Vibration dampening for horizontal stabilizers

Systems and methods provide for the mitigation of vibrational forces acting on a horizontal stabilizer of an aircraft. According to one aspect, a damper is coupled to a front portion of a horizontal stabilizer to dampen vibrations in a first degree of freedom, with another damper coupled to a mounting point of the horizontal stabilizer to dampen vibrations in a second degree of freedom. The dampers may be passive, operating independently to mitigate vibrational forces, or active, applying a mitigating force to the horizontal stabilizer based on real-time or estimated vibration states.

BACKGROUND

Horizontal stabilizers of aircraft are often subjected to turbulent airflow and flight characteristics that induce vibrations throughout the horizontal stabilizers. These vibrations are currently absorbed and distributed throughout the horizontal stabilizer and airframe to which the stabilizer is attached with no or little negative impact. As aircraft manufacturers strive to produce more fuel-efficient aircraft, the use of fuel-efficient engines, such as high bypass ducted fans, will increase. Due to the typical orientation of a high bypass ducted fan engine, the turbulent discharge air (jet-wash) from the engine commonly flows on and over the horizontal stabilizer. As the sizes of high bypass ducted fan engines increase, the quantity of jet-wash will similarly increase. The impact of the resulting increase in vibrational forces caused by the additional jet-wash on the horizontal stabilizer will also increase. In some cases, the increased quantity of jet-wash may be sufficient to induce measurable vibrations in the horizontal stabilizer that are transferred to the fuselage and ultimately felt by the passengers and crew. Long-term effects of the vibrations may include fatigue in the aircraft structure, which may reduce the lifespan of the aircraft or increase maintenance costs.

SUMMARY

Apparatus and methods described herein provide for the mitigation of vibrations in a horizontal stabilizer of an aircraft. According to one aspect, a vibration dampening system for a horizontal stabilizer includes at least two dampers. A first damper is coupled to a front portion of the horizontal stabilizer and is configured to dampen a vibrational force in a first degree of freedom. A second damper is coupled to the horizontal stabilizer proximate to a mounting point of the stabilizer. The second damper is configured to dampen the vibrational force in a second degree of freedom.

According to another aspect, a method for mitigating vibration in a horizontal stabilizer of an aircraft is provided. According to the method, a vibration is received at a first damper that is coupled to a front portion of the horizontal stabilizer and at a second damper coupled to the stabilizer at a pivot point. The vibration is dampened in a first degree of freedom with the first damper, and is dampened in a second degree of freedom with the second damper.

According to yet another aspect, a vibration dampening system for a horizontal stabilizer of an aircraft is provided. The vibration dampening system includes at least three visco-elastic dampers. The first damper is coupled to a front portion of the horizontal stabilizer and is configured to dampen a vibrational force in a first degree of freedom. The second and third dampers are coupled to the horizontal stabilizer at two pivot points, both of which pivot the stabilizer around a pitch axis of the horizontal stabilizer, and are both configured to dampen the vibrational force in a second degree of freedom.

DETAILED DESCRIPTION

The following detailed description is directed to a vibration mitigation system and corresponding method that utilizes dampers to mitigate vibrations associated with a horizontal stabilizer of an aircraft. As discussed above, increasing aircraft fuel efficiency is a substantial concern for aircraft and engine manufacturers and corresponding customers. High bypass ducted fan engines, for example, have proven to offer increased efficiency, however, as the size of these engines increases, the corresponding distortion or turbulence associated with the prop wash may create undesirable vibrational forces on the horizontal stabilizer of an aircraft. These vibrations may translate to the airframe, potentially creating excessive noise as well as structural fatigue.

Utilizing the concepts and technologies described herein, a vibration dampening system utilizes a number of dampers to absorb and mitigate vibrational forces in multiple directions. Dampers may be coupled to a horizontal stabilizer of an aircraft at a mounting point, such as a mid-box attachment point or a pivot point, to mitigate vibrations in a first degree of freedom, or z-direction, while one or more dampers coupled to a front portion of the horizontal stabilizer mitigate vibrations in a second degree of freedom, or x-direction. The dampers may be passive, such as visco-elastic dampers, which optimally operate to target mitigation of vibrational forces having a particular frequency. The number, type, or characteristics of the dampers may be selected or designed to mitigate vibrations in at least one frequency or at multiple frequencies. The dampers may additionally or alternatively be active dampers, utilizing linear actuators to induce a force in a desired direction at a desired frequency or frequencies to mitigate corresponding vibrations. According to alternative embodiments, the active dampers may operate via altering a pressure of a fluid within a fluid chamber of a visco-elastic damper rather than utilizing a linear actuator. The induced motion created by the active dampers may be based on an actual real-time vibration state of the horizontal stabilizer as measured by one or more sensors or accelerometers, based on an estimated vibration state as predicted according to one or more aircraft parameters, or a combination thereof.

In the following detailed description, references are made to the accompanying drawings that form a part hereof, and which are shown by way of illustration, specific embodiments, or examples. Referring now to the drawings, in which like numerals represent like elements through the several figures, a vibration mitigation system and method for employing the same according to the various embodiments will be described.

