Abstract:
A method for installing a mounting hole through a wall of a fan case of a gas turbine engine is disclosed. The method may comprise: 1) installing a pilot hole through the wall of the fan case at an angle perpendicular to an outer surface of the wall, 2) inserting a boss through the pilot hole, 3) bonding the boss to the wall of the fan case, and 4) installing the mounting hole through the boss at an off-axis angle with respect to the angle perpendicular to the outer surface of the wall.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a non-provisional claiming priority under 35 USC §119(e) to U.S. Provisional Patent Application Ser. No. 62/001,955 filed on May 22, 2014. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure generally relates to gas turbine engines, and more specifically, relates to systems for mounting instrumentation on fan containment cases of gas turbine engines. 
       BACKGROUND 
       [0003]    Gas turbine engines are internal combustion engines typically used to provide thrust to an aircraft or to provide power for land-based operations. A gas turbine engine may consist of a fan section surrounded by a fan containment case, a compressor section, one or more combustors, and a turbine section. Air may be drawn into the engine and accelerated by the rotating blades of the fan section, and it may be subsequently compressed/pressurized in the compressor section prior to entry into the combustor(s). Once in the combustor(s), the air may be mixed with fuel and combusted to generate hot combustion gases. Energy may then be extracted from the combustion gas products in the turbine section to power the turbine section, the compressor section, and the fan section. The combustion gas products may then be expelled through an exhaust nozzle of the engine to provide forward thrust for aerospace applications, or to provide power for other applications. 
         [0004]    During the testing and development of new gas turbine engines, various types of instrumentation may be mounted on the fan containment case for testing engine operation. Some testing systems may require the use of arrays of instruments mounted on the fan case, with each of the instruments extending into the outer flowpath of the fan section. For example, in one testing system, a pair of probes may be mounted on the fan case forward and aft of a fan blade to monitor the position and rotation of the blade. Due to the curved shape of the fan blades, optimal readings in this system often requires the instruments to be inserted through the wall of the fan case at off-axis angles that deviate from perpendicular to the outer surface of the fan case. However, as many recent gas turbine engine designs use composite-based fan containment cases, the formation of off-axis cuts through the composite fan cases for mounting the instruments has presented a significant challenge as off-axis cuts may damage the composite materials of the fan case. 
         [0005]    To enable the mounting of instrumentation features on composite fan cases, U.S. Patent Application Publication Number 2013/0336773 describes the use metallic rails with instrument holders mounted on a band of sacrificial composite material circumscribing the outer surface of the fan case. The sacrificial composite material reinforces the fan case and allows the drilling of a hole through the composite fan case. While effective, it is not disclosed whether the metallic rails support the mounting of instruments at off-axis angles. Moreover, space may be limited for such metallic rails on fan cases due to instrumentation trays or other external components. 
         [0006]    Clearly, there is a need for improved systems for mounting instruments on composite fan cases of gas turbine engines. 
       SUMMARY OF THE DISCLOSURE 
       [0007]    In accordance with one aspect of the present disclosure, a method for installing a mounting hole through a wall of a fan case of a gas turbine engine is disclosed. The method may comprise: 1) installing a pilot hole through the wall of the fan case at an angle perpendicular to an outer surface of the wall, 2) inserting a boss through the pilot hole, 3) bonding the boss to the wall of the fan case, and 4) installing the mounting hole through the boss. 
         [0008]    In another refinement, installing the mounting hole through the boss may include installing the mounting hole through the boss at an off-axis angle with respect to the angle perpendicular to the outer surface of the wall. 
         [0009]    In another refinement, bonding the boss to the wall of the fan case may include adhesively bonding the boss to the wall of the fan case. 
         [0010]    In another refinement, the boss may comprise a base and an insertion tab extending from a bottom surface of the base, and inserting the boss through the pilot hole may include inserting the insertion tab through the pilot hole. 
         [0011]    In another refinement, bonding the boss to the wall of the fan case may further include fixing a width of an adhesive bond between the bottom surface and the outer surface of the wall using at least one bond bump extending from the bottom surface of the base. 
         [0012]    In another refinement, bonding the boss to the wall of the fan case may further include fixing a width of an adhesive bond between the insertion tab and a wall of the pilot hole using at least one o-ring encircling the insertion tab. 
         [0013]    In another refinement, inserting the boss through the pilot hole may further include centering the boss in the pilot hole using the at least one o-ring. 
