Abstract:
A rotor in a gas turbine engine has a rotor body with at least one slot receiving a blade. The blade has an outer surface formed of a first material and an airfoil extending from a dovetail. The dovetail is received in the slot. A grounding element is in contact with a portion of the dovetail formed of a second material that is more electrically conductive than the first material. The grounding element is in contact with a rotating element that rotates with the rotor and is formed of a third material. The first material is less electrically conductive than the third material. The grounding element and rotating element together form a ground path from the portion of the dovetail into the rotor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application relates to a structure for electrically grounding blades for use in a gas turbine engine. 
         [0002]    Gas turbine engines are known, and typically include a fan delivering air into a compressor section. In the compressor section, the air is compressed and then delivered into a combustion section. The compressed air is mixed with fuel and burned in the combustion section. Products of this combustion pass downstream to drive turbine rotors. 
         [0003]    The fan blades are subject to a large volume of air moving across an airfoil, and this can build up a large static electric charge. Conventionally, the fan blades were formed of a conductive metal that was grounded to a hub that mounts the fan blade. As such, the charge would dissipate. 
         [0004]    More recently, fan blades have become larger. One factor driving the larger fan blades is the use of a gear reduction between a turbine driven spool which drives the fan blade and the spool. The gear reduction allows a single turbine rotor to drive both a compressor section and the fan, but at different speeds. 
         [0005]    As the size of the fan blade has increased, its weight has also increased. As such, efforts have been made to reduce the weight of fan blades. One modification is to change the material for the fan blade from titanium to an aluminum. The aluminum fan blades have been covered with a polyurethane coating and fabric wear pads to protect the aluminum. These materials have insulation qualities and, thus, the blade may not be electrically grounded to a rotor. 
       SUMMARY OF THE INVENTION 
       [0006]    In a featured embodiment, a rotor for use in a gas turbine engine has a rotor body with at least one slot receiving a blade. The blade has an outer surface formed of a first material and an airfoil extending from a dovetail. The dovetail is received in the slot. A grounding element is in contact with a portion of the dovetail formed of a second material that is more electrically conductive than the first material. The grounding element is in contact with a rotating element that rotates with the rotor. The rotating element is formed of a third material. The first material is less electrically conductive than the third material. The grounding element and rotating element together form a ground path from the portion of the dovetail into the rotor. 
         [0007]    In another embodiment according to the previous embodiment, the first material includes an outer coating that is relatively non-conductive compared to the second and third materials. 
         [0008]    In another embodiment according to any of the previous embodiments, the grounding element is formed of a material that is more electrically conductive than the first material. 
         [0009]    In another embodiment according to any of the previous embodiments, the rotating element is separate from the rotor. 
         [0010]    In another embodiment according to any of the previous embodiments, the rotating element is a lock ring which secures the blade within the rotor. The grounding element contacts the lock ring. The lock ring contacts the rotor to provide the grounding path. 
         [0011]    In another embodiment according to any of the previous embodiments, the grounding element has a radially outwardly tang extending into a hole in the dovetail. 
         [0012]    In another embodiment according to any of the previous embodiments, the hole is formed in a radially inner surface of the dovetail. The tang extends radially outwardly into the hole. 
         [0013]    In another embodiment according to any of the previous embodiments, the tang is secured in a relatively conductive adhesive deposited in the hole. 
         [0014]    In another embodiment according to any of the previous embodiments, the grounding element has a contact face that is polygonal, with angled sides associated with angled sides of the platform. 
         [0015]    In another embodiment according to any of the previous embodiments, the first material includes a protective coating formed on the blade. The second material is aluminum. 
         [0016]    In another featured embodiment, a gas turbine engine has a fan section, a compressor section, a combustor section, and at least one turbine rotor. The at least one turbine rotor drives a compressor rotor. The at least one turbine rotor also drives a rotor of the fan or compressor section through a gear reduction. The blade has an outer surface formed of a first material and an airfoil extending from a dovetail. The dovetail is received in the slot. A grounding element is in contact with a portion of the dovetail formed of a second material that is more electrically conductive than the first material. The grounding element is in contact with a rotating element that rotates with the rotor. The rotating element is formed of a third material. The first material is less electrically conductive than the third material. The grounding element and rotating element together form a ground path from the portion of the dovetail into the rotor. 
         [0017]    In another embodiment according to the previous embodiment, the first material includes an outer coating that is relatively non-conductive compared to the second and third materials. 
         [0018]    In another embodiment according to any of the previous embodiments, the grounding element is formed of a material that is more electrically conductive than the first material. 
         [0019]    In another embodiment according to any of the previous embodiments, the rotating element is separate from the rotor. 