FIG. 1shows a side view of a vibration mitigation system100for mitigating vibrations associated with the horizontal stabilizer102of an aircraft. Looking atFIG. 1, a vibration dampening system100includes dampers104. According to this embodiment, the dampers104include two passive dampers, passive damper105A and passive damper105B (collectively referred to as “passive dampers105”). Although only two dampers104are shown in this view, additional dampers104may be used. In this example, each passive damper105A and105B is a visco-elastic damper, having a viscous damper106and an elastic element such as a spring114. The viscous damper106includes a viscous fluid chamber108(double acting) and a damper piston110having a number of orifices113. The damper piston110exerts a force on a viscous fluid112within the viscous fluid chamber108. Because the viscous fluid112is substantially incompressible, when a downward force is applied by the damper piston110, the viscous fluid112is pressed through a number of orifices113in the damper piston110. The characteristics of the viscous fluid112and of the orifices113determine the dampening characteristics of the viscous damper106, which can be quantified by a damping coefficient (c), which is described in greater detail below. The spring114is compressed with the downward force applied to the damper piston110, further mitigating the downward force, while providing a return force that moves the damper piston110upward toward a starting position after downward motion is stopped.

The opposite process is also true. When an upward force is applied to the damper piston110, the viscous fluid112in a top portion of the viscous fluid chamber108is pressed through the orifices113, slowing the piston movement and mitigating the upward force. The spring114is stretched to create a tension force that acts to mitigate the upward force while providing a return force that moves the damper downward after upward movement is stopped. It should be appreciated that the number of viscous dampers106within a passive damper105may be smaller, greater, or equal to the number of elastic elements, or springs114. Throughout this disclosure, the elastic element will be referred to as a spring114, however, any suitable elastic element may be utilized without departing from the scope of this disclosure. For example, the elastic element may include, but not be limited to, any conventional type of spring, compressed gaseous spring, or passive electric linear actuator that is permanently exerting a predetermined force. It should also be appreciated that although the various embodiments with respect to passive dampers105are described as having viscous dampers106utilizing viscous fluid112, the present disclosure may alternatively be implemented using hydro-pneumatic dampers or other dampers utilizing compressible fluids.

The forces applied to the damper piston110originate from vibration forces in the horizontal stabilizer102since the passive dampers104are coupled to the horizontal stabilizer102. According to one embodiment, the vibration mitigation system100includes a passive damper105A that is coupled to a front portion116of the horizontal stabilizer102and at least one passive damper105B coupled to the horizontal stabilizer102at a pivot point118. For the purposes of this disclosure, “front portion” may include any portion of the horizontal stabilizer102forward of the pivot points118and corresponding pivot axis around which the horizontal stabilizer102rotates for trimming purposes. For example, according to some embodiments, the front portion116may be on or proximate the leading edge of the horizontal stabilizer102, while according to other embodiments, the front portion116may include a front horizontal stabilizer spar, which could be positioned between the leading edge and the pivot points118. In some implementations, the front portion may be near or proximate to the pivot points118. Although only one passive damper105B is shown at the pivot point118in the side view ofFIG. 1, according to the examples described and shown throughout the drawings, there may be two passive dampers105positioned at pivot points around a pivot axis. This configuration is best shown and further described below with respect toFIG. 3. It should be understood that the disclosure herein is not limited to any particular number of dampers.

It is common for horizontal stabilizers102of commercial aircraft to pivot around a pivot axis in order to trim the aircraft pitch to accommodate different center of gravity positions of the aircraft during different phases of flight. To provide for this pivoting capability, the horizontal stabilizer102of an aircraft is typically mounted to the fuselage or vertical stabilizer (depending on tail configuration) via pivot points, while controlling the pitch using a jack screw or suitable actuator coupled to the front portion116of the horizontal stabilizer102. By raising and lowering the jack screw, the front portion116of the horizontal stabilizer102is raised and lowered, pivoting the entire horizontal stabilizer102around the pivot axis intersecting the pivot points to which the horizontal stabilizer102is mounted. For the purpose of this disclosure, the vibration mitigation system100may be shown and described as being mounted to a structure120. While not specifically shown inFIG. 1(best seen inFIG. 3and discussed below), the structure120may be spars within the vertical stabilizer and the horizontal stabilizer102.

According to various embodiments, the passive damper105A that is coupled to the front portion116of the horizontal stabilizer102is configured to dampen vibrational forces in a first degree of freedom, while the passive damper105B coupled to the horizontal stabilizer102at the pivot point118is configured to dampen vibrational forces in a second degree of freedom. More specifically, as seen inFIG. 1, the vibrational forces being mitigated in the first degree of freedom by the passive damper105A may be oriented in a fore-aft direction substantially parallel to the x-axis or longitudinal axis of the aircraft. The vibrational forces being mitigated in the second degree of freedom by the passive damper105B may be oriented in an up-down direction substantially parallel to the z-axis, or substantially normal to the longitudinal axis of the aircraft. It should be noted that the dampers104additionally operate to dampen vibrational forces in a third degree of freedom, which is around a roll axis, as indicated by the roll label around the x-axis indicator inFIG. 1. The roll dampening exists in situations where the horizontal stabilizer102on opposing sides of the vertical stabilizer (shown inFIGS. 2 and 3) are subjected to different vibrational forces. The different vibrations may occur due to engine thrust asymmetry, yaw, or asymmetric wind gusts. By mitigating these vibrational forces on the horizontal stabilizer102on both sides of the vertical stabilizer, roll-inducing forces may be mitigated.

