Rotor hub and blade root fairing apparatus and method

A fairing system may be assembled about a rotor of a rotorcraft to present an aerodynamically quasi-static region that rotates in an airstream, as well as certain extensions that sweep through the airstream as the rotor hub passes through the air. A spherical interface between the extensions on the rotor hub fairing and the base or root portion of each blade fairing provides three degrees of freedom permitting lead-lag, flapping, and blade pitch pivoting in the blade, while still maintaining an aerodynamic profile that will minimize drag.

BACKGROUND

1. The Field of the Invention

This invention relates to rotating wing aircraft, and, more particularly to rotating wing aircraft relying on autorotation of a rotor to provide lift, and systems and methods for improving aerodynamics of a rotorcraft airframe.

2. The Background Art

Rotating wing aircraft rely on a rotating wing to provide lift. In contrast, fixed wing aircraft rely on air flow over a fixed wing to provide lift. Fixed wing aircraft must therefore achieve a minimum ground velocity on takeoff before the lift on the wing is sufficient to overcome the weight of the plane. Fixed wing aircraft therefore generally require a long runway along which to accelerate to achieve this minimum velocity and takeoff.

In contrast, rotating wing aircraft can take off and land vertically or along short runways inasmuch as powered rotation of the rotating wing provides the needed lift. This makes rotating wing aircraft particularly useful for landing in urban locations or undeveloped areas without a proper runway.

The most common rotating wing aircraft in use today are helicopters. A helicopter typically includes a fuselage, housing an engine and passenger compartment, and a rotor, driven by the engine, to provide lift. Forced rotation of the rotor causes a reactive torque on the fuselage. Accordingly, conventional helicopters require either two counter rotating rotors or a tail rotor in order to counteract this reactive torque.

Another type of rotating wing aircraft is the autogyro. An autogyro aircraft derives lift from an unpowered, freely rotating rotor or plurality of rotary blades. The energy to rotate the rotor results from a windmill-like effect of air passing through the underside of the rotor. The forward movement of the aircraft comes in response to a thrusting engine such as a motor driven propeller mounted fore or aft.

During the developing years of aviation aircraft, autogyro aircraft were proposed to avoid the problem of aircraft stalling in flight and to reduce the need for runways. The relative airspeed of the rotating wing is independent of the forward airspeed of the autogyro, allowing slow ground speed for takeoff and landing, and safety in slow-speed flight. Engines may be tractor-mounted on the front of an autogyro or pusher-mounted on the rear of the autogyro.

Airflow passing the rotary wing, alternately called rotor blades, which are tilted upward toward the front of the autogyro, act somewhat like a windmill to provide the driving force to rotate the wing, i.e. autorotation of the rotor. The Bernoulli effect of the airflow moving over the rotor surface creates lift.

Various autogyro devices in the past have provided some means to begin rotation of the rotor prior to takeoff, thus further minimizing the takeoff distance down a runway. One type of autogyro is the “gyrodyne,” which includes a gyrodyne built by Fairey aviation in 1962 and the XV-1 convertiplane first flight tested in 1954. The gyrodyne includes a thrust source providing thrust in a flight direction and a large rotor for providing autorotating lift at cruising speeds. To provide initial rotation of the rotor, jet engines were secured to the tip of each blade of the rotor and powered during takeoff, landing, and hovering.

Although rotating wing aircraft provide the significant advantage of vertical takeoff and landing (VTOL), they are much more limited in their maximum flight speed than are fixed wing aircraft. The primary reason that prior rotating wing aircraft are unable to achieve high flight speed is a phenomenon known as “retreating blade stall.” As the fuselage of the rotating wing aircraft moves in a flight direction, rotation of the rotor causes each blade thereof to be either “advancing” or “retreating.”

That is, in a fixed-wing aircraft, all wings move forward in fixed relation, with the fuselage. In a rotary-wing aircraft, the fuselage moves forward with respect to the air. However, rotor blades on both sides move with respect to the fuselage. Thus, the velocity of any point on any blade is the velocity of that point, with respect to the fuselage, plus the velocity of the fuselage. A blade is advancing if it is moving in the same direction as the flight direction. A blade is retreating if it is moving opposite the flight direction.

