Patent Publication Number: US-9429103-B2

Title: Variable area fan nozzle with wall thickness distribution

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present disclosure is a Division of U.S. patent application Ser. No. 13/363,219, filed Jan. 31, 2012. 
    
    
     BACKGROUND 
     The present disclosure relates to gas turbine engines and, more particularly, to a variable area fan nozzle of a gas turbine engine. 
     A typical gas turbine engine includes a fan section that is driven by a core engine. The fan section drives air through an annular bypass passage. The air is discharged through a fan nozzle. In some designs, the fan nozzle is moveable to selectively change a nozzle exit area of the fan nozzle and influence operation of the fan section, for example. 
     SUMMARY 
     A method of controlling flutter of a fan nozzle according to an aspect of the present disclosure includes, for a given design of a fan nozzle, determining a vibration mode that causes a flutter characteristic of the fan nozzle and, in response to the determined vibration mode, establishing a wall thickness distribution of at least one wall of the fan nozzle which includes local thick portions and local thin portions that alter the flutter characteristic. 
     In a further non-limiting embodiment of any of the foregoing examples, the vibration mode includes a vibration frequency and a strain mode that is selected from a group consisting of bending strain and torsion strain. 
     In a further non-limiting embodiment of any of the foregoing examples, the flutter characteristic includes at least one of an amount of flutter and a location of flutter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of the disclosed examples will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
         FIG. 1  schematically illustrates an example gas turbine engine. 
         FIG. 2  illustrates a perspective view of the gas turbine engine of  FIG. 1 . 
         FIG. 3  illustrates a perspective, isolated view of a variable area fan nozzle. 
         FIG. 4  illustrates a cross-section through a variable area fan nozzle. 
         FIG. 5  illustrates a representation of a wall thickness distribution that includes local thick portions and local thin portions. 
         FIG. 6  illustrates an example fiber-reinforced polymer matrix composite material of a variable area fan nozzle. 
         FIGS. 7A and 7B  illustrate finite element analysis of fan nozzles under a bending strain mode. 
         FIGS. 8A and 8B  illustrate another finite element analysis of fan nozzles under a torsion strain mode. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  schematically illustrates an example gas turbine engine  20 , and  FIG. 2  illustrates a perspective view of the gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-spool turbofan that generally incorporates a fan section  22  and a core engine CE that includes a compressor section  24 , a combustor section  26  and a turbine section  28  generally disposed along an engine central longitudinal axis A. Although depicted as a turbofan gas turbine engine in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines including three-spool architectures. 
     The engine  20  includes a low pressure spool  30  and a high pressure spool  32  mounted for rotation about the engine central longitudinal axis A relative to an engine static structure  36  via several bearing systems  38 . It should be understood that various bearing systems  38  at various locations may alternatively or additionally be provided. 
     The low speed spool  30  generally includes an inner shaft  40  that typically couples a fan  42 , a low pressure compressor  44  and a low pressure turbine  46 . In the illustrated embodiment, the inner shaft  40  is connected to the fan  42  through a geared architecture  48  to drive the fan  42  at a speed different than the low speed spool  30 , in this case slower than the spool  30 . The high speed spool  32  includes an outer shaft  50  that couples a high pressure compressor  52  and high pressure turbine  54 . An annular combustor  56  is arranged between the high pressure compressor  52  and the high pressure turbine  54 . The inner shaft  40  and the outer shaft  50  are concentric and rotate via bearing systems  38  about the engine central longitudinal axis A which is typically collinear with their longitudinal axes. 
     A fan nacelle  58  extends around the fan  42 . A core nacelle  60  extends around the core engine CE. The fan nacelle  58  and the core nacelle  60  define a bypass passage or duct B therebetween. A variable area fan nozzle (VAFN)  62  extends at least partially around the central longitudinal axis A and defines an exit area  64  of the bypass passage B. The VAFN  62  is selectively movable in a known manner to vary the exit area  64 . 
     The compressor section  24  moves air along a core flowpath for compression and presentation into the combustor section  26 , then expansion through the turbine section  28 . The core airflow is compressed by the low pressure compressor  44  and the high pressure compressor  52 , mixed and burned with fuel in the combustor  56 , then expanded over the high pressure turbine  54  and low pressure turbine  46 . The turbines  46 ,  54  rotationally drive the respective low pressure spool  30  and high pressure spool  32  in response to the expansion. 
     In a further example, the engine  20  is a high-bypass geared aircraft engine that has a bypass ratio that is greater than about six (6), with an example embodiment being greater than ten (10), the gear assembly  48  is an epicyclic gear train, such as a planetary or star gear system or other gear system, with a gear reduction ratio of greater than about 2.3:1 or greater than about 2.5:1 and the low pressure turbine  46  has a pressure ratio that is greater than about 5. Low pressure turbine  46  pressure ratio is pressure measured prior to inlet of low pressure turbine  46  as related to the pressure at the outlet of the low pressure turbine  46  prior to an exhaust nozzle. It should be understood, however, that the above parameters are only exemplary. 
