Rotorcraft vibration suppression system in a four corner pylon mount configuration

The vibration suppression system includes a vibration isolator located in each corner in a four corner pylon mount structural assembly. The combination of four vibration isolators, two being forward of the transmission, and two being aft of the transmission, collectively are effective at isolating main rotor vertical shear, pitch moment, as well as roll moment induced vibrations. Each opposing pair of vibration isolators can efficiently react against the moment oscillations because the moment can be decomposed into two antagonistic vertical oscillations at each vibration isolator. A pylon structure extends between a pair of vibration isolators thereby allowing the vibration isolators to be spaced a away from a vibrating body to provide increased control.

BACKGROUND

Technical Field

The present application relates in general to vibration control. More specifically, the present application relates to systems for isolating mechanical vibrations in structures or bodies that are subject to harmonic or oscillating displacements or forces. The systems of the present application are well suited for use in the field of aircraft, in particular, helicopters and other rotary wing aircraft.

Description of Related Art

For many years, effort has been directed toward the design of an apparatus for isolating a vibrating body from transmitting its vibrations to another body. Such apparatuses are useful in a variety of technical fields in which it is desirable to isolate the vibration of an oscillating or vibrating device, such as an engine, from the remainder of the structure. Typical vibration isolation and attenuation devices (“isolators”) employ various combinations of the mechanical system elements (springs and mass) to adjust the frequency response characteristics of the overall system to achieve acceptable levels of vibration in the structures of interest in the system. One field in which these isolators find a great deal of use is in aircraft, wherein vibration-isolation systems are utilized to isolate the fuselage or other portions of an aircraft from mechanical vibrations, such as harmonic vibrations, which are associated with the propulsion system, and which arise from the engine, transmission, and propellers or rotors of the aircraft.

Vibration isolators are distinguishable from damping devices in the prior art that are erroneously referred to as “isolators.” A simple force equation for vibration is set forth as follows:
F=m{umlaut over (x)}+c{dot over (x)}+kx

A vibration isolator utilizes inertial forces (m{umlaut over (x)}) to cancel elastic forces (kx). On the other hand, a damping device is concerned with utilizing dissipative effects (c{dot over (x)}) to remove energy from a vibrating system.

One important engineering objective during the design of an aircraft vibration-isolation system is to minimize the length, weight, and overall size including cross-section of the isolation device. This is a primary objective of all engineering efforts relating to aircraft. It is especially important in the design and manufacture of helicopters and other rotary wing aircraft, such as tilt rotor aircraft, which are required to hover against the dead weight of the aircraft, and which are, thus, somewhat constrained in their payload in comparison with fixed-wing aircraft.

Another important engineering objective during the design of vibration-isolation systems is the conservation of the engineering resources that have been expended in the design of other aspects of the aircraft or in the vibration-isolation system. In other words, it is an important industry objective to make incremental improvements in the performance of vibration isolation systems which do not require radical re-engineering or complete redesign of all of the components which are present in the existing vibration-isolation systems.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring toFIG. 1in the drawings, a rotorcraft11is illustrated. Rotorcraft11has a rotor system13with a plurality of rotor blades21. Rotorcraft11further includes a fuselage15, landing gear17, and an empennage19. A main rotor control system can be used to selectively control the pitch of each rotor blade21in order to selectively control direction, thrust, and lift of rotorcraft11. It should be appreciated that even though rotorcraft11is depicted as having certain illustrated features, it should be appreciated that rotorcraft11can take on a variety of implementation specific configurations, as one of ordinary skill in the art would fully appreciate having the benefit of this disclosure. Further, it should be appreciated that rotorcraft11can have variety of rotor blade quantities. It should be understood that the systems of the present application may be used with any aircraft on which it would be desirable to have vibration isolation, including unmanned aerial vehicles that are remotely piloted.

The systems of the present application may also be utilized on other types of rotary wing aircraft. Referring now toFIGS. 2A and 2Bin the drawings, a tilt rotor aircraft111according to the present application is illustrated. As is conventional with tilt rotor aircraft, rotor assemblies113aand113bare carried by wings115aand115b, and are disposed at end portions116aand116bof wings115aand115b, respectively. Tilt rotor assemblies113aand113binclude nacelles120aand120b, which carry the engines and transmissions of tilt rotor aircraft111, as well as, rotor hubs119aand119bon forward ends121aand121bof tilt rotor assemblies113aand113b, respectively.

