Patent Publication Number: US-2022235841-A1

Title: Two-mode tuned vibration absorber

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/295,561, entitled Two-Mode Tuned Vibration Absorber and filed Mar. 7, 2019, which claims the benefit of U.S. Provisional Patent Application Ser. No. 62/639,692, entitled Two-Mode Tuned Vibration Absorber and filed Mar. 7, 2018, the disclosures of which are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     1. Field of the Disclosure 
     Embodiments of this disclosure relate generally to structural vibration mitigation, and more specifically to a tuned vibration absorber (TVA) for propeller-driven aircraft. 
     2. Description of the Related Art 
     Many tuned vibration absorbers (TVAs) have been described in the prior art; however, these are generally designed to attenuate vibrations at one particular frequency. U.S. Pat. No. 8,511,601 to Dandaroy et al. discloses an elastomer-type tuned vibration absorber for attenuating a single frequency mode. 
     SUMMARY 
     In an embodiment, a dual-frequency vibration-reduction apparatus includes a beam having a longitudinal axis and a transverse axis perpendicular to the longitudinal axis, an attachment mechanism for mechanically coupling a portion of the beam to a structure, and one or more masses attached to the beam such that the dual-frequency vibration-reduction apparatus vibrates bi-modally at a first frequency and a second frequency for reducing vibrations of the structure at the first frequency and the second frequency. 
     In another embodiment, a paired dual-frequency vibration reducer includes two dual-frequency vibration-reduction apparatuses. A first dual-frequency vibration-reduction apparatus is mounted opposite a structure from a second dual-frequency vibration-reduction apparatus. Torsion loads from the first vibration-reduction apparatus are substantially cancelled by the second vibration-reduction apparatus to avoid imposing torsion loads onto the structure. 
     In yet another embodiment, a tuned vibration absorber for attenuating a primary frequency vibration of a structure and a secondary frequency vibration of the structure is provided. The tuned vibration absorber includes a beam having an attachment bracket for attaching the beam to the structure, a first mass adjacent a first end portion of the beam, and a second mass adjacent a second end portion of the beam. The second end portion is opposite the first end portion along a longitudinal axis of the beam. The first mass and the second mass are adapted to provide a bending mode of the beam for attenuating the primary frequency vibration of the structure. The first mass has an uneven mass distribution along a transverse axis of the beam, which is perpendicular to the longitudinal axis. The second mass also has an uneven mass distribution along the transverse axis. The uneven mass distributions of the first mass and the second mass are adapted to excite a torsional mode of the beam, based on the bending mode. The torsional mode is adapted for attenuating the secondary frequency vibration of the structure. 
     In another embodiment, a tuned vibration absorber for attenuating vibrations of a structure at a primary frequency and a secondary frequency is provided. The tuned vibration absorber includes a first beam having an attachment bracket for attaching the first beam to the structure. A second beam is adjacent a first end portion of the first beam and has a first mass and a second mass near opposite ends of the second beam. A third beam is adjacent a second end portion of the first beam. The second end portion is opposite the first end portion along a longitudinal axis of the first beam. The third beam has a third mass and a fourth mass near opposite ends of the third beam. The second beam and the third beam are positioned about the first beam to provide a bending mode vibration of the first beam for attenuating a vibration of the structure at the primary frequency. The first mass and the second mass are positioned about the second beam, and the third mass and the fourth mass are positioned about the third beam, to provide a bending mode vibration of the second beam and the third beam, respectively, for providing a torsion mode vibration of the first beam for attenuating a vibration of the structure at the secondary frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments of the present disclosure are described in detail below with reference to the attached drawing figures, which are incorporated by reference herein and wherein: 
         FIG. 1  is a perspective view of an embodiment of a tuned vibration absorber (TVA) that provides bi-modal vibrations adapted for attenuating vibrations of a structure at a primary frequency and a secondary frequency; 
         FIG. 2  is a perspective view of another embodiment of a TVA that provides bi-modal vibrations adapted for attenuating vibrations of a structure at a primary frequency and a secondary frequency; and 
         FIG. 3  shows the TVA of  FIG. 1  mounted to an exemplary aircraft structure. 
     
    
    
     DETAILED DESCRIPTION 
     Propeller-driven aircraft produce loud noise and vibration at frequencies corresponding to a frequency of the blades of a rotating propeller, known as a blade pass frequency (BPF), and at harmonic frequencies of the BPF. On aircraft having more than one propeller, the propellers typically have the same number of blades and rotate at the same rate such that the propellers have the same BPF. Also, aircraft usually operate at a consistent cruise speed with the same propeller rate. Therefore, the BPF for a given aircraft is usually a consistent and known frequency that may be targeted for vibration reduction of structures of the aircraft. 
