Abstract: A modification of a 727 airplane, including a change in wing profile, to affect flight characteristics and the engines to lessen noise, such that the takeoff and landing procedure is modified so that engine power may be reduced, permitting the modified aircraft to meet the latest noise requirements without substantially reducing pay load.

Abstract: An active noise control system for minimizing undesirable acoustic noise in a defined volume, wherein the undesirable acoustic noise is generated by high frequency structural vibrations emanating from a vibration source structurally coupled to the defined volume at a structural interface. The active noise control system comprises a sensor subsystem disposed in combination with the defined volume for sensing the undesirable acoustic noise in the defined volume, an actuator subsystem disposed proximal to the structural interface, and a controller functionally interconnecting the sensor subsystem to the actuator subsystem, the controller being operative to receive input from the sensor subsystem and to transmit command signals to the actuator subsystem in response thereto for generating selected high frequency counter-vibrations that are interactive with the high frequency structural vibrations to minimize the undesirable acoustic noise in the defined volume.

Abstract: An outer cowl panel (24) for an engine nacelle (20). The outer cowl panel (24) includes integral track fairings (30) at aft side edges of the outer cowl panel. The outer cowl panel (24) also includes a chamfered leading edge having outer face sheets (88A, 88B) that extend over the leading edges of a stepped stack of prepreg sheets (88C, 88D, 88E, 88F).

Abstract: An outflow valve for a cabin pressure control system of an aircraft cabin includes an aft door that is modified to disrupt or increase speed of cabin air flowing past the aft door and out of the outflow valve. The aft door may be modified by notches formed in its leading edge. In the alternative, the aft door may be modified by through-holes formed behind the leading edge. The outflow valve also includes a forward door. Bolt heads or a baffle may protrude from an inner surface of the forward door to disrupt or slow down cabin air flowing over the inner surface of the forward door and out of the outflow valve.

Abstract: An active noise control system for a helicopter for minimizing undesirable acoustic noise in a cabin defined by a cabin structure, wherein the undesirable acoustic noise is generated by high frequency structural vibrations emanating from a gearbox, and wherein the gearbox has a plurality of attachment feet structurally coupling the gearbox to the cabin structure at a plurality of structural interfaces.

Abstract: A tunable, mechanical and acoustical damping system is integrated into a lightweight composite aerospace vehicle structure. In one embodiment, the damping system (22) is incorporated into the wall of a spacecraft payload fairing (12) and includes an outer skin (24), a grid structure (26), an inner skin (28) and a tuning conduit (30). The grid structure (26) forms a number of cells (36) some of which may be interconnected. The cell volumes and the tuning conduit (30) are effective as a Helmholz resonator to reduce potentially damaging noise in the payload compartment (20) of the fairing (12). By appropriate selection of the conduit's dimensions in conjunction with the cell dimensions, the resulting resonator can be tuned to damp a selected frequency or frequencies. For example, a resonant frequency of the fairing (12) with its payload (18). A viscoelastic material (34) is provided between the grid structure (26) and inner skin (28) for mechanical damping.

Abstract: A method and system are provided for reducing flow-induced resonance in a cavity of a structure. At least one pair of diaphragms are mounted in a housing and separated from one another such that a chamber is formed therebetween within the housing. A first diaphragm of the pair of diaphragms is oscillated in accordance with a time-varying function. A second diaphragm of the pair of diaphragms is oscillated inversely to the first diaphragm so that pressure in the chamber formed therebetween changes in correspondence with the time-varying function. The chamber is vented at a position flush with the surface of the structure along the leading edge of the cavity. As a result of such venting, the pressure in the chamber that changes in correspondence with the time-varying function introduces a corresponding time-varying disturbance into the shear layer along the leading edge of the cavity.

Abstract: A wall element has a noise attenuating characteristic and is suitable for use as an aircraft cabin interior panel element that is mounted directly on the aircraft fuselage structure. The wall element includes a rigid lightweight composite panel having a core arranged between two cover layers, and an outer layer arranged adjacent and spaced away from one of the cover layers. The core and the cover layers of the composite panel are air permeable in a direction through the thickness of the panel, while the outer layer is non-air-permeable and is relatively soft and flexible. The outer layer is connected to the composite panel preferably only along the perimeter edge thereof. The non-permeable outer layer faces the aircraft cabin space, while the permeable composite panel is mounted on the fuselage structure. With this structure, acoustic vibrations of the fuselage, to which the wall element is attached, are attenuated and do not result in substantial noise radiation from the outer layer into the cabin.