Tilt rotor assemblies113aand113bmove or rotate relative to wing members115aand115bbetween a helicopter mode in which tilt rotor assemblies113aand113bare tilted upward, such that tilt rotor aircraft111flies like a conventional helicopter; and an airplane mode in which tilt rotor assemblies113aand113bare tilted forward, such that tilt rotor aircraft111flies like a conventional propeller driven aircraft. InFIG. 2A, tilt rotor aircraft111is shown in the airplane mode; and inFIG. 2B, tilt rotor aircraft111is shown in the helicopter mode. As shown inFIGS. 2A and 2B, wings115aand115bare coupled to a fuselage114. Tilt rotor aircraft111also includes a vibration isolation system according to the present application for isolating fuselage114or other portions of tilt rotor aircraft111from mechanical vibrations, such as harmonic vibrations, which are associated with the propulsion system and which arise from the engines, transmissions, and rotors of tilt rotor aircraft111.

Referring toFIGS. 3-5, a vibration suppression system601is illustrated. System601, also termed a vibration isolator system, includes a vibration isolator401located in each corner in a four corner pylon mount structural assembly. The combination of four vibration isolators401, two being forward of transmission607, and two being aft of transmission607, collectively are effective at isolating main rotor vertical shear, pitch moment, as well as roll moment induced vibrations. For example, rotor hub induced pitch moment vibrations, which can become relatively large in high-speed forward flight, can be effectively isolated with the four vibration isolators, corner located as shown inFIGS. 3 and 4. Locating isolators401away from the transmission is an improvement over legacy configurations which typically couple the transmission directly to the isolator. However, this is not the case in the present application.

The four corner pylon mount structural assembly includes a first pylon structure615a, second pylon structure615b, a first roof beam603a, a second roof beam603b, a forward cross member201a, and an aft cross member201b. Structural adapters can be used to structurally couple roof beams603aand603bwith cross members201aand201b. In the illustrated embodiment, roof beams603aand603bare coupled to an airframe605, while pylon structures615aand615bare coupled to isolators401. First pylon structure615ais mounted with a first vibration isolator401aand a second vibration isolator401b, while a second pylon structure615bis mounted with a third vibration isolator401cand a fourth vibration isolator401d. Each vibration isolator401a-dis mounted substantially vertical, as illustrated inFIG. 5. Transmission607is coupled to pylon structures615aand615bas opposed to direct coupling to the isolators. A driveshaft609carries mechanical power from an engine611to transmission607. It should be appreciated that embodiments of pylon system601may employ any practical number of engines and transmissions. Furthermore, it is contemplated that any plurality of pylon structures and vibration isolators may be used in a variety of orientations spaced fore, aft, and even outboard of transmission607.

As seen inFIGS. 4 and 5, isolators401a-dare mounted away from transmission607. For example, isolators401a-dare mounted forward and aft of transmission607. Additionally, isolators401a-dare mounted outboard from transmission607. As depicted inFIG. 4, isolators401a-dare mounted sufficiently outboard so as to be located further outboard than the point of coupling606between transmission607and pylon structures615aand615b. The point of coupling606is inboard between roof beams603a,603b. In so doing, two isolators401a,401care positioned above roof beams603a,603bforward of transmission607. Likewise two isolators401b,401dare positioned above roof beams603a,603baft of transmission607. Isolators401a-dare spaced away from the point of coupling between pylon structures615aand615band the transmission in fore, aft, and outboard directions in the preferred embodiment. However, it is understood that other embodiments may adjust the spacing do affect dynamics from different aircraft or transmissions.

Pylon structures615a,615bare configured to correlate motion of the transmission between a plurality of isolators401simultaneously by suspending a portion of transmission607between a plurality of isolators located on opposing ends of the pylon structure. The use of pylon structures615a,615bpermits an aircraft to space the location of isolators401a-dto an infinite number of locations independent of transmission607. Locating isolators forward and aft of transmission permits the pylon mount structural assembly minimizes the size of each isolator401a-dand avoids the use of additional elements to control the dynamics of transmission607. For example, the pylon mount structural assembly is springless in that the assembly does not use a spring mounted externally beneath the transmission to control dynamics of the transmission. The pylon mount structural assembly is configured to control pitch and roll dynamics by spacing of isolators401a-dand the use of pylon structures615aand615b.

Further, implementing active vibration isolators, such as piezoelectric vibration isolators, can be effective for vibration isolation for a multiple RPM rotorcraft. It should be appreciated that other active actuation methods can be used as well, such as hydraulic, electromagnetic, electromechanical, to name a few. Active vibration isolators can also achieve better vibration isolation by overcoming damping losses, and adjusting the frequency response characteristics. Further, each opposing pair of vibration isolators401can efficiently react against the moment oscillations because the moment can be decomposed into two antagonistic vertical oscillations at each vibration isolator401.

