Abstract:
A gas turbine engine includes first and second shafts rotatable about a common axis. A first turbine section is supported on the first shaft. Second compressor and turbine sections are supported on the second shaft. The gas turbine engine includes a fan. A first compressor section is arranged in an axial flow relationship with the second compressor and the first and second turbines. A geared architecture operatively connects the first shaft and the fan. An inducer operative couples to the gear train.

Description:
BACKGROUND 
       [0001]    This disclosure relates to a geared architecture for a gas turbine engine. 
         [0002]    One type of geared turbofan engine includes a two-spool arrangement in which a low spool, which supports a low pressure turbine section, is coupled to a fan via a planetary gear train. A high pressure spool supports a high pressure turbine section. Low and high pressure compressor sections are respectively supported by the low and high spools. 
         [0003]    The planetary gear train includes a planetary gear set surrounding and intermeshing with a centrally located sun gear that is connected to the low spool. A ring gear circumscribes and intermeshes with the planetary gears. A fan shaft supports the fan. The fan shaft is connected to either the planetary gears or the ring gear, and the other of the planetary gears and ring gear is grounded to the engine static structure. This type of planetary gear arrangement can limit the design speeds of and configuration of stages in the low and high pressure turbine sections. 
       SUMMARY 
       [0004]    In one exemplary embodiment, a gas turbine engine includes first and second shafts rotatable about a common axis. A first turbine section is supported on the first shaft. Second compressor and turbine sections are supported on the second shaft. The gas turbine engine includes a fan. A first compressor section is arranged in an axial flow relationship with the second compressor and the first and second turbines. A geared architecture operatively connects the first shaft and the fan. An inducer operative couples to the gear train. 
         [0005]    In a further embodiment of any of the above, the gas turbine engine includes a bypass flow path and a core flow path. The first and second compressor and turbine sections are arranged in the core flow path, and the fan extends into the bypass flow path. The inducer is arranged in the core flow path outside of the bypass flow path and upstream from the first compressor section. 
         [0006]    In a further embodiment of any of the above, first and second shafts respectively provide low and high spools. The first compressor and turbine sections are low pressure compressor and turbine sections. The second compressor and turbine sections are high pressure compressor and turbine sections. 
         [0007]    In a further embodiment of any of the above, the geared architecture includes first and second gear trains. The first gear train is an epicyclic gear train, and the second gear train is configured to provide a speed reduction. 
         [0008]    In a further embodiment of any of the above, the inducer is coupled to the first gear train. The epicyclic gear train is a differential gear train that includes a sun gear. Planetary gears are arranged about and intermesh with the sun gear. A ring gear circumscribes, and intermeshes with the planetary gears. 
         [0009]    In a further embodiment of any of the above, the inducer is rotationally fixed relative to the ring gear. 
         [0010]    In a further embodiment of any of the above, the inducer is rotationally fixed relative to the star gear. 
         [0011]    In a further embodiment of any of the above, the inducer is coupled to the second gear train. 
         [0012]    In a further embodiment of any of the above, the inducer is rotationally fixed relative to the fan. 
         [0013]    In a further embodiment of any of the above, the inducer is configured to rotate at a different rotational speed than the first compressor section. 
         [0014]    In a further embodiment of any of the above, the inducer is configured to rotate at a different rotational speed than the fan. 
         [0015]    In a further embodiment of any of the above, the inducer is configured to rotate at a different rotational speed than the fan. 
