Abstract:
A method for damping aeroelastic modes, including limit cycle oscillations (LCO), is implemented by determining a mass for a tuned mass damper (TMD) based on an modal frequency for a mode having a potentially positive growth rate and attaching a TMD to at least one attachment point with significant motion such that a damping axis of the tuned mass damper is substantially oriented in a direction aligned with the local modal deflection.

Description:
BACKGROUND INFORMATION 
       [0001]    1. Field 
         [0002]    Embodiments of the disclosure relate generally to the field of vibration reduction systems for aircraft and more particularly to a plurality of tuned mass dampers having viscous damping and mounted in multiple locations on the airframe with directional orientation determined to maximize damping of primary modes of aeroelastic limit cycle oscillation. 
         [0003]    2. Background 
         [0004]    Large modern commercial jet aircraft are designed with consideration of the aeroelastic stability of the aircraft. However, in certain cases aeroelastic designs may be subject to resonant oscillations created under certain aerodynamic conditions and at various speeds. Such oscillations can be localized in certain portions of the airframe or may be whole airframe aeroelastic modes including limit cycle oscillations (LCO) involving the nacelles, wing and fuselage. 
         [0005]    To minimize LCO, prior art aeroelastic solutions include payload and/or fuel restrictions, active modal suppression using control surfaces, adding ballast, vortex generators to change aerodynamic flow characteristics and structural changes (such as adding wing stiffness). Payload or fuel restrictions will typically reduce capability of the aircraft while active modal suppression requires extensive design and experimentation resulting in extended design lead time and may also affect performance. Use of ballast results in a significant increase in weight which may affect performance and may drive structural changes and inherent structural changes for stiffness also typically add weight. Vortex generators, while often effective for localized oscillation suppression are not typically effective for full airplane LCO 
         [0006]    It is therefore desirable to provide modal damping to satisfy aeroelastic stability and vibration requirements with low cost, simple design elements with minimized weight increase and no performance impact. 
       SUMMARY 
       [0007]    Embodiments disclosed herein provide a method for damping aeroelastic modes including whole airframe limit cycle oscillations (LCO) implemented by determining a mass for a tuned mass damper (TMD) based on a LCO aeroelastic mode frequency having a potentially positive growth rate and attaching a TMD to an attachment point such that a damping axis of the tuned mass damper is substantially oriented in a direction aligned with the modal deflection at a location having significant motion. 
         [0008]    In an example embodiment for the TMD, a tuned mass assembly incorporating a primary mass and tuning masses is concentrically mounted on a shaft with opposing concentric springs with a viscous damper for the tuned mass. The viscous damper includes magnets mounted on the tuned mass assembly and a case having a conductive, non-magnetic, surface mounted concentrically to the shaft adjacent the magnets for generation of eddy currents. The shaft is supported by end caps mounted to the aircraft attachment point. In one embodiment, one or more TMDs are mounted in aircraft nacelles for reciprocation on an axis oriented in an inboard and outboard direction. 
