Aeronautics and astronautics – Aircraft – heavier-than-air – Helicopter or auto-rotating wing sustained – i.e. – gyroplanes
Reexamination Certificate
2001-09-14
2003-11-11
Carone, Michael J. (Department: 3641)
Aeronautics and astronautics
Aircraft, heavier-than-air
Helicopter or auto-rotating wing sustained, i.e., gyroplanes
C244S00100R, C188S378000, C248S550000
Reexamination Certificate
active
06644590
ABSTRACT:
BACKGROUND
The present invention relates generally to an active system and method for mounting a vibrating component to a support structure for reducing the transmission of vibration and noise passing from the vibrating component to the support structure and, more particularly, to an active vibration and noise reduction system and method for use on a rotary wing aircraft.
Significant effort has been devoted to reducing the vibratory and acoustic loads on aircraft, particularly rotary wing aircraft such as helicopters, and the resulting vibration and noise that develops within the aircraft. A primary source of vibratory and acoustic loads in a helicopter is the main rotor system.
The main rotor system of a helicopter includes rotor blades mounted on a vertical shaft that projects from a transmission, often referred to as a gearbox. The gearbox comprises a number of gears which reduce the rotational speed of the helicopter's engine to the much slower rotational speed of the main rotor blades. The gearbox has a plurality of mounting “feet” which are connected directly to structure in the airframe which supports the gearbox.
The main rotor lift and driving torque produce reaction forces and moments on the gearbox. All of the lift and maneuvering loads are passed from the main rotor blades to the airframe through the mechanical connection between the gearbox feet and the airframe. The airframe structure which supports the gearbox is designed to react to these primary flight loads and safely and efficiently transmit the flight loads to the airframe.
In addition to the nearly static primary flight loads, the aircraft is also subjected to vibratory loads originating from the main rotor blades and acoustic loads generated by clashing of the main rotor transmission gears. The vibratory loads are strongest at a frequency equal to the rotational speed of the main rotor blades (P), which is generally between about 4 and about 5 Hz, multiplied by the number of rotor blades, typically 2 or 4. The product of the main rotor blades rotational speed and the number of blades is called the “fundamental”. Tonals of decreasing vibratory strength occur at multiples of two, three and sometimes four of the fundamental. For example, for a 4 bladed rotor, this would correspond to 8P, 12P, and 16P.
The acoustic loads generated by the transmission gears are at a frequency that the gear teeth mesh with and contact each other, and are thus related to the type of construction and gear ratios used in the transmission. The acoustic loads also include a fundamental and tonals of decreasing strength at integer multiples of the fundamental. Typically, the noise generated by gear clashing is in the range of about 500 Hz to about 3 kHz.
The vibratory and acoustic loads produce vibrations and audible noise that are communicated directly to the helicopter airframe via the mechanical connection between the gearbox and the airframe. This mechanical connection becomes the “entry point” for the unwanted vibration and noise energy into the helicopter cabin. The vibrations and noise within the aircraft cabin cause discomfort to the passengers and crew. In addition, low frequency rotor vibrations are a primary cause of maintenance problems in helicopters.
In the past, “passive” solutions have been tried for reducing the vibratory and acoustic loads on aircraft and the resulting vibration and noise that develops within the aircraft. For noise reduction, passive systems have employed broadband devices such as absorbing blankets or rubber mounts. However, broadband passive systems have generally proven to be heavy and, consequently, not structurally efficient for aircraft applications where weight is paramount. Additionally, broadband passive systems are not very effective at reducing low frequency vibration. A passive technique for reducing vibration involves the installation of narrowband, low frequency vibration absorbers around the aircraft that are tuned to the vibration frequency of interest, typically the fundamental. These narrowband, passive vibration reduction systems are effective, but the number of vibration tonals present in a helicopter may require a number of these systems which then adds significant weight. Additionally, narrowband passive systems work best when placed at ideal locations about the helicopter airframe, many of which may be occupied by other equipment.
More recently, “active” vibration and noise reduction solutions are being employed since active systems have a much lower weight penalty and can be effective against both low frequency vibration and higher frequency noise. Active systems utilize sensors to monitor the status of the aircraft, or the vibration producing component, and a computer-based controller to command countermeasures to reduce the vibration and noise. The sensors are located throughout the aircraft and provide signals to the adaptive controller. The controller provides signals to a plurality of actuators that are located at strategic places within the aircraft. The actuators produce controlled forces or displacements which attempt to minimize vibration and noise at the sensed locations.
Low frequency motion (i.e., vibration) behaves according to rigid body rules and structural models can be used to accurately predict the nature and magnitude of the motion. Since low frequency motion is easily modeled, its negative effects can be cancelled with an active system of moderate complexity. High frequency motion (i.e., noise) at the transmission gear clash frequencies does not obey the rigid body rules present at low vibration frequencies. The use of riveted airframes in combination with the complex mode shapes present at high frequencies make structural models much less accurate. As a result, active systems for high frequency energy reduction become more complex, requiring large numbers of actuators and sensors to counter the more complex modal behavior of the airframe structure.
Some active systems utilize hydraulic actuation systems and hydraulic actuators to reduce vibration and noise. The hydraulic actuation system is preferred since the hydraulic system provides the necessary control bandwidth and authority to accommodate the frequencies and high loads typically experienced in an aircraft such as a helicopter. Additionally, aircraft typically have hydraulic power sources with spare capacity which can be utilized or augmented.
Two methods of actuator placement are frequently used in active systems: (1) distribute the actuators over the airframe, or (2) co-locate the actuators at, or near, the vibration or noise entry point. The co-location approach places the actuators at or near the structural interface between the transmission and airframe, stopping vibration and noise near the entry point before it is able to spread out into the aircraft. This has the advantage of reducing the number of actuators and the complexity of the control system. Active systems using this approach employ actuators mounted in parallel or in series with the entry point to counteract the vibration and noise.
The distributed actuator approach requires a large number of actuators for controlling noise due to the high frequencies, and their associated short spatial wavelength. The large number of actuators can drive up weight and add significantly to control system complexity. One distributed actuator active noise reduction system for use in a helicopter application uses more than 20 actuators to control transmission noise. Distributed actuators for low frequency vibration will be less numerous and are effective at reducing vibration at the sensor locations, but can drive vibration at other areas of the aircraft to levels exceeding those already present.
The parallel actuator approach is effective for low frequency vibration but can produce counteracting forces in the supporting structural elements which can exceed the design limit of the elements and lead to premature failure. Additionally, the parallel approach is not effective at reducing noise because the parallel actuator provide a direct path for n
Terpay Gregory Weston
Welsh William
Zipfel George G.
Carone Michael J.
General Dynamics Advanced Information Systems Inc.
Johnston Michael G.
Moore & Van Allen PLLC
Sukman Gabriel S.
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