         [0014]    In another refinement, inserting the boss through the pilot hole may further include aligning a curvature of the bottom surface of the base with a curvature of the outer surface of the wall of the fan case by aligning an indicating feature of the boss with an indicating feature of the fan case. 
         [0015]    In another refinement, the method may further comprise installing at least one fastener hole through the boss. 
         [0016]    In another refinement, the method may further comprise inserting an instrument through the mounting hole, and fastening the instrument to the boss using at least one fastener. 
         [0017]    In accordance with another aspect of the present disclosure, a fan case for a gas turbine engine is disclosed. The fan case may comprise a circumferentially-extending wall formed at least in part from a composite material. The wall may have at least one pilot hole extending therethrough at an angle perpendicular to an outer surface of the wall. The fan case may further comprise a boss bonded to the fan case. The boss may comprise: 1) a base, 2) an insertion tab extending from a bottom surface of the base and through the pilot hole, and 3) a mounting hole extending through the boss. 
         [0018]    In another refinement, the mounting hole may extend through the boss at an off-axis angle with respect to the angle perpendicular to the outer surface of the wall. 
         [0019]    In another refinement, the boss may be adhesively bonded to the wall of the fan case. 
         [0020]    In another refinement, the boss may be formed from aluminum. 
         [0021]    In another refinement, the insertion tab may comprise at least one o-ring encircling the insertion tab. 
         [0022]    In another refinement, the at least one o-ring may center the boss in the pilot hole. 
         [0023]    In another refinement, the bottom surface of the base may include at least one bond bump that may fix a width of an adhesive bond between the bottom surface of the base and the outer surface of the wall of the fan case. 
         [0024]    In another refinement, the bottom surface of the base may further include a curvature matching a curvature of the outer surface of the wall. 
         [0025]    In another refinement, the boss may further include at least one indicating feature to indicate an alignment of the curvature of the bottom surface of the base with the curvature of the outer surface of the wall. 
         [0026]    In accordance with another aspect of the present disclosure, a boss for mounting an instrument on a composite fan case of a gas turbine engine is disclosed. The boss may comprise a base and an insertion tab extending from a bottom surface of the base. The insertion tab may be configured to insert through a pilot hole that extends through a wall of the composite fan case at an angle perpendicular to an outer surface of the wall. The boss may further comprise a mounting hole extending through the boss at an off-axis angle with respect to the angle perpendicular to the outer surface of the wall. 
         [0027]    These and other aspects and features of the present disclosure will be more readily understood when read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a perspective view of a fan case of a gas turbine engine, constructed in accordance with the present disclosure. 
           [0029]      FIG. 2  is perspective view of detail  2  of  FIG. 1 , illustrating a boss attached to the fan case, constructed in accordance with the present disclosure. 
           [0030]      FIG. 3  is a cross-sectional view through the section  3 - 3  of  FIG. 2 , illustrating a mounting hole through the boss at an off-axis angle, constructed in accordance with the present disclosure. 
           [0031]      FIG. 4  is a cross-sectional view similar to  FIG. 3 , but having an instrument inserted in the mounting hole, constructed in accordance with the present disclosure. 
           [0032]      FIG. 5  is a bottom perspective view of the boss prior to installation in the fan case, constructed in accordance with the present disclosure. 
           [0033]      FIG. 6  is a side view of the fan case in a vertical position to prior to installation of the boss, constructed in accordance with the present disclosure. 
           [0034]      FIG. 7  is a cross-sectional view through the section  7 - 7  of  FIG. 6 , schematically illustrating the formation of a pilot hole in the fan case, in accordance with a method of the present disclosure. 
           [0035]      FIG. 8  is a cross-sectional view similar  FIG. 7 , but schematically illustrating the installation of the boss in the pilot hole, in accordance with a method of the present disclosure. 
           [0036]      FIG. 9  is a cross-sectional view similar to  FIG. 8 , but schematically illustrating the installation of the mounting hole through the boss, in accordance with a method of the present disclosure. 
           [0037]      FIG. 10  is a cross-sectional view similar to  FIG. 9 , but schematically illustrating the installation of a fastener hole in the boss, in accordance with a method of the present disclosure. 
           [0038]      FIG. 11  is a cross-sectional view similar to  FIG. 10 , but schematically illustrating the insertion of the instrument in the mounting hole, in accordance with a method of the present disclosure. 
           [0039]      FIG. 12  is a cross-sectional view similar to  FIG. 11 , but schematically illustrating the fastening of the instrument to the boss, in accordance with a method of the present disclosure. 
