Abstract:
A method enables a gas turbine engine variable bypass valve system to be assembled. The method comprises positioning a unison ring circumferentially within the gas turbine engine such that the unison ring is radially outward from a structural frame, coupling at least one bellcrank to the unison ring, such that the unison ring is radially supported only by said at least one bellcrank, and coupling the at least one bellcrank to a bellcrank support that is coupled to the structural frame.

Description:
BACKGROUND OF THE INVENTION 
     This application relates generally to gas turbine engines and, more particularly, to variable bypass valve systems used with gas turbine engines. 
     At least some known gas turbine engines include a fan upstream from a core engine that includes, in serial flow relationship, a low pressure compressor, or a booster, and a high pressure compressor. The low pressure compressor and the high pressure compressor compress airflow entering the combustor through the fan, with the airflow moving through an inlet section and a discharge section of the booster and then through an inlet section and a discharge section of the high pressure compressor. The booster is designed to facilitate providing optimal airflow to the high pressure compressor above a specific design throttle setting. At throttle settings off design, the booster may supply more air than the high pressure compressor can flow, which can lead to unsteady airflow in the engine. 
     To facilitate mitigating the effects of unsteady airflow, at least some known gas turbine engines include variable bleed valve (VBV) systems which rotate bypass doors open during unsteady airflow conditions to allow booster airflow to exit and bypass the high pressure compressor. The bleed doors and valves facilitate protecting the booster and high pressure compressor from eventual aerodynamic stall. At least some known VBV systems include a unison ring that is supported by dedicated ring supports extending from the booster frame structure. The structural supports facilitate preventing the unison ring from deforming excessively during operational loading. Excessive deformation of the unison ring may cause high stresses to be induced into the VBV system, which over time may lead to premature failure of the VBV system. 
     The unison ring is connected to two actuator bellcranks and ten door bellcranks. Rotation of the actuator bellcranks by action of a hydraulic actuator causes the ring to rotate, and thus subsequent rotation of the doors. The bellcranks are connected to the doors through clevis and spherical bearing attachments. In the ring to bellcrank connections, ring bushings extend between each bellcrank and the unison ring, and are positioned relative to the bellcranks such that a pre-determined gap is defined between each ring bushing and each bellcrank. 
     Assembling a known VBV system that includes dedicated ring supports may be a time consuming and tedious process, as the width of the gap between the ring ID and the dedicated supports is facilitated to be minimized. As the gap becomes smaller, assembly of the booster, including the dedicated supports, becomes more difficult as the supports must be inserted inside an inner diameter of the unison ring. However, if the gap is excessively large, the ring will not be adequately supported to minimize system stresses. Additionally in at least some known systems, the width of the bellcrank to bushing gap is predefined to be large enough to facilitate minimizing contact between the ring bushings and each bellcrank during operation. However, widening the gap increases an overall size of the VBV system which increases a cost and weight of the VBV system. 
     Assembling the VBV system may be time consuming and tedious process, as the width of the gap is optimized. Specifically, in at least some known systems, the width of the gap is predefined to be large enough to facilitate minimizing contact between the ring bushings and each bellcrank during operation. However, widening the gap increases an overall size of the VBV system which increases a cost and weight of the VBV system. On the other hand, although minimizing the width of the gap increases the effectiveness of the dedicated structural supports, the decreased width makes assembly of the VBV much more difficult, and also increases the chances for undesirable contact between the bellcrank and the ring bushings. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, a method for assembling a gas turbine engine variable bypass valve system is provided. The method comprises positioning a unison ring circumferentially within the gas turbine engine such that the unison ring is radially outward from a structural frame, coupling at least one bellcrank to the unison ring, such that the unison ring is radially supported only by said at least one bellcrank, and coupling the at least one bellcrank to a bellcrank support that is coupled to the structural frame. 
     In another aspect of the invention, a variable bypass valve (VBV) system for a gas turbine engine is provided. The VBV system comprises a plurality of bellcranks spaced circumferentially within the gas turbine engine, and an annular unison ring coupled to the plurality of bellcranks for controlling the position of the plurality of bellcranks. The unison ring is radially supported only by the plurality of bellcranks. 
