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Browsing by Author "Nash, Grant"

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    Utilizing Distributed Vibration Absorbers to Reduce Noise Transmission Through the Windshield of a Cessna 150
    Nash, Grant (Virginia Tech, 2004-07-26)
    High levels of noise have historically been present in the interior of aircrafts. This causes passenger, pilot, and other aircraft employee fatigue as well as speech interference and discomfort issues. In general aviation aircraft, little has been done to reduce the noise in the interior of fuselages. Noise transmitting into the cabin of a single engine aircraft is complex. Researchers have identified areas of noise transmission in general aviation aircraft to include the windshield, exhaust, side windows, and little through the engine firewall. Sources originally noted that structure-borne and airborne noise transmission paths were equal contributors to interior energy penetration of single-engine aircraft. However, additional studies found that airborne noise transmission paths dominate interior energy transmission for single-engine aircraft. Energy transmits mostly through the front of the aircraft via the windshield. Little goes through the engine firewall, which contradicts the original assumption that engine vibration contributed to large noise transmission into the cabin of the aircraft via the firewall. Airborne noise and some exhaust noise transmit through the firewall, but not near as much as noise that transmits through the windshield. Reducing interior aircraft noise levels is a complicated joint effort, combining propeller radiation control; fuselage wall reduction methods; exhaust emission regulation, management of air turbulence; some propeller, wake-induced vibration control; and a little engine vibration restraint. For minimum propeller acoustic propagation, it is important to control propeller radiation by using techniques such as increasing the number of blades, altering blade airfoil (especially using a felix or grooved design); applying small angle of attack; utilizing swept blades; decreasing blade diameter; lowering tip speed; and reducing the load on a propeller (i.e. by controlling the blade thickness, tip volume, and blade shape). Controlling the vibration in the fuselage skin can also help to reduce interior noise. Some early attempts were made using ribs/stiffeners, tuned dampers, and a limp mass double wall. More recently, dynamic vibration absorbers have been utilized, quite successfully, to reduce fuselage skin vibration and thus, interior noise levels. Attempting to control the exhaust emission and induced vibration from air turbulence has contributed to lower airplane cabin noise levels as well. For large aircrafts, the strategic location of luggage compartments and bathrooms help in keeping the interior quiet. Most importantly for small single-engine aircraft, the windshield has been found to contribute heavily to aircraft interior noise levels. Currently, the use of active control methods (especially the active structural acoustic control methods) and the utilization of dynamic vibration absorbers (a form of passive noise control) are the most popular techniques to reduce interior aircraft noise levels. In small general aviation aircraft, the blade passage frequency (bpf) and the first few harmonics have been found to be the largest contributor to noise transmitting into the fuselage. This project analyzes a two degree-of-freedom (DOF) dynamic vibration absorber in hopes of reducing windshield vibration of a Cessna 150 fuselage at the fundamental blade passage frequency of approximately 87 Hz and thus, reducing noise transmitting into the interior of the aircraft. This research project is unique in several ways. First, numerous passive noise control techniques have been utilized to control vibration and acoustics on an aircraft, but none have used the two degree of freedom Distributed Vibration Absorbers (DVA) employed in this project, as a noise reduction method on the windshield of an aircraft. Secondly, little research has been done on analyzing noise transmission into small, single engine general aviation aircraft, which is conducted in the work here. Third, little work has been done on analyzing and reducing noise propagation through the windshield of a small engine aircraft, which is also analyzed in this project. Finally, the modal analysis conducted on the windshield of the small engine plane is one of the few modal decompositions that has been conducted on a small general aircraft windshield.