           [0040]      FIG. 13  is a flowchart illustrating a sample sequence of steps which may be involved in mounting the instrument to the fan case using the boss, in accordance with a method of the present disclosure. 
       
    
    
       [0041]    It should be understood that the drawings are not necessarily drawn to scale and that the disclosed embodiments are sometimes illustrated schematically and in partial views. It is to be further appreciated that the following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses thereof. In this regard, it is to be additionally appreciated that the described embodiment is not limited to use for instrument mounting on fan containment cases of gas turbine engines. Hence, although the present disclosure is, for convenience of explanation, depicted and described as certain illustrative embodiments, it will be appreciated that it can be implemented in various other types of embodiments and in various other systems and environments. 
       DETAILED DESCRIPTION 
       [0042]    Referring now to the drawings, and with specific reference to  FIG. 1 , a fan case  10  for a gas turbine engine is depicted. The fan case  10  may form a containment case for a fan section of the gas turbine engine. It may have a wall  12  circumferentially extending about a central axis  13  and having one or more bosses  14  affixed thereto for mounting equipment on the fan containment case. As a non-limiting possibility, the bosses  14  may be configured to mount one or more testing instruments on the fan containment case  10 , such as during the testing or development of the gas turbine engine. For example, for monitoring the position or rotation of the fan blades of the fan section, pairs of bosses  14  may be positioned co-axially with respect to each other on the fan case  10 , with one boss  14  being positioned forward of the fan blade of interest and the other boss  14  being positioned aft of the fan blade of interest. However, various alternative arrangements of the bosses  14  may be used depending on the measuring requirements of the instruments. In addition, the bosses  14  may be used to mount other types of equipment on the fan case as well. 
         [0043]    The wall  12  of the fan case  10  may be at least partially formed from a composite material such as a carbon epoxy composite or another type of type of composite material. The bosses  14  may be formed from a material that is more easily machined compared to the composite material of the fan case  10 . For example, the bosses  14  may be formed from aluminum, an aluminum alloy, or another type of readily machinable metal or material. Due to their readily machinable construction, mounting holes  16  for instruments or other types of equipment may be drilled into the bosses  14  at various angles, including “off-axis angles” that deviate with respect an angle perpendicular to an outer surface  18  of the fan case (see  FIGS. 2-3  and further details below). Accordingly, off-axis cuts may be formed through the bosses  14 , rather than through the composite material of the fan case  10 , to reduce or eliminate the risk of damage to the fan case  10 . 
         [0044]    Turning now to  FIGS. 2-3 , a single boss  14  attached to the fan case  10  is shown. The boss  14  may have a base  20  and an insertion tab  22  extending from a bottom surface  24  of the base  20 , as best shown in  FIG. 3 . Although shown as cylindrical in shape, the base  20  and the insertion tab  22  may have various other shapes as well. The insertion tab  22  may extend through a pilot hole  26  formed in the wall  12  of the fan case  10 , and the bottom surface  24  of the base  20  may face the outer surface  18  of the fan case  10 . The boss  14  may be adhesively bonded to the fan case  10  with a suitable elastomeric or other bond-thickness tolerant adhesive such as, but not limited to, a polysulfide adhesive or a room temperature vulcanizing (RTV) adhesive. In particular, the bottom surface  24  may be adhesively bonded to the outer surface  18 , and the insertion tab  22  may be adhesively bonded to the wall of the pilot hole  26 . 
         [0045]    The boss  14  may have one or more mounting holes  16  formed through the body of the boss and extending through the wall  12  of the fan case  10 , as best shown in  FIG. 3 . The mounting hole  16  may be oriented at various angles (α) with respect to an angle perpendicular to the outer surface  18  of the wall  12 , and the angle (α) may be dictated by the mounting or measuring requirements of the instrument/equipment. For example, the angle (α) may range from about 0° to about 15°, although angles outside of this range may also be used in some circumstances. The boss  14  may also have one or more fastener holes  27  formed on an upper surface  28  of the base  20  for fastening the instrument/equipment to the boss  14  (see  FIG. 2 ). 
         [0046]    Turning now to  FIG. 4 , an instrument  30  or other type of equipment may be inserted through the mounting hole  16  of the boss  14  to support the instrument  30  on the fan case  10  at a desired angle (α). As one possibility, the instrument  30  may be involved in monitoring the position/rotation of fan blades of the gas turbine engine. In this case, the instrument  30  may extend through both the body of the boss  14  and the wall  12  of the fan case  10  to be placed in contact with an outer airflow path  32 . In addition, the instrument  30  may also extend through any flowpath liners (not shown) that exist between the inner surface of the wall  12  and the outer airflow path  32 . However, various other mounting arrangements may also apply depending on the type of instrument/equipment used. 