         [0016]    In one exemplary embodiment, a gas turbine engine includes first and second shafts rotatable about a common axis. A first turbine section is supported on the first shaft. Second compressor and turbine sections are supported on the second shaft. The gas turbine engine includes a fan. A first compressor section is arranged in an axial flow relationship with the second compressor and the first and second turbines. A geared architecture operatively connects the first shaft and the fan. An inducer is operatively coupled to the gear train. The gas turbine engine includes a bypass flow path and a core flow path. The first and second compressor and turbine sections are arranged in the core flow path, and the fan extends into the bypass flow path. The inducer is arranged in the core flow path outside of the bypass flow path and upstream from the first compressor section. First and second shafts respectively provide low and high spools. The first compressor and turbine sections are low pressure compressor and turbine sections, and the second compressor and turbine sections are high pressure compressor and turbine sections. The geared architecture includes first and second gear trains. The first gear train is an epicyclic gear train, and the second gear train is configured to provide a speed reduction. 
         [0017]    In a further embodiment of any of the above, the inducer is coupled to the first gear train. The epicyclic gear train is a differential gear train that includes a sun gear. Planetary gears are arranged about and intermesh with the sun gear. A ring gear circumscribes and intermeshes with the planetary gears. 
         [0018]    In a further embodiment of any of the above, the inducer is rotationally fixed relative to the ring gear. 
         [0019]    In a further embodiment of any of the above, the inducer is rotationally fixed relative to the star gear. 
         [0020]    In a further embodiment of any of the above, the inducer is coupled to the second gear train. 
         [0021]    In a further embodiment of any of the above, the inducer is rotationally fixed relative to the fan. 
         [0022]    In a further embodiment of any of the above, the inducer is configured to rotate at a different rotational speed than the first compressor section. 
         [0023]    In a further embodiment of any of the above, the inducer is configured to rotate at a different rotational speed than the fan. 
         [0024]    In a further embodiment of any of the above, the inducer is configured to rotate at a different rotational speed than the fan. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The disclosure can be further understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
           [0026]      FIG. 1  schematically illustrates a gas turbine engine embodiment. 
           [0027]      FIG. 2  is a schematic view of a geared architecture embodiment for the engine shown in  FIG. 1 . 
           [0028]      FIG. 3  is a schematic view of another geared architecture embodiment. 
           [0029]      FIG. 4  is a schematic view of a geared architecture embodiment with an inducer. 
           [0030]      FIG. 5  is a schematic view of yet another geared architecture embodiment. 
           [0031]      FIG. 6  is a schematic view of another geared architecture embodiment with an inducer. 
           [0032]      FIG. 7  is a schematic view of yet another geared architecture embodiment with an inducer. 
           [0033]      FIG. 8  is a schematic view of still another geared architecture embodiment with an inducer. 
           [0034]      FIG. 9A  is a schematic view of an epicyclic gear train having a first example geometry ratio. 
           [0035]      FIG. 9B  is a schematic view of an epicyclic gear train having a second example geometry ratio. 
           [0036]      FIG. 9C  is a schematic view of an epicyclic gear train having a third example geometry ratio. 
           [0037]      FIG. 10  is a nomograph depicting the interrelationship of speeds of epicyclic gear train components for a given geometry ratio. 
           [0038]      FIG. 11A  is a schematic view of an epicyclic gear train having the first geometry ratio with a carrier rotating in the opposite direction to that shown in  FIG. 9A . 
           [0039]      FIG. 11B  is a schematic view of an epicyclic gear train having the second geometry ratio with a carrier rotating in the opposite direction to that shown in  FIG. 9B . 
           [0040]      FIG. 11C  is a schematic view of an epicyclic gear train having the third geometry ratio with a carrier rotating in the opposite direction to that shown in  FIG. 9C . 
       