Abstract: A display and user interface incorporated into an on-board engine vibration monitoring and trim balance system. The interface consists of an alphanumeric display and a number of push buttons integrally formed with a front panel of the system. The user operates the interface to obtain and alter information pertaining to engine vibration data, engine trim balance solutions, and balance part configurations. Push buttons are used to scroll through an elaborate menu structure for receiving information and making selections among various available functions. The menu structure may in particular provide for the user to review particular installed balance part location data prior to asking the system to compute an optimum balance solution. Once the balance solution is calculated, the user may review a list of balance parts to be removed from and installed on the engine.

Abstract: An apparatus and method for reducing noise within the fuselage of a propeller driven aircraft. In one embodiment the propeller blades are spaced apart and in front of an offset inlet. The inlet provides air to a gas turbine engine powering the propeller, and also to a scavenge duct. The propeller blades and inlet are spaced apart such that during cruise conditions for the aircraft, the trailing edge of the propeller blades are spaced apart from the inlet by more than one-fourth of the chord length of the propeller blades, and less than about the chord length of the propeller blades. This spacing reduces the aerodynamic interaction between the propeller blades and a bubble of partially stagnated air in front of the inlet. In another embodiment, the scavenge duct is constructed such that it flows an increased amount of air, thus allowing the inlet duct to flow an increased amount of air.

Abstract: A flex-plate tuned vibration absorber (35) including a tuned mass (43) flexibly suspended by one or more flex plates (58). In one aspect the tunable vibration absorber (TVA) (35) is adaptive in that the position of the mass is adaptively moveable with respect to its initial position, thereby effectuating a change in the TVA's resonant frequency. In one embodiment, the tuned mass (43) moves along a rigid frame (53). In another, movement of the mass (43) statically stresses a preferably disc-shaped flexible plate (58) thereby changing its stiffness. Preferably, a reducer (78) gears down the speed of the motor (64). In another aspect, a TVA system includes a base sensor (40) providing a base vibration signal, a mass sensor (38) for providing a vibration signal of the tuned mass (43), and an electronic controller (42) for generating a control signal to the TVA (35). The TVA finds application for absorbing vibration in vibrating structures, such as in aircraft.

Abstract: The present invention relates to a noise supressor assembly for a jet engine comprised of a plurality of conduit assemblies proximate the trailing edges of the wall members which capture a portion of an ambient gaseous stream flowing in an ambient flow for introduction into the hot core gaseous flow to thereby deform the interface between the hot core flow and the fan flow or ambient flow when there is no fan between the fan flow and the ambient flow or between fully or partially mixed core and fan flows and the ambient flow thereby mixing such gaseous streams to thereby reduce velocity of the hot core and fan flows and thus jet noise level.

Abstract: The invention is a supersonic aircraft. In detail, the aircraft is in the form of a flying wing having a generally flat upper surface, a generally cosine shaped lower surface and a swept back leading edge. A propulsion system is mounted in the aircraft for providing forward thrust and is adapted to provide a source of pressurized air. A plenum is mounted under and behind the leading edge of the flying wing in a spaced relationship thereto, the plenum having a swept back leading edge and a length substantially equal to the length of the flying wing and a trailing edge in the form of an exhaust nozzle; the exhaust nozzle extending substantially over the entire length of the plenum. A duct system is coupled between the at least one engine and the plenum such that pressurized air from the engine can be provided to the plenum for ejection out the nozzle in the form of a sheet of pressurized air under the wing.

Abstract: The present invention is embodied in an aircraft engine noise absorption system having a resonator cavity for absorbing incident noise except for a residue noise signal having a predominant frequency, the system comprising an actuator providing an actuator acoustic signal, a noise sensor for sensing the predominant frequency, and a controller for setting the actuator acoustic signal to the predominant frequency and varying one of a phase and an amplitude of the actuator acoustic signal to decrease the residue noise signal.