Referring now also toFIGS. 6 and 7in the drawings, isolator401comprises an upper housing403and a lower housing405. An upper reservoir housing427and a lower reservoir housing429are coupled to end portions of upper housing403and a lower housing405, respectively. Each upper reservoir housing427and a lower reservoir housing429define an upper fluid chamber407and a lower fluid chamber409, respectively. A piston spindle411includes a cylindrical portion that is at least partially disposed within the interior of upper housing403and lower housing405. A plurality of studs417rigidly couple together upper housing403and a lower housing405via an upper ring439and a lower ring441, respectively, so that upper housing403and lower housing405function as a single rigid body. Studs417extend through piston spindle411within apertures sized to prevent any contact between studs417and piston spindle411during operation. Further, piston spindle411is resiliently coupled to upper housing403and lower housing405via an upper elastomer member413and a lower elastomer member415, respectively. Upper elastomer member413and lower elastomer member415each function similar to a journal bearing, as further discussed herein.

Piston spindle411is coupled to a vibrating body, such as a transmission of an aircraft via a pylon assembly, such as a pylon assembly601. A spherical bearing assembly425is coupled to lower housing405. Spherical bearing assembly425includes an attachment member431configured for coupling the spherical bearing assembly425to a body to be isolated from vibration, such as a roof beam of an airframe in an aircraft, such as roof beam603. In such an arrangement, the airframe serves as the body to be isolated from vibration, and the transmission of the aircraft serves as the vibrating body. Spherical bearing assembly425includes a spherical elastomeric member433having an elastomeric material bonded between a non-resilient concave member and a non-resilient convex member. Spherical elastomeric member433is configured to compensate for misalignment in loading between the pylon assembly601and roof beam603through shearing deformation of the elastomeric material. Spherical elastomeric member433is partially spherical shaped with a rotational center point445that lies on a centerline plane443of attachment member431. Furthermore, spherical bearing assembly425is positioned and located to reduce an overall installation height of vibration isolator401, as well is provide optimized performance of pylon assembly601and related propulsion components.

Upper elastomer member413and lower elastomer member415seal and resiliently locate piston spindle411within the interior upper housing403and lower housing405. Upper housing403and lower housing405can each be coupled to piston spindle411with an upper adapter435and lower adapter437, respectively. Upper elastomer member413and lower elastomer member415function at least as a spring to permit piston spindle411to move or oscillate relative to upper housing403and lower housing405. Upper elastomer member413and lower elastomer member415can be a solid elastomer member, or alternatively can be alternating layers of non-resilient shim members and elastomer layers.

Isolator401further includes an elongated portion419integral with piston spindle411, the elongated portion419being configured to define a tuning passage421. Tuning passage421axially extends through elongated portion419to provide for fluid communication between upper fluid chamber407and lower fluid chamber409. The approximate length of tuning passage421preferably coincides with the length of elongated portion419, and is further defined by L1. Tuning passage421is generally circular in cross-section and can be partially tapered longitudinally in order to provide efficient fluid flow.

A tuning fluid423is disposed in upper fluid chamber407, lower fluid chamber409, and tuning passage421. Tuning fluid423preferably has low viscosity, relatively high density, and non-corrosive properties. For example, tuning fluid423may be a proprietary fluid, such as SPF I manufactured by LORD CORPORATION. Other embodiments may incorporate hydraulic fluid having suspended dense particulate matter, for example.

The introduction of a force into piston spindle411translates piston spindle411and elongated portion419relative to upper housing403and lower housing405. Such a displacement of piston spindle411and elongated portion419forces tuning fluid423to move through tuning passage421in the opposite direction of the displacement of piston spindle411and elongated portion419. Such a movement of tuning fluid423produces an inertial force that cancels, or isolates, the force from piston spindle411. During typical operation, the force imparted on piston spindle411is oscillatory; therefore, the inertial force of tuning fluid423is also oscillatory, the oscillation being at a discrete frequency, i.e., isolation frequency.

The isolation frequency (fi) of vibration isolator401can be represented by the following equation:

In the above equation, R represents the ratio of the functional area Apof piston spindle411to the total area ATinside the tuning passage421. As such, R=Ap/ATMass of tuning fluid423is represented by mt. The combined spring rate of elastomer members413and415is represented by K.

It should be appreciated that isolator401is merely exemplary of a wide variety of vibration isolators that may be used. For example, vibration isolator401is illustrated as a passive vibration isolator; however, it should be fully appreciated that vibration isolator401can also be of an active isolator. An active isolator is configured so that the isolation frequency can be selective changed during operation. For example, an active vibration isolator is illustrated in U.S. Patent Application Publication No. US 2006/0151272 A1, titled “Piezoelectric Liquid Inertia Vibration Eliminator”, published 13 Jul. 2006, to Michael R. Smith et al., which is hereby incorporated by reference.

