Composite Flexbeam Joint

1. Technical Field

This invention relates to composite flexbeams for helicopter rotors and more particularly to a flanged composite flexbeam in which the flanges are continuously wrapped around fittings at each end of the beam to provide, with the flexbeam web, encapsulated end joints which are an integral part of the beam structure.

2. Background Art

A current method of obtaining articulation on state-of-the-art helicopter rotors is the bearingless approach whereby rotor blades are mounted on composite flexbeams rigidly attached to the rotor shaft. There is a joint between the flexbeam and the shaft and between the flexbeam and the blade. The flexbeams are tailored to meet the required bending stiffness at a minimum torsional stiffness and are believed to be the preferred approach for forthcoming generation rotors.

Typically root end joints of composite flexbeams are fabricated by building up the beam laminates in the joint area to near-solid sections and to bolt through them. These types of joints are inefficient and difficult to fabricate for large rotors due to the high laminate thickness required, and result in highdrag structures and the numerous ply endings cause shear stress problems.

Roman et al. U.S. Pat. No. 4,111,605 shows a crossbeam rotor structure having C-shaped straps with thrubolt attachment of the rotor blades at the outer ends. Delker U.S. Pat. No. 3,434,372 shows a helicopter rotor system employing a laminated coupler with end fittings between a rotor blade and a hub. Mouille U.S. Pat. No. 4,104,003 shows a helicopter rotor system employing a beam carrying two opposite blades.

An object of the present invention is the provision of a composite flexbeam construction for helicopter bearingless rotors which is relatively easy to fabricate and which provides efficient joints for the attached members.

Another object of the invention is to provide a composite flexbeam for helicopter rotors which has a minimum thickness in the joint area and requires no time consuming, difficult to inspect and manufacture, laminate build-ups.

A further object of the invention is to provide a composite flexbeam for helicopter rotors using continuous windings and minimizing the problem of ply endings.

Still another object of the invention is to provide a bolt attachment for a composite flexbeam which can transfer loads by means of an encased fitting without a direct structural load path between the bolts and the composite.

The foregoing and other objects, features and advantages will be apparent from the specification and claims and from the accompanying drawings which illustrate an embodiment of the invention.

FIG. 1 is a fragmentary, sectional view of a helicopter rotor hub showing connections to flexbeams in accordance with this invention.

FIG. 2 is an enlarged fragmentary section of one end of a flexbeam.

FIG. 3 is a transverse section through a hoop wrapped end of a flexbeam.

FIG. 4 is a load transfer path diagram for an axial (centrifugal) loading.

FIG. 5 is a load transfer path diagram for a flapwise moment (out-of-plane) loading.

FIG. 6 is a load transfer path diagram for a flapwise shear loading.

FIG. 7 is a load transfer path diagram for a chordwise shear (inplane) loading when a hoop winding has been employed with the flexbeam structure.

In FIG. 1, a helicopter rotor shaft 10 is shown which is connected to and drives hub 12 to which are attached pairs of flexbeams at 90.degree. spacing, one pair of the flexbeams being shown at 14 and 16. A rotor blade would be attached to the outer end of each pair of flexbeams, but this is not shown. Each flexbeam is attached to a cuff 18 on the hub by thru-bolts 20.

The details of one of the flexbeams, such as flexbeam 16, are shown in FIG. 2. Each flexbeam is essentially of C-section and consists of a web, a top flange, a bottom flange, and a fitting at either end encased within the top and bottom flanges and the web. Web 22 is a composite structure made up of 0.degree. and .+-.45.degree. plies and carries shear loads. Top flange 24 and bottom flange 26, which carry primary loads, are constructed with essentially low angle plies in the direction of their length with their ends being joined as at 28 with a continuous wrap-around forming an arc or curved section. Fitting 30, basically a rectangular block with a rounded end, and typically metallic, is surrounded on its top and bottom surface and its rounded end by the continuous wrap of the top and bottom flanges and on one side surface by web 22. There is another fitting 32 similarly disposed at the other end of the flexbeam. The web, flanges and fittings are cured together as a unitary member to act as a complete structure. Holes 34 are provided through each end of the flexbeam for receiving attachment bolts, such as bolts 20.

Since the low angle plies at the ends of the flexbeam are continuous around the fittings, a degree of continuity and redundancy is provided in the end attachment areas. Also, with no ply endings associated shear stresses are eliminated. The plies in the web are tapered off at each end of the flexbeam along the side of the end fitting so as to shear gradually through the fitting.

To more positively contain the fitting within the flexbeam, particularly against higher inplane loads in either direction, a thin hoop wrap may be performed around the fitting at the completion of the fabrication process as shown at 36 in FIG. 3. Here fitting 38 is dome shaped to present a smoother perimeter for the hoop wrap. It also may be desirable in some instances to wrap plies from the web around an end 28 of flexbeam 16, the plies being intermixed with flange plies. This integration of plies around the fitting provides for better load sharing.

While the flexbeam has been described and shown as essentially a C-section member, it is possible that two flexbeams could be located web-to-web to constitute an I-section with web and flange plies being intermixed to some degree to form a unitary structure. For example, flexbeams 14 and 16 could be moved together to eliminate the spacing shown and the ply wrappings intermixed to form a unitary member.

The flexbeam construction is such that the transfer of bending moments, the axial forces and shear loads are accommodated without a direct structural load path between the composite flexbeam and the clamping bolts. The load on the flexbeam is essentially compressive which is a major advantage of the joint since composites have relatively good compression strength. Typically joints use shear as a major load reaction which is an inefficient way in composite materials.

The loads imposed upon the composite flexbeam of a bearingless rotor are axial (centrifugal), inplane, out-of-plane bending and shear plus a torsional twisting load. With respect to an axial (centrifugal) load imposed upon the rotor hub end of a flexbeam, as shown in FIG. 4, the flexbeam load A would be reacted, as at B, by the rounded end of fitting 30, the fitting load would be reacted, as at C, by bolt 20, or bolts, and the bolt load would be reacted, as at D, by hub cuff 18. There is thus no direct reaction between the composite structure of the flexbeam and the bolt. Similarly, flap moment, flap shear (lift) and chordwise (inplane) loads are reacted with no direct reaction between the flexbeam and the bolt. This is shown in FIGS. 5, 6 and 7.

FIGS. 5 and 6 show the load transfer from the composite flexbeam A to the fitting B into the bolts C and then reacted D by the hub cuff, for both flapwise moment and flapwise shear (lift) loading. In the case of flapwise shear loading, the transfer is essentially into the upper cuff with some shear being through the web of the flexbeam. FIG. 7 shows the reaction of chordwise shear loading with a hoop wrap fitting with a pair of flexbeams 14 and 16. As shear loading is applied to the right, the right flexbeam 16 transfers the load by direct bearing B on fitting 30 which in turn transfers the load C to bolts 20 and the load is reacted D by the hub cuffs. In the left flexbeam 14, the load A from the beam is transferred, as at E, into hoop winding 40 and through the hoop winding, as at F, into fitting 42, the loading then being reacted by bolts 20, as at C, and finally by the hub cuffs as at D.

It is to be understood that the invention is not limited to the particular embodiment shown and described herein, but that various changes and modifications may be made without departing from the spirit or scope of this concept as defined by the following claims.