# Patent

## Title
Main rotor blade with removable tip cap

## Abstract

A main rotor blade assembly includes a tip cap removably mountable to an outboard end section of a forward tip spar section and an aft tip spar section.

## Background
The present invention is a continuation of U.S. patent application Ser. No. 11/953,440, filed Dec. 10, 2007.


STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
This invention was made with government support with the United States Navy under Contract No.: N00019-06-C-0081. The government therefore has certain rights in this invention.


BACKGROUND OF THE INVENTION
The present invention relates to a rotary wing aircraft main rotor blade, and more particularly to a main rotor blade tip arrangement.
Rotary-wing aircraft rotor blades often include an anhedral tip section of a selected sweep, taper, and form to increase blade performance. Anhedral tip sections increase hover performance and lift capabilities of a rotary wing aircraft. Rotor blade tip sections are subjected to significant stresses/strains due to aerodynamic forces, and concomitantly experience significant structural degradation due to wear and abrasion. These aerodynamic forces may be increased upon a tip section with an anhedral form due to the loads induced thereby.
Conventional anhedral tip sections require numerous components to carry the loads induced by the anhedral form. Conventional anhedral tip sections may include two structural anhedral tip skins, two structural cores along with various non-structural components and redundant fasteners.
SUMMARY OF THE INVENTION
A main rotor blade assembly according to an exemplary aspect of the present invention includes: a forward tip spar section which defines a non-straight form; an aft tip section which defines a non-straight form, the aft tip section mounted to the forward tip spar; and a tip cap removably mountable to an outboard end section of the forward tip spar section and the aft tip spar section, the outboard end section comprises a reduced perimeter stepped section to receive the end cap at a tip cap interface.
A main rotor blade assembly according to an exemplary aspect of the present invention includes: a tip spar which defines an outboard end section which comprises a reduced perimeter stepped section; and a tip cap removably mountable to the tip spar at the outboard end section to receive the tip cap at a tip cap interface.



BRIEF DESCRIPTION OF THE DRAWINGS
The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently disclosed embodiment. The drawings that accompany the detailed description can be briefly described as follows:
FIG. 1 is a perspective view of a rotary wing aircraft for use with the present invention;
FIG. 2A is a perspective view of a main rotor blade;
FIG. 2B is an expanded perspective view of a tip section of the rotor blade of FIG. 2A;
FIG. 3 is an exploded view of a main rotor blade;
FIG. 4 is an exploded view of a removable tip cap for a main rotor blade;
FIG. 5A is a plan view of a tip section of a main rotor blade;
FIG. 5B is a top plan view of a main rotor blade;
FIG. 5C is an exploded view of a tip section of the main rotor blade;
FIG. 5D is an expanded view of a distal end of a tip spar assembly without a tip cap mounted thereto;
FIG. 5E is an expanded section view of a tip spar bonded to a main blade spar with a tip cap installed to the tip spar; and
FIG. 6 is a chart illustrating one manufacturing plan of a main rotor blade in accordance with one non-limiting embodiment of the present invention.



DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENT
FIG. 1 schematically illustrates a rotary-wing aircraft 10 having a main rotor system 12. The aircraft 10 includes an airframe 14 having an extending tail 16 which mounts a tail rotor system 18, such as an anti-torque system. The main rotor assembly 12 is driven about an axis of rotation A through a main gearbox (illustrated schematically at T) by one or more engines E. The main rotor system 12 includes a multiple of rotor blade assemblies 20 mounted to a rotor hub H. Although a particular helicopter configuration is illustrated and described in the disclosed non-limiting embodiment, other configurations and/or machines, such as high speed compound rotary wing aircraft with supplemental translational thrust systems, dual contra-rotating, coaxial rotor system aircraft, turbo-props, tilt-rotors and tilt-wing aircraft, will also benefit from the present invention.
Referring to FIG. 2A, each rotor blade assembly 20 of the rotor assembly 12 generally includes a root section 22, an intermediate section 24, a tip section 26 and a tip cap 28. Each rotor blade section 22, 24, 26, 28 may define particular airfoil geometries to particularly tailor the rotor blade aerodynamics to the velocity increase along the rotor blade span. The rotor blade tip section 26 includes an anhedral form 27 (FIG. 2B), however, any angled and non-angled forms such as cathedral, gull, bent, and other non-straight forms will benefit from the present invention. The anhedral form 27 as defined herein may include a rotor blade tip section 26 which is at least partially not contained in a plane defined by the intermediate section 24.
The rotor blade sections 22-28 define a span R of the main rotor blade assembly 20 between the axis of rotation A and a distal end 30 of the tip cap 28 such that any radial station may be expressed as a percentage in terms of a blade radius x/R. The rotor blade assembly 20 defines a longitudinal feathering axis P between a leading edge 32 and a trailing edge 34. The distance between the leading edge 32 and the trailing edge 34 defines a main element chord length Cm.
Referring to FIG. 3, the rotor blade assembly 20 generally includes a main blade assembly 40 and a tip assembly 42. The main blade assembly 40 includes an upper skin 44 a main core 46, a main spar 48, a lower skin 50, and a leading edge assembly 52. The main spar 48, main core 46 and skins 44, 50 are generally referred to as a pocket assembly, the forward portion of which is closed out by the leading edge assembly 52.
The leading edge assembly 52 generally includes a main sheath laminate 54 upon which is mounted a wear-resistant material such as a titanium erosion strip 56 and a nickel erosion strip 58 to provide abrasion protection. Alternatively, the sheath laminate 54 may include a single erosion strip 60 manufactured of AM355 which replaces the titanium erosion strip 56 and the nickel erosion strip 58. It should be understood that a variety of a wear-resistant materials may alternatively or additionally be provided for the leading edge assembly 52. Additional structures such as weight cups 62, blade tie down fastener structures 64 and trim tab systems 66 may also be provided, further details of which need not be further elaborated herein.
The tip assembly 42 generally includes a tip spar 48T, a tip core 46T, a tip leading edge assembly 68 and the tip cap 28. The tip cap 28 may be removably attached to the tip spar 48T though hardware 70 in a cap interface 78 (FIG. 4) to close-out the end of the tip assembly 42. The cap interface 78 may be defined at an inboard radial station 77 of approximately ninety eight percent (98% x/R) and end at an outboard radial station 79 which defines the distal end of the tip spar 48T, however other locations may alternatively be provided. It should be understood that other overlap lengths, locations and interfaces may alternatively or additionally be provided.
The tip leading edge assembly 68 may also include a splice cap laminate 54T and a tip erosion strip 72 to provide abrasion protection. The splice cap laminate 54T abuts the main sheath laminate 54 at a tip interface 84 while the tip erosion strip 72 may overlap the tip interface 84 therebetween. The tip cap 28 also includes an erosion cap 76 which may overlap the tip cap interface 78 between the tip assembly 42 and the tip cap 28. The tip erosion strip 72 and the erosion cap 76 may include AM355, titanium, nickel, or a variety of other wear-resistant materials or combinations thereof.
Referring to FIG. 5A, the main spar 48 and the tip spar 48T extend along the length of the sections 22-26. The main spar 48 and the tip spar 48T are structural members manufactured of a high strength non-metallic composite material and/or high strength metallic material such as titanium. The main spar 48 is attached to the tip spar 48T in a male-female overlap relationship 80. The upper skin 44 and the lower skin 50 extend outboard of the overlap relationship 80 which allows a contiguous composite skin laminate from the root section 22 through the tip section 26 to define the aerodynamic shape of the blade (FIG. 5B). That is, no separate main skins and tip skins are utilized. In one non-limiting embodiment, the upper skin 44 and the lower skin 50 extend from the root section 22 to the inboard radial station 77 of the cap interface 78 of the tip spar 48T which abuts the tip cap 28. In other words, the upper skin 44 and the lower skin 50 extend past the overlap relationship 80. Lift properties, for example, are significantly increased over conventional rotor blades with separate tip skins due in part to the increased strength of the one-piece composite skin laminate. Should a blade strike occur, the tip assembly 42 will still separate from the main blade assembly 40 at the tip interface 82 such that only the tip spar 48T and related tip assembly 42 components may break away which thereby allows the main blade 20 defined by the main spar 48 to remain flightworthy, albeit with reduced lift capabilities.
The tip spar 48T and the tip core 46T define the anhedral form or other angled and non-angled forms such as cathedral, gull, bent, and others. Furthermore, the tip core 46T may be hot formed to specifically include the anhedral form as well as be of similar or different weight per cubic foot (pcf) as compared to the main core 46 to provide particular lift and strength properties. The anhedral form may be located at a radial station of approximately ninety six percent (96% x/R). It should be understood that the anhedral form or other angled and non-angled forms may be located at other radial stations.
An end section ST of the tip spar 48T is bonded over an end section SM of the main spar 48 (FIG. 5C) to define the overlap relationship 80. An adhesive material, such as epoxy film adhesive, is one example of a bonding agent. The main spar end section SM may be of a reduced perimeter such that a flush outer mold line (OML) spar surface is formed therebetween. That is, the end section SM of the main spar 48 defines a jogged or stepped area to receive the end section ST of the tip spar 48T. The overlap relationship 80 may begin at an inboard radial station 82 of approximately eighty nine percent (89% x/R) and end at an outboard radial station 84 of approximately ninety two percent (92% x/R). It should be understood that other overlap length, locations and interfaces may alternatively or additionally be provided.
Referring to FIG. 5C, the tip spar 48T may include a forward tip spar section 48Tf and an aft tip spar section 48Ta. The forward tip spar section 48Tf includes an open aft section 90 with a stepped inner mold line to receive the open aft section 90 of the forward tip spar section 48Tf of FIG. 5D. The aft tip spar section 48Ta includes an open forward section 92 with a stepped outer mold line. It should be understood that other overlap length, locations and interfaces may alternatively or additionally be provided. The forward and aft split arrangement facilitates assembly of the forward tip spar section 48Tf to the aft tip spar section 48Ta and the end section ST of the tip spar 48T over the end section SM of the main spar 48 (FIG. 5E). Assembly and manufacture of the main rotor blade assembly 20 may generally follow the manufacturing plan as schematically illustrated in FIG. 6.
It should be understood that relative positional terms such as “forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like are with reference to the normal operational attitude of the vehicle and should not be considered otherwise limiting.
It should be understood that although a particular component arrangement is disclosed in the illustrated embodiment, other arrangements will benefit from the instant invention.
Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined unless otherwise indicated and will still benefit from the present invention.
The foregoing description is exemplary rather than defined by the limitations within. Many modifications and variations of the present invention are possible in light of the above teachings. The disclosed embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.

