Abstract:
A hollow fan blade for turbo fan gas turbine engines is formed of two separate detail halves. Each detail half has a plurality of cavities and ribs machined out to reduce weight. These detail halves are subsequently bonded and given an airfoil shape in the forming operation. The present invention provides a hollow fan blade with internal cavity and rib geometry with improved durability that permits the bonding and forming to be performed without the need for gas pressurization. The ability to form the hollow fan blade of the present invention without gas pressurization is a result of the cavity and rib geometry and the orientation of the ribs. The ribs are tapered and transition into a compound radius of the floor that simulates the classical arch design element. The orientation of the ribs is generally in a parallel plane with the load vector that results from forming loads during the pre-form and final form operations. This orientation provides compressive stress transfer into the ribs and away from the concave and convex skins. The rib spacing is controlled where ribs can not be held in a parallel plane with the load vector.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to gas turbine engines and more particularly to an improved hollow fan blade for a gas turbine engine. 
   A gas turbine engine, such as a turbo fan engine for an aircraft, includes a fan section, a compression section, a combustion section and a turbine section. An axis of the engine is centrally disposed within the engine and extends longitudinally through the sections. The primary flow path for working medium gases extends axially through the sections of the engine. A secondary flow path for working medium gases extends parallel to and radially outward of the primary flow path. 
   The fan section includes a rotor assembly and a stator assembly. The rotor assembly of the fan includes a rotor disc and plurality of radially extending fan blades. The fan blades extend through the flow path and interact with the working medium gases and transfer energy between the fan blades and working medium gases. The stator assembly includes a fan case, which circumscribes the rotor assembly in close proximity to the tips of the fan blades. 
   During operation, the fan draws the working medium gases, more particularly air, into the engine. The fan raises the pressure of the air drawn along the secondary flow path, thus producing useful thrust. The air drawn along the primary flow path into the compressor section is compressed. The compressed air is channeled to the combustion section where fuel is added to the compressed air and the air/fuel mixture is burned. The products of combustion are discharged to the turbine section. The turbine section extracts work from these products to power the fan and compressed air. Any energy from the products of combustion not needed to drive the fan and compressor contributes to useful thrust. 
   In order to reduce weight, the fan blades in some gas turbine engines are hollow. Each fan blade is made by combining two separate detail halves. Each half includes a plurality of cavities and ribs machined out to reduce the weight while forming a structurally sound internal configuration. These halves are subsequently bonded to form the hollow fan blade. The hollow fan blade is then subjected to forming operations at extremely high temperatures at which time it is given an airfoil shape and geometry. During the forming operation, the two detail halves are twisted and cambered under high temperatures to the desired shape. Inherent to the hollow fan blade design is a set of “skins” on the convex and concave side of the airfoil. These skins undergo significant compressive loading during the bonding and forming operations. At elevated temperatures, these skins do no possess the robustness to withstand this loading, and deform by sagging or drooping inward toward the center of the blade. To prevent collapse of the cavities during the forming process, the cavities are filled with high-pressure gas to maintain their geometry during the forming operation. 
   To a large extent, the internal geometry of the hollow fan blades has been designed to provide bird-impact capabilities. The previous hollow fan blades had an internal geometry comprising numerous machined internal cavities and associated ribs primarily running radially with secondary ribs running chord-wise. 
   There are several drawbacks to the known hollow fan blades. First, using the high-pressure gas required during forming operation increases time and cost of the operation. Additionally, the intersecting ribs in the hollow fan blades require numerous different diameter cutters and numerous cutting operations to achieve the small fillets that the objectives dictate. This also increases the time and cost of manufacturing the hollow fan blades. 
   SUMMARY OF THE INVENTION 
   The present invention provides a hollow fan blade with internal cavity and rib geometry with improved durability that permits the bonding and forming to be performed without the need for gas pressurization. This reduces the time and cost of manufacturing the hollow fan blade. 
   The ability to form the hollow fan blade of the present invention without gas pressurization is a result of the cavity and rib geometry and the orientation of the ribs. The ribs are tapered and transition into a compound radius of the floor that simulates the classical arch design element. The orientation of the ribs is generally in a parallel plane with the load vector that results from forming loads during the pre-form and final form operations. This orientation provides compressive stress transfer into the ribs and away from the concave and convex skins. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages of the present invention can be understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
       FIG. 1  is a sectional view of an axial flow, turbo fan gas turbine engine with the hollow fan blades of the present invention. 
