Beech 1900D flight test to characterize propeller noise on the fuselage exterior

This report documents a flight test conducted on the experimental Beech 1900D, operated by Raytheon Aircraft. The flight test was conducted primarily to gain insight into the nature of the air-borne fuselage excitation due to the propellers. A secondary goal was to record wing spar vibrations as a measure of the contribution of structure-borne excitations from the wing and power plant to interior noise. A coarse interior noise survey was also conducted in the passenger section of the fuselage. Twenty flush-mounted microphones were attached to the fuselage exterior to monitor the air-borne excitation in the vicinity of the propellers. Four accelerometers recorded the fore and aft wing spar vibrations at the attachment to the fuselage. A reference microphone and a moveable, 8microphone array were used to record interior noise. Data analysis has revealed that the point of highest sound pressure on the exterior lies forward of the propeller plane during all flight conditions. The magnitude of the excitation tends to decrease in proportion with the inverse of distance from the point of highest sound pressure. The relevance of this observation is that the excitation from a propeller can be very easily modeled in analysis. 1.0 Introduction: Goals of the Flight Test Many flight tests have been performed with the goal of studying interior noise in flight. Of particular interest to the authors are other flight tests with the Beech 1900D. For instance, Williams et al. used near-field acoustical holography to determine the fuselage normal velocity over a large portion of the aircraft interior. The reconstructed velocity reveals air-borne sound Received March 26, 2001. Copyright €> 2001 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. *Professor and Chair, Aerospace Engineering, 2004 Learned Hall. Senior Member AIAA. Graduate Research Assistant, Mechanical Engineering, 3013 Learned Hall. Student Member AIAA. ^Senior Aerodynamics Engineer, Nacelle and Thrust Reverser Systems. Formerly Assistant Professor, Mechanical Engineering, 3013 Learned Hall, University of Kansas. Member AIAA transmission paths from the fuselage to the interior space. Palumbo et al. demonstrated in-flight interior noise reduction using active structural acoustic control on the Beech 1900D experimental aircraft. They accomplished both single frequency and multiple frequency attenuation with a filtered-x LMS controller implemented in the principal component domain. In both studies, knowledge of the exterior noise field was not needed. During the flight test reported here, the exterior noise was characterized in the vicinity of the propellers. Other such tests are hard to find in the open literature, although it is known that Lyle' performed similar flight tests of the XV-15 tilt-rotor vehicle in order to validate rotor codes. The aircraft was instrumented with microphones placed on the fuselage exterior, and sound pressure levels due to the propeller rotation were measured and compared with the levels predicted by the rotor codes. The current authors wanted to characterize the exterior noise field to construct the acoustic excitation function for structural acoustic analysis. From the outset, the hope was that a simple characterization of the field, which captures the essential qualities of the excitation, would be possible. A simple model of the field would simplify analysis, thereby enabling meaningful analyses in the preliminary design phase. Although structural-acoustic analysis can be meaningful using only air-borne excitations, accounting for structure-borne excitation is definitely preferred. As such, vertical accelerations of the wing spars just inside the fuselage were recorded as a measure of wing vibration. Preliminary analysis shows that there are indications of a causal relationship between wing vibration and interior noise. To support structural acoustic analyses of the vehicle in flight, a crude interior noise survey was also conducted. The overall goal, then, is to use air-borne and structureborne excitations to predict interior noise and to compare with measured noise levels. Such analyses, and observations of the relative roles of air-borne and structure-borne excitation in causing interior noise, are the subject of on-going research. c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. 2.0 Experimental Setup 2.1 Exterior Microphones The 20 exterior microphones were PCB type V103 A/025, which, being ICP-type sensors, require the same type of signal conditioning as the interior microphones and accelerometers. These sensors were chosen for exterior pressure measurement due to the high allowable sound pressure levels (-178 db) and the fact that the exterior microphones could be mounted on the skin of the aircraft. The microphones were attached to the fuselage via double-sided tape. Then, a silicon fairing was placed over each microphone and taped in place with speed tape provided by Raytheon. Lead wires were also covered with tape. The limitation that only 20 external microphones were available (i.e., affordable) resulted in the decision to use 16 on the port (left) side and 4 on the starboard side. To compare port to starboard variations, 4 microphones were positioned on each side at the same fuselage station (longitudinal position), namely the propeller plane, and at the same water line (vertical) positions. On the port side, an additional 9 microphones were positioned such that, with 3 of the 4 propeller plane sensors, a 4 by 3 grid was established. The final 3 microphones were placed aft of the 4 by 3 grid at the water line of the point of closest approach (PC A) of the propeller to the fuselage. The locations of the sensors are shown in Figures 1 and 2. Fig. 1. Microphones installed on port (left) side. Figure 2 shows the relevant dimensions of the exterior microphone setup. Note that all vertical spacing is 18 inches and all horizontal spacing is 15 inches. T 18 in.