While the dampers104are shown throughout the figures as being oriented substantially parallel with the x-axis and z-axis, it should be understood that any one or more of the dampers104described and shown with respect to any of the various embodiments disclosed herein may be oriented at an angle with respect to the x or z axis. For example, if primary vibrational forces were determined to be transferred to the structure120at a particular angle with respect to the x-axis through the front portion116of the horizontal stabilizer102, the passive damper105A may be oriented at a corresponding angle that allows the vibrational forces to be absorbed substantially linearly along the direction of travel of the damper piston110and spring114.

FIG. 2shows a side view of a vibration dampening system100installed in an aircraft202having a low-tail configuration200. The low-tail configuration is a conventional configuration in which the horizontal stabilizer102is mounted within an aft portion of the fuselage204with the vertical stabilizer206extending upwards from the fuselage204above the horizontal stabilizer102. For various well-known reasons, a conventional low-tail configuration200is desirable over other alternative configurations, several of which are discussed below. However, the low positioning of the horizontal stabilizer102exposes the horizontal stabilizer102to distortion from the exit airflow210from one or more engines208. Although the engines208may be of any type and quantity, as discussed above, the engines208may be high bypass ducted fan engines that produce a significant amount of exit airflow210distortion. Other examples of engines208include, but are not limited to, propeller driven systems, turbofan systems, turbo-propeller systems, electric drive propulsion systems, hybrid electric systems, and open-rotor propulsion systems.

The aerodynamic lift loads associated with a horizontal stabilizer102are generally transferred to the fuselage204through the pivot points118of the horizontal stabilizer102, so the dampers104positioned at the pivot points118will target the frequency by which the lift force will be varying due to the vibrational forces associated with the exit airflow210. The exit airflow210distortion varies the drag forces associated with the horizontal stabilizer102, which are primarily transferred to the fuselage204via the jack screw coupled to the front portion116of the horizontal stabilizer102, which is used to rotate the horizontal stabilizer102around the pivot points118for trimming purposes. Accordingly, the damper104positioned at the front portion116of the horizontal stabilizer102will target the frequency by which the drag forces will vary due to the vibrational forces associated with the nature of the distortion induced in the exit airflow210. Additionally, any pitching moments induced around the pivot points118of the horizontal stabilizer102will be substantially transmitted to the jack screw at the front portion116of the horizontal stabilizer102. Any varying forces associated with changes to the pitching moment may also be mitigated by the damper104positioned at the front portion116of the horizontal stabilizer102according to various embodiments described herein.

The frequencies targeted by the dampers104at each location may vary depending on the flight characteristics or phase of flight of the aircraft202. For example, the exit airflow, as well as ambient airflow, over the horizontal stabilizer102may differ during the climb phase, cruise phase, and descent phase of any given flight. According to various embodiments, the vibration dampening system100may be tuned to mitigate undesirable vibrational forces during a particular phase of flight. For example, due to the disproportionate length of time that the aircraft202may spend in cruise flight compared to other phases of flight, the dampers104may be designed or selected according to vibration frequencies most likely encountered during cruise flight characteristics. As will be discussed below, a number of design considerations are utilized in tuning or adapting the vibration dampening system100to the particular aircraft202for maximum dampening effect.

As previously suggested, the structure120to which the dampers104are mounted may include structural components of the fuselage204, including a jack screw commonly used at the front portion116of the horizontal stabilizer102for trimming purposes. Turning now toFIG. 3, further detail with respect to the vibration dampening system100and the structure120to which it is attached will be described according to various embodiments.FIG. 3is a perspective view of a vibration dampening system100installed in an aircraft202having a low-tail configuration200. With this perspective view, it can be seen that up and down movement of the jack screw302moves the front portion116of the horizontal stabilizer102up and down, which pivots the horizontal stabilizer102around the pivot axis310intersecting the pivot points118A and118B.

The vibration dampening system100of this embodiment includes three dampers104. Specifically, passive damper105A is coupled to the jack screw302at the front portion116of the horizontal stabilizer102and is configured to dampen vibrational forces in the first degree of freedom substantially parallel to the x-axis, or fore and aft. The front portion116of this example includes a horizontal stabilizer front spar304such that the jack screw302is coupled to the horizontal stabilizer front spar304via the passive damper105A. The passive dampers105B and105C are coupled to the pivot points118A and118B, respectively, at a horizontal stabilizer rear spar306and are configured to dampen vibrational forces in the second degree of freedom substantially parallel to the z-axis, or up and down.