Abstract: Method and apparatus for a reinforced skin structure such as an aircraft fuselage which has reduced interior cabin noise. The reinforced skin structure according to the present invention, includes a frame assembly which has a plurality of spaced apart frame members and an outer skin mounted on the frame assembly. A plurality of inner panels, which generally defines an interior compartment, are incorporated within the skin structure. Each of the inner panels is typically supported in a spaced apart fashion from the frame members. A septum is disposed between the outer skin and at least some of the plurality of inner panels. The septum acts to provide an air barrier and increases the air resonant frequency of the cavity between the outer skin and the inner panels. A layer of insulation is placed on the inner side of the septum removing the need to place insulation against the outer skin.

Abstract: A technique for reducing the acoustic response of a spacecraft electronics equipment module that is structurally and thermally independent of a core spacecraft structure to which it can be mounted. The module takes the form of a thermal radiation panel on which electronic components are directly mounted. The panel is adjusted in stiffness to reduce its dynamic vibration response to acoustics and launch transients. These adjustments are facilitated by the external mounting of the panel to the core structure, preferably using a statically determinate mount that renders the module even less susceptible to vibration transferred from the core structure. External mounting removes the panel from the primary structure load path, which provides the freedom to adjust the stiffness as desired.

Abstract: A constrained damping layer assembly includes a first constraining layer adapted for being secured to a first portion of an aircraft frame, and a second constraining layer adapted for being secured to a second portion of the aircraft frame. A damping material is sandwiched between the first constraining layer and the second constraining layer. The damping material includes a first side, which is adapted for slidably contacting the first constraining layer, and further includes a second side, which is adapted for slidably contacting the second constraining layer. The first constraining layer is adapted for sliding in a first direction upon introduction of a sheer load onto the interior panel, and the second constraining layer is adapted for sliding in a second direction upon introduction of a sheer load onto the interior panel.

Abstract: The passive damping wedge for reducing sound radiation from reinforced structures excited by fluid flow over the outer surface of the structure. The damping wedge is generally attached at the trailing edge of the skin structure on the inner surface. The unique three-dimensional design of the damping wedge provides increased damping which absorbs vibrational energy in the structure. The wedge is made of viscoelastic material with a high damping capacity to maximize the ability to reflect vibrations within. The wedge generally has an elongated base which extends upwards from a flat lower surface. An upper surface tapers downwards from the base into a plurality of spaced apart front end members which extend away from the base. The wedge also has a plurality of pairs of sides, with each pair of sides tapering inwardly to generally define each of the front end members.

Abstract: A method for reducing the total operating noise and/or the total vibration of at least two generally-identical rotating machines. In one embodiment, the noise (or vibration) contribution of each machine at a predetermined reference relative phase angle is estimated. Optimum phase angles are calculated by minimizing a cost function which includes such estimated noise (or vibration) contributions and each rotating machine's relative phase angle. In another embodiment, the gradient of a weighted sum of a measure of the magnitude of the acoustic pressure amplitude of the total operating noise (or the amplitude of the total vibration) is measured. Each machine phase angle is adjusted by a predetermined increment opposite in sign to its associated gradient. The measuring and adjusting are repeated until the gradient is within a limit.

Abstract: An aircraft engine has a nozzle shaped to reduce the volume of Mach diamonds being formed in the exhaust plume. A notch or recess is formed in the discharge edge of the nozzle. The recess provides a forward discharge edge that causes additional Mach diamonds to occur at a regular spacing from the forward discharge edge. These additional Mach diamonds are axially staggered with other Mach diamonds, which occur at regular spacing from the rearward edge of the nozzle discharge edge. Each Mach diamond has a volume that is substantially less than one-half the volume of a Mach diamond created by a conventional nozzle. This results in less high temperature areas per axial increment in the plume than the prior art exhaust plumes. Because Mach diamonds are the primary cause of high infrared emissions, as well as acoustic noise, a reduction in the total volume of Mach diamonds in the exhaust plume thus reduces infrared emissions, as well as the noise.

Abstract: A sonic control device that reduces the effects of shock waves generated by an aircraft traveling at supersonic speeds. The control device includes a control surface located at or near the nose section of the aircraft. The position of the control surface can be moved between a retracted position and an extended position. When in a deflected position, the control surface increases the air pressure at the nose section. The increase in air pressure at the nose section decreases both the pressure amplitude and the slope of the overall shock wave as the wave travels toward the ground. Additionally, the deflection of the control surface may induce a downward directed pressure increase which creates less of a drag penalty than a truly blunt nose. When shock control is not desired, the control surface is moved back to the retracted position to reduce the drag on the plane. The moving control device allows a supersonic aircraft to efficiently travel above both land and water.