       FIG. 2  is a plan view of one detailed half of one of the hollow fan blades of  FIG. 1 . 
       FIG. 3  is a sectional view through three of the cavities of the detail half of  FIG. 2  and through a cutter for forming the cavities. 
       FIG. 4  is a sectional view through an assembled fan blade corresponding to the fan blade detail half of  FIG. 3 . 
       FIG. 5  is a rear view of the assembled fan blade of  FIG. 4 , illustrating two section lines. 
       FIG. 6  is an enlarged, top perspective view of the fan blade of  FIG. 5 . 
       FIG. 7  is a front view of the fan blade of  FIG. 4 , after the twisting and cambering operation. 
       FIG. 8  is an enlarged view of the upper portion of the fan blade of  FIG. 7 . 
       FIG. 9  is top view of the fan blade of  FIG. 7 . 
       FIG. 10  is a plan view of an alternate detail half for the fan blades shown in  FIG. 1 . 
       FIG. 11  is an enlarged view of an alternate rib for the detail half of  FIG. 10 . 
       FIG. 12  shows one arrangement for a plurality of the alternate ribs of  FIG. 11 . 
       FIG. 13  shows another arrangement for a plurality of the alternate ribs of  FIG. 11 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A gas turbine engine  10 , such as a turbofan gas turbine engine, circumferentially disposed about an engine centerline, or axial centerline axis  12  is shown. The engine  10  includes a fan  14 , a compressor  16 , a combustion section  18  and a turbine  20 . As is well known in the art, air compressed in the compressor  16  is mixed with fuel which is burned in the combustion section  18  and expanded in turbine  20 . The air compressed in the compressor and the fuel mixture expanded in the turbine  20  can both be referred to as a hot gas stream flow  28 . The turbine  20  includes rotors  22  which rotate in response to the expansion, driving the compressor  16  and fan  14 . The turbine  20  comprises alternating rows of rotary airfoils or blades  24  and static airfoils or vanes  26 . 
   The fan  14  is surrounded by a fan case  27  and includes a rotor assembly. The rotor assembly includes a rotor disk  29  and a plurality of fan blades  30 . Each fan blade  30  extends radially outwardly from the rotor disk  29  across the working medium flow paths into proximity with the fan case  27 . The fan blades  30  are hollow fan blades and include a first hollow fan blade detail half  30   a  and a second hollow fan blade detail half  30   b.    
   A first embodiment of one fan blade detail half  30   a  is shown in  FIG. 2 . The other fan blade detail half  30   b  would be complementary. The fan blade detail half  30   a  comprises a substrate  31 , preferably Titanium, having a root edge  32  opposite a tip  34  and a leading edge  36  opposite a trailing edge  38 . The fan blade detail half  30   a  includes Region A, which is approximately the radially inner-most third adjacent the root edge  32 . Region B extends from Region A toward the tip  34 , excluding a corner area adjacent the tip  34  and trailing edge  38 , which is Region C. 
   In order to reduce weight while still maintaining the necessary stiffness and strength, a plurality of elongated continuous cavities  40   a–d  are machined into the interior surface of the substrate  31 . The cavities  40   a–d  are spaced from one another to form a plurality of continuous, non-intersecting ribs  42   a–d . Alternatively (or additionally), the ribs  42   a–d  are superplastically formed. Throughout this description, the reference numeral  40  may be used to refer to the cavities  40  generically, while for specific subsets of cavities  40 , the reference numeral  40  will be appended with one or more of the letters a–d. Similarly, the reference numeral  42  may be used generically for the ribs  42   a–d.    
   The ribs  42  are oriented and biased in order to provide stiffness where needed, both during forming and during use in the turbine engine  10  of  FIG. 1 . Further, the ribs  42  curve and change direction to eliminate any long, straight cavities  40 , which would have low inertia. Preferably, the cavities  40  do not continue in any direction for lengths greater than half the blade chord. 