         [0009]    The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic pictorial view of a TMD for aeroelastic mode damping; 
           [0011]      FIG. 2  is a detailed schematic cutaway of the TMD of  FIG. 1 ; 
           [0012]      FIG. 3A  is a rear right isometric view of an example embodiment of an adjustable TMD confirmation tool as mounted on the nacelle inlet bulkhead; 
           [0013]      FIG. 3B  is a rear left isometric view of the TMD of  FIG. 3A  with the outer case removed; 
           [0014]      FIG. 3C  is a partial exploded view of the TMD of  FIG. 3A ; 
           [0015]      FIG. 4  is an isometric view of the primary mass; 
           [0016]      FIG. 5  is an isometric view of the primary mass assembly; 
           [0017]      FIG. 6  is an isometric view of the translating mass buildup; 
           [0018]      FIG. 7A  is an isometric view of the left end cap; 
           [0019]      FIG. 7B  is an isometric view of the right end cap showing the shaft capture bushing; 
           [0020]      FIG. 8  is an end view of the TMD as installed showing the support blade and link assembly; 
           [0021]      FIG. 9  is a rear section view of the nacelle bulkhead with the TMD installed; 
           [0022]      FIG. 10  is a side partial section view of the nacelle showing the TMD location as mounted and the section view line  FIG. 9-FIG .  9 ; 
           [0023]      FIG. 11  is a detailed rear view of the installed TMD from view ring 
           [0024]      FIG. 11  shown in  FIG. 9 ; 
           [0025]      FIG. 12  is a bottom view of the installed TMD; 
           [0026]      FIG. 13  is a top view of an example aircraft in which the TMD is installed for LCO suppression; 
           [0027]      FIG. 14  is a graph of aeroelastic growth rate vs. speed with suppression by the TMD illustrated; 
           [0028]      FIG. 15  is a graph of the increase in aeroelastic damping with respect to viscous damping in the TMD; 
           [0029]      FIG. 16  is graph of flutter mode damping for three selected TMD masses; and, 
           [0030]      FIG. 17  is a flowchart of the method for whole airframe limit cycle oscillation damping using the TMD embodiments described. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    Embodiments disclosed herein provide a tuned mass damper (TMD) to dampen aeroelastic modes involving the complete primary structure including whole airframe limit cycle oscillation (LCO) vibration involving the powerplant, wing and fuselage. In an example embodiment, the tuned mass damper is attached to the nacelle in the region of the lower fan case. In alternative embodiments, the damper may be attached to one or more of the airplane nacelles, (or other locations on the airframe). For the embodiment described in detail subsequently, the TMD is located in the lower forward nacelle inlet cowl in a horizontal position with inboard and outboard motion of the mass to maximize effectiveness for an airplane LCO mode which has significant displacement at this location and direction. The TMD frequency is equal to the modal frequency for which suppression is desired. The mass is mounted to move with minimal Coulomb friction and is provided with an optimized amount of viscous damping (proportional to velocity of the mass in the TMD and ˜2 to 5% for the example embodiment). The viscous damping may be obtained by means of pneumatics, hydraulics, or as in this embodiment, magnetic braking. 
         [0032]      FIG. 1  shows a production TMD  2  as implemented for a particularly defined aeroelastic mode and direction. The TMD  2  is mounted to an inlet bulkhead  3  in an engine nacelle for a large commercial aircraft. TMD  2  incorporates a primary mass  4  mounted on a shaft  5 . The mass  4  is constrained by springs  6  for reciprocal motion with the combination of weight of the mass and the spring constant of the springs defining a tuned frequency. Viscous damping is achieved with a magnetic element  7  which reacts with a conductive surface  8  adjacent and parallel to the directional motion of the mass creating eddy currents. In alternative embodiments, hydraulic or pneumatic systems associated with the mass for creation of the desired viscous damping. 
         [0033]    The overall position of the TMD  2  in the aircraft  1  is shown in  FIG. 2  with relative positioning of the inlet bulkhead  3  and showing the inboard and outboard direction of oscillation of the TMD with arrow  9   
         [0034]    As shown in  FIG. 3A  for an example embodiment, a configuration confirmation tool TMD  10  is mounted to an inlet bulkhead  3  in an engine nacelle for a large commercial aircraft. The features described herein provide a test and evaluation tool for confirmation of the configuration, sizing, damping and orientation requirements to achieve desired aeroelastic modal damping. A case  14  houses the TMD operating components and various brackets are employed to mount the TMD to the bulkhead as will be described in greater detail subsequently. As shown in  FIGS. 3B and 3C , the TMD  10  includes a translating mass buildup  16  which is supported for reciprocal oscillation on a shaft  18 . Orientation of the shaft establishes a damping axis  19  for the TMD. Springs  20 , concentric to the shaft and constrained with inner spring seats  22  and outer spring seats  24 , resiliently constrain the translating mass buildup  16  for resonant motion response. The shaft  18  is supported by left and right end caps  26  which mount to the bulkhead  3 . Case  14  has two separable halves  14   a  and  14   b  which include slotted reliefs  28  which allow upper and lower accelerometer posts  30  to protrude. 