     Embodiments of the present disclosure provide a tuned vibration absorber (TVA) that decreases vibrations at two frequencies (e.g., at the BPF and at one harmonic frequency of the BPF). The TVA is mounted to a structure, and vibration of the TVA is tuned to counteract vibrations of the structure. By vibrating at two modes, the TVA “absorbs” (i.e., reduces) vibrations of the structure corresponding to frequencies of the two modes. 
     Generally, a bending mode type of TVA includes a mass (e.g., a weight) on the end of a beam. The beam dimensions and material, as well as the amount and position of a mass attached to the end of the beam, together determine a natural frequency at which bending of the beam resonates. 
     In some embodiments, the TVA may be adapted to reduce vibration at the BPF and at a harmonic frequency of the BPF that dominates the acoustic spectra (together with the BPF) for a particular propeller aircraft. In certain embodiments, the TVA may be adapted to reduce vibration for different combinations of the BPF with various harmonic tones (e.g., by adjusting the amount and position of masses coupled to a beam of the TVA, as described below). In some embodiments, the TVA is adapted for reducing vibration at other low frequency tones (e.g., at frequencies not necessarily associated with a BPF). In other embodiments, the TVA is adapted for reducing vibration at BPF frequencies during different regimes of flight. For example, the TVA may be adapted to provide noise and vibration reduction during the climb and cruise phases of flight, or during both long range and high-speed cruise (e.g., situations where different propeller rotational speeds are used). 
       FIG. 1  is a perspective view of an exemplary TVA  100  having a secondary torsional mode vibration reducer. TVA  100  provides bi-modal vibrations including a bending mode vibration at a primary frequency and a torsional mode vibration at a secondary frequency for reducing vibrations of a structure at the primary and secondary frequencies, respectively. The torsion mode is characterized by twisting of a beam  110  around a longitudinal axis of the beam. In the example of a propeller-driven aircraft, the primary frequency is substantially matched with the BPF and the secondary frequency is substantially matched with a harmonic of the BPF. 
     TVA  100  includes beam  110 , an attachment bracket  115 , a first mass  121  located at or near a first end  111  of beam  110 , and a second mass  122  located at or near a second end  112  of beam  110 . The first and second ends  111 ,  112  are located at opposite ends of beam  110  along a longitudinal axis, as depicted in  FIG. 1 . Attachment bracket  115  is an example of an attachment mechanism adapted to enable a portion of TVA  100  to be mounted to a structure (not shown) for reducing vibrations of the structure. In a propeller-driven aircraft, the structure may include but is not limited to frames, stringers, skin portions, engine beam mounts, rudder pedals, and heads-up display (HUD) assemblies (see e.g.,  FIG. 3 ). Alternative attachment mechanisms (e.g., rivets, bolts, welding, and clamps) may be used without departing from the scope hereof. Attachment bracket  115  enables a mechanical coupling of beam  110  to the structure while allowing portions of the beam to flex or bend. In certain embodiments, attachment bracket  115  is located substantially near the middle of beam  110 . 
     First mass  121  and second mass  122  are adapted to provide a bending mode vibration of beam  110  at a target frequency. In certain embodiments, the target frequency substantially matches the BPF. For example, the BPF may be in the range from about 20 Hz to about 40 Hz for a helicopter and less than 200 Hz for a propeller driven aircraft. In certain embodiments, the BPF is from about 28-32 Hz for a helicopter and about 150 Hz or less for a propeller driven aircraft. The length, material, and cross-sectional dimensions of beam  110 , as well as the amount and location of first and second masses  121 ,  122  with respect to attachment bracket  115 , contribute to the bending mode vibration frequency of beam  110 . The amount of mass in first and second masses  121 ,  122  may each be independently adjusted (e.g., by swapping out objects that have different masses). Also, the positions of first and second masses  121 ,  122  may each be independently adjusted along the longitudinal axis of beam  110  for adjusting the bending mode vibration frequency of beam  110 . Thus, TVA  100  may be adapted for achieving a variety of target frequencies. 