FIGS. 4 and 5show side and perspective views, respectively, of a vibration dampening system100installed in an aircraft202having a cruciform tail configuration400according to various embodiments described herein. With a cruciform tail configuration400, the horizontal stabilizer102is mounted between a tip402and a root404of the vertical stabilizer206. As seen inFIG. 5, a jack screw302couples a front portion116the horizontal stabilizer102to a vertical stabilizer front spar504. A damper104is installed between the jack screw302and the vertical stabilizer front spar504(or structure120), or between the jack screw302and the front portion116of the horizontal stabilizer102. According to various embodiments, this damper104may be a passive damper105A as discussed above, or an active damper described in further detail below with respect toFIGS. 10-12.

This example of a cruciform tail configuration400includes two pivot points118A and118B defining a pivot axis310around which the horizontal stabilizer102rotates for trimming via the jack screw302. Two dampers, passive dampers105B and105C in this example, are mounted to the horizontal stabilizer102between the horizontal stabilizer rear spar306and a vertical stabilizer rear spar506at the pivot points118A and118B. The dampers operate in the same manner described above with respect to the low-tail configuration200. In particular, the passive damper105A is configured to dampen vibrational forces in the first degree of freedom substantially parallel to the x-axis, or fore and aft, while the passive dampers105B and105C are configured to dampen vibrational forces in the second degree of freedom substantially parallel to the z-axis, or up and down.

FIGS. 6 and 7show side and perspective views, respectively, of a vibration dampening system100installed in an aircraft202having a T-tail configuration600according to various embodiments described herein. Although the precise structure of the horizontal stabilizer102and vertical stabilizer206to which the vibration dampening system100is attached in the T-tail configuration600may differ slightly from that of the cruciform tail configuration400described with respect toFIGS. 4 and 5below, the configuration and operation of the dampers104themselves is substantially the same in the T-tail configuration600shown inFIGS. 6 and 7. The main difference with the T-tail configuration600being that the horizontal stabilizer102is mounted at or proximate to the tip402of the vertical stabilizer206rather than at a location between the tip402and root404.

Turning now toFIG. 8, and alternative embodiment of the vibration dampening system100will be described. In the example shown inFIG. 8, the vibration dampening system100includes a damper104, configured as a passive damper105A, coupled to the front portion116of the horizontal stabilizer102, and a bank802of parallel dampers104coupled to a pivot point118of the horizontal stabilizer102. The bank802of dampers104replaces a single damper104in the various embodiments described above. The bank802of this example includes two dampers104A and104B. Each of the dampers104A and104B of the bank802is a visco-elastic damper, having a viscous damper106and a spring114. Each visco-elastic damper may be tuned or configured to mitigate vibrations at a particular frequency, which may differ from other targeted frequencies within the bank802of dampers104. In doing so, readily available visco-elastic dampers may be selected according to a desired target frequency and aggregated to create a bank802that mitigates vibrations at a frequency or frequencies that may differ from those of the individual dampers104within the bank802. Each damper104within a bank802may affect the operation of other dampers104within the bank802, however, the affect may be determined using known engineering techniques such that the bank802may be designed accordingly. The number and type of dampers104within a bank802may vary without departing from the scope of this departure. Although the bank802is shown inFIG. 8to be coupled to the pivot point118with a single passive damper105A coupled to the front portion116of the horizontal stabilizer102, a bank802may be utilized at any or all damper positions within the vibration dampening system100.

FIG. 9is a cross-sectional view of a visco-elastic damper (or passive damper105) having a concentric arrangement900according to various embodiments. The concentric arrangement900includes a plurality of concentrically arranged springs114A and114B positioned around a viscous damper106. According to this example, a first spring114A abuts a fixed bottom damper wall906at a first end902of the first spring114A. The first spring114A abuts a moveable top damper wall908at a second end904of the first spring114A. The moveable top damper wall908is connected to a damper piston110within a viscous fluid chamber108. The moveable top damper wall908is also connected to a horizontal stabilizer102at a connection point920and to a structure120at the fixed bottom damper wall906. Vibrational forces from the horizontal stabilizer102at the connection point920translate the damper piston110upwards and downwards, as indicated by the open arrows. Linear translation of the damper piston110is resisted upon by viscous fluid112within the viscous fluid chamber108while compressing the first spring114A. The second spring114B is positioned within the first spring114A and abuts a top surface910of the viscous fluid chamber106at a first inner spring end912and the moveable top damper wall908at a second inner spring end914. The visco-elastic damper104mitigates the vibrational forces in a manner described above. However, because of the different characteristics of the first spring114A and the second spring114B, the visco-elastic damper104may be tuned to target different vibration frequencies.