     Most of the thrust is provided through the bypass passage B due to the high bypass ratio. The fan section  22  of the engine  20  is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft, with the engine at its best fuel consumption—also known as “bucket cruise Thrust Specific Fuel Consumption (‘TSFC’)”—is the industry standard parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point. “Low fan pressure ratio” is the pressure ratio across the fan blade alone, without a Fan Exit Guide Vane (“FEGV”) system. The low fan pressure ratio as disclosed herein according to one non-limiting embodiment is less than about 1.45. “Low corrected fan tip speed” is the actual fan tip speed in ft/sec divided by an industry standard temperature correction of [(Tambient deg R)/518.7)^0.5]. The “Low corrected fan tip speed” as disclosed herein according to one non-limiting embodiment is less than about 1150 ft/second. 
       FIG. 3  illustrates a perspective, isolated view of selected portions of the VAFN  62 . As shown, the VAFN  62  is a bifurcated design that includes a first VAFN section  62   a  and a second VAFN section  62   b . In general, each of the VAFN sections  62   a  and  62   b  are semi-circular and extend around the central longitudinal axis A of the engine  20 . 
       FIG. 4  schematically illustrates a cross-section through the first VAFN section  62   a . It is to be understood that the geometry of the first VAFN section  62   a  is exaggerated for the purpose of this description and is not a limitation to the disclosed geometry. It is to be further understood that the second VAFN section  62   b  is of similar construction and geometry as the first VAFN section  62   a . In this example, the VAFN section  62   a  includes a body  70  that extends at least partially around the central longitudinal axis A of the engine  20 . The body  70  includes a radially outer wall  72  and a radially inner wall  74  that together form the overall shape of the body  70  and thus the VAFN section  62   a . In this example, the body  70  generally has an airfoil cross-sectional shape. That is, the walls  72  and  74  of the body  70  form a wing-like shape to provide a reaction force via Bernoulli&#39;s principle with regard to air flow over the walls  72  and  74 . 
     In this example, the VAFN section  62   a  is a hollow structure. Thus, the radially inner wall  74  is radially-inwardly spaced from the radially outer wall  72  such that there is an open space  76  between the walls  72  and  74 . Optionally, the VAFN section  62   a  includes supports  78  extending between walls  72  and  74  from wall  72  to wall  74  to stiffen and strengthen the structure. 
     In operation, the first VAFN section  62   a  and the second VAFN section  62   b  are selectively moveable to vary the exit area  64  of the engine  20 . For example, the VAFN sections  62   a  and  62   b  are movable between at least a stowed position and a deployed position such that in the deployed position a greater exit area  64  is provided. 
     Airflow through the bypass passage B flows over the radially inner wall  74  and, at least when the VAFN  62  is in the deployed position, also over the radially outer wall  72 . The airflow over the VAFN  62  causes vibrations in the VAFN sections  62   a  and  62   b . Depending upon, for example, the weight of the VAFN  62 , certain vibration modes (i.e., frequencies), can cause the VAFN sections  62   a  and  62   b  to flutter. Flutter is an aeroelastic event where the aerodynamic forces due to vibration, in combination with the natural mode of vibration, produce a significant and periodic motion in the VAFN sections  62   a  and  62   b . The flutter can, in turn, elevate stresses at certain locations, cause the VAFN  62  to contact the fan nacelle  58  or damage the VAFN  62 . As will be described in more detail below, the disclosed VAFN  62  includes a strategic wall thickness distribution to reduce flutter and thereby enhance the durability of the VAFN  62  and engine  20 . 
       FIG. 5  shows a representation of a wall thickness distribution  80  of the radially outer wall  72 , the radially inner wall  74  or both of the first VAFN section  62   a . That is, the walls  72  and  74  may have equivalent or similar wall thickness distribution  80  or, alternatively, have dissimilar wall thickness distributions  80 . In that regard, in one embodiment, the radially outer wall  72  has a first wall thickness distribution and the radially inner wall  74  has a second wall thickness distribution that is different than the first wall thickness distribution. 
     The wall thickness distribution  80  is represented by a plurality of thickness zones  82 . As an example, the walls  72  and  74  are made of a fiber-reinforced polymer matrix material and the thickness zones  82  represent one or more layers or plies in a multi-layered structure of the material. In that regard, each of the layers or plies that represents the thickness zone  82  is selected to have predetermined thickness such that when the layers of all of the thickness zones  82  are stacked and formed into the wall  72  or  74 , the difference in the individual thicknesses of the layers produce local thick portions  84 / 86 , and local thin portions  88 / 90 . 