    
    
     DETAILED DESCRIPTION 
       [0041]      FIG. 1  schematically illustrates a gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-spool turbofan that generally incorporates a fan section  22 , a compressor section  24 , a combustor section  26  and a turbine section  28 . Alternative engines might include an augmentor section (not shown) among other systems or features. The fan section  22  drives air along a bypass flowpath B while the compressor section  24  drives air along a core flowpath C for compression and communication into the combustor section  26  then expansion through the turbine section  28 . 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. 
         [0042]    The engine  20  generally includes a low speed spool  30  and a high speed spool  32  mounted for rotation about an 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. 
         [0043]    The low speed spool  30  generally includes an inner shaft  40  that interconnects a fan  42 , a low pressure (or first) compressor section  44  and a low pressure (or first) turbine section  46 . The inner shaft  40  is connected to the fan  42  through a geared architecture  48  to drive the fan  42  at a lower speed than the low speed spool  30 . The high speed spool  32  includes an outer shaft  50  that interconnects a high pressure (or second) compressor section  52  and high pressure (or second) turbine section  54 . A combustor  56  is arranged between the high pressure compressor  52  and the high pressure turbine  54 . A mid-turbine frame  57  of the engine static structure  36  is arranged generally between the high pressure turbine  54  and the low pressure turbine  46 . The mid-turbine frame  57  supports one or more bearing systems  38  in the turbine section  28 . 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 collinear with their longitudinal axes. As used herein, a “high pressure” compressor or turbine experiences a higher pressure than a corresponding “low pressure” compressor or turbine. 
         [0044]    The core airflow C is compressed by the low pressure compressor  44  then 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 mid-turbine frame  57  includes airfoils  59  which are in the core airflow path. The turbines  46 ,  54  rotationally drive the respective low speed spool  30  and high speed spool  32  in response to the expansion. 
         [0045]    The engine  20  in one example is a high-bypass geared aircraft engine. In a further example, the engine  20  bypass ratio is greater than about six (6), with an example embodiment being greater than ten (10), the geared architecture  48  is an epicyclic gear train, such as a star gear system or other gear system, with a gear reduction ratio of greater than about 2.3 and the low pressure turbine  46  has a pressure ratio that is greater than about 5. In one disclosed embodiment, the engine  20  bypass ratio is greater than about ten (10:1), the fan diameter is significantly larger than that of the low pressure compressor  44 , and the low pressure turbine  46  has a pressure ratio that is greater than about 5:1. 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 of one embodiment of a geared architecture engine and that the present invention is applicable to other gas turbine engines including direct drive turbofans. 
         [0046]    A significant amount of thrust is provided by the bypass flow 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 per hour divided by lbf of thrust the engine produces at that minimum point. “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 [(Tram 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. 
         [0047]    An example geared architecture  48  for the engine  20  is shown in  FIG. 2 . Generally, the engine static structure  36  supports the inner and outer shafts  40 ,  50  for rotation about the axis A. The outer shaft  50  supports the high pressure compressor section  52  and the high pressure turbine section  54 , which is arranged upstream from the mid turbine frame  59 . 
         [0048]    The inner shaft  40  is coupled to the geared architecture  48 , which is an epicyclic gear train  60  configured in a differential arrangement. The gear train  60  includes planetary gears  64  supported by a carrier  62 , which is connected to the inner shaft  40  that supports the low pressure turbine  46 . A sun gear  66  is centrally arranged relative to and intermeshes with the planetary gears  64 . A ring gear  70  circumscribes and intermeshes with the planetary gears  64 . In the example, a fan shaft  72 , which is connected to the fan  42 , is rotationally fixed relative to the ring gear  70 . The low pressure compressor  44  is supported by a low pressure compressor rotor  68 , which is connected to the sun gear  66  in the example. 
         [0049]    The carrier  62  is rotationally driven by the low pressure turbine  46  through the inner shaft  40 . The planetary gears  64  provide the differential input to the fan shaft  72  and low pressure compressor rotor  68  based upon the geometry ratio, which is discussed in detail in connection with  FIGS. 9A-10 . 
         [0050]    Another example geared architecture  148  for the engine  120  is shown in  FIG. 3 . The engine static structure  136  supports the inner and outer shafts  140 ,  150  for rotation about the axis A. The outer shaft  150  supports the high pressure compressor section  152  and the high pressure turbine section  154 , which is arranged upstream from the mid turbine frame  159 . 
         [0051]    The inner shaft  140  is coupled to the geared architecture  148 , which is an epicyclic gear train  160  configured in a differential arrangement. The gear train  160  includes planetary gears  164  supported by a carrier  162 , which is connected to the inner shaft  140  that supports the low pressure turbine  146 . A sun gear  166  is centrally arranged relative to and intermeshes with the planetary gears  164 . A ring gear  170  circumscribes and intermeshes with the planetary gears  164 . In the example, a fan shaft  172 , which is connected to the fan  142 , is rotationally fixed relative to the ring gear  170 . The low pressure compressor  144  is supported by a low pressure compressor rotor  168 , which is connected to the sun gear  166  in the example. 