     First mass  121  and second mass  122  each include an uneven distribution of mass along a transverse axis of beam  110 , the transverse axis being perpendicular to the longitudinal axis, as depicted in  FIG. 1 . The uneven distribution of mass may be provided in a variety of ways, including but not limited to objects of uneven shape or uneven density, or by forming a mass from a subset of smaller masses that are unevenly distributed (e.g., in number, size, density, and/or location). In the embodiment depicted in  FIG. 1 , first mass  121  includes a small portion  121 A and a large portion  121 B. Similarly, second mass  122  includes a small portion  122 A and a large portion  122 B. The large and small portions  121 A,  121 B of first mass  121  may be oriented oppositely along the transverse axis compared to the large and small portions  122 A,  122 B of second mass  122 , as depicted in  FIG. 1 ; however, in some embodiments, the uneven distribution of mass may be matched at both ends of beam  110 . In other words, both small portions  121 A,  122 A are on the same transverse side of beam  110  and both large portions  121 B,  122 B are both on the opposite side, transversely speaking, of beam  110 . 
     As depicted in  FIG. 1 , a first gap  121 C is provided separating small portion  121 A from large portion  121 B. Similarly, a second gap  122 C separates small portion  122 A from large portion  122 B. First and second gaps  121 C,  122 C are optional. Increasing the width of the gaps increases the moment of inertia for a given amount of mass, which alters the frequency of vibration. Alternatively, the width of the gaps is increased while the masses are modified to maintain a desired frequency of vibration. This enhances the effectiveness of the torsion mode. 
     The unevenly distributed masses of first mass  121  and second mass  122  provide a torsion mode vibration of beam  100  at a secondary frequency, which may be excited by the bending mode vibration at the primary frequency. The cross-sectional dimensions of beam  110  are adapted to provide the torsional mode at the secondary frequency using the same unevenly distributed masses (e.g., first mass  121  and second mass  122 ) that provide the bending mode vibration at the primary frequency. The torsion mode is predominately determined by the cross-section of beam  110 . Thus, beam  110  may be adapted to provide both a torsion mode and a bending mode at different frequencies. 
     First and second masses  121 ,  122  and beam  110  include slots for easily adjusting positions of the first and second masses  121 ,  122  along the longitudinal axis for tuning the bending mode to the primary frequency, and along the transverse axis for tuning the torsion mode to the secondary frequency. Thus, TVA  100  provides vibration absorber modes at two frequencies that are each with a broad frequency range. TVA  100  may be tuned to each of the two frequencies by adjusting the positions of the masses  121 ,  122  along the beam  110 . Additionally, first and second masses  121 ,  122  may be replaced with heavier or lighter masses to tune TVA  100 . 
     The bending mode frequency of TVA  100  may be adjusted by moving first and second masses  121 ,  122  closer to, or further away from, the middle of beam  110  along the longitudinal axis. For example, moving first and second masses  121 ,  122  closer to the middle of beam  110  (e.g., towards mounting bracket  115 ) provides a higher frequency bending-mode vibration. Conversely, moving first and second masses  121 ,  122  further away from the middle of beam  110  provides a lower frequency bending-mode vibration. Similarly, moving the small portions  121 A,  122 A and large portions  121 ,  122 B closer together along the transverse axis provides a higher frequency torsional-mode vibration, whereas moving the small portions  121 A,  122 A and large portions  121 ,  122 B further apart along the transverse axis provides a lower frequency torsional-mode vibration. 
     TVA  100  provides a dual-frequency vibration-reduction apparatus for attenuating low frequency vibrations (e.g., &lt;1 kHz) on any structure needing vibration reduction at two frequencies, which is particularly prevalent in propeller aircraft and helicopters. In certain embodiments, the primary frequency range is from about 20 Hz to about 200 Hz and the overall frequency range (primary and secondary frequencies) is from about 20 Hz to about 500 Hz. In some embodiments, the primary frequency range is from about 28 Hz to about 150 Hz and the overall frequency range is from about 28 Hz to about 450 Hz. Advantages of TVA  100  include that it is simple to tune for reducing vibrations at the primary and secondary frequencies and may be adjusted for treating many different frequency vibration problems, such as the BPF and the first harmonic, the BPF and the second harmonic, the BPF and the third harmonic, etc. By modifying first and second masses  121 ,  122 , and their positions, TVA  100  may be tuned to a wide range of frequencies. This enables the same TVA  100  to be used on a variety of structures and at a variety of locations for increasing the effectiveness of attenuating structural vibration. 
       FIG. 2  is a perspective view of an exemplary TVA  200  that provides bi-modal vibrations including a primary bending mode at a primary frequency and a secondary bending mode at a secondary frequency. TVA  200  includes a second TVA  220  and a third TVA  230  used as mass components of a first TVA  210 . Each of the first, second, and third TVAs  210 ,  220 ,  230  include a beam, a mounting bracket positioned substantially near the middle of each beam, and a pair of masses positioned substantially near the ends of each beam. In the case of first TVA  210 , the pair of masses positioned substantially near its ends are second TVA  220  and third TVA  230 . 