         [0047]    Referring now to  FIG. 5 , additional features of the boss  14  will now be described. Prior to installation on the fan case  10 , the boss  14  may be provided without a mounting hole and the insertion tab  22  may have a solid construction, such that the mounting hole(s)  16  may be machined/drilled through the boss  14  upon installation on the fan case  10  (see further details below). The insertion tab  22  may have one or more o-rings  34  encircling a periphery  35  of the insertion tab  22  that may serve to center the boss  14  in the pilot hole  26 . The insertion tab  22  may also have one or more apertures  37  configured to retain the o-ring(s)  34  in position (see  FIG. 3 ). The o-ring  34  may be formed from an electrically insulating material (e.g., silicone, etc.) that may provide galvanic isolation between the boss  14  and the composite material of the fan case  10 , thereby assisting to prevent galvanic corrosion. 
         [0048]    The o-ring  34  may also assist in fixing a width of the adhesive bond between the insertion tab  22  and the wall of the pilot hole  26  (also see  FIG. 3 ). Similarly, one or more bond bumps  36  may extend from the bottom surface  24  of the base  20  to assist in fixing a width of an adhesive bond between the bottom surface  24  and the outer surface  18  of the fan case  10 . The bond bumps  36  may consist of raised portions on the bottom surface  24  that extend in a radial direction from the insertion tab  22 . The height of the bond bumps  36 , as well as the thickness of the o-ring(s)  34 , may be configured to provide an optimized adhesive bond gap spacing between the fan case  10  and the boss  14  according to the optimal bond gap spacings of the selected adhesive. 
         [0049]    The bottom surface  24  and the bond bumps  36  may have a curvature that matches or at least closely compliments a curvature of the outer surface  18  of the fan case  10 . The boss  14  may also have an indicating feature  38  which may be aligned with an indicating feature on the fan case  10  to ensure curvature alignment between the curvature of the bottom surface  24 /bond bumps  36  and the curvature of the outer surface  18  (also see  FIG. 2 ). As one possibility, the indicating feature  38  may consist of a flattened tab  40  which may be aligned with a horizontal line  42  of the fan case  10  when the fan case is disengaged from the gas turbine engine and oriented vertically (see  FIG. 6  and further details below). 
         [0050]    The boss  14  may be installed on the fan case  10  while the fan case is disconnected from the rest of the gas turbine engine and mounted vertically as shown in  FIG. 6 , such as at a tilting/rotating machining station (not shown).  FIGS. 7-10  schematically depict a series of steps that may be involved in installing the boss  14  on the fan case  10 . As shown in  FIG. 7 , the pilot hole  26  may first be drilled, machined, or otherwise formed through the wall  12  of the fan case  10  at the desired mounting location. The pilot hole  26  may have a diameter matching a diameter of the insertion tab  22 , and it may be installed at an angle perpendicular to the outer surface  18  of the fan case  10  so as to avoid off-axis cuts that could damage the composite material of the fan case  10 . 
         [0051]    After applying a selected adhesive along the bottom surface  24  of the base  20  and around the periphery  35  of the insertion tab  22 , the boss  14  may be inserted through the pilot hole  26  as shown in  FIG. 8 . More specifically, with the indicating feature  38  properly aligned with an indicating feature on the fan case  10  (e.g., the horizontal line  42 ), the insertion tab  22  may be inserted through the pilot hole  26  until the bond bump(s)  36  abut against the outer surface  18  of the fan case  10 . This may cause excess adhesive to flow out of the bonding interface, thereby fixing the width of the adhesive bond between the bottom surface  24  and the outer surface  18  of the fan case  10 . In addition, the o-ring(s)  34  may center the boss  14  in the pilot hole  26 , while fixing the width of the adhesive bond between the insertion tab  22  and the wall of the pilot hole  26 . The adhesive may then be allowed to cure to affix the boss  14  to the fan case  10 . If necessary, the upper surface  28  of the boss  14  may also be machined at an angle that is perpendicular to the desired angle (α) of the mounting hole  16 . 
         [0052]    The mounting hole  16  may be drilled, machined, or otherwise formed through the boss  14  at the desired angle (α), as depicted in  FIG. 9 . In addition, as shown in  FIG. 10 , one or more fastener holes  27  may be drilled or machined through the upper surface  28  of the boss  14  to allow the fastening of the instrument  30  to the boss  14 . The fastener holes  27  may be threaded holes or other types of fastener holes. In some cases, the steps depicted in  FIGS. 7-10  may be repeated as necessary to provide an array of bosses  14  attached to the fan case  10  at desired positions. 