         [0035]    The translating mass buildup  16  includes a primary mass  32  shown in  FIG. 4  which is employed in a primary mass assembly  33  shown in  FIG. 5 . The primary mass  32  has a central boss  34  with symmetrical cylindrical extensions  36 . The central boss includes flats  38  for mounting of the accelerometer posts  30 . Additionally, the central boss may include machined weight adjustment pockets  39 . Flanged grooves  40  in the central boss receive magnetic rings  42  for damping to be described in greater detail subsequently. For the embodiment shown, the magnetic rings are semi-cylindrical halves  42   a  and  42   b  joined with screws  43  for mounting within the flanged grooves  40 . Shoulders  44  on the primary mass receive the inner spring seats  22 . The primary mass  32  has a central axial bore  46  which incorporates a low friction bearing  48  receiving the shaft  18  (as seen in  FIGS. 3B and 3C ). 
         [0036]    The primary mass assembly  33  includes the primary mass  32  with magnetic rings  42  mounted in the flanged grooves  40 . Interconnecting half cylindrical ring magnet spacers  50  constrain the magnetic rings in the flanged grooves and provide physical spacing of the magnets from the case halves  14   a  and  14   b  in which the translating mass buildup oscillates. Additionally, outboard faces  52  of shoulders  44  incorporate threaded bores which receive studs  54 . 
         [0037]    The translating mass buildup  16  is shown in detail in  FIG. 6  includes tuning masses  56   a ,  56   b ,  56   c  and  56   d  which are removably mounted on the studs  54  concentrically over the cylindrical extensions  36  on each side of the primary mass. The tuning masses in the configuration confirmation tool version of the TMD provide adjustment for exact frequency matching in the TMD to the desired frequency of the aeroelastic mode to be damped. Lock washers  58  and jam nuts  60  secure the tuning masses to the studs. 
         [0038]      FIGS. 7A and 7B  show the end caps  26  which support the shaft  18  and mount the TMD to the aircraft nacelle inlet bulkhead. Bores  62  in lateral flanges  64  receive threaded ends  66  of the shaft  18  (as shown in  FIG. 3C ). As shown in  FIG. 7B  bushings  68  are inserted in the bores  62  to closely receive the shaft ends  66  which are constrained by nuts  69  and associated washers  70  (also seen in  FIG. 3 ). Transverse brackets  71  extend from the lateral flanges  64  for mounting to the bulkhead. As seen in  FIG. 7B , a raised disc  72  on inner surfaces  74  of the end cap receive and locate the outer spring seats  24  (as best seen in  FIG. 3 ). 
         [0039]    Mounting of the TMD employing the end caps is shown in  FIG. 8 . Transverse brackets  71  are attached to a strengthening plate  76  using fasteners  78 . The plate  76  then mounts to bulkhead  3 . Additional stability of the TMD is provided through blades  80  which extend from and are attached to the lateral flanges  64 . Attachment of the blades  80  to link fittings  82  with link assemblies  84  provides torsional stability for the cantilevered TMD. 
         [0040]    Details of the location and orientation of the TMD mounting for the example embodiment are shown in  FIGS. 9-13 .  FIG. 9  shows the inlet bulkhead  3  as a section view  FIG. 9-FIG .  9  in the engine nacelle  90  seen in  FIG. 10 . For the embodiment shown, the TMD mass translates inboard and outboard with respect to the aircraft as represented by arrow  92  in  FIGS. 9 and 11 . The TMD is a damped resonant oscillator with the resonant frequency established by the total mass of the translating mass assembly  16  and the spring constants of the springs  20 . Very precise tuning of the resonance can be achieved by variation of the tuning masses  56   a - 56   d  previously described. Viscous damping is accomplished for the embodiment shown by means of eddy currents developed by magnet rings  42  attached to the moving translating mass assembly  16  close to a stationary conductive metal surface of the case  14 . The magnet rings are replaceable in the mass buildup for altering the eddy current interaction with the conductive surface to adjust the viscous damping level. For the described embodiment viscous damping of approximately 2-5% is achieved. In alternative embodiment, damping could be achieved by fluid flow or other means. 