         [0020]    In another embodiment according to any of the previous embodiments, the rotating element is a lock ring which secures the blade within the rotor. The grounding element contacts the lock ring. The lock ring contacts the rotor to provide the grounding path. 
         [0021]    In another embodiment according to any of the previous embodiments, the grounding element has a radially outwardly tang extending into a hole in the dovetail. 
         [0022]    In another embodiment according to any of the previous embodiments, the hole is formed in a radially inner surface of the dovetail. The tang extends radially outwardly into the hole. 
         [0023]    In another embodiment according to any of the previous embodiments, the tang is secured in a relatively conductive adhesive deposited in the hole. 
         [0024]    In another embodiment according to any of the previous embodiments, the grounding element has a contact face that is polygonal, with angled sides associated with angled sides of the platform. 
         [0025]    In another embodiment according to any of the previous embodiments, the first material includes a protective coating formed on the blade. The second material is aluminum. 
         [0026]    These and other features of the invention will be better understood from the following specifications and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1A  shows an exemplary gas turbine engine. 
           [0028]      FIG. 1B  shows an aluminum blade. 
           [0029]      FIG. 1C  shows the aluminum blade mounted into a rotor. 
           [0030]      FIG. 2  shows details of a grounding arrangement. 
           [0031]      FIG. 3  is another view of the  FIG. 2  embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 1A  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. 
         [0033]    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. 
         [0034]    The low speed spool  30  generally includes an inner shaft  40  that interconnects a fan  42 , a low pressure compressor  44  and a low pressure turbine  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 compressor  52  and high pressure turbine  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  further supports 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. 
         [0035]    The core airflow 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. 
         [0036]    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 planetary 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. The geared architecture  48  may be an epicycle gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about 2.5:1. 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. 
         [0037]    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 (“TSFCT”)”—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. 
         [0038]    A fan blade  120  is illustrated in  FIG. 1B  having an airfoil  118  extending radially outwardly from a dovetail or root  124 . A leading edge  21  and a trailing edge  22  define the forward and rear limits of the airfoil  118 . Fan blade  120  may be used in an engine such as engine  20 . 
         [0039]    As shown in  FIG. 1C , a fan rotor  116  receives the dovetail  124  to mount the fan blade  120  with the airfoil  118  extending radially outwardly. As the rotor is driven to rotate, it carries the fan blade  120  with it. 
         [0040]    A lock ring  100  locks the blades  120  within the rotor  116  and rotates with the rotor  116 . 
         [0041]    As mentioned above, the lock ring  100  and rotor  116  may be formed of titanium or a titanium alloy, while the blade  120  may be formed of aluminum, but coated with a non-conductive coating, such as polyurethane coating  125  (see  FIG. 3 ), or including fabric pads. As such, the fan blade  120  is not grounded. 
         [0042]    A grounding element  130  is thus associated with the fan blade  120 . 
         [0043]    As can be seen in  FIG. 2 , the grounding element  130  has a contact or forward face  132  which is generally polygonal, with a radially outer face  121 , sides  202  extending at an angle, and associated with sides  200  of the dovetail  124  of the blade  120 , and a bottom surface  131 . A upwardly extending grounding tang  134  is spaced into the plane of  FIG. 2 , and extends into a hole  136  formed in a bottom of the dovetail  124 . 
         [0044]    As shown in  FIG. 3 , the lock ring  100  is in contact with the grounding element  130  along with the forward face  132 . The bottom radially inner portion  131  extends rearwardly to the tang  134  which extends radially outwardly into the hole  136  formed in the bottom of the dovetail. The tang  134  is shown secured in a conductive adhesive  138 . 
         [0045]    As can be appreciated, the lock ring  100  contacts the rotor  116 , The lock ring  100  also contacts the grounding element at forward face  132 , and provides an electrical connection through the tang  134  into the adhesive  138 . The adhesive  138  can be read as being part of the grounding element. A surface  139  in the dovetail hole  136  is the underlying aluminum substrate, and thus provides a good conductive surface such that static electricity may be drained from the fan blade  120 , and to the rotor  116 . The location of the contact is such that it is generally protected from the elements such that there is unlikely to be corrosion at the connection. 
         [0046]    As can be appreciated, the coating material  125  is less electrically conducive than the aluminum at surface  139 , or the lock ring  100 . However, the provision of the grounding element  130  still provides the grounding connection and through the coating layer  125 . 
         [0047]    While the disclosed embodiment provides contact between the grounding element  130  and the lock ring  100 , it is also possible to have the grounding element contact the rotor  116  directly. While the specified disclosure is to a fan blade, other blades, such as compressor and turbine blades, may benefit from these teachings. 
         [0048]    Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.