The rotor blades are airfoils that provide lift that depends on the speed of air flow thereover. The advancing blade therefore experiences much greater lift than the retreating blade. One technical solutions to this problem is that the blades of the rotors are allowed to “flap.” That is, the advancing blade is allowed to fly or flap upward in response to the increased air speed thereover such that its blade angle of attack is reduced. This reduces the lift exerted on the blade. The retreating blade experiences less air speed and tends to fly or flap downward such that its blade angle of attack is increased, which increases the lift exerted on the blade.

Flap enables rotating wing aircraft to travel in a direction perpendicular to the axis of rotation of the rotor. However, lift equalization due to flapping is limited by a phenomenon known as “retreating blade stall.” As noted above, flapping of the rotor blades increases the angle of attack of the retreating blade. However, at certain higher speeds, the increase in the blade angle of attack required to equalize lift on the advancing and retreating blades results in loss of lift (stalling) of the retreating blade.

A second limit on the speed of rotating wing aircraft is the drag at the tips of the rotor. The tip of the advancing blade is moving at a speed equal to the speed of the aircraft and relative to the air, plus the speed of the tip of the blade with respect to the aircraft. That is equal to the sum of the flight speed of the rotating wing aircraft plus the product of the length of the blade and the angular velocity of the rotor. In helicopters, the rotor is forced to rotate in order to provide both upward lift and thrust in the direction of flight. Increasing the speed of a helicopter therefore increases the air speed at the rotor or blade tip, both because of the increased flight speed and the increased angular velocity of the rotors required to provide supporting thrust.

The air speed over the tip of the advancing blade can therefore exceed the speed of sound even though the flight speed is actually much less. As the air speed over the tip approaches the speed of sound, the drag on the blade becomes greater than the engine can overcome. In autogyro aircraft, the tips of the advancing blades are also subject to this increased drag, even for flight speeds much lower than the speed of sound. The tip speed for an autogyro is typically smaller than that of a helicopter, for a given airspeed, since the rotor is not driven. Nevertheless, the same drag increase occurs eventually.

A third limit on the speed of rotating wing aircraft is reverse air flow over the retreating blade. As noted above, the retreating blade is traveling opposite the flight direction with respect to the fuselage. At certain high speeds, portions of the retreating blade are moving rearward, with respect to the fuselage, slower than the flight speed of the fuselage. Accordingly, the direction of air flow over these portions of the retreating blade is reversed from that typically designed to generate positive lift. Air flow may instead generate a negative lift, or downward force, on the retreating blade. For example, if the blade angle of attack is upward with respect to wind velocity, but wind is moving over the wing in a reverse direction, the blade may experience negative lift.

The ratio of the maximum air speed of a rotating wing aircraft to the maximum air speed of the tips of the rotor blades is known as the “advance ratio. The maximum advance ratio of rotary wing aircraft available today is less than 0.5, which generally limits the top flight speed of rotary wing aircraft to less than 200 miles per hour (mph). For most helicopters, that maximum achievable advance ratio is between about 0.3 and 0.4.

In view of the foregoing, it would be an advancement in the art to provide a rotating wing aircraft capable of vertical takeoff and landing and flight speeds in excess of 200 mph.

At high advance ratios, the drag on the rotor blades near their roots, as well as the drag on the various apparatus within the rotor hub itself become even more significant. In helicopters and autogyros, rotor drag can be a significant fraction of the overall drag on the aircraft. As advance ratios increase, the drag caused by the rotor may be a significant consumer of power. Up to 25 percent of the overall drag on the aircraft is contemplated.

Accordingly, it would be an advance in the art to provide a mechanism for reducing the drag on a rotor, particularly near the root of each rotor blade, as well as for the rotor hub at the center of the rotor. It would further be an advance in the art to provide a minimized drag coefficient, while still supporting or containing all of the necessary functionality and mechanisms required to operate the rotor blades. It would be a further advance in the art to provide a reduction in aerodynamic drag for support systems, actuators, connectors, and the like transferring any materials, forces, information, actuation, or the like from an aircraft, through a rotor hub, and out to a rotor blade.

BRIEF SUMMARY OF THE INVENTION

The invention has been developed in response to the present state of the art and, in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available apparatus and methods. The features and advantages of the invention will become more fully apparent from the following description and appended claims, or may be learned by practice of the invention as set forth hereinafter.