According to one embodiment, the displacement length of one of either the first spring114A or114B may be selected such that the moveable top damper wall908may translate a desired distance with resistance from one spring before the other spring is engaged. The successive engagement of the springs114A and114B as a function of the displacement of the damper piston110allows a variable natural frequency for the visco-elastic damper as a function of damper displacement. This aspect of the visco-elastic damper105can be tailored to passively absorb only the most critical vibration frequencies at different flight conditions or engine settings. As will be described in greater detail below, this passive damper105may be combined with a variable, or active damper, if a different damping coefficient is desired at an alternative flight condition.

As an example illustrating the functionality of a passive damper105having the concentric arrangement900shown and described above, the oscillation frequency of the horizontal stabilizer102exposed to prop wash at takeoff is most likely higher (due to higher fan/engine RPM) and the magnitude of the oscillatory displacements are larger than that of a cruise condition due to potentially faster exit airflow210and corresponding prop wash. A sequential spring system can be designed to operate at a high frequency/high displacement condition at takeoff during which both springs114A and114B might be engaged due to the higher displacements of the horizontal stabilizer102in this condition. The same system will also be able to operate optimally during cruise flight for which the horizontal stabilizer102is subjected to a lower frequency vibration and lower displacement due to different engine settings.

FIGS. 1-9have described operation of a vibration dampening system100utilizing passive dampers105. One advantage to using passive dampers is that they are relatively simple, inexpensive, and reliable. However, according to alternative embodiments, any or all dampers104within a vibration dampening system100described herein may be active dampers. For the purposes of this disclosure, “active dampers” are dynamically adaptable to vary damping characteristics according to real-time or estimated vibration states associated with the horizontal stabilizer102. According to some embodiments, active dampers utilize linear actuators to induce motion in a desired direction at a desired frequency or frequencies to mitigate corresponding vibrations. According to alternative embodiments, the active dampers may operate via altering a pressure of a fluid within a fluid chamber of a visco-elastic damper rather than utilizing a linear actuator. The induced motion created by the active dampers may be based on an actual real-time vibration state of the horizontal stabilizer as measured by one or more sensors or accelerometers, based on an estimated vibration state as predicted according to one or more aircraft parameters, or a combination thereof.

As described previously, the aircraft202operates at various flight/engine conditions which results in a constantly changing forcing function for various forces and moments acting on a horizontal stabilizer102subjected to distortion from the exit airflow210or other vibrational forces. An active dampening system has the advantage of being adaptable to the operating conditions and therefore providing a superior vibrational dampening behavior. It should be noted that active dampening system can be designed in a way to allow for redundancy of the systems (multiple identical, independent systems), as well as a potential passive backup system.

Looking atFIG. 10, a vibration dampening system100includes an active dampening system1000utilizing active dampers1002. The active dampers1002may include electric linear actuators1004that may be selectively activated via an actuator command1012from a vibration dampening computer1006to translate up and down or fore and aft to apply a mitigating force that counters the vibrational forces experienced by the horizontal stabilizer102. In this example, sensors1008are positioned on the horizontal stabilizer102and/or the fuselage204and corresponding structure120. The sensors1008may include any type and number of accelerometers or position sensors operative to measure and provide real-time vibration states associated with the vibrational forces to the vibration dampening computer1006as sensor input1010. It should be appreciated that “real-time vibration state” may be raw acceleration and position data or some resulting data calculated using the real-time acceleration and position data, such as frequency and amplitude of vibrations in the horizontal stabilizer102. The sensors1008take measurements along two principle axes of arbitrary orientation as long as the axes are substantially normal to each other and their offset from the horizontal stabilizer102or fuselage204is known.

Using the sensor input1010, the vibration dampening computer1006determines the optimal force (if the active dampening system1000is a purely electric damping system as described here with respect toFIG. 10) or natural frequency/damping ratio (if the active dampening system1000is a semi-passive system as discussed below with respect toFIG. 12) to be executed by the active dampers1002. The vibration dampening computer1006then provides the optimal force/position determination as an actuator command1012to the active dampers1002for actuation. Each active damper1002relays back a position versus time input signal as actuator feedback1014to the vibration dampening computer1006to ensure a closed control loop.

FIG. 11illustrates an alternative embodiment of an active dampening system1000utilizing active dampers1002. This example is similar to that described above with respect toFIG. 10, with the primary difference being the lack of sensor input1010. With the system described above, sensors1008are used to measure and provide real-time vibration states associated with the vibrational forces on the horizontal stabilizer102to the vibration dampening computer1006. However, in this alternative embodiment, rather than utilizing sensors1008, the vibration dampening computer1006utilizes one or more aircraft parameters1102corresponding to a current state of the aircraft202or one or more aircraft systems in order to determine an estimated vibration state. The vibration dampening computer1006utilizes the estimated vibration state in the same manner described above with respect to the real-time vibration state in order to determine an actuator command that mitigates the estimated vibrations. For the purposes of this embodiment, the broken lines corresponding to the sensors1008and associated sensor input1010are used to indicate an optional inclusion of sensors1008to be discussed in further detail below with respect to yet another alternative embodiment.

