Fiber reinforced composite spar for a rotary wing aircraft and m

Fluid reaction surfaces (i.e. – impellers) – Specific blade structure – Laminated – embedded member or encased material

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Details

416229R, 298897, F04D 2938

Patent

active

057555583

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to fiber reinforced resin matrix composites, and more particularly, to an improved composite spar for a rotary wing aircraft having enhanced structural properties and improved damage tolerance with minimal weight, and a method for facilitating the manufacture thereof.


BACKGROUND OF THE INVENTION

A rotor blade spar is the foremost structural element of a helicopter rotor blade assembly inasmuch as its primary function is to transfer combined flapwise, edgewise, torsional and centrifugal loads to/from a central torque drive hub member. Typically, a leading edge sheath and trailing edge pocket assembly mount to and envelop the spar thereby yielding the desired airfoil contour. The spar typically extends the full length of the rotor blade and mounts at its inboard end to a cuff assembly or fitting which facilitates mounting to the hub member. Due to the extreme operational loading environment of the rotor blade, high strength, high density materials such as aluminum or titanium have, in the past, been the materials of choice for spar construction.
More recently, however, fiber reinforced resin matrix composite materials, e.g., graphite and fiberglass, have been employed due to their advantageous strength to weight ratio and improved damage tolerance. Regarding the latter, the structural fibers of composite materials can be viewed as a plurality of redundant load paths wherein damage to one or more fibers can be mitigated by the load carrying capability of adjacent fibers.
Despite the inherent weight and strength advantages of advanced composites, the widespread use thereof has been impeded by the high cost of associated fabrication methods. Blending the desired structural characteristics with a low cost manufacturing process, i.e., one which reduces labor intensive process steps yet maintains laminate quality, has been an ongoing and continuous challenge for designers of composite structures.
Primary structural items to be considered by the designer include: the selection of fiber reinforcement, i.e., materials having the requisite mechanical properties, resin binder, fiber matrix orientation, fiber continuity, alleviation of stress concentrations due to ply drop-offs or joint configurations, and reduction of thermally induced stresses. To maximize the benefits of composites it is essential that fiber orientation be optimally tailored to meet the strength and stiffness requirements for a particular application. That is, composites can be tailored to be anisotropic (capable of carrying load in a particular direction) rather than quasisotropic (equal strength in all directions); hence, orienting the fibers in the direction of the load will optimally result in the most weight efficient solution. Similarly, by varying the use of available matrix reinforcement materials (e.g., graphite, fiberglass, aramid fibers), the designer is able to control such parameters as vibratory and steady bending strength, stiffness, and toughness. In addition to the selection of materials and/or optimum fiber orientation, the continuity or discontinuity of fibers, and methods of joining discontinuous plies will significantly impact component strength. Generally, it is desirable to maintain fiber continuity and stagger joints so as to prevent stress concentrations and/or the build-up thereof in a particular region. Still other considerations relate to the thermal induced stresses which may result in microcracking. Microcracking is a phenomena wherein thermally induced stresses cause small cracks to develop in the binder material due to the thermal incompatibility of adjacent composite material. Generally, it is preferable to use the same material throughout the laminate or materials of similar thermal coefficient to reduce this effect.
These considerations are weighed and balanced against the cost and complexity of a particular fabrication technique. Typically, the manufacturing approach should: minimize cutting operations and material scrap, facilitate ease of handling, minimize the pr

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