Abstract: The present invention is directed toward a unique lift-generated noise reduction apparatus. This apparatus includes a plurality of tip fences that are secured to the trailing and leading assemblies of the high-lift system, as close as possible to the discontinuities where the vortices are most likely to form. In one embodiment, these tip fences are secured to some or all of the outboard and inboard tips of the wing slats and flaps. The tip fence includes a generally flat, or an aerodynamically shaped plate or device that could be formed of almost any rigid material, such as metal, wood, plastic, fiber glass, aluminum, etc. In a preferred embodiment, the tip fences extend below and perpendicularly to flaps and the slats to which they are attached, such that these tip fences are aligned with the nominal free stream velocity of the aircraft.In addition to reducing airframe noise, the tip fence tends to decrease drag and to increase lift, thus improving the overall aerodynamic performance of the aircraft.

Abstract: An actively controlled acoustic treatment panel for suppressing noise in a gas turbine engine nacelle including a backsheet formed by a planar matrix of adjacent individually controllable elements, where each of the controllable elements has a transducer and a honeycomb cell enclosing the transducer. A facesheet is also bonded to the controllable elements. A plurality of sensors are positioned within the acoustic treatment panel for sensing acoustic pressure of the noise propagated against the facesheet. A driver is provided for electrically driving each of the transducers to effect displacement thereof in a direction substantially perpendicular to the facesheet. Circuitry is also operatively connected to the pressure sensors and the driver for controlling velocity magnitude and phase of the transducers during displacement, wherein a resulting acoustic impedance at the facesheet achieves a desired acoustic impedance boundary condition at the nacelle.

Abstract: An aircraft cavity acoustic resonance suppression system is described which comprises a small diameter, substantially cylindrically shaped member disposed substantially parallel to and spaced up to a distance corresponding to about three airflow boundary layer thicknesses from the surface of the aircraft near the leading edge of the cavity and substantially transverse of airflow thereacross, and an actuator operatively connected to the member providing selective adjustment of the spacing between the member and the aircraft surface.

Abstract: The application of active noise control to influence the fan-noise components in engine ducts. The fan is used as a source of counter noise by introducing flow distortions upstream or downstream of the fan. Microphone arrays sense the noise and produce a control output signal for controlling a distortion producer. The control output signal for the distortion producer comes from a control algorithm utilizing the output of the microphone signals sampled at a defined rate based on the fan blade passing frequency.

Abstract: A truss type extruded aluminum section (1) is formed of a pair of planar plates (2)(3) and ribs (4)(5)(6), wherein hollow portions (7)(8)(9) are formed within the section (1) by the ribs (4)(5)(6) and the planar plates (2)(3). Vibration-damping resin is provided on the inner surfaces of hollow portions (7)(8)(9), and particularly on the inner surface of the planar plate (3) and on a single surface of the inclined rib (4). With this arrangement, the vibration energy bending the planar plate (3) and the rib (4) is converted into a heat energy. In the case of manufacturing such truss type extruded aluminum vibration-damping section (1), for making easy the insertion of the vibration-damping resin in the hollow portion, the vibration-damping resin is stuck on a plastic film (11) to form a long planar body. The plastic film (11) serves as a cover or an adhesive.

Abstract: An active noise control subassembly for reducing noise caused by a source (such as an aircraft engine) independent of the subassembly. A noise radiating panel is bendably vibratable to generate a panel noise canceling at least a portion of the source noise. A piezoceramic actuator plate is connected to the panel. A back plate is spaced apart from the first plate and the panel with the panel positioned between the source noise and the back plate. A pair of spaced-apart side walls each generally abut the panel and the back plate so as to generally enclose a back cavity. A mechanism is provided for varying the panel resonating frequency by varying the state of the back cavity (such as by varying its fluid pressure and/or volume) while the panel is undergoing bending vibrations.

Abstract: Acoustic noise resulting from the interaction of a periodic pressure and velocity fluctuation induced by a periodically passing blade wake interacting with a downstream airfoil (10)is attenuated by locating an antisymmetric pressure wave generator (30) adjacent the leading edge (18) of the vane (10).