   A first subset of cavities  40   a  and ribs  42   a  extend continuously from the root edge  32  toward the leading edge  36 . The cavities  40   a  and ribs  42   a  extend from the root edge  32  initially radially outward (i.e. toward the tip  34 ) in Region A and then curve slightly away from and then toward the leading edge  36  at substantially a 45 degree angle but in a curved path in Region B. Region A is an area of significant blade pull (i.e. P/A). The portions of the ribs  42   a  in Region A help carry the load on the blade half  30   a . The radially-extending portions of ribs  42   a  also minimize any stress concentration from the fillets. The slight curves in Region A prevent cavity  40   a  collapse during the forming process, when the fan blade detail half  30   a  is formed to its desired shape. 
   In Region B, a second subset of cavities  40   b  and ribs  42   b  extend continuously from the leading edge  36  toward the trailing edge  38  and curve downwardly slightly toward the root edge  32  at approximately a 45 degree angle, but in a curved path. These portions of the ribs  42   a  and ribs  42   b  in Region B extend substantially chordwise (at approximately a 45 degree angle) at the leading edge to provide bird strike stiffness. 
   A third subset of cavities  40   c  and ribs  42   c  extend continuously along a curve approximately 45 degree chordwise path and then sharply curve perpendicularly to extend substantially radially toward the tip  34  and trailing edge  38  at approximately a 60 degree angle. A fourth subset of cavities  40   d  and ribs  42   d  extend continuously along a curved path substantially radially and toward the tip  34  and the trailing edge  38  at an approximately 60 degree angle. In Region C, these ribs  42   c  and  42   d  are oriented transversely to the tip  34  to provide strength in the event of a tip  34  rub on the inner surface of the fan housing. Rib orientation in Region C is close to perpendicular to rib orientation of Region B where they meet in order to minimize mass of the fillets, which are a result of the cutter radius. 
   Generally, near the tip  34 , stiffness is needed in the radial direction for tip rub events. Diagonal stiffness is needed in the corners adjacent the tip  34  and leading edge  36  and adjacent the tip  34  and trailing edge  38 . 
   The cavities  40  are all formed in the substrate  31  between the root edge  32  and the tip  34 , and between the leading edge  36  and trailing edge  38 . Along each edge  32 ,  34 ,  36 ,  38  is a frame  44  that is substantially equal to the thickness of the ribs  42 . Each of the ribs  42  is contiguous with the frame  44  at both ends. Each of the cavities  40  begins and terminates adjacent the frame  44 . The termination points occur in regions where the airfoil thickness is relatively low, which reduces the depth the cutters have to plunge into the part to start machining. 
     FIG. 3  is a sectional view of the detail half  30   a  being machined by a cutter  54 . Each cavity  40  has a floor  48  between opposite wall interior surfaces  50 , some of which define the ribs  42 . Each cavity  40  further includes a radius  42  transition between the wall interior surface  50  and the floor  48 . As shown, the floor  48  and both wall interior surfaces  50  are preferably cut simultaneously in a single pass by the cutter  54 . Because the cavities  40  are continuous and the ribs  42  do not intersect, each cavity  40  is formed in a single pass with a single cutter. Alternatively, the cavities  40  may each be formed in a single rough cut and a second, finish cut, but this is still a significant reduction in the number of cuts and cutters required. Additionally, because floor radius is relatively large and approximately follows the curvature of the external surface of the cutter  54  can be operated by a 3-axis machine  55  (shown schematically), instead of the previously-required 5-axis machine. In addition, because there are no transversely-extending ribs intersecting the ribs  42  the number of cutters of different diameters required is greatly reduced. A detail half could conceivably be done with a single form cutter, including both rough and finish passes. The other fan blade detail half  30   b  would be made in a similar manner. 
     FIG. 4  is a sectional view of a portion of the fan blade  30 . The ribs  42  of fan blade detail half  30   a  are aligned and joined with the ribs  42  of the fan blade detail half  30   b . To provide increased strength during forming and during use, the ribs  42  are tapered and transition into a compound radius (including radius  52  and the floor  48 ) that simulates the classical arch design element. The two radii (of the radius  52  and floor  48 ) should be selected such that the transition between each other and the tapered wall geometry are smooth and gradual. The sizing will depend upon the required load transitioning and carrying capabilities. Preferably, the ratio of the width w of the cavity at the rib wall fillet run out to the thickness t of the floor  48  should be less than ten, but can be larger if the rib can be aligned more parallel to the load. 