The aircraft parameters1102may include any number and type of information that may be applicable in determining the forces acting on the horizontal stabilizer102at any given time using known analytical techniques. For example, the aircraft parameters1102may include, but are not limited to, one or more engine settings, flight characteristics, aircraft characteristics, flight control settings, ambient parameters, or a combination thereof. Non-limiting examples of these illustrative aircraft parameters will now be provided. Engine settings may include engine revolutions per minute (RPM) of various spools, thrust settings, blade pitch or engine pitch (if variable). Flight characteristics may include air speed, angle of attack, pitch attitude, roll attitude, yaw attitude, and flight path angle. Aircraft characteristics may include aircraft weight and center of gravity. Flight control settings may include horizontal stabilizer incidence angle and elevator and trim-tab effective angles. Ambient parameters may include ambient pressure, temperature, and relative humidity.

Using these aircraft parameters1102, the vibration dampening computer1006can analyze the predicted vibrational forces acting on the horizontal stabilizer102and use the resulting estimated vibration state rather than the real-time vibration state measured by the sensors1008discussed with respect to the embodiment ofFIG. 10above to determine an appropriate actuator command1012. Referring now to the active dampening system1000ofFIG. 11, including the broken lines associated with the sensors1008, a third implementation of the active dampening system1000will be described.

In this third embodiment of the active dampening system1000, the vibration dampening computer1006utilizes aircraft parameters1102to determine the estimated vibration state and corresponding actuator command1012. In addition, the vibration dampening computer1006receives sensor input1010from the sensors1008measuring the real-time vibration state of the horizontal stabilizer102. Utilizing the real-time vibration state of the horizontal stabilizer102, the vibration dampening computer1006may determine a corresponding actuator command1012, which can be used to adjust the actuator command1012provided from the estimated vibration state. In this manner, the active dampening system1000may provide actuator commands1012based on predicted vibration states determined from numerous aircraft parameters, while measuring actual vibration states as they occur and making corrections accordingly. This type of dual-input system may be more complex than those previously described, but may operate faster and more accurately.

FIG. 12illustrates yet another alternative embodiment for an active dampening system1000. This system utilizes active visco-elastic dampers1202A and1202B (collectively and generally referred to as “active visco-elastic dampers1202”) to mitigate vibrational forces applied to the horizontal stabilizer102. This embodiment utilizes the vibration dampening computer1006to actively apply a mitigating force to the horizontal stabilizer102via the active visco-elastic dampers1202, similar to the systems discussed above with respect to the electric linear actuators1004ofFIGS. 10 and 11above. However, instead of employing electric linear actuators, the active dampening system1000of this example utilizes active visco-elastic dampers1202. Like the passive dampers105described above, the active visco-elastic dampers1202have an elastic element such as a spring114and a viscous damper, which is a variable viscous damper1206. The difference between the viscous damper106of previously described passive systems and the variable viscous damper1206of this embodiment that results in an active system is that the vibration dampening computer1006is operative to alter the pressure within the viscous fluid chamber108to move the damper piston110up and down as needed to actively mitigate the vibrational forces. Because the variable viscous damper1206allows for the pressure of the viscous fluid112within to be altered, the chamber within the variable viscous damper1206is referred to as a variable coefficient dampening element1224.

The active dampening system1000ofFIG. 12operates via the vibration dampening computer1006controlling the pressure within one or more associated variable coefficient dampening elements1224. This is done by actuating a variable flow valve, i.e. a flow control valve,1214within a fluid accumulator circuit1204A or1204B (referred to collectively and generally as a “fluid accumulator circuit1204”) to add or remove viscous fluid112and corresponding pressure from the variable viscous damper1206. Altering the pressure within the variable coefficient dampening element1224alters the damping coefficient (c), which in turn alters the dampening characteristics of the active visco-elastic damper1202.

To control the pressure of the viscous fluid112, a fluid accumulator circuit1204may be used. For illustrative purposes, the fluid accumulator circuit1204B that is fluidly coupled to the active visco-elastic damper1202B will be described. The fluid accumulator circuit1204B is outlined with a dashed line inFIG. 12. Similarly, a second fluid circuit, or fluid accumulator circuit1204A, is associated with the active visco-elastic damper1202A and is outlined with a dotted and dashed line for clarity purposes.

The fluid accumulator circuit1204B includes pressure sensors1208in series with a variable flow valve1214. The pressure sensors1208provide the vibration dampening computer1006with sensor input1210regarding the pressure within the variable coefficient dampening element1224and within the fluid accumulator circuit1204B. Actuation of the variable flow valve1214moves fluid from the accumulator1216to the variable coefficient dampening element1224and similarly releases pressure from the variable coefficient dampening element1224, allowing the vibration dampening computer1006to manage the dampening coefficient (c) within the active visco-elastic damper1202. The fluid accumulator circuit1204B additionally includes a reservoir1220for storage of viscous fluid112and a pump1218for charging the system.

The vibration dampening computer1006in this embodiment receives vibration state input1222, which is used to determine the pressure command1212for the variable flow valve1214. The vibration state input1222may include a real-time vibration state or corresponding acceleration and position data, which may be measured in real-time by a number of accelerometers and position sensors as described above with respect toFIG. 10. Alternatively, the vibration state input1222may include aircraft parameters1102, which may be used by the vibration dampening computer1006to determine an estimated vibration state as described above with respect toFIG. 11.

