Abstract:
A turbine airfoil includes at least one spar arrangement having a length less than an associated turbine airfoil length. During operation, the turbine airfoil has an outer skin surface which operates at a substantially higher temperature than that of an internal supporting parted spar arrangement. The parted spar arrangement permits the turbine airfoil outer skin surface to thermally expand between spar arrangements, thus preventing self-constraining thermal stresses from forming within the spar arrangement or the airfoil skin surfaces.

Description:
GOVERNMENT RIGHTS 
     The government has rights in this invention pursuant to Contract No. F33615-97C-2778 awarded by the Department of the Air Force. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to airfoils and, more particularly, to turbine airfoils with parted spars. 
     Turbine airfoils include a blade tip, a blade length, and a blade root. Typically, a cooling system supplies pressurized air internally to the airfoil blade. The internal pressures created by the cooling system generate ballooning stresses at an outer skin of the airfoil blade. To prevent the internal pressures from damaging the airfoil blade, typically the outer skin is supported with a rigid spar which extends along the length of the airfoil. 
     External surfaces of turbine airfoils are subjected to high temperature gas flows during operation. Cooling a turbine airfoil prolongs the turbine airfoil useful life and improves turbine airfoil performance. Increasing the turbine airfoil performance enhances efficiency and performance of an associated turbine engine. As engine performance is enhanced, turbine airfoils are subjected to increased aerodynamic loading and higher temperature gas flows. To withstand such loads and temperatures, turbine airfoils may be fabricated using composite materials. Although such composite materials can withstand the loads and high temperatures, such materials usually are not as resistive to high temperature gradients as other known materials. 
     During operation, turbine airfoils are cooled internally with a pressurized cooling system. Accordingly, continuous spars operate at temperatures which are substantially less than the operating temperatures of the turbine airfoil outer skin surfaces. A temperature gradient between the continuous spar and the outer skin surfaces creates opposing thermal strains in both the continuous spar and the outer skin surfaces. The thermal strain mismatch created by the temperature gradient causes the continuous spar operating at a lower temperature to be in tension, and the outer skin surfaces to be in compression. Composite materials, such as ceramics, maintain a high modulus of elasticity and a low ductility at high temperatures, and the thermal stresses may cause cracks to develop within the continuous spars leading to failure of the turbine airfoil. 
     BRIEF SUMMARY OF THE INVENTION 
     In an exemplary embodiment, a turbine airfoil includes a parted spar arrangement which reduces thermal stresses within the turbine airfoil. The turbine airfoil includes a blade tip, a blade root, and a blade span extending between the blade tip and the blade root. The blade span includes a skin covering extending over the blade span, and at least one spar arrangement having a length less than a length of the blade span and positioned between the blade root and the blade tip. The spar arrangement includes a plurality of spars including at least a first spar having a first side and a second side. 
     During operation, the turbine airfoil is cooled internally such that an outer skin covering surface operates at higher temperatures than that of the parted spar arrangement and temperature gradients develop between the parted spars and the outer skin covering surface. Because the airfoil uses parted spar arrangements, the turbine airfoil skin surfaces are permitted to thermally expand between parted spar arrangements which prevents thermal stresses from developing as a result of the outer skin surfaces operating at higher temperatures. Accordingly, the outer skin coverings and the parted spar arrangements are not subjected to the potentially damaging thermal strains of known turbine airfoils and may be fabricated from low strength and low ductility materials to provide a turbine airfoil which includes a spar arrangement that is reliable and cost-effective. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a turbine airfoil including a parted spar arrangement; 
     FIG. 2 is a cross-sectional view of the turbine airfoil along line  2 — 2  shown in FIG. 1; 
     FIG. 3 is a cross-sectional view of an alternative embodiment of a turbine airfoil including a parted spar arrangement; 
     FIG. 4 is a perspective view of a high pressure vane including a parted spar arrangement; and 
     FIG. 5 is a perspective view of a strut leading edge extension including a parted spar arrangement. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a perspective view of a turbine airfoil  10  including a parted spar arrangement  11 . Turbine airfoil  10  includes a blade root  12 , a blade tip  14 , and a blade span  16  extending between blade root  12  and blade tip  14 . Blade span  16  has a length  18  and includes a skin covering  20  which extends over blade span  16  from blade root  12  to blade tip  14 . Skin covering  20  includes an outer skin surface  21  and an inner skin surface (not shown in FIG.  1 ). Blade length  18  extends between blade root  12  and blade tip  14  along a line  22 . In one embodiment length  18  is approximately 2.0 inches. Turbine airfoil  10  extends from a mounting feature  24  which is configured to anchor turbine airfoil  10  to an associated turbine engine (not shown). In one embodiment, mounting feature  24  is a dovetail key. 
