Fluid reaction surfaces (i.e. – impellers) – Specific working member mount – Turbo machine
Reexamination Certificate
2002-02-20
2003-12-23
Nguyen, Ninh H. (Department: 3745)
Fluid reaction surfaces (i.e., impellers)
Specific working member mount
Turbo machine
C416S230000, C416S239000, C416S248000
Reexamination Certificate
active
06666651
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to propeller blades for airplanes, and more particularly to a fiber-reinforced resin composite blade attached to a unitary metal ferrule and a method of manufacture.
BACKGROUND OF THE INVENTION
Modern propellers for small airplanes and airboats typically consist of an assembly of two, three, or more blades attached symmetrically around a rotatable hub. The blade is often machined from aluminum, or may be fabricated from fiber-reinforced resin, such as graphite fibers embedded in an epoxy matrix.
Aluminum blades are generally strong. A disadvantage is that the edges of an aluminum blade usually get heavily nicked and pitted by gravel and other objects, requiring remachining of the edges. Another disadvantage is that metal blades are heavy, compared to synthetic composite blades. All materials for airplanes are preferably light, but it is especially desirable for the propeller to be light, so that the center of gravity of the airplane is not near the nose of the airplane. Another disadvantage is the cost of the material and the cost of machining the blade.
Resin/fiber composite blades are lighter than aluminum blades. An advantage of composite materials is that the fibers can be selected and oriented to yield a blade with more stiffness where needed and more ductility where needed. For example, glass fiber is more ductile than graphite; graphite fiber is stiffer than glass and has greater tensile strength. So that articles of consistent quality and strength can be fabricated from resin/fiber composites, fabricators often use “pre-preg,” that is, a fibrous material pre-impregnated with resin. Typically, the fibrous material is saturated with liquid resin and heated very gently to cause the resin to gel to what is called the “B-stage,” but not to cure.
B-stage resin contains very little solvent and the molecules of polymer are close together but not cross-linked. The resin cannot flow at room temperature, but generally does soften when heated. B-stage pre-preg material up to several thousandths of an inch in thickness can be cut with scissors and feels like stiff paper or manila stock.
Other methods are also used, such as “wet lay-up,” wherein the fibrous material is saturated with liquid resin by the fabricator and laid up into the mold, without and intermediate B-stage.
The orientation of the fibers within the resin matrix has a large affect on the stiffness of the resulting resin/fiber composite. Fibers may be woven or knit before impregnation, or may be aligned parallel to each other. When the fibers are aligned parallel, the resulting pre-preg is called “unidirectional.” Unidirectional resin/fiber composite is flexible if bent parallel to the fibers and stiff if bent across the fibers. Woven and knit textiles also have characteristic flexing properties.
A relatively well-known method of fabricating propeller blades, called “compression molding,” starts with machining a mold having two halves, each having a cavity the shape of one side of the blade. Uncured fiber/resin composite material is arranged in the cavities according to a design plan called a “lay-up schedule.” The lay-up schedule specifies the shape and fiber orientation of the pieces, which are overlapped to yield the desired tapering shape. Splitting the mold into two halves is generally preferred for molding an article having bilateral symmetry; an article with a higher degree of symmetry might preferably be molded from a mold divided into a higher number of segments.
Typically, uncured resin/fiber composite is laid up in each mold half and an insert of wood or polyurethane foam is placed in one of the halves. Strengthening inserts, typically machined from titanium or aluminum, are also generally included. The halves of the mold are brought together and clamped. This “mold assembly,” consisting of metal mold halves and inserts, is heated to the curing temperature, such as by being placed in an oven.
During curing, the resin flows to join the uncured resin/fiber composite into a fairly uniform mass, which adheres to the insert. The resin eventually crosslinks and becomes rigid. After the resin is cured, it will not soften again upon heating.
One disadvantage of using compression molding to make propeller blades is the high cost. Both materials and labor for compression molded propeller blades are expensive. For example, U.S. Pat. No. 4,810,167 of Spoltman et al. discloses a propeller blade that contains multiple machined components. Some of the components are laid up in the mold assembly; others are glued on after molding. The base of the molded blade must be precision machined to accept these glued-on components.
This method also has a large indirect cost, which is inflexibility of mold use. Because there are critical mating surfaces on the foam and metal inserts, the post-assembled components (components attached to the molded article after cure), as well as the mold cavities, each mold is dedicated to making a single design of blade. Even the ability to change the lay-up schedule to make a blade of the same shape, but different stiffness characteristics, is seriously limited. Because molds are typically the most expensive and longest lead-time part of new design, it is undesirable to have to make a new mold to accommodate every small change.
To avoid these high costs of machined parts, complicated lay-up, and single-design molds, the method known as “internal pressure molding,” or “IPM,” has been used to make propeller blades. IPM uses a mold cavity to define the outer shape of the article being molded, as does compression molding, but an inflatable, stretchable bladder compresses the resin/fiber composite from the inside. Blades made using IPM are hollow; no foam or wood insert is needed. Eliminating the insert decreases machining cost, assembly cost, and weight of the finished article. The bladder conforms to the profile of the composite material, thus one mold can accommodate nearly any lay-up schedule that yields the same external shape. New lay-up schedules can be tested very cheaply and fewer molds are needed in the shop.
A major disadvantage of conventional IPM composite blades is that they are not as strong as solid aluminum or compression molded blades having metal inserts and glued-on reinforcing components. The root of the blade, especially the portion that attaches to the hub, must withstand high dynamic force and fatigue. The root of the blade can be reinforced by attaching two halves of a split collar around it after it is molded. This strengthens the blade, but neutralizes some of the cost savings of IPM. A split collar is also not as strong as a unitary collar or ferrule is. As a result, the market for IPM airplane blades has been limited to relatively low-performance craft. There is a need for a simple, inexpensive IPM airplane propeller blade that is strong enough to be used on the most demanding airplanes, such as competitive aerobatic planes.
SUMMARY OF THE INVENTION
This invention is an IPM composite blade assembly that is suitable for competitive aerobatic airplanes, or for other uses, such as airboats. The blade assembly includes a fiber-reinforced resin blade, means for mounting the assembly onto a propeller hub, and a unitary ferrule for reinforcing the root of the blade.
The unitary ferrule is co-molded with the blade and preferably forms a part of the mold. Assembly of the ferrule into the lay-up is very simple and quick. Because the unitary ferrule does not have to be split to go over the molded root of the blade, it is very strong.
The uncured composite material is laid up into a standard segmented mold. An inflatable bladder is laid between the halves of the mold before the mold is closed. Each cavity segment includes a recess segment that accommodates half of the ferrule and the end of the recess segment is open at the end. The uncured composite material laid into the ferrule recess forms the root of the blade.
The mold segments are mated together with the cavity segments facing each other. The laid-up uncured composite materi
Nguyen Ninh H.
Redman Mary Jo
Tervo Calif
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