         [0052]    The carrier  162  is rotationally driven by the low pressure turbine  146  through the inner shaft  140 . The planetary gears  164  provide the differential input to the fan shaft  172  and low pressure compressor rotor  168  based upon the geometry ratio. The geared architecture  148  includes an additional speed change device  74  interconnecting the inner shaft  140  and the gear train  160 . Higher low pressure turbine section rotational speeds are attainable with the additional speed change device  74 , enabling the use of fewer turbine stages in the low pressure turbine section. The speed change device  74  may be a geared arrangement and/or a hydraulic arrangement for reducing the rotational speed from the low pressure turbine section  146  to the fan  142  and low pressure compressor section  144 . 
         [0053]    Another example geared architecture  248  for the engine  220  is shown in  FIG. 4 . The engine static structure  236  supports the inner and outer shafts  240 ,  250  for rotation about the axis A. The outer shaft  250  supports the high pressure compressor section  252  and the high pressure turbine section  254 , which is arranged upstream from the mid turbine frame  259 . 
         [0054]    The inner shaft  240  is coupled to the geared architecture  248 , which is an epicyclic gear train  260  configured in a differential arrangement. The gear train  260  includes planetary gears  264  supported by a carrier  262 , which is connected to the inner shaft  240  that supports the low pressure turbine  246 . A sun gear  266  is centrally arranged relative to and intermeshes with the planetary gears  264 . A ring gear  270  circumscribes and intermeshes with the planetary gears  264 . In the example, a fan shaft  272 , which is connected to the fan  242 , is rotationally fixed relative to the ring gear  270 . The low pressure compressor  244  is supported by a low pressure compressor rotor  268 , which is connected to the sun gear  266  in the example. 
         [0055]    The carrier  262  is rotationally driven by the low pressure turbine  246  through the inner shaft  240 . The planetary gears  264  provide the differential input to the fan shaft  272  and low pressure compressor rotor  268  based upon the geometry ratio. The geared architecture  248  includes an additional speed change device  274  interconnecting the inner shaft  240  and the gear train  260 . 
         [0056]    An inducer  76  is fixed for rotation relative to the ring gear  270 . The inducer  76  is arranged in the core flow path C to provide some initial compression to the air before entering the low pressure compressor section  244 . The inducer  76  rotates at the same rotational speed as the fan  242  and provides some additional thrust, which is useful in hot weather, for example, where engine thrust is reduced. 
         [0057]    Another example geared architecture  348  for the engine  320  is shown in  FIG. 5 . The engine static structure  336  supports the inner and outer shafts  340 ,  350  for rotation about the axis A. The outer shaft  350  supports the high pressure compressor section  352  and the high pressure turbine section  354 , which is arranged upstream from the mid turbine frame  359 . 
         [0058]    The inner shaft  340  is coupled to the geared architecture  348 , which is an epicyclic gear train  360  configured in a differential arrangement. The gear train  360  includes planetary gears  364  supported by a carrier  362 , which is connected to the inner shaft  340  that supports the low pressure turbine  346 . A sun gear  366  is centrally arranged relative to and intermeshes with the planetary gears  364 . A ring gear  370  circumscribes and intermeshes with the planetary gears  364 . In the example, a fan shaft  372  is connected to the fan  342 . The low pressure compressor  344  is supported by a low pressure compressor rotor  368 , which is rotationally fixed relative to the ring gear  370  in the example. 
         [0059]    The carrier  362  is rotationally driven by the low pressure turbine  346  through the inner shaft  340 . The planetary gears  364  provide the differential input to the fan shaft  372  and low pressure compressor rotor  368  based upon the geometry ratio. The geared architecture  348  includes an additional speed change device  374  interconnecting the inner shaft  340  and the gear train  360 . The speed change device  374  receives rotational input from the sun gear  366  and couples the fan shaft  372  to the gear train  360 , which enables slower fan speeds. 
         [0060]    Another example geared architecture  448  for the engine  420  is shown in  FIG. 6 . The engine static structure  436  supports the inner and outer shafts  440 ,  450  for rotation about the axis A. The outer shaft  450  supports the high pressure compressor section  452  and the high pressure turbine section  454 , which is arranged upstream from the mid turbine frame  459 . 
         [0061]    The inner shaft  440  is coupled to the geared architecture  448 , which is an epicyclic gear train  460  configured in a differential arrangement. The gear train  460  includes planetary gears  464  supported by a carrier  462 , which is connected to the inner shaft  440  that supports the low pressure turbine  446 . A sun gear  466  is centrally arranged relative to and intermeshes with the planetary gears  464 . A ring gear  470  circumscribes and intermeshes with the planetary gears  464 . In the example, a fan shaft  472  is connected to the fan  442 . The low pressure compressor  444  is supported by a low pressure compressor rotor  468 , which is rotationally fixed relative to the ring gear  470  in the example. 