   After the halves  30   a,b  are bonded, the fan blade  30  is given an airfoil shape in a forming operation, which is illustrated in  FIGS. 5–9 . During the forming operation, the two detail halves are twisted and cambered to the desired shape under high heat. Because of the orientation and shape, as well as the spacing, of the ribs  42  as described and shown, the cavities  40  do not require a high pressure gas to increase their strength and prevent cavity collapse during the forming operation. This reduces the time and expense of the forming operation. 
     FIG. 5  is a rear view of the assembled fan blade  30  of  FIG. 4 , illustrating two section lines, a and b, through the fan blade  30 .  FIG. 6  is an enlarged, top perspective view of the fan blade of  FIG. 5 . As can be seen in  FIG. 6 , the section line a extends from a position generally near the corner of the tip  34  and leading edge  36  rearwardly to the trailing edge  38  and downwardly toward the root edge  32  ( FIG. 5 ). The section lines a, b assist in visualizing the twisting and cambering of the fan blade  30  relative to the ribs  42  and cavities  40 . 
     FIG. 7  is a front view of the fan blade of  FIG. 4  after the forming operation, showing the resulting locations of sections a and b.  FIG. 8  is an enlarged view of the upper portion of the fan blade of  FIG. 7 .  FIG. 9  is top view of the fan blade of  FIG. 7 . As can be seen by referencing  FIG. 5  with respect to  FIGS. 7–9 , the orientation of the ribs  42  should be in a parallel plane with the load vector that results from forming loads during the pre-form and final form operations. This orientation presents the optimum configuration for load carrying capability and compressive stress transfer into the ribs  42  and away from the concave and convex skins. The specific orientation of the ribs  42  is therefore dependent upon the final shape of the fan blade  30 , which will vary from one engine to another. Obviously, there are trade-offs and balancing among the various requirements as described herein, such that the ribs  42  cannot always be completely in a parallel plane with the load vector. For this reason ribs totally straight are avoided. 
     FIG. 10  illustrates a second embodiment of a fan blade detail half  60   a  that could be used in the turbine engine  10  of  FIG. 1 . Again, the other fan blade detail half (not shown) would be complementary. The fan blade detail half  60   a  comprises a substrate  61 , preferably Titanium, having a root edge  62  opposite a tip  64  and a leading edge  66  opposite a trailing edge  68 . A plurality of elongated continuous cavities  70  are machined into or superplastically or otherwise formed on the interior surface of the substrate  61  in the manner described above with respect to  FIGS. 3 and 4 . The cavities  70  are spaced from one another to form a plurality of continuous non-intersecting ribs  72 ,  73 . In this embodiment the spacing between ribs is held constant throughout. 
   In the second embodiment, the cavities  70   a–d  extend continuously alongside ribs  72   a–e  and ribs  73 . The cavities  70   a–d  also extend around freestanding ends  74  of some of the ribs  72   a–e , thereby reducing the number of cavities  70   a–d.    
   The fan blade detail half  60   a  includes Region A, which is approximately the radially inward half. Region B comprises approximately a quarter adjacent the leading edge  66  and the tip  64 . Region C comprises approximately a quarter adjacent the trailing edge  68  and the tip  64 . A transition region is the central area substantially defined among the Regions A, B and C. 
   A first subset of cavities  70   a  and ribs  72   a  extend continuously from the root edge  62  initially radially outward in Region A, and then curving toward the leading edge  36  at substantially a 45 degree angle but in a curved path in Region B. In Region B, each cavity  70   a  extends continuously around a freestanding end  74  of one of the ribs  72   a , thereby reducing the number of cavities  70   a . Ribs  73  extend continuously parallel to the ribs  72   a  from the root edge  62  to the leading edge  66  between adjacent cavities  70   a.    
   A second subset of cavities  70   b  and ribs  72   b  extend continuously from the root edge  62  initially radially outward toward the tip  64  in Region A adjacent the trailing edge  68 , and curving slightly toward the leading edge  66  in the transition region and then toward the trailing edge  68  at approximately a 45 degree angle in a slightly curved path in Region C. Each cavity  70   b  also extends continuously around a freestanding end  74  of one of the ribs  72   b , thereby reducing the number of cavities  70   b . Ribs  73  are defined between adjacent cavities  70   b  and are parallel to the ribs  72   b.    