FIG. 12illustrates alternative embodiments for controlling multiple active visco-elastic dampers1202within an active dampening system1000. First, each active visco-elastic damper1202may be coupled to a separate fluid accumulator circuit1204. Ignoring the dotted lines inFIG. 12, active visco-elastic damper1202A is coupled to the fluid accumulator circuit1204A, while active visco-elastic damper1202B is coupled to the fluid accumulator circuit1204B. The vibration dampening computer1006controls both fluid accumulator circuits1204A and1204B. Alternatively, components of a fluid accumulator circuit1204that may be selected or designed to provide fluid to multiple active visco-elastic dampers1202may be shared. Looking atFIG. 12including the components drawn with a dotted line, but ignoring the fluid accumulator circuit1204A, a smaller fluid accumulator circuit supplying viscous fluid112to both active visco-elastic dampers1202A and1202B can be seen. It should be appreciated that the drawings have been simplified for clarity purposes and should not be considered limiting. There may be additional or fewer components than those shown inFIG. 12and the other drawings.

Turning toFIG. 13, a method for determining damper characteristics for mitigating vibration in a horizontal stabilizer102of an aircraft202according to various embodiments presented herein will be described. It should be appreciated that more or fewer operations may be performed than shown in the figures and described herein. These operations may also be performed in parallel, or in a different order than those described herein.FIG. 13shows a routine1300for determining the characteristics of the dampers of a vibration dampening system100. The routine1300begins at operation1302, where the inputs for the analysis are determined. The inputs may include a number of parameters, including but not limited to, the aircraft geometry and overall structural arrangement of the aircraft202, the material properties for the horizontal stabilizer102, engine core and fan performance parameters, a range of operating conditions corresponding to the design mission profile (i.e., Mach number and altitude), and weighting factors to be provided for the mission profile. The weighting factors indicate the relative significance of the various parameters from a noise and fatigue perspective.

From operation1302, the routine1300continues to operation1304, where an unsteady, powered computational fluid dynamics (CFD) analysis is performed using the analysis inputs determined at operation1302for a number of flight conditions attainable in a mission. The routine1300continues in parallel to operations1306and1308. At operation1306, the vibrational forces acting on the horizontal stabilizer102are determined by post-processing the CFD results. This process models the unsteady lift, drag, and pitching moments acting upon the horizontal stabilizer102immersed in the exit airflow210. In other words, operation1306estimates the lift, drag, and pitching moments as a function of time acting on the horizontal stabilizer102experiencing vibrational forces from the turbulent prop wash.

At operation1308, a dynamic finite element method (FEM) model is constructed using the horizontal stabilizer102geometry and mechanical properties determined in operation1302. From operations1306and1308, the routine1300continues to operation1310, where the dynamic FEM model is run for a range of vibrational forces (lift, drag, and pitching moments) obtained from the CFD analysis. At operation1312, a matrix of excitation frequency (ω0) at the root of the horizontal stabilizer102(i.e., at the pivot points118) for the range of flight conditions provided.

The routine1300continues to operation1314, where the linear ordinary differential equations of motion are solved to identify an adequate damping coefficient and damping ratio such that the vibration dampening system100would cause the vibrations to dampen substantially, within the requirements and technical limitations associated with a given design problem and what the general requirements of the aircraft may prescribe. According to one non-limiting implementation, the linear ordinary differential equations of motion are solved to identify an adequate damping coefficient (c) and damping ratio (ζ) such that (ζ) is greater than 1. At operation1316, the dampers104are selected or designed according to the damping coefficient (c) determined at operation1314, and the routine1300ends.

FIG. 14shows a routine1400for mitigating vibration in a horizontal stabilizer102of an aircraft202utilizing passive dampers105according to various embodiments presented herein. The routine1400begins at operation1402, where vibrational forces are received at the passive dampers105of a vibration dampening system100. Operations1404and1406occur in parallel. At operation1404, the elastic elements of a visco-elastic damper are compressed. As discussed above, depending on the configuration of the passive dampers105, there may be a single elastic element, such as a spring114, multiple elastic elements, and/or concentrically arranged springs114. In some embodiments, such as with the concentric arrangement900, the spring compression may be sequential, with a first spring compressing to a predetermined displacement before the second spring is engaged.

At operation1406, the damper pistons110associated with the viscous dampers108are moved from the applied vibrational forces. This movement presses the viscous fluid112through orifices113in the damper pistons110, which operates to resist or slow the movement of the damper pistons110. From operations1404and1406, the routine1400continues to operation1408, where the damper pistons110are returned in a direction of their starting positions via the force exerted by the compressed elastic elements. This resistant oscillatory movement of the passive dampers105effectively mitigates the vibrational forces applied to the horizontal stabilizer102.

