Since the inception of rotor-craft there has been a desire and need to control the interior noise created by impinging vortices, grinding transmissions, and boundary layer noise. This has been of particular concern in helicopters due to their configuration which makes it difficult to passively control the often overwhelming noise. In the past noise control in helicopters was considered an expensive and unnecessary luxury, especially in the military. It was not that the high noise levels were not a concern, but the technology to efficiently control noise did not exist, until now.
Due to the advancement of smart structures and, perhaps more importantly, the development of control architectures which can efficiently cope with the huge number of required calculations in a strongly coupled system, noise control in helicopters has become an attainable goal. The objective of the AMSL Acoustic Research Group is manyfold. First, the project requires an accurate characterization of the system dynamics that will be encountered within the fuselage that have direct or indirect influence over the interior acoustics. Second, a control system needs to be developed which can compensate for the aforementioned dynamics using existing technology and hardware, namely smart structures. Third, local control methodologies need to be explored which can accurately and reliably carry out the orders of the main control system. Last, experiments need to be carried out which test the involved technologies. The experiments will involve both a high degree of modelling and physical experimentation on a testbed within an anechoic chamber which is physically representative of the dynamics encountered in an actual helicopter fuselage. The combination of these different steps will provide for a solution to the helicopter acoustic control problem that will utilize the state of the art in all related fields to ensure the best possible performance currently attainable.
Interior noise is can cause several problems: Vibration levels affect mechanical reliability; Pilot fatigue and passenger comfort; Broadband noise; and Affect interior acoustic environment by controlling structural vibrations.
In the past several years an interest in aircraft interior noise has developed. Anyone who has sat in the aft portion of a MD-80 or DC-9 will testify that the noise near the engines is no small annoyance. Even worse, the noise in most helicopters is down right painful. Military helicopters, in particular, have little passive noise control, due partly to strict weight constraints and to the fact that little emphasis is given to passenger comfort. But, extreme noise can cause much more damage than mere discomfort. A pilots abilities can be severely affected by noise fatigue in longer flights, sacrificing the safety of crew and the mission. In addition, communication between occupants becomes greatly hindered without the use of specially designed headphones, which are impractical for all occupants to wear. In the past, these problems were accepted because no solution existed which conformed to other constraints required by military helicopters. Noise control in helicopters poses a particularly difficult challenge for several reasons. The noise encountered in helicopters is generally much higher, ranging from 10 to 30 dBA above the recommended level of 80 dBA. The reason for the higher noise level is largely due to a helicopter's configuration. The three primary sources of noise, the main rotor, engine and transmission, are in close proximity to the usually small cabin. Making matters worse is the fact that these noise sources are closest to the occupants heads. The noise is also very broadband, covering a wide range of frequencies and thus requiring many actuators and sensors. In addition, a high degree of coupling occurs between disturbances, both structurally and acoustically.
Techniques such as Active Noise Cancellation using speakers has proven to be inadequate and implausible due to the size of helicopter cabins and the number of speakers required for significant noise reduction. Until recently, neither the hardware or the technology required to control a large number of actuators had existed to effectively combat helicopter noise.
Experimental Testbed & ModellingIn designing an experimental testbed it was necessary to balance the complexity of the testbed with what could be reasonably modelled. It was absolutely necessary to represent the dynamic and acoustic complexity of an actual helicopter cabin to assure that the problem being solved was equivalent to the real world system and the results and knowledge gained could be easily translated to an actual cabin. The physical features of a helicopter that were seen to be important were the rib and stringer construction combined with the thin skin, which effectively forms an array of plates or shells throughout the cabin. The actual shape of the testbed was seen as less important than the volume of the cabin, as volume is the greatest factor in determining acoustic modal complexity. It was thus determined that a cylinder formed from rings, stringers and a thin skin would accurately represent the system dynamics. Due to space and monetary restrictions, the testbed needed to be scaled down. The scaling also allows the testbed to fit within an anechoic chamber.
To model this complicated system it is necessary to use a sophisticated commercial code, capable of modelling the structural dynamics, as well as the acoustics and the piezo-actuators. Currently, ANSYS is bei explored to determine its capability, but other codes will also be evaluated. Once confidence has been established in the software, many simulations will be run on the virtual testbed to determine which control types and architectures best conform to the system at hand.
By combining the various technologies at hand we are confident that an applicable solution will be found to the problem of interior helicopter acoustic control. During this process a great deal of knowledge will be gained concerning the control of large, highly coupled systems, especially as it pertains to smart structures. In addition, it will be demonstrated that multiplefields of study can efficiently come together to serve greater purposes than could be accomplished individually.