Aerodynamic drag is a principle of momentum transfer. In order for a fluid, such as air, to pass by a solid object, the air must change its path to circumnavigate the object. As a direct result, the direction and velocity of the air change. Therefore, a momentum transfer must occur between the solid object and the fluid flow, such as the air.

That momentum transfer amounts to force imposed by the air stream against the solid object. Likewise, the solid object resists with that same force. Accordingly, the force, acting through some time period, changes the momentum of the airflow. Meanwhile, the airflow by being exposed to that force, and resisting with its own force, transfers a force to the object.

This force transferred to a solid object, whether fixed or moving, is related directly to the relative velocity existing between the solid object and the airstream. For example, an airstream may be moving, while an object is fixed in that airstream. Likewise, an object may be moving through still air, thus imposing a relative velocity on the surrounding still air with respect to the solid object. Similarly, an object may be moving in a moving airstream.

Accordingly, the significant velocity is the relative velocity. Relative velocity the difference in velocities between the absolute velocity of the solid object, and the absolute velocity of the air.

In aircraft, aerodynamic drag is a significant concern. For example, on fixed wing aircraft, the wing shape, designed principally to provide lift, must always be designed with the consideration of aerodynamic drag. Since a wing cannot typically have zero projected area exposed to an airstream, it will have some amount of drag. When flaps are added, drag increases, although the effective chord of the wing in increases, and thus lift is increased. At low speeds, fixed wing aircraft employ flaps, such as during landing operations. At low speed, the drag is not so significant, because the velocity is reduced. Thus, the tradeoff between increased chord for increased lift, and the increased frontal projected area causing increased drag, provides in landing a net benefit with the use of flaps to increase lift.

Rotorcraft face similar issues. The rotary wings or blades are exposed to the relative velocity between themselves and the surrounding airflows. Likewise, rotor blades or rotary wings in a rotorcraft provide lift, but also experience drag.

One area of drag that is surprisingly great is the drag caused by the passage of the rotor hub during flight. The airframe may be covered up with a skin, to form a fuselage that passes through the air with less drag. However, the various mechanisms that operate the collective and cyclic pitch in rotorcraft, such as a helicopter for example, must continue to operate. Likewise, in a sophisticated heliplane or autogyro, drag forces through the rotor hub may be significant.

Typically, various shafts, lines, actuators, connectors, plates, and other components may be part of the rotor hub. Likewise, the root of each rotor blade must somehow connect to the hub. Connection of a rotor blade operating as an airfoil to a hub with its mechanical and control connections is a serious mechanical engineering enterprise. Providing the support for the blade, as well as the freedom of motion, with the other infrastructure may cause the root of the rotor blade to be shaped non-aerodynamically. Inasmuch as the portion of the rotor blade structure nearest the hub is not providing as significant a contribution to lift as other parts of the blade, the aerodynamics for contribution to lift may not be particularly valuable.

In one embodiment of an apparatus and method in accordance with the invention, a rotor cannot actually be completely enveloped in a static aerodynamic profile. For example, the drag on a sphere at the location of the hub would not significantly change when that sphere or rotor hub is rotating. Symmetry precludes such changes. Meanwhile, however, the blades are constantly changing their presented area while rotating. Accordingly, a system in accordance with the invention may provide a substantially static profile for at least a portion of the hub. A portion of the hub may be enveloped in a cowling or a fairing that effectively closes off the mechanical structures of the hub to the flow of air therethrough and past the hub components. Some portions may provide a static aerodynamic profile. Others may change presented (e.g., projected) area contributing to drag.

Likewise, since rotor blades are not static profiles, but rotate through the air, present a continually changing profile, they cannot have completely static aerodynamic profiles. Likewise, since they radiate outward, each position on a radius from the center of the hub outward acts at a different profile, and with a different velocity than every other location at almost all times. Moreover, due to the lead-lag tendencies of rotorcraft blades, a certain amount of flexure will occur in the lead-lag direction.

Meanwhile, the centrifugal forces within a blade tend to straighten the blade out by pulling all rotating portions of the blade away from the hub. This axial load on each blade also tends to counteract the tendency of each blade to flap.

Even where rigid materials and substantially rigid connections are used on a rotor blade, the rotor blade may still bend during its rotating cycle. Accordingly, a certain degree of vertical “flapping” motion exists between a root of a rotor blade, or the region of the blade near the root as a result of the flapping tendency of rotorcraft blades.

Finally, a rotor blade must pivot about a longitudinal axis in order to change its collective pitch, cyclic pitch, or both. Accordingly, that longitudinal axis or feathering axis will be an axis of motion, in a circumferential direction thereabout, by the blades. All of these motions must be, and are accommodated in a fairing system in accordance with the invention.

The fairing provides three degrees of freedom in an interface between a fairing secured to a blade near the root, and a fairing connected to a rotor hub. By providing a spherical interface between the substantially static profile of the fairing on the hub, and each moving blade fairing, the three degrees of freedom required in moving a blade can be accommodated without sacrificing aerodynamic contouring.

In certain embodiments, the blade pitch angles of rotation or angles of pivoting may be accommodated by a circular opening through which components of the blade root portion may secure to the hub. Meanwhile, the maximum beam bending and chord bending corresponding to flapping and lead-lag motion, respectively may be accommodated by the spherical interface at which the base or root of the blade fairing interfaces with a matching, spherical face on the hub fairing. At the interface between the hub fairing and the root fairing, a seal may provide flexibility and support relative motion between the root fairing or blade fairing and the hub fairing. The seal may be one of several types, typically a flexible seal, such as a blade or face seal contacting an inner surface of the root portion of the blade fairing. In other embodiments, a bellows seal or other mechanism may provide for stopping a flow through the joint or the spherical interface between the blade fairing and the root fairing. In this manner, the clearance, whatever it be, between the blade fairing and the hub fairing does not regularly pass airflow into the hub, and thus maintains a substantially static, thin region of air between the mated spherical surfaces.

In one embodiment of an apparatus and method in accordance with the invention, a plurality of rotor blades, typically two, four, or more may extend from a rotor hub radially outward. The hub may have a fairing surrounding its components. As each of the structural components connected to each blade root extend away from the hub, they may be covered with a projection or extension that leads to a blade root fairing on the blades.

At the extreme outermost radially extent of each projection or extension from the rotor hub fairing, a spherical surface is formed. At one location on that spherical surface, a frustum is created in which a seal holding ring may be formed. The opening will typically have a circular cross section as if a plane intersected with and removed a portion of the spherical surface.

In other embodiments, the spherical surface may be formed such that it only extends as a frustum as far as the seal holding ring. Through this seal-holding ring, all the components that extend outside the rotor hub fairing and pertain to the blade pass through the seal ring or seal opening at the end of the extension without contacting the fairing.

The seal between the rotor hub fairing and the blade fairing need not be a high velocity, rotating seal. Rather, the seal may be quasi-static, subject to motion, but not at particularly high rates of speed. Relative motion is due to cyclic pitch changes, flapping, and lead-lag motion of a blade. Thus, the seal ring is fitted with a seal, and the fairing for the blade is in contact with the seal. The spherical surfaces of the hub fairing and blade fairing may be spaced apart with no other seal therebetween. They will typically maintain a slight clearance as engineered (e.g. for material properties and tolerances) between the spherical surface of the rotor hub fairing and the internal spherical surface at the root of the blade fairing.

In one embodiment, the clearance between the spherical surfaces of the rotor fairing or rotor hub fairing and the blade fairing may be on the order of thousandths of a inch. In certain embodiments, the clearance may be between two and ten thousandths of an inch. In one presently contemplated embodiment, a clearance of about six thousandths has been deemed suitable as an offset between the spherical surfaces.

Likewise, a similar clearance may be provided between the innermost diameter of the seal ring and the components passing therethrough. Inasmuch as comparatively tight tolerances may be maintained, clearances may be engineered according to the aerodynamics desired and the tolerances provided for the interfacing components.

In certain embodiments currently contemplated, the seal between the blade fairing and the rotor hub fairing may actually be a bellows. For example, inasmuch as a blade might be configured to turn inside a blade fairing, a bellows may provide lead-lag as well as flapping flexure in the seal between the blade fairing and the hub fairing. In other embodiments, a sliding “wiper” seal surface may simply contact the inner surface of the spherical portion of a blade fairing, being secured around the seal ring portion of the hub fairing.