     FIG. 2 is a partial perspective view of turbine airfoil  10  including a parted spar arrangement  11 . Turbine airfoil  10  includes a suction side  52  and a pressure side  54 . Pressure side  54  has more curvature than suction side  52 . When turbine airfoil  10  is exposed to an airflow, the increased curvature of pressure side  54  causes an area of low pressure to form adjacent suction side  52  of turbine airfoil  10  and an area of high pressure to form adjacent pressure side  54  of turbine airfoil  10 . 
     Turbine airfoil  10  is manufactured such that spar arrangement  11  is integrally connected with skin covering  20  and extends from skin covering  20 . Accordingly, suction side  52  of turbine airfoil  10  includes outer skin surface  21  and an inner skin surface  56 , and pressure side  54  of turbine airfoil  10  includes outer skin surface  21 , and an inner skin surface  60 . Pressure side  54  and suction side  52  are connected to spar arrangement  11  and define a turbine airfoil leading edge  64  and a trailing edge  66 . Leading edge  64  is smooth and extends between suction side  52  and pressure side  54 . Leading edge  64  has a width  70  which is greater than a width  72  of trailing edge  66 . 
     Parted spar arrangement  11  includes a first spar  80  and a second spar  82  positioned between first spar  80  and trailing edge  66 . First spar  80  has a first side  84  and a second side  86 . A first cavity  88  is formed between leading edge  64  and first spar first side  84 . First spar  80  extends from suction side inner skin surface  56  to pressure side inner skin surface  60  for a width  90 . First spar  80  also has a length  92  which extends from a first side  93  of spar arrangement  11  in a direction substantially parallel to line  22  to a second side (not shown) of spar arrangement  11 . In one embodiment, width  90  is approximately 0.5 inches and length  92  is approximately 0.25 inches. 
     Second spar  82  has a first side  94  and a second side  96 . A second cavity  98  is formed between first spar second side  86 , second spar first side  94 , pressure side inner skin surface  60  and suction side inner skin surface  56 . Suction side inner skin surface  56  and pressure side inner skin surface  60  are connected and form a trailing edge wall  100 . Suction side outer skin surface  21  and pressure side outer skin surface  21  extend from trailing edge wall  100  to form trailing edge  66 . A third cavity  110  is formed between suction side inner skin surface  56 , pressure side inner skin surface  60 , trailing edge wall  100 , and second spar second side  96 . Second cavity  98  is positioned between first cavity  88  and third cavity  110 . 
     Second spar  82  has a length  112  which extends from first side  93  of spar arrangement  11  to the second side of spar arrangement  11 . Second spar  82  also has a width  114  which extends from suction side inner skin surface  56  to pressure side inner skin surface  60 . In one embodiment, length  112  is substantially equal to length  92  of first spar  80 . Alternatively, length  112  of second spar  82  is different than length  92  of first spar  80 . In another embodiment, first spar  80  is offset from second spar  82  in direction  22 . In a further embodiment, length  112  is approximately 0.3 inches, width  114  is approximately 0.3 inches, and first spar  80  is offset approximately 0.1 inches in direction  22  from second spar  82 . 
     During operation, outer skin surface  21  is subjected to high temperature gas flows. To cool turbine airfoil  10 , a cooling system (not shown) supplies a pressurized airflow internally to turbine airfoil  10 . Because of the pressurized airflow supplied by the cooling system, spar arrangement  11  operates at a substantially cooler temperature than skin covering  20  including outer skin surface  21 , pressure side inner skin surface  60 , and suction side inner skin surface  56 . Accordingly, a temperature gradient is created between skin covering  20  and spar arrangement  11 . 
     Spar arrangement spars  80  and  82  have lengths  92  and  112  respectively, which permit pressure side  54  and suction side  52  to thermally expand without developing thermal strains in spar arrangement  11 . As a result, spar arrangement  11  can be constructed from low strength and low ductility material. In one embodiment, spar arrangement  11  is constructed from SiC—SiC Ceramic Matrix Composite material. Alternatively, spar arrangement  11  is constructed from a monolithic ceramic material. 
     Alternatively, turbine airfoil  10  may be fabricated with additional spar arrangements  120 . Spar arrangements  120  are constructed substantially similarly to spar arrangement  11  and include a first spar  122  and a second spar  124 . Spar arrangements  120  are positioned between spar arrangement  11  and blade tip  14  and spars  122  and  124  are located a distance  126  and  128  respectively from spar arrangement  11 . In one embodiment, spar arrangements  120  are located approximately 0.1 inches from spar arrangement  11 . In another embodiment, first spar  122  is offset from first spar  80  in a direction  129  and second spar  124  is offset from second spar  82  in direction  129 . In one embodiment, spars  122  and  124  are offset from spars  80  and  82  respectively, approximately 0.1 inches in direction  129 . 