## Claims

1. A main rotor blade assembly comprising:
a tip spar;
a tip cap removably mountable to said tip spar at a tip cap interface; and
a tip leading edge assembly having a splice cap laminate and a tip erosion strip mounted adjacent to said splice cap laminate, said tip leading edge assembly at least partially overlaps said tip cap interface.

2. The main rotor blade assembly as recited in claim 1, further comprising:
an upper blade skin; and
a lower blade skin, said tip leading edge assembly at least partially overlaps said upper blade skin and said lower blade skin.

3. A main rotor blade assembly comprising:
a forward tip spar section which defines a non-straight form;
an aft tip section which defines a non-straight form, said aft tip section mounted to said forward tip spar; and
a tip cap removably mountable to an outboard end section of said forward tip spar section and said aft tip spar section, said outboard end section comprises a reduced perimeter stepped section to receive said end cap at a tip cap interface; and
a tip leading edge assembly having a splice cap laminate and a tip erosion strip mounted adjacent to said splice cap laminate, said tip leading edge assembly at least partially overlaps said tip cap interface.

4. The main rotor blade assembly as recited in claim 3, further comprising hardware within said cap interface to removably attach said tip cap.

5. The main rotor blade assembly as recited in claim 3, wherein said tip cap interface is defined between an inboard radial station of approximately ninety eight percent (98% x/R) and an outboard radial station which defines a distal end of said tip spar.

6. The main rotor blade assembly as recited in claim 3, wherein said forward tip spar section comprises an open aft section with a stepped inner mold line to receive said aft tip spar section which comprises an open forward section with a stepped outer mold line.

7. The main rotor blade assembly as recited in claim 3, wherein said non-straight form of said forward tip spar section and said aft tip spar section is defined at an approximately ninety six percent a radial station (96% x/R).

8. The main rotor blade assembly as recited in claim 3, further comprising an erosion cap mounted to said tip cap, said erosion cap at least partially overlaps said tip cap interface.

9. The main rotor blade assembly as recited in claim 8, wherein said erosion cap abuts said tip erosion strip.

10. A main rotor blade assembly comprising:
a tip spar which defines an outboard end section which comprises a reduced perimeter stepped section;
a tip cap removably mountable to said tip spar at said outboard end section to receive said tip cap at a tip cap interface; and
a tip leading edge assembly having a splice cap laminate and a tip erosion strip mounted adjacent to said splice cap laminate, said tip leading edge assembly at least partially overlaps said tip cap interface.

11. The main rotor blade assembly as recited in claim 10, wherein said tip cap is attached to said tip spar in an overlap relationship.

12. The main rotor blade assembly as recited in claim 10, wherein said tip cap interface is defined at an inboard radial station of approximately ninety eight percent (98% x/R) and ends at an outboard radial station which defines a distal end of said tip spar.

13. The main rotor blade assembly as recited in claim 10, further comprising a main spar, said tip spar attached to said main spar in an overlap relationship.

14. The main rotor blade assembly as recited in claim 13, wherein said overlap relationship comprises an end section of said main spar.

15. The main rotor blade assembly as recited in claim 14, wherein said end section of said main spar comprises a reduced perimeter stepped section to receive said tip spar.

16. The main rotor blade assembly as recited in claim 15, wherein said overlap relationship extends between an inboard radial station of approximately eighty nine percent (89% x/R) and an outboard radial station of approximately ninety two percent (92% x/R).

17. The main rotor blade assembly as recited in claim 13, further comprising
a main core adjacent said main spar; and
a tip core adjacent said tip spar.

18. The main rotor blade assembly as recited in claim 10, wherein said tip spar defines a non-straight form.

19. The main rotor blade assembly as recited in claim 18, wherein said tip spar defines an anhedral form.

20. The main rotor blade assembly as recited in claim 10, further comprising hardware within said cap interface to removably attach said tip cap.