     In a further aspect, a gas turbine engine is provided. The gas turbine includes a structural frame and a variable bypass valve (VBV) system for selectively controlling air flowing through at least a portion of the engine. The structural frame extends circumferentially within the gas turbine engine. The variable bypass valve (VBV) system includes at least one bellcrank rotatably coupled to the structural frame, and an annular unison ring that is coupled to the bellcrank such that the unison ring is radially supported only by the bellcrank. The unison ring is for selectively controlling actuation of the bellcrank. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a gas turbine engine; 
     FIG. 2 is a partial view of a variable bypass valve system that may be used with the engine shown in FIG. 1; and 
     FIG. 3 is an enlarged cross-sectional view of a portion of the variable bypass valve system shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic illustration of a gas turbine engine  10  including a low-pressure compressor  12 , a high-pressure compressor  14 , and a combustor  16 . Engine  10  also includes a high-pressure turbine  18 , and a low-pressure turbine  20 . Compressor  12  and turbine  20  are coupled by a first rotor shaft  24 , and compressor  14  and turbine  18  are coupled by a second rotor shaft  26 . In one embodiment, engine  10  is a GE90 engine available from General Electric Aircraft Engines, Cincinnati, Ohio. In an alternative embodiment, gas turbine engine  10  is a CF6 engine available from General Electric Aircraft Engines, Cincinnati, Ohio. 
     In operation, air flows through low pressure compressor  12  and compressed air is supplied from low pressure compressor  12  to high pressure compressor  14 . Compressed air is then delivered to combustor  16  and airflow from combustor  16  drives turbines  18  and  20 . 
     FIG. 2 is a partial view of a variable bypass valve (VBV) system  50  that may be used with engine  10 . FIG. 3 is an enlarged cross-sectional view of a portion of variable bypass (VBV) system  50 . VBV system  50  includes a unison ring  52 , a plurality of bellcranks  54 , a plurality of bellcrank supports  56 , and a plurality of bypass doors  60 . More specifically, in the exemplary embodiment, system  50  includes twelve bellcranks  54  spaced circumferentially within engine  10 . Each bellcrank  54  includes an upstream end  70 , a downstream end  72 , and a body  74  extending therebetween, and having an inherent radial stiffness between ends  70  and  72 . Bellcranks  54  are positioned such that body  74  extends substantially parallel to a centerline axis of symmetry (not shown) of engine  10 . 
     Each bellcrank upstream end  70  includes a bearing race  80  extending between a radially outer side  82  and a radially inner side  84  of bellcrank  54 . Bearing race  80  contains a bearing element  85  which has an inside diameter  86  that is sized to receive a fastener  88  therethrough. In the exemplary embodiment, fastener  88  is threaded. Each bellcrank downstream end  72  is coupled to each respective bellcrank support  56  which is fixedly coupled to an engine fan frame  90  and is rotatably coupled to at least one actuator  89  by actuator devises  91 . Door devises  92  are coupled between bellcrank  54  and each respective bypass door  60 , such that actuation of bellcrank  54  causes subsequent rotation of bypass doors  60 . 
     Engine fan frame  90  extends circumferentially within engine  10  around a flow passage  100  extending between low pressure compressor  12  and high pressure compressor  14  (shown in FIG.  1 ). Bypass doors  60  are selectively operable to regulate the airflow entering high pressure compressor  14  during idle, low power operation, and transient load conditions. More specifically, in a closed position, as shown in FIG. 2, doors  60  are in a sealing contact with fan frame  90  and substantially prevent air from bypassing high pressure compressor  14 . 
     Bellcranks  54  extend upstream from each bellcrank support  56  to couple with unison ring  52 . Unison ring  52  is annular and includes a radially outer portion  110  and a radially inner portion  112 . In the exemplary embodiment, portions  110  and  112  are substantially identical and are substantially parallel. Each portion  110  and  112  includes a plurality of openings  114  and  116  respectively extending therethrough. Portions  110  and  112  are positioned such that each set of openings  114  and  116  are substantially concentrically aligned. Unison ring portions  110  and  112  are spaced a distance  120  apart which is wider than a thickness t 1  of bellcrank upstream end  70 . 