         [0041]    As seen in  FIG. 11 , the TMD with end caps  26  is mounted to strengthening plate  76  which is attached to the bulkhead  3 . For the example embodiment, secondary retention of the TMD under destructive load conditions that might result in expulsion of the TMD from the nacelle inlet is provided by retention cables  94  which attach to the blades  80  on each side of the TMD and are routed to retention fittings  96  connected to the bulkhead  3 . Additionally, cabling  98  for electrical connection to accelerometers mounted in the accelerometer posts  30  is routed through wire brackets  100 . 
         [0042]      FIG. 13  shows the mounting location of the TMDs of the example embodiment in the nacelles  90  extending from the wings  101  of an example aircraft  102 . For an aircraft in which whole airframe LCO with significant lateral (inboard/outboard) motion of the nacelles is present, the TMD of the described embodiment has been demonstrated to effectively reduce the growth rate of chosen aeroelastic modes and provide sufficient aeroelastic modal damping for acceptable aircraft flight characteristics. 
         [0043]    For the example aircraft, the aeroelastic mode of interest is shown in  FIG. 14 . The flutter mode at a principal resonant frequency, trace  120 , shows potential positive growth rates in the operating airspeed regime  122  and required reduction for acceptable aircraft performance. Implementation of the TMD as described for the embodiment disclosed provided a significant improvement in the aeroelastic mode growth rate  124  as shown in trace  126  in  FIG. 14 . 
         [0044]    Adjustment of the viscous damping in the TMD allows enhancement of the flutter mode damping as shown in  FIG. 15 . Curve  128  shows increasing flutter mode damping over a range of between 0.04 to 0.28 g with a maximum increase in the flutter mode damping at approximately 0.11 g viscous damping. 
         [0045]      FIG. 16  shows the flutter mode damping provided for various masses in the TMD of 75 lbs, trace  130 , 100 lbs, trace  132  and 150 lbs, trace  134 . A 100 lb mass provides an acceptable damping level over a full range of operating mass damper viscous damping values of 0.05 to 0.5 g. 
         [0046]    A method for adding aeroelastic damping by employing the embodiments described herein is shown in  FIG. 17 . Critical aeroelastic modes with potential undesirable growth rates are determined, step  1700 , and locations/directions that have significant modal deflections are indentified, step  1702 . An initial mass and stroke for at least one tuned mass damper is determined, step  1703 . Using a TMD configuration confirmation tool, the mass is adjusted with tuning masses, step  1704 . The tuned mass assembly is then concentrically mounted on a shaft for reciprocal motion, step  1706 , constrained by opposing springs, step  1708 . Removable viscous damping magnets are attached to the tuned mass assembly, step  1710 , and a metallic case is attached concentrically surrounding the tuned mass assembly and magnets for eddy current generation, step  1712 . Ends of the shaft are received in end caps to create the TMD, step  1714 , and the end caps are secured in a direction aligned with modal deflection at a location having significant modal deflection, step  1716 . One TMD attached to each nacelle inlet bulkhead on the aircraft with inboard/outboard orientation of the shafts to establish the damping axis for reciprocal oscillation of the masses are employed for the embodiments disclosed. Support for cantilever loads on the TMD may be added, step  1718 , and assembly lanyard retention cables to avoid expulsion of the TMD may be attached, step  1720 . Upon confirmation of the desired damping of LCO by the configuration confirmation tool TMD, a production TMD with a mass from the optimum tuned mass and optimized viscous damping of the configuration confirmation tool is defined, step  1722 . The production TMD is then mountable at the locations and orientations determined by the configuration confirmation tool TMD for LCO damping, step  1724 . 
         [0047]    Having now described various embodiments of the disclosure in detail as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present disclosure as defined in the following claims.