Abstract: An active noise control subassembly array for reducing noise caused by a source (such as an aircraft engine). A pair of subassemblies each has a piezoceramic-driven noise radiating panel to generate a panel noise canceling at least some of the source noise. A front plate is spaced apart from the panel and has a sound exit port. A first pair of spaced-apart side walls each generally abuts the panel and the front plate so as to generally enclose a front cavity to define a resonator. A back plate is spaced apart from the panel by a second pair of side walls. The pair of subassemblies are spaced apart to create a space between their adjacent side walls. A third subassembly is similar to the pair of subassemblies but may have a different resonant frequency and is positioned such that the back plates of the subassembly pair serve as the front plate of the third subassembly with the space between the subassembly pair serving as the sound exit port of the third subassembly.

Abstract: A motor operated restrictor valve including a valve assembly for preventing noise at lavatories and galleys of an aircraft while permitting waste water to pass overboard through the drain mast.

Abstract: Aircraft cabin noise and engine vibration are monitored at selected cabin and engine locations (51a/51b), respectively. An optimizing equation uses aircraft cabin noise information to separately determine for each engine a balance solution (60) that minimizes aircraft cabin noise at the selected cabin locations over the engine RPM range of interest. Next, the balance solutions are used to predict the engine vibration levels (63) that will be produced if the balanced solution is implemented. Then a test (65) is made to determine if the predicted engine vibration levels are acceptable, i.e., below a predetermined level. If acceptable, the balance solutions are used to select balance weights suitable for the engines being balanced (66) and the result displayed (67) for implementation by engine maintenance personnel. If the predicted vibration level is unacceptable, a new balance solution is determined for each engine (68) using the optimizing equation constrained by the allowable vibration level.

Abstract: An active noise control subassembly for reducing noise caused by a source (such as an aircraft engine) independent of the subassembly. A noise radiating panel is bendably vibratable to generate a panel noise canceling at least a portion of the source noise. A piezoceramic actuator plate is connected to the panel. A front plate is spaced apart from the panel and the first plate, is positioned generally between the source noise and the panel, and has a sound exit port. A first pair of spaced-apart side walls each generally abut the panel and the front plate so as to generally enclose a front cavity to define a resonator.

Abstract: The blades of the rotor which can rotate in the transverse duct have an angular distribution according to an uneven azimuth modulation given by the following sinusoidal law:.THETA.n=n.times.360.degree./b+.DELTA..THETA. sin(m.times.n.times.360.degree./b)where .THETA.n is the angular position of the nth of the blades counted in series from an arbitrary origin, b is the number of blades, m is a whole number chosen from 1 to 4, which is not prime with the number b of blades, chosen from 6 to 12, and .DELTA..THETA. is chosen to be greater than or equal to a minimum value .DELTA..THETA. min, which is such that the product .DELTA..THETA. min.times.b is chosen within a range of values extending from 1.5 radian to 1 radian.

Abstract: A noise deduction for a rotorcraft includes a unit to produce compressed air, a drive shaft provided with a first air supply passage in which the compressed air from the air-producing unit flows, and rotor blades provided with second air supply passages in which the air from the first air supply passage flows. In order to eject the compressed air supplied from the rotor blades, ejection nozzles having an ejection adjusting mechanism therein are arranged at respective blade tips of the rotor blades and respective trailing edges thereof in the rotational direction. In operation, the ejection nozzles eject the compressed air to the radial direction of the rotor blades, so that, since the blade tip vortex in the vicinity of the blade tip of the preceding rotor blade is relocated outside the blade tip of the following rotor blade, a direct collision thereof on the vortex can be avoided.

Abstract: An active noise control subassembly for an aircraft engine. An aircraft engine noise radiating panel is bendably vibratable to generate a canceling noise generally opposite in phase to at least a portion of the discrete tonal noise produced by the engine. A piezoceramic actuator plate is vibratable by an applied electric AC signal. The plate is connected to the panel such that vibrations in the plate cause bending vibrations in the panel and such that the plate is compressively prestressed along the panel when the panel is free of bending vibrations. The compressive prestressing increases the amplitude of the canceling noise before the critical tensile stress level of the plate is reached. Preferably, a positive electric DC bias is also applied to the plate in its poling direction to increase the amplitude of the canceling noise before the sum of the AC signal and DC bias exceeds the depolarization voltage in a direction opposite to the poling direction.