Vibration suppression system601is configured such that transmission607is “soft mounted” with a vibration isolator401a-dlocated at each end of a pylon structure615. During operation, each vibration isolator401a-dallows each pylon structure615a,615bto float relative to roof beams603a,603bthrough the deformation of upper elastomer member413, lower elastomer member415, and spherical elastomeric member433. If coupling613is required to compensate for a large amount of axial and angular misalignment, then the size and complexity of coupling613is undesirably large. Further, it is desirable to minimize the size and complexity of aircraft components in order to minimize weight and expense of the aircraft, thereby maximizing performance and reducing manufacturing associated expenditure. As such, vibration isolators401a-dare uniquely configured to reduce the size and complexity of drive system components, such as coupling613. More specifically, spherical bearing assembly425is configured so that centerline plane443of attachment member431lies on or near a waterline plane of driveshaft axis617so as to reduce a moment arm that could otherwise contribute to axial (chucking) misalignment. An undesirable moment arm could be produced if centerline plane443of attachment member431were to lie a significant moment arm distance, as measured in the waterline direction, from driveshaft axis617. Chucking occurs essentially when engine611and transmission translate towards or away from each other. Further, the location of spherical bearing assembly425circumferentially around lower housing405reduces the overall height of vibration isolators401a-d. A compact pylon system601improves performance by reducing moment arms that can react between components.

Referring briefly toFIG. 8in the drawings, a mechanical equivalent model701for vibration isolator401ofFIGS. 4 and 5is illustrated. In mechanical equivalent model701, a box703represents the mass of the fuselage Mfuselage; a box705represents the mass of the pylon assembly Mpylon; and a box707represents the mass of the tuning mass Mt, in this case, the mass of tuning fluid423. A vibratory force F·sin(ωt) is generated by the transmission and propulsion system. Force F·sin(ωt) is a function of the frequency of vibration of the transmission and propulsion system.

Force F·sin(ωt) causes an oscillatory displacement upof the pylon assembly; an oscillatory displacement of the fuselage uf; and an oscillatory displacement of the tuning mass ut. Elastomer members413and415are represented by a spring709disposed between the fuselage Mfuselageand the pylon assembly Mpylon. Spring709has a spring constant K.

In mechanical equivalent model701, tuning mass Mtfunctions as if cantilevered from a first fulcrum711attached to the pylon assembly Mpylon, and a second fulcrum713attached to the fuselage Mfuselage. The distance a from first fulcrum711to second fulcrum713represents the cross-sectional area of tuning passage421, and the distance b from first fulcrum711to the tuning mass Mtrepresents the effective cross-sectional area of piston spindle411, such that an area ratio, or hydraulic ratio, R is equal to the ratio of b to a. Mechanical equivalent model701leads to the following equation of motion for the system:

As is evident, no means for actively tuning vibration isolator401is available. Once the cross-sectional areas of tuning passage421and piston spindle411are determined, and the tuning fluid is chosen, the operation of vibration isolator401is set. However, an embodiment of vibration isolator401can be configured such that the isolation frequency can be selectively altered and optimized by the removing and replacing elongated portion419from piston spindle411with another elongated portion419having a different diameter tuning passage421. As such, vibration isolator401can be adaptable to treat a variety of isolation frequencies, as well as being adaptable for variances in stiffness K of upper and lower elastomer members413and415.

Referring now also toFIG. 9, an active vibration control system801is illustrated. System801can includes a plurality of vibration feedback sensors803a-803din communication with a vibration control computer (VCC)805. VCC805is in communication with each active vibration isolator in system601so that the isolation frequency of each active vibration isolator can be actively modified during operation. The vibration control system is configured to detect and convey vibration data through a plurality of feedback sensors803a-803dto regulate the isolation frequency of at least one vibration isolator401a-d.

The vibration suppression system of the present application provides significant advantages, including: 1) efficient and effective vibration suppression rotor induced vertical hub shear forces, hub pitch moments, and hub roll moments; 2) improved occupant ride quality; 3) improved life of life critical rotorcraft components; 4) decreased size of isolators; and 5) ability to control the roll, pitch, and shear without the assistance of externally mounted systems to the transmission.

It is apparent that embodiments with significant advantages have been described and illustrated. Although the embodiments in the present application are shown in a limited number of forms, it is not limited to just these forms, but is amenable to various changes and modifications without departing from the spirit thereof.