         [0062]    The carrier  462  is rotationally driven by the low pressure compressor  446  through the inner shaft  440 . The planetary gears  464  provide the differential input to the fan shaft  472  and low pressure compressor rotor  468  based upon the geometry ratio. The geared architecture  448  includes an additional speed change device  474  interconnecting the inner shaft  440  and the gear train  460 . The speed change device  474  receives rotational input from the sun gear  466  and couples the fan shaft  472  to the gear train  460 , which enables slower fan speeds. 
         [0063]    The inducer  476  is fixed for rotation relative to the fan shaft  472 . The inducer  476  is arranged in the core flow path C to provide some initial compression to the air before entering the low pressure compressor section  444 . The inducer  476  rotates at the same rotational speed as the fan  442 . 
         [0064]    Another example geared architecture  548  for the engine  520  is shown in  FIG. 7 . The engine static structure  536  supports the inner and outer shafts  540 ,  550  for rotation about the axis A. The outer shaft  550  supports the high pressure compressor section  552  and the high pressure turbine section  554 , which is arranged upstream from the mid turbine frame  559 . 
         [0065]    The inner shaft  540  is coupled to the geared architecture  548 , which is an epicyclic gear train  560  configured in a differential arrangement. The gear train  560  includes planetary gears  564  supported by a carrier  562 , which is connected to the inner shaft  540  that supports the low pressure turbine  546 . A sun gear  566  is centrally arranged relative to and intermeshes with the planetary gears  564 . A ring gear  570  circumscribes and intermeshes with the planetary gears  564 . In the example, a fan shaft  572  is connected to the fan  542 . The low pressure compressor  544  is supported by a low pressure compressor rotor  568 , which is rotationally fixed relative to the ring gear  570  in the example. 
         [0066]    The carrier  562  is rotationally driven by the low pressure turbine  546  through the inner shaft  540 . The planetary gears  564  provide the differential input to the fan shaft  572  and low, pressure compressor rotor  568  based upon the geometry ratio. The geared architecture  548  includes an additional speed change device  574  interconnecting the inner shaft  540  and the gear train  560 . The speed change device  574  receives rotational input from the sun gear  566  and couples the fan shaft  572  to the gear train  560 , which enables slower fan speeds. 
         [0067]    The inducer  576  is fixed for rotation relative to the fan shaft  572 . The inducer  576  is arranged in the core flow path C to provide some initial compression to the air before entering the low pressure compressor section  544 . In one example, the sun gear  566  rotates at the same speed as one of the fan shaft  572  and the inducer  576 , and the other of the fan shaft  572  and the inducer  576  rotate at a different speed than the sun gear  566 . In another example, the inducer  576 , sun gear  566  and fan shaft  572  rotate at different rotational speeds than one another through the speed change device  574 , which is another epicyclic gear train, for example. 
         [0068]    Another example geared architecture  648  for the engine  620  is shown in  FIG. 8 . The engine static structure  636  supports the inner and outer shafts  640 ,  650  for rotation about the axis A. The outer shaft  650  supports the high pressure compressor section  652  and the high pressure turbine section  654 , which is arranged upstream from the mid turbine frame  659 . 
         [0069]    The inner shaft  640  is coupled to the geared architecture  648 , which is an epicyclic gear train  660  configured in a differential arrangement. The gear train  660  includes planetary gears  664  supported by a carrier  662 , which is connected to the inner shaft  640  that supports the low pressure turbine  646 . A sun gear  666  is centrally arranged relative to and intermeshes with the planetary gears  664 . A ring gear  670  circumscribes and intermeshes with the planetary gears  664 . In the example, a fan shaft  672  is connected to the fan  642 . The low pressure compressor  644  is supported by a low pressure compressor rotor  668 , which is rotationally fixed relative to the ring gear  670  in the example. 
         [0070]    The carrier  662  is rotationally driven by the low pressure turbine  646  through the inner shaft  640 . The planetary gears  664  provide the differential input to the fan shaft  672  and low pressure compressor rotor  668  based upon the geometry ratio. The geared architecture  648  includes an additional speed change device  674  interconnecting the inner shaft  640  and the gear train  660 . The speed change device  674  receives rotational input from the sun gear  666  and couples the fan shaft  672  to the gear train  660 , which enables slower fan speeds. 
         [0071]    The inducer  676  is arranged in the core flow path C to provide some initial compression to the air before entering the low pressure compressor section  644 . The inducer  676  is fixed to the sun gear  666  for rotation at the same rotational speed. 
         [0072]    In the arrangements shown in  FIGS. 2-8 , the relative rotational directions are shown for each of the fan, low pressure compressor section, high pressure compressor section, high pressure turbine section, low pressure turbine section and inducer. The geared architectures may be configured in a manner to provide the desired rotational direction for a given engine design. 
         [0073]    The example geared architectures enable large fan diameters relative to turbine diameters, moderate low pressure turbine to fan speed ratios, moderate low pressure compressor to low pressure turbine speed ratios, high low pressure compressor to fan speed ratios and compact turbine section volumes. The low pressure turbine section may include between three and six stages, for example. 
         [0074]    The rotational speeds of the sun gear, ring gear and carrier are determined by the geometry ratio of the differential gear train. The interrelationship of these components can be expressed using the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         X 
                         carrier 
                       
