The present invention pertains to gas turbine rotor apparatus and, more particularly, to rotor blade platforms for use therein.
Rotor blades of dynamic machines, such as axial flow gas turbine engine compressors and fans, employ platforms which extend generally laterally of the blades to partially define the aerodynamic flow path between adjacent blades. Where the rotor blades are metallic and are retained by their roots in a rotatable hub, as by the well-known dovetail and slot arrangement, the platforms are often integrally formed as cantilevered elements on one or both sides of the blade airfoil portions. Adjacent blade platforms cooperate to define the aerodynamic flow path.
The current trend is toward incorporation of composite blades within gas turbine engines due to their inherent light weight, which makes them economically attractive. Composite blades are those blades formed by laminating multiple plies of elongated, small diameter filaments of high modulus of elasticity embedded in a lightweight matrix. Typical examples are the nonmetallic composites such as graphite filaments in an epoxy resin, and the metallic composites represented by boron filaments embedded in an aluminum matrix. The manner of lamination and filament orientation is well known in the art, though development is continuing. However, to date no practical configuration has been found by which blade platforms can be molded integrally with the blades during the blade lamination process. Accordingly, attention has been directed toward bonding a composite platform to the blade after the blade forming operation, an approach which has been generally unsuccessful for several reasons. First, complete bonding which is capable of withstanding delamination or separation under centrifugal loading is difficult to obtain. For example, in the case of a metallic composite, diffusion bonding of the platform and blade require pressures and temperatures of such magnitude that the relatively brittle filaments within the ply matrix can be crushed. Secondly, fastening by such conventional means as bolts and rivets is undesirable due to stress concentration at the holes and the fact that the holes necessarily sever the high-strength elongated filaments.
Accordingly, it becomes necessary to develop a platform for a composite blade which overcomes the aforementioned difficulties. Preferably, the platform configuration should be adaptable to blades of the increasingly popular variable pitch variety (i.e., those that are rotatable about their longitudinal axis). Furthermore, since composite blades and platforms are particularly susceptible to foreign object damage due to the relatively soft matrix materials and relatively brittle filament materials employed, it becomes desirable to provide a rotor apparatus wherein the blade or platform, or both, are easily replaceable.