     First TVA  210  includes a first mounting bracket  215  for attaching a first beam  216  to a structure (not shown) as described above for TVA  100 . Second TVA  220  includes a first mass  221 , a second mass  222 , a second mounting bracket  225 , and a second beam  226 . First and second masses  221 ,  222  are preferably equal and adapted to produce a bending mode vibration of second beam  226  at the secondary frequency. Similarly, third TVA  230  includes a third mass  233 , a fourth mass  234 , a third mounting bracket  235 , and a third beam  236 . Third and fourth masses  233 ,  234  are preferably equal and adapted to produce a bending mode vibration of third beam  236  also at the secondary frequency. In embodiments, first, second, third, and fourth masses  221 ,  222 ,  233 ,  234  are equivalent in amount and position such that second TVA  220  and third TVA  230  each vibrate at the same frequency (e.g., the secondary frequency). 
     Second TVA  220  and third TVA  230  are adapted as masses for first beam  216  for providing a bending mode vibration along the longitudinal axis of first beam  216  to substantially match the primary frequency. Effectively, second TVA  220  and third TVA  230  act as a dead mass with respect to first TVA  210  at their respective ends of first TVA  210 , which allows the bending mode of first TVA  210  to be tuned to the primary frequency (e.g., the BPF). 
     In the example of a propeller-driven aircraft, the primary frequency is substantially matched with the BPF and the secondary frequency is substantially matched with a dominant harmonic of the BPF. 
     The bending mode frequency of first TVA  210  may be adjusted by moving second TVA  220  and third TVA  230  closer to, or further away from, the middle of first beam  216  along its longitudinal axis. For example, moving second TVA  220  and third TVA  230  closer to the middle of first beam  216  (e.g., towards first mounting bracket  215 ) provides a higher frequency vibration. Conversely, moving second TVA  220  and third TVA  230  further away from the middle of first beam  216  provides a lower frequency vibration. First and second masses  221 ,  222  may be closer to, or further away from, the middle of second beam  226  for tuning second TVA  220 . Similarly, third and fourth masses  233 ,  234  may be moved closer to, or further away from, the middle of third beam  236  for tuning third TVA  230 . 
     TVA  200  provides a dual-frequency vibration-reduction apparatus for attenuating low frequency vibrations (e.g., &lt;1 kHz) on any structure needing vibration reduction at two frequencies, which is particularly prevalent in propeller aircraft and helicopters. Advantages of TVA  200  include that it is simple to tune for reducing vibrations at the primary and secondary frequencies and may be adjusted for treating many different frequency vibration problems, such as the BPF and the first harmonic, the BPF and the second harmonic, the BPF and the third harmonic, etc. By modifying first, second, third, and fourth masses  221 ,  222 ,  233 ,  234 , and their positions, and by modifying positions of second TVA  220  and third TVA  230 , TVA  200  may be tuned to a wide range of frequencies (e.g., from about 20 Hz to about 500 Hz). This enables the same TVA  200  to be used on a variety of structures and at a variety of locations for increasing the effectiveness of attenuating structural vibration. In certain embodiments, the primary frequency range is from about 20 Hz to about 200 Hz and the overall frequency range (first and second frequencies) is from about 20 Hz to about 500 Hz. In some embodiments, the primary frequency range is from about 28 Hz to about 150 Hz and the overall frequency range is from about 28 Hz to about 450 Hz. 
       FIG. 3  shows TVA  100  mounted to a structure in an exemplary environment. The structure could be any of frames, stringers, skin portions, engine beam mounts, rudder pedals, and heads-up display (HUD) assemblies. As depicted in  FIG. 3 , two of TVA  100  are mounted as a pair on opposing sides of an aircraft frame  340  via bracket  115 . Mounting TVA  100  in pairs assists with preventing torsion loads being imposed on the frame. Alternatively, a single TVA  100  may be mounted to frame  340  with a frame stiffener mounted to the opposite side of frame  340  (not shown). 
     Many different arrangements of the various components depicted, as well as components not shown, are possible without departing from the spirit and scope of the present disclosure. Embodiments of the present disclosure have been described with the intent to be illustrative rather than restrictive. Alternative embodiments will become apparent to those skilled in the art that do not depart from its scope. A skilled artisan may develop alternative means of implementing the aforementioned improvements without departing from the scope of the present disclosure. 
     It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations and are contemplated within the scope of the claims. Not all operations listed in the various figures need be carried out in the specific order described.