     A plurality of ring bushings  130  are inserted in each portion opening  114  and  116 . More specifically, each ring bushing  130  includes a substantially cylindrical portion  132  and an annular lip  134  that extends radially outwardly from portion  132 . Accordingly, external diameters D 1  and D 5  of lip  134  are larger than external diameter D 2  and D 6  of cylindrical portion  132 . Furthermore, lip external diameter D 1  and D 5  are larger than respective diameters D 3  and D 4  of each unison ring opening  114  and  116 , and cylindrical portion diameters D 2  and D 6  are slightly larger than unison ring openings D 3  and D 4 . Accordingly, each ring bushing cylindrical portion  132  is at least partially received within each unison ring opening  114  and  116  such that each bushing annular lip  134  contacts an external surface  140  of each unison ring portion  110  and  112 . Each ring bushing  130  also includes an opening  142  extending therethrough and sized to receive fastener  88  therethrough. 
     Bellcrank bearing  150  is inserted within each bellcrank upstream end bearing race  80 . In the exemplary embodiment, bearing  150  is a spherical bearing and has a high axial load capability. Spherical bearing  150  has a height h 1  that is taller than bellcrank upstream end thickness t 1  and accordingly, bearing  150  extends a distance  154  outwardly from each bellcrank side  82  and  84  to a respective bearing radially outer contact surface  156  and a bearing radially inner contact surface  158 . 
     A gap  160  is defined between each ring bushing  130  and each respective bearing contact surface  156  and  158 . Gaps  160  facilitate accommodating component tolerance stack-ups during assembly and permit a nominal amount of radial movement of bellcrank bearing  150 . 
     During engine operation, VBV system  50  is actuated to permit airflow to exit and bypass high pressure compressor  14 . Specifically, selective operation of bypass doors  60  facilitates preventing aerodynamic stalls within engine  10 . More specifically, as unison ring  52  is actuated, bellcrank  54  is rotated to cause rotation of bypass doors  60  from an open position to a closed position, or vice-versa depending on the actuation direction of unison ring  52 . Furthermore, as ring  52  is actuated, bellcrank spherical bearing radially outer contact surface  156  or bearing radially inner contact surface  158  contacts a respective ring bushing  130  to facilitate providing radial support for unison ring  52  without additional dedicated structural supports. More specifically, depending on loading induced to VBV system  50 , spherical bellcrank bearing  150 , gap  160  may be minimized to enable bearing  150  to “bottom out” against a ring bushing  130  and provide additional radial support for unison ring  52 . 
     As a result, a load path  170  through VBV system is changed from being transmitted directly into a booster flange, as is known in the art, to being transmitted into fan frame  90  through bellcranks  54  and bellcrank supports  56 . Furthermore, because ring  52  is radially supported by twelve bellcranks  54  which extend downstream from a perimeter of unison ring  52 , loading transmitted to ring  52  is facilitated to be more evenly distributed through VBV system  50  as compared to other known VBV systems, and thus, less stresses are induced through system  50 . 
     During assembly of VBV system  50 , because dedicated supports are non-existent to interfere with unison ring  52 , assembly time of engine  10  is facilitated to be reduced and less labor intensive as components installed within engine  10  after unison ring  52  is installed do not require navigation around the dedicated supports. Furthermore, because the dedicated supports are not required, VBV system  50  facilitates reducing assembly costs and an overall engine weight. 
     The above-described variable bypass valve system for a gas turbine engine is cost-effective and reliable. The variable bypass valve system includes a unison ring that is such that the unison ring is radially supported only by the bellcranks. Accordingly, the variable bypass valve system does not include any dedicated unison ring supports, and as such, assembly costs and an overall weight of the engine are reduced. Furthermore, because the variable bypass valve system includes twelve bellcranks, the system is radially supported by the bellcranks such that loading transmitted from the unison ring during actuation is facilitated to be evenly distributed through the unison ring. As a result, the variable bypass valve system facilitates extending a useful life of the engine in a cost effective and reliable manner. 
     Exemplary embodiments of variable bypass valve systems are described above in detail. The systems are not limited to the specific embodiments described herein, but rather, components of each assembly may be utilized independently and separately from other components described herein. Each variable bypass valve component can also be used in combination with other variable bypass valve components 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.