Background of the Invention 1. Field of the Invention

This invention relates generally to fluid actuators and more particularly to a composite cylinder for use in hydraulic actuators intended for aircraft flight controls.

2. Prior Art

Fluid actuators of the type used in flight controls for aircraft and other high pressure applications are required to withstand high impulse pressures without failing over extended periods of time. In the prior art, to insure that the actuator had the necessary strength, common practice has been to construct the actuator cylinder out of a monolithic metal piece. Such structure adds considerably to the weight of the actuator.

To provide actuator cylinders having lighter weight than those constructed with a monolithic metal piece but at the same time providing adequate strength, the use of a composite cylinder has been suggested. Typical of such composite cylinders are those disclosed in prior art U.S. Patents Nos. 4,685,384, 4,697,499, 4,802,404, and 4,773,306. The composite cylinders

disclosed in these patents include a metal liner which is wound with hoop windings made of a suitable composite fiber such as a graphite filament impregnated with a suitable resin. The filaments, in addition to being hoop wound, have also been helically wound, and in some instances, disposed in longitudinal winding form. The combination of the hoop, helical and longitudinal windings provide the ability for the composite cylinder to react to circumferential loads, axial loads and co pressive loads generated in the cylinder during the operation of the hydraulic actuator.

Although such composite cylinders have functioned quite well for the purposes intended, difficulties have been experienced in that the windings are incapable of supporting the loads encountered in all instances. Such difficulties have been generated because of the manner of retaining the wound filaments on the liner and particularly on the metal end glands necessary for receiving end plates which in turn receive the piston rods or clevis attachment members necessary for proper operation of the hydraulic actuator.

Summary of the Invention A composite cylinder for use in a hydraulic actuator and comprising a cylindrical liner having an outer surface which is dome shaped adjacent each end thereof. A plurality of layers of resin impregnated carbon filaments are wound on the outer surface of the liner and over the dome shaped portions. At least two of the plurality of layers are helically wound at an angle with respect to the axis of the cylindrical liner such that radial stress as applied during actuator operation will not tend to displace the helically wound filaments on the dome shaped portions of the liner.

Brief Description of the Drawings FIGURE 1 is a perspective view illustrating a composite cylinder constructed in accordance with the principles of the present invention; and FIGURE 2 is a fragmented partial cross sectional view illustrating the end portions of the composite cylinder as shown in FIGURE 1 and taken about . the lines 2-2 thereof.

Detailed Description

The composite cylinder constructed in accordance with the principles of the present invention is adapted for use as an integral part of a hydraulic actuator used for the flight control surfaces on aircraft. Such actuators may be used either as single or as dual tandem actuators as is well known in the prior art. Such actuators include a cylinder which receives a piston and piston rod with the ends of the cylinder being closed by a tail stock and a gland which typically are threaded into opposite ends of the cylinder. Appropriate fluid inlet and outlet passageways and mounting means are included as would be position indicators such as linear variable differential transformers (LVDT) . The entire apparatus is not illustrated nor described in detail herein since such is well known to those skilled in the art. For the sake of completeness, the disclosures of U.S. Patents Nos. 4,685,384, 4,697,499, 4,773,306, and 4,803,404 are incorporated herein by reference. The composite cylinder of the present invention includes an integral thin walled liner which terminates at each end in threaded end sections to receive the threaded tail stock and gland. The integral liner has a dome shaped outer surface at each end and is wound with a plurality of layers of impregnated carbon

fibers to provide the overall additional strength required in a hydraulic actuator used to position aircraft flight controls while at the same time providing a substantial weight reduction. Through utilization of a composite cylinder constructed in accordance with the principles of the present invention, an overall actuator weight reduction of approximately fifteen percent (15%), as compared to metal actuators, has been realized. The metal liner is preferably constructed of stainless steel (15-5PH) and may include a central barrel with separate end fittings which are assembled upon a mandrel for winding. The filament used in winding is preferably a carbon fiber which has been impregnated with an epoxy resin including an appropriate curing agent and a curing accelerator as is well known to those skilled in the art. The windings are placed on the outer surface of the integral liner and are preferably helically wound but may also include layers of hoop wound filaments interspersed between the helically wound layers. Helically wound layers are disposed particularly over the dome shaped portions of the cylindrical liner in such a manner that radial stress applied during actuator operation will not tend to displace the helically wound filaments on the dome shaped portions of the liner. Preferably, the layers of filament would be wound continuously without cutting or breaking the filament.

A composite cylinder constructed in accordance with the present invention is shown in the drawings at . 10 and includes a liner 12 having threaded end portions 14 which include an enlarged portion 16 providing a dome shaped outer surface 18. Threads 20 are formed in the outer end sections of the liner to threadably receive the tail stock and end gland.

As is illustrated more particularly in FIGURE 2, the integral liner may include a central barrel 22 which is assembled with a pair of end fittings 24 as is shown by the shoulder 26 after which the windings are disposed upon the outer surface thereof. As is also shown particularly in FIGURE 2, the composite cylinder includes a plurality of windings generally designated at 30. These windings preferably are helical and are disposed during winding such that the filament is at a relatively shallow angle with respect to the longitudinal axis 32 of the cylinder. Although the plurality of layers 30 are preferably helically wound, they may also include hoop wound layers interspersed between the helical layers should such be desired. As above pointed out, the helical windings are disposed upon the dome shaped outer surface 18 in such a manner that radial stresses when applied during actuator operation will not tend to displace the helical wound filaments on the dome shaped portion. This is accomplished by winding the filament in a geodesic manner over the dome shaped outer surface 18. The preferred angle of the filament with respect to the longitudinal axis of the cylinder at the commencement of the winding depends upon the distance the outer surface of the dome portion 18 is displaced from the center line 32 as compared to the outer surface of the remainder of the liner. As is shown in FIGURE 2, the radius from the longitudinal axis 32 to the apex 34 of the dome portion 18 is a major radius as shown at 36. A minor radius as shown at 38 is the distance from the longitudinal axis ₄ 32 to the outer surface of the liner immediately adjacent the dome shaped portion 16 as noted at 40 which is at the greatest thickness of the remainder of the liner other than the dome shaped portion. The angle at

which the filament would be placed to commence winding at the point 42 would be determined by the formula — sin ^"1 R minor R major where = winding angle R minor = minor radius R major = major radius

It will be recognized by those skilled in the art that as the layers of filament are wound upon the outer surface of the liner, the overall diameter of the liner continuously increases. The angle at which the helically wound filaments are wound will decrease as each layer is displaced further from the longitudinal axis by establishing the commencement winding angle as above described. The helically wound filaments are disposed over the dome shaped portions in a geodesic manner and thus will retain their positions upon the dome shaped portions irrespective of the operational stresses applied to the actuator during flight.

As is well known to those skilled in the art, once the epoxy impregnated carbon filaments have been wound upon the outer surface of the cylindrical liner, which itself has been positioned upon a winding mandrel, the entire unit, mandrel included, is cured in an oven at an elevated temperature. The curing temperature must be higher than the highest operational temperature to which the actuator will be subjected. After the curing process, the cylinder is separated from the mandrel and is then ready for use or alternatively for further operations preparatory to final assembly for use as the case may be.