Abstract: An active mount for fixed wing applications. One aspect of the invention provides decoupling of two tones which are close in frequency by positioning the mount actuators and error sensors in the primary transmission path of the disturbance vibration and by providing adequate spatial separation between the two sets of error sensors to reduce or eliminate cross-coupling of the signals. Another aspect of the invention utilizes orthogonally positioned actuators with corresponding actuators of paired mounts being focalized for each engine.

Abstract: System for reducing vibrations in the cabin of an aircraft driven by two or more propellers each having n blades (n being an integer equal or larger than 2) of which the relative phase angle can be adjusted, said system comprising synchrophase means for adjusting said relative phase angle such that a predetermined relative phase angle is maintained. The system furthermore comprises means supplying information about the vibration level at one or more positions within the cabin or thereto related information and for generating a thereto corresponding level signal, and a control unit which in response to said level signal supplies an error signal to the synchrophase means causing the synchrophase means to adjust the blades of the various propellers such that a new relative phase angle is maintained which differs m.(2.pi./n) radians (m being an integer) from the previous relative phase angle.

Abstract: The spacecraft comprises a fuselage (1), a wing (2), a power unit incorporating two liquid-propellant launching rocket engines (3), two liquid-propellant boost rocket engines (4), six transverse-thrust rocket engines (5) located in the spacecraft fuselage (1) on a rotatable ring (6), solid-propellant emergency deceleration rocket engines (7), and solid-propellant additional boosting rocket engines (8), a payload compartment (9), a crew compartment (10), a tail unit with two vertical fin struts (11) a bottom tailplane (12), and a top tailplane (13). The fuselage (1) is provided with a movable center conical body (14). The spacecraft landing gear has a swivelling tail wheel (21). The crew compartment (10) is interposed between the fin struts (11) under the top tailplane (13). The spacecraft is provided with an orbital maneuvering system whose final control elements are in fact low-thrust rocket engines (22) and (23), and gyrodynes.

Abstract: An active noise and vibration cancellation system with broadband control capability. A broadband disturbance signal detector positioned within a closed compartment such as an aircraft cabin or vehicle passenger compartment provides a signal representative of the frequency spectrum and corresponding relative magnitude of a broadband signal emanating from a vibrational energy source to a controller. The controller receives the broadband disturbance signal as well as error signals from error sensors which, by virtue of adaptive filters within the controller, enhance the cancellation capability of the control signals produced by one or more actuators positioned within the compartment.

Abstract: A method and an apparatus for active structural acoustic control (ASAC) of sound radiation from and noise transmission through a structure. The method derives acoustic far-field information, corresponding to a `virtual` planar array of microphones, from a vibrational field induced on the structure. The far-field information is used to compute error signals which are provided to an adaptive controller which controls actuators mounted on or embedded in the structure so as to suppress the flexural modes of structure vibration generating acoustic energy. The method is generally applicable to any structure and set of environment boundary conditions. An implementation of the method includes an array of accelerometers mounted on one face of a planar panel, an array of piezoceramic actuators mounted on an opposite panel face, and a frequency-domain adaptive controller utilizing a set of wavenumber-space algorithms.

Abstract: An active liner for attenuating noise includes a rigid backplate supporting a piezoelectric panel. A pressure transducer is disposed in the panel for sensing acoustic pressure of noise being propagated against a face surface of the panel. A controller includes a predetermined schedule of acoustic impedance for controlling a displacement driver joined to the panel to effect a displacement velocity of the panel face surface for obtaining a predetermined acoustic impedance at the sensed acoustic pressure for attenuating noise.

Abstract: Blade-vortex interaction noises, sometimes referred to as "blade slap", are avoided by increasing the absolute value of inflow to the rotor system of a rotorcraft. This is accomplished by creating a drag force which causes the angle of the tip-path plane of the rotor system to become more negative or more positive.

Abstract: An "adaptive synchrophaser" is disclosed for modifying the phase angle relationship between aircraft propellers to reduce cabin noise and/or vibration. Rather than use a constant pre-selected angle for a specific passenger cabin configuration during a particular flight mode (e.g., during liftoff or cruise), the synchrophaser periodically monitors actual operating conditions and modifies the phase angle accordingly. In the preferred embodiment, a plurality of transducers (microphones) are installed at several cabin locations to sample noise periodically. The signals are then transmitted via a signal conditioner, a multiplexer and an analog-to-digital converter to a signal processor. The processor calculates the maximum acoustic noise at each microphone location for all possible phase angles. It then identifies the optimum phase angle that resulted in the lowest maximum noise anywhere in the cabin, and signals a synchrophaser to set that angle.

