Abstract:
A propeller blade includes a foam core having a groove formed therein, a fibrous material filling at least a portion of the groove and a structural layer that surrounds the fibrous material and at least a portion of the foam core.

Description:
PRIORITY CLAIM 
     This application claims priority to European Patent Application No. 12305590.7, filed May 29, 2012, and all the benefits accruing therefrom under 35 U.S.C. §119, the contents of which in its entirety are herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to propellers and, in particular, to propeller blades that include a rib in the spar core of the propeller blade. 
     Modern propeller blades typically include root portions which extend into the hub arm of the hub of the propeller system and which are secured to and rotatable relative to the hub arm via a retention assembly. Typically the retention assembly includes one or a plurality of ball bearing assemblies which permit the rotation of the blade in the hub arm for accomplishing pitch change of the blade for altering the speed of the propeller and accordingly, the aircraft. 
     The blades are typically formed by surrounding a foam spar core with a resin impregnated fabric. Leading and trailing edges of the blade are then formed over the fabric and surrounded by, for example, a Kevlar sock. Such blades are light and effective for their intended purposes. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one embodiment, a propeller blade that includes a foam core having a groove formed therein, a fibrous material filling at least a portion of the groove and a structural layer that surrounds the fibrous material and at least a portion of the foam core is disclosed. 
     In another embodiment, a method of forming a propeller blade that includes: forming a foam core, the form core including a groove formed therein; disposing a fibrous material in the groove; and forming a structural layer that surrounds fibrous material and a portion of the foam core is disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a plan-view of a propeller blade according to one embodiment of the present invention; 
         FIG. 2  is a cross-section of the propeller blade shown in  FIG. 1 ; 
         FIG. 3  is a plan-view of a spar core having a groove formed therein; and 
         FIG. 4  is a plan-view of the spar core of  FIG. 3  with the groove filled with a fibrous material. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIGS. 1 and 2 , plan and cross-section views of a propeller blade  100  according to one embodiment of the present invention are illustrated. For simplicity, the direction shown by arrow X shall be referred to as the span wise direction herein, the direction shown by arrow Y shall be referred to as the chord wise direction herein and the direction shown by arrow Z shall be referred to as the thickness direction or simply, the thickness. The cross-section shown in  FIG. 2  is taken along line A-A. 
     The blade  100  is formed by first forming a spar  102 . The spar  102  includes a spar foam core  104  (core) surrounded by a structural layer  106 . The core  104  is typically formed of a foam material that is injected into a mold to form a particular shape. In other embodiments, the core  104  could be machined to define the desired shape. The mold can include a layer of fiberglass or carbon (pre-preg) on the walls thereof to which the foam of the core  104  adheres. As such, the core  104  can be surrounded by a layer (not shown) of fiberglass or carbon in one embodiment but this is not required. The foam that forms the core  104  can be selected from one of: polyurethane (PU), polyisocyanurate, or polymethacrylimide (PMI). 
     According to one embodiment, the blade  100  includes one or more ribs  105  formed in the spar foam core  104 . A more detailed explanation of the ribs  105  is provided below but, in general, the ribs  105  serve to provide stiffness in the thickness direction (e.g., from the camber  160  to the face  162  sides) of the propeller blade  100 . In more detail, a propeller blade with high activity factor and large chord wise lengths, has a lower ratio of stiffness in the chord wise direction as compared to stiffness in the span wise direction. When the curvatures of the camber  160  and face  162  sides of the blade  100  are high, secondary bending moments may be generated. The consequences of these secondary bending moments are large deformations and loadings in the direction perpendicular to the chord (e.g., in direction Z in  FIG. 2 ). The ribs  105  can provide structure to reduce or avoid these deformations by locally raising inertia and stiffness of the chord wise direction Y of the spar  104 . 
     The structural layer  106  is typically formed of a dry braided carbon fiber which is subsequently resin injected or a resin-impregnated fabric material (e.g. resin impregnated carbon fiber fabric) and disposed such that it surrounds the core  104  and the ribs  105  (and the fiberglass layer if it is included) by, for example, a braiding process. In one embodiment, the structural layer  106  is impregnated with a resin. In some cases, the spar  102  is heated to set the resin in the structural layer  106 . With the inclusion of the ribs  105 , the respective leading and trailing edges  115 ,  116  of the spar  102  are kept in a fixed relation to one another. As such, the possibility of the core  104  cracking may be reduced. 
     In some instances, the spar  102  is formed such that a portion of it surrounds a root portion  108  that allows the blade  100  to be connected to a hub (not shown). Rotation of the hub causes the blade  100  to rotate and, consequently, causes the generation of thrust to propel an aircraft. In the following discussion, it shall be assumed that the blade  100  rotates in the clockwise direction. The root portion  108  is sometimes referred to as a “tulip” in the industry and is typically formed of a metal. 
     After the spar  102  is formed, leading edge foam  112  and trailing edge foam  114  are formed on the leading and trailing edges  115 ,  116  respectively of the spar  102 . The leading edge foam  112 , trailing edge foam  114  and the spar  102  can then be encased in an outer layer  118 . The outer layer  118  can be formed of Kevlar and be in the form of a sock that is pulled over the assembly that includes the leading edge foam  112 , trailing edge foam  114  and the spar  102 . Of course, the outer layer  118  could be formed in other manners as well. 
       FIG. 3  is a plan view of the camber side  160  of the spar core  104  that forms blade  100 . The spar core  104  includes a groove  300  formed therein. The groove  300  includes a width (w) and a depth (d) that extends into the page. The groove  300  can be formed in a many different manners. For instance, the groove  300  could be part of the mold into which the foam forming the spar core  104  is formed. In another embodiment, the spar core  104  is initially formed without the groove  300  and then the groove  300  is machined or otherwise formed in the core  104 . As illustrated, the groove  300  causes depressions in the leading and trailing edges  301 ,  303  of the core  104 . Of course, the groove could be formed such that depressions are only formed in the camber  160  and face  162  sides. 
     Referring now to  FIG. 4 , the groove  300  has had one or more layers of a fibrous material  302  disposed therein. In one embodiment, the fibrous material  302  is a carbon fiber cloth. In one embodiment, the fibrous material  302  is formed of the same material as is used to form the structural layer  106  (e.g., a resin impregnated cloth or dry braided carbon fiber or cloth). The resin in the fibrous material  302  is eventually cured and the cured combination of the resin and the fibrous material  302  defines the ribs  105  shown in  FIG. 1 . 
     The fibrous material  302  could be placed in the groove  300  such that the camber  160  and face  162  sides (including the fibrous material  302 ) are substantially smooth. That is, the fibrous material  302  can fill the depth (d) of the groove  300  in one embodiment. It shall be understood, that the fibrous material  302  could be formed, for example, by braiding of dry carbon fibers in one embodiment. In another embodiment, the fibrous material  302  is a fibrous cloth and may include resin in it or not. 
     Only one groove  300  has been shown in  FIGS. 3-4  but that is by way of example, not limitation. The number and location of the grooves  300  is a matter of design choice that may be decided by the skill artisan after examination of this disclosure. 
     In the manner described above, after the fibrous material  302  has been placed, the spar core  104  shown in  FIG. 4  can have the structural layer  106  formed over it by first braiding a dry carbon fiber over the spar core  104  and fibrous material  302 . A resin can then be injected into the structural layer  106  and the fibrous material  302 . In this manner, the material can be made rigid and become the spar ribs  105  described above. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while the various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.