         [0053]    Once the desired number of bosses  14  is affixed to the fan case  10 , the fan case  10  may be delivered to and assembled with the gas turbine engine. The instrument  30  may then be mounted on the fan case  10  according to the steps depicted in  FIGS. 11-12 . The instrument  30  may be inserted through the mounting hole  16  and through any flowpath liners to place at least a portion of the instrument  30  in contact with the outer airflow path  32 , as depicted in  FIG. 11 . In addition, one or more fastener holes  44  formed in a flange  46  of the instrument  30  may be aligned with the fastener hole(s)  27  of the boss  14 , as shown. The instrument  30  may then be fastened to the boss  14  using one or more fasteners  48  (see  FIG. 12 ). The fastener(s)  48  may be threaded fasteners, although other types of fasteners or fastening arrangements may also be used in some circumstances. Once mounted and fastened to the boss  14 , the instrument  30  may perform measurements while the gas turbine engine is running at a testing station or on an aircraft. When the measurements are completed, the instrument  30  may be removed from the boss  14  and a plug may be inserted and affixed to the mounting hole  16  to permit the use of the gas turbine engine in various aerospace or land-based applications. 
         [0054]      FIG. 13  summarizes the steps that may be involved in installing the boss  14  and the instrument  30  on the fan case  10 . Beginning with a first block  100 , the fan case  10  may be mounted vertically at a machining station (see  FIG. 6 ). Next, a pilot hole  26  may be installed through the wall  12  of the fan case  10  at an angle perpendicular to the outer surface  18  of the fan case according to a next block  102  (see  FIG. 7 ). A suitable adhesive may then be applied to the bottom surface  24  of the boss  14  and to the periphery  35  of the insertion tab  22  according to a block  104 . According to the next blocks  106  and  108 , the boss  14  may be inserted through the pilot hole  26  with the indicating feature aligned with an indicating feature on the fan case  10  (such as the horizontal line  42 ). Once the adhesive bond width is fixed by abutting the bond bumps  36  against the outer surface  18  of the fan case  10 , the adhesive may be allowed to cure according to a next block  110 . In addition, the upper surface  28  of the boss  14  may be machined at an angle perpendicular to the desired mounting hole angle (α) (block  112 ). 
         [0055]    The mounting hole  16  may be installed through the boss at the desired angle (α) according to the measuring requirements of the instrument  30  (block  114 / FIG. 9 ), and one or more fastener holes may be drilled/machined through the upper surface  28  of the boss  14  (block  116 / FIG. 10 ). At this stage, the fan case  10  may be delivered to and assembled with the gas turbine engine to perform the desired measurements (block  118 ). The instrument  30  may then be inserted through the mounting hole  16  and fastened to the boss  14  according to blocks  120  and  122  ( FIGS. 11-12 ), and the desired measurements may be performed (block  124 ). It will be understood that the steps disclosed above may be repeated as necessary to carry out all required measurements. In addition, it will also be understood that variations in the sequence of steps depicted in  FIG. 13  may be necessitated under various circumstances. Variations such as these are also encompassed within the scope of the present disclosure. 
       INDUSTRIAL APPLICABILITY 
       [0056]    In general, it can therefore be seen that the technology disclosed herein has industrial applicability in a variety of settings including, but not limited to, the development and testing of gas turbine engines. The boss disclosed herein allows the mounting of instruments or other equipment through the walls of composite fan cases at off-axis angles for various testing purposes, without requiring the drilling of off-axis cuts through the composite material of the fan case. In this way, the technology of the present disclosure may reduce or eliminate the risk of damage to composite fan cases caused by the drilling or machining of off-axis cuts through the fan case wall. The boss may be constructed from readily available materials which may be readily machined to install mounting holes at various angles. In addition, the bosses disclosed herein may replace instrumentation rails of the prior art when space limitations on the fan case precludes their use. Other advantageous features of the boss include an o-ring for centering the boss in the pilot hole, bond bumps for fixing the adhesive bond width between the boss and the fan case, and an indicating feature for assisting the alignment of the curvature of the boss with the curvature of the outer surface of the fan case. It is expected that the technology disclosed herein may find wide industrial applicability in a wide range of areas such as, but not limited to, aerospace technologies.