Abstract: Blade-vortex interaction noises, sometimes referred to as "blade slap", are avoided by increasing the absolute value of inflow to the rotor system of a rotorcraft. This is accomplished by creating a drag force which causes the angle of the tip-path plane of the rotor system to become more negative or more positive.

Abstract: A noise source for an aircraft engine active noise cancellation system in which the resonant frequency of noise radiating structure is tuned to permit noise cancellation over a wide range of frequencies. The resonant frequency of the noise radiating structure is tuned by a plurality of drivers arranged to contact the noise radiating structure. Excitation of the drivers causes expansion or contraction of the drivers, thereby varying the edge loading applied to the noise radiating structure. The drivers are actuated by a controller which receives input of a feedback signal proportional to displacement of the noise radiating element and a signal corresponding to the blade passage frequency of the engine's fan. In response, the controller determines a control signal which is sent to the drivers, causing them to expand or contract. The noise radiating structure may be either the outer shroud of the engine or a ring mounted flush with an inner wall of the shroud or disposed in the interior of the shroud.

Abstract: A noise source for an aircraft engine active noise cancellation system in which the resonant frequency of a noise radiating element is tuned to permit noise cancellation over a wide range of frequencies. The resonant frequency of the noise radiating element is tuned by an expandable ring embedded in the noise radiating element. Excitation of the ring causes expansion or contraction of the ring, thereby varying the stress in the noise radiating element. The ring is actuated by a controller which receives input of a feedback signal proportional to displacement of the noise radiating element and a signal corresponding to the blade passage frequency of the engine's fan. In response, the controller determines a control signal which is sent to the ring, causing the ring to expand or contract. Instead of a single ring embedded in the noise radiating panel, a first expandable ring can be bonded to one side of the noise radiating element, and a second expandable ring can be bonded to the other side.

Abstract: Apparatus for the interference and manipulation of vortex formation and control of fluid flow to reduce noise and increase flow stability is provided. A specific embodiment is directed to a flexible jet noise suppressor for aircraft, particularly supersonic transport. The jet noise suppressor comprises at least one flexible filament having a length that is about 3 to 10 nozzle diameters. The filament(s) trail behind the nozzle and may be let out for take-off and climb and reeled in while the aircraft is in flight or on the ground. Surprisingly, the use of a single flexible filament eliminates the well-known "screech noise" at about 3,000 Hz and reduces the screech noise by about 50 dB while further reducing the broad band noise up to 15 dB over the entire sound spectrum.

Abstract: A flade exhaust nozzle for a high thrust jet engine is configured to form an acoustic shield around the core engine exhaust flowstream while supplementing engine thrust during all flight conditions, particularly during takeoff. The flade airflow is converted from an annular 360.degree. flowstream to an arcuate flowstream extending around the lower half of the core engine exhaust flowstream so as to suppress exhaust noise directed at the surrounding community.

Abstract: A noise source for an aircraft engine active noise cancellation system in which the resonant frequency of a noise radiating element is tuned to permit noise cancellation over a wide range of frequencies. The resonant frequency is tuned by adjusting the size of a frame which encloses the noise radiating element. One or more expandable elements are disposed in the frame to produce expansion and contraction of the frame. The elements are actuated by a controller which receives input of a feedback signal proportional to displacement of the noise radiating element and a signal corresponding to the blade passage frequency of the engine's fan. In response, the controller determines a control signal which is sent to the elements and causes the frame to expand or contract.

Abstract: A noise source for an aircraft engine active noise cancellation system in which the resonant frequency of a noise radiating element is tuned to permit noise cancellation over a wide range of frequencies. The resonant frequency of the noise radiating element is tuned by a plurality of force transmitting mechanisms which contact the noise radiating element. Each one of the force transmitting mechanisms includes an expandable element and a spring in contact with the noise radiating element so that excitation of the element varies the spring force applied to the noise radiating element. The elements are actuated by a controller which receives input of a signal proportional to displacement of the noise radiating element and a signal corresponding to the blade passage frequency of the engine's fan. In response, the controller determines a control signal which is sent to the elements and causes the spring force applied to the noise radiating element to be varied.