FIG. 15shows a routine1500for mitigating vibration in a horizontal stabilizer102of an aircraft202utilizing active dampers1002according to various embodiments presented herein. It should be appreciated that the logical operations described herein may be implemented (1) as a sequence of computer implemented acts or program modules running on a computing system and/or (2) as interconnected machine logic circuits or circuit modules within the computing system. The implementation is a matter of choice dependent on the performance and other operating parameters of the computing system. Accordingly, the logical operations described herein are referred to variously as operations, structural devices, acts, or modules. These operations, structural devices, acts, and modules may be implemented in software, in firmware, hardware, in special purpose digital logic, and any combination thereof. It should also be appreciated that more or fewer operations may be performed than shown in the figures and described herein. These operations may also be performed in parallel, or in a different order than those described herein.

The routine1500begins at operation1502, where vibrational forces are received at the active dampers1002. At operation1504, the vibration dampening computer1006receives sensor input1010from the sensors1008if real-time vibration states are measured. If sensors1008are not used to measure real-time vibration states, then the vibration dampening computer1006receives aircraft parameters1102from the various aircraft systems on the aircraft202. As discussed above, according to one embodiment, the vibration dampening computer1006receives both sensor input1010and aircraft parameters1102.

From operation1504, the routine1500continues to operation1506, where the vibration state is determined, based either on real-time measurements from the sensors1008or estimated according to the received aircraft parameters1102. At operation1508, the vibration dampening computer1006determines an actuator command1012or a pressure command1212based on the vibration state, and provides the appropriate command to the active dampers1002or the variable flow valve1214. The vibration dampening computer1006receives feedback from the active dampers1002at operation1510, and the routine1500ends.

FIG. 16shows an illustrative computer architecture1600of a vibration dampening computer1006described above, capable of executing the software components described herein mitigating vibration in a horizontal stabilizer102in the manner presented above. The computer architecture1600includes a central processing unit1602(CPU), a system memory1608, including a random access memory1614(RAM) and a read-only memory1616(ROM), and a system bus1604that couples the memory to the CPU1602.

The CPU1602is a standard programmable processor that performs arithmetic and logical operations necessary for the operation of the computer architecture1600. The CPU1602may perform the necessary operations by transitioning from one discrete, physical state to the next through the manipulation of switching elements that differentiate between and change these states. Switching elements may generally include electronic circuits that maintain one of two binary states, such as flip-flops, and electronic circuits that provide an output state based on the logical combination of the states of one or more other switching elements, such as logic gates. These basic switching elements may be combined to create more complex logic circuits, including registers, adders-subtractors, arithmetic logic units, floating-point units, and the like.

The computer architecture1600also includes a mass storage device1610for storing an operating or control system1618, as well as specific application modules or other program modules, such as a vibration mitigation module1624operative to provide actuator commands1012and pressure commands1212to the dampers104according to the various embodiments described above. The mass storage device1610is connected to the CPU1602through a mass storage controller (not shown) connected to the bus1604. The mass storage device1610and its associated computer-readable media provide non-volatile storage for the computer architecture1600.

The computer architecture1600may store data on the mass storage device1610by transforming the physical state of the mass storage device to reflect the information being stored. The specific transformation of physical state may depend on various factors, in different implementations of this description. Examples of such factors may include, but are not limited to, the technology used to implement the mass storage device1610, whether the mass storage device is characterized as primary or secondary storage, and the like. For example, the computer architecture1600may store information to the mass storage device1610by issuing instructions through the storage controller to alter the magnetic characteristics of a particular location within a magnetic disk drive device, the reflective or refractive characteristics of a particular location in an optical storage device, or the electrical characteristics of a particular capacitor, transistor, or other discrete component in a solid-state storage device. Other transformations of physical media are possible without departing from the scope and spirit of the present description, with the foregoing examples provided only to facilitate this description. The computer architecture1600may further read information from the mass storage device1610by detecting the physical states or characteristics of one or more particular locations within the mass storage device.

Although the description of computer-readable media contained herein refers to a mass storage device, such as a hard disk or CD-ROM drive, it should be appreciated by those skilled in the art that computer-readable media can be any available computer storage media that can be accessed by the computer architecture1600. By way of example, and not limitation, computer-readable media may include volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules, or other data. For example, computer-readable media includes, but is not limited to, RAM, ROM, EPROM, EEPROM, flash memory or other solid state memory technology, CD-ROM, digital versatile disks (DVD), HD-DVD, BLU-RAY, or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the computer architecture1600.

According to various embodiments, the computer architecture1600may operate in a networked environment using logical connections to other aircraft systems and remote computers through a network, such as the network1620. The computer architecture1600may connect to the network1620through a network interface unit1606connected to the bus1604. It should be appreciated that the network interface unit1606may also be utilized to connect to other types of networks and remote computer systems. The computer architecture800may also include an input-output controller1622for receiving and processing input from a number of other devices, including a control display unit, a keyboard, mouse, electronic stylus, or touch screen that may be present on a connected display1612. Similarly, the input-output controller1622may provide output to the display1612, a printer, or other type of output device.