed for separation of individual acoustic modes. Tests were conducted at Mach 0.2, which is typical of takeoff conditions. Figure 1(a) is a photograph of the model installed in the wind tunnel. Acoustic data were obtained from an axially-translating microphone probe. "Flyover" directivities could be obtained from these acoustic probe surveys. Fixed microphones were also located near the tunnel wall in the same approximate plane as the translating probe; however, data from the fixed microphones are not included in this report.