   A third subset of ribs  72   c  extend from the leading edge  66  toward the trailing edge  68  and curve downwardly slightly toward the root edge  62  in a curved path in Region B. At least one cavity  70   c  extends continuously in a serpentine path around one free end  74  of one rib  72   c , around the opposite free end  74  of the next rib  72   c  and again around the opposite free end  74  of the next rib  72   c . The serpentine path further reduces the number of cavities  70  needed to define the ribs  72 . 
   A fourth set of ribs  72   d  are each at least partially defined by a single cavity  70   d . The cavity  70   d  and a rib  72   d  extend continuously from the root edge  62  radially outward (toward the tip  64 ) in Region A, then curve slightly toward the leading edge  66  in the transition region, and then slightly toward the trailing edge  68  at an approximately 60 degree angle in Region C. Near the tip  64 , the cavity  70   d  extends continuously around the free end  74  of the rib  72   d  and then around alternating free ends  74  of two more ribs  72   d  oriented approximately 60 degrees toward the tip  64 . The cavity  70   d  then extends continuously into Region B around alternating free ends  74  of a plurality of ribs  72   e  oriented substantially parallel to the ribs  72   c , i.e. substantially chordwise, approximately 30 degrees and curved slightly. This long serpentine path of cavity  70   d  further reduces the number of cavities  70  necessary to create ribs  72 . 
   Again, Region A is an area of significant blade pull. The portions of the ribs  72   a, b, d  in Region A help carry the load on the blade half  60   a . The radially-extending portions of ribs  72   a, b, d  also minimize any stress concentration from the fillets. The substantially chordwise orientation of the portions of ribs  72   a, c, d  in Region B provide bird strike strength. The substantially radial orientation of the portions of the ribs  72   b, d  in Region C provide strength to the tip in the event of tip rub on interior of the fan housing. 
   By machining contiguous cavities  70  around freestanding ends  74  of the ribs  72 , fewer cavities  70  are required, thereby reducing time and cost. However, the freestanding ends  74  of the ribs  72  may cause a variation in the stiffness, which can act as a stress concentration when the blade sees externally applied loads, such as from bird impact or heavy bending moments from a released neighboring blade during a blade out event. Therefore,  FIG. 11  shows an enlarged view of an alternate rib  72 ′ for the detail half  60   a  of  FIG. 10 . The alternate rib  72 ′ would also be used in the complementary detail half (not shown). The alternate rib  72 ′ has a freestanding end  74 ′ that is flared such that it has a larger width than the rest of the rib  72 ′. The cavity  70  extends continuously around the free, flared end  74 ′ of the rib  72 ′. The ribs  72 ′ are tapered and have a radius  82  transition into the floor  78 . The radius  82  extends around the flared end  74 ′. The ribs  73 ′ between adjacent cavities  70  have a generally constant width. The flared end  74 ′ increases the strength of the bond joint in that location. It also increases the footprint at the base of the fillet, which improves the stiffness in the vicinity of the flared end  74 ′, which reduces the load on the bond joint. 
   Because it would be impractical to vary the width of the cutter and the cavity  70 ,  FIG. 12  shows one possible arrangement of a plurality of the ribs  72 ′ with the flared ends  74 ′. Incorporating the flared ends  74 ′ is achievable with minimum weight impact if the cavity  70  ends can be staggered as shown in  FIG. 12 . In  FIG. 12 , the ribs  72 ′ and the cavities  70  that extend continuously around the flared ends  74 ′ of the ribs  72 ′ are staggered, such that the increased width of a flared end  74 ′ of one rib  72 ′ is not aligned with the flared ends  74 ′ of the adjacent ribs  72 ′ (referring to “alignment” in a direction perpendicular to the ribs  72 ′). Oblique ribs  72 ′ are a good way to stagger the ends  74 ′. 
   Where stagger is not feasible, the thickness changes needs to be gradual or they force curvature into successive neighbors as shown in  FIG. 13 . In  FIG. 13 , the flared ends  74 ′, ribs  72 ′ and cavities  70  are not staggered, but are aligned. In this case, the increase in thickness at the flared ends  74 ′ is taken from a decreased thickness in adjacent ribs  73 ″ between adjacent cavities  70 . 
   In accordance with the provisions of the patent statutes and jurisprudence, exemplary configurations described above are considered to represent a preferred embodiment of the invention. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope. Alphanumeric identifiers for steps in the method claims are for ease of reference by dependent claims, and do not indicate a required sequence, unless otherwise indicated.