     FIG. 3 is a partial perspective view of a turbine airfoil  130  including a parted spar arrangement  132 . In one embodiment, turbine airfoil  130  is a frame strut. Turbine airfoil  130  includes a blade tip (not shown), a blade root (not shown), and has a blade span  136  which extends between the blade root and the blade tip. Turbine airfoil  130  further includes a first side  140  and a second side  142 . Turbine airfoil  130  includes an outer skin covering surface  144  which extends over blade span  136 . First side  140  includes outer skin covering surface  144  and an inner skin surface  146 . Second side  142  of turbine airfoil  130  includes outer skin surface  144  and an inner skin surface  148 . First side  140  and second side  142  are connected to spar arrangement  132  and define a turbine airfoil leading edge  150 . Leading edge  150  is smooth and extends between first side  140  and second side  142 . Outer skin surface  144  extends from leading edge  150  to a trailing edge  152 . Turbine airfoil first side  140  has a curvature extending from leading edge  150  to trailing edge  152  that is substantially the same as a curvature extending over second side  142 . In one embodiment turbine airfoil  130  is a symmetrical airfoil. 
     Parted spar arrangement  132  includes a first spar  160  and a second spar  162  positioned between first spar  160  and trailing edge  152 . First spar  160  has a first side  164  and a second side  166 . A first cavity  168  is formed between leading edge  150  and first spar first side  164 . First spar  160  extends from first side inner skin surface  146  to second side inner skin surface  148  for a width  170 . First spar  160  also has a length  172  extending from a first side  173  of spar arrangement  132  to a second side (not shown) of spar arrangement  132 . 
     Second spar  162  has a first side  180  and a second side  182 . A second cavity  184  is formed between first spar second side  166 , second spar first side  180 , first side inner skin surface  146  and second side inner skin surface  148 . A third cavity  185  is formed between second spar second side  182 , first side inner skin surface  146 , trailing edge  152 , and second side inner skin surface  148 . Second spar  162  has a length  188  which extends from first side  173  of spar arrangement  132  to the second side of spar arrangement  132 . Second spar  162  also has a width  190  which extends from second side inner skin surface  148  to first side inner skin surface  146 . 
     FIG. 4 is a perspective view of a high pressure vane  200  including a parted spar arrangement  202 . Vane  200  includes a vane root  204 , a vane tip  206 , and a vane span  208  extending between vane root  204  and vane tip  206 . Vane span  208  has a length  210  and includes a skin covering  212  which extends over vane span  208  from vane root  204  to vane tip  206 . Skin covering  212  includes an outer skin surface  214  and an inner skin surface (not shown). High pressure vane  200  extends from a mounting feature  220  which is configured to anchor vane  200 . 
     Parted spar arrangement  202  includes a first spar  222  and a second spar  224 . First spar  222  is positioned between a first cavity  230  and a second cavity  228 . Second spar  224  is positioned between cavity  228  and a third cavity  226 . 
     FIG. 5 is a perspective view of a strut leading edge extension  250  including a parted spar arrangement  252 . Strut leading edge extension  250  has a first end  254 , a second end (not shown), and an extension span  256  extending between first end  254  and the second end. A skin covering  258  extends over extension  250  from first end  254  to the second end and defines a leading edge  260  and a trailing edge  262 . Trailing edge  262  extends to a mounting feature  264  configured to anchor strut leading edge extension  250  to a strut (not shown). In one embodiment, mounting feature  264  is a dovetail key. 
     Parted spar arrangement  252  includes a first spar portion  270 . First spar portion  270  has a first side  272 , a second side  273  and a length  274 . First spar portion  270  is parted along span  256  of strut leading edge extension by a parting distance  276  and has a second portion  278 . First side  272  bounds a first cavity  279  and second side  273  bounds a second cavity  280 . First spar  270  is formed integrally with skin covering  258  and extends from a first side  282  of strut leading edge extension  250  to a second side  284  of strut leading edge extension  250 . Thus, a total spar length of parted spar arrangement  252  is equal to a sum of the length of second portion  278  and length  274  of first portion  270 , and this total spar length is less than span  256 . 
     The above-described turbine airfoil includes parted spar arrangements that are cost-effective and reliable. The turbine airfoil includes at least one spar arrangement which has an overall length less than that of a turbine airfoil blade length and which includes a plurality of spars to support the airfoil skin from the internal pressures generated by the cooling system. Furthermore, the spar arrangement permits the outer skin surfaces of the turbine airfoil to thermally expand. Such expansion prevents thermal strains within the turbine airfoil and permits the spar arrangement to be constructed from a low strength and low ductility material. Accordingly, a cost effective and accurate airfoil spar arrangement is provided. 
     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.