                       
                         X 
                         ring 
                       
                     
                     = 
                     
                       GR 
                       
                         1 
                         + 
                         GR 
                       
                     
                   
                   , 
                   where 
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0075]    X carrier  is the nomograph distance of the planetary rotational axis from the sun gear axis, 
         [0076]    X ring  is the nomograph radius of the ring gear, and 
         [0077]    GR is the geometry ratio. 
         [0000]    Thus, for a geometry ratio of 3.0, 
         [0000]    
       
         
           
             
               
                 X 
                 carrier 
               
               
                 X 
                 ring 
               
             
             = 
             
               0.75 
               . 
             
           
         
       
     
         [0078]    The relative sizes amongst the sun gear, planetary gears and ring gear for several different geometry ratios are schematically depicted in  FIGS. 9A-9C . Referring to  FIG. 9A , the epicyclic gear train  760  includes a sun gear  766 , planetary  764 , carrier  762  and ring gear  770  that are sized to provide a geometry ratio of 3.0. Referring to  FIG. 9B , the epicyclic gear train  860  includes a sun gear  866 , planetary  864 , carrier  862  and ring gear  870  that are sized to provide a geometry ratio of 2.0. Referring to  FIG. 9C , the epicyclic gear train  960  includes a sun gear  966 , planetary  964 , carrier  962  and ring gear  970  that are sized to provide a geometry ratio of 1.5. In the examples, the ring gear radius remains constant. 
         [0079]      FIG. 10  graphically depicts effects of the geometry ratio on the rotational speeds and directions of the sun and ring gears and the carrier. The upper, lighter shaded bars relate to  FIG. 9A-9C . Assuming a rotational input from the low pressure turbine to the carrier of 10,000 RPM, the sun gear would be driven at 15,000 RPM and the ring gear would be driven at 8,333 RPM for a geometry ratio of 3.0. In an arrangement in which the fan is coupled to the ring gear and the sun gear is coupled to the low pressure compressor, like the arrangement shown in  FIG. 2 , the following speed ratios would be provided: LPT:fan=1.2, LPC:LPT=1.5, and LPC:fan=1.8. 
         [0080]    The lower, darker shaded bars relate to  FIGS. 11A-11C . The carrier and ring gear rotate in the opposite direction than depicted in  FIG. 9A-9C . 
         [0081]    Although an example embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.