     In this example, each of the thickness zones  82  is represented as a percent thickness, X %, Y % or Z %, of a preset maximum thickness of the thickness zones  82 . As an example, X %&lt;Y %&lt;Z %. In a further example, X % is less than 40%, Y % is from 40-60% and Z % is greater than 60%. In a further example, the present maximum thickness of one thickness zone  82  is 0.5 inches (1.27 centimeters) or less. In embodiments, the thickness of a layer or ply, and thus the percent thickness, is established by changing the fiber density, fiber volume percent or area weight of polymer of the layer or ply. Alternatively, each layer or ply is made up of sub-layers or sub-plies, and the number of sub-layers or sub-plies is changed to alter percent thickness. 
     For a given location or portion of the wall  72  or  74 , the overall thickness, as represented in  FIG. 5 , is determined by the sum of the thicknesses of the thickness zones  82  in the particular location. Thus, the local thick portions  84 / 86  have thicknesses represented at  84   a / 86   a , and the local thin portions  88 / 90  have thicknesses represented at  88   a / 90   a . That is, the thickness  84   a  is the sum of the thickness zones  82  (in the vertical column) of X %, Y %, Z %, Y % and X %. Similarly, the thicknesses  86   a ,  88   a  and  90   a  are determined by the sum of the thickness zones  82  in the respective vertical columns at those locations. 
     In a further example, the local thin portions  88 / 90  have a minimum thickness, thickness  88   a , and the local thick portions  84 / 86  have a maximum thickness, thickness  84   a . The minimum thickness  88   a  is 90% or less of the maximum thickness  84   a . In a further example, the minimum thickness  88   a  is 80% or less of the maximum thickness  84   a . In another embodiment, the minimum thickness  88   a  is 70% or less of the maximum thickness  84   a , and in a further example the minimum thickness  88   a  is 60% or less of the maximum thickness  86   a . Additionally, in a further example, the arrangement of the local thick portions  84 / 86  and the local thin portions  88 / 90  with respect to location from the leading end to the trailing end of the VAFN section  62   a  is a repeating pattern or symmetric pattern. 
     The individual thicknesses of the zones  82 , and thus the local thick portions  84 / 86  and local thin portions  88 / 90 , are selected to control a flutter characteristic of the VAFN  62 . In one embodiment, for a given design of a fan nozzle, which may be a fan nozzle or a variable area fan nozzle, a vibration mode is determined that causes a flutter characteristic of the fan nozzle. As an example, the flutter characteristic includes an amount of flutter, location of flutter or both. The vibration mode, as used herein, includes at least one of a vibration frequency and a strain mode, such as bending strain or torsion strain. Thus, for a given vibration frequency and a given strain mode, the given design of the fan nozzle can be analyzed, such as by using finite element analysis, to determine one or more flutter characteristics of the fan nozzle. 
     In response to the determined vibration mode, the wall thickness distribution  80  is established such that the radially outer wall  72 , the radially inner wall  74  or both include local thick portions  84 / 86  and local thin portions  88 / 90  that alter the flutter characteristic. Without being bound to any particular theory, at a given location, the local thickness of the respective wall  72  and/or  74  influences the flutter characteristic at that location. In general, at each local location, the local wall thickness is reduced or minimized to alter the flutter characteristic and thus also reduce or minimize the overall weight. 
     In a further example, the fiber-reinforced polymer matrix material of the walls  72  and  74  of the VAFN  62  are made of a multi-layered structure, wherein each layer includes unidirectionally oriented fibers. In one example, the multi-layered structure includes 0°/90° cross-oriented layers and +/−45° cross-oriented layers. As shown in  FIG. 6 , the radially outer wall  72  in a further example includes a region  92  of 0°/90° cross-oriented layers and a region  94  of +/−45° cross-oriented layers. It is to be understood that the disclosed example is also representative of the radially inner wall  74 . 
       FIG. 7A  illustrates an example finite element vibration mode analysis of a given VAFN section (V) that does not include the above-described wall thickness distribution  80 , represented as a two-dimensional projection. At a given vibration mode frequency, the contours  96  represent regions of differing strain energy. In this example, the strain energy is a bending strain. In general, there is a relatively high amount of bending strain at a leading edge LE of the VAFN section (V). 
       FIG. 7B  illustrates the first VAFN section  62   a  with the wall thickness distribution  80 , represented as a two-dimensional projection. As shown, there is less bending strain energy at the leading edge LE and thus less flutter than in the given design (V). 
     Similarly,  FIGS. 8A and 8B  show the given VAFN design (V) and the first VAFN section  62   a  at a given vibration mode frequency under torsional strain. In the given VAFN design (V) shown in  FIG. 8A , there is a significant gradient of torsional strain energy from the leading edge LE to the trailing edge TE. However, as shown in  FIG. 8B , the first VAFN section  62   a  that has the wall thickness distribution  80  reduces the gradient from the leading edge to the trailing edge. Thus, in the examples shown in  FIGS. 7A and 7B , the disclosed wall thickness distribution  80  alters the location of the flutter characteristic, and in the examples shown in  FIGS. 8A and 8B , the disclosed wall thickness distribution  80  alters the amount and location of the flutter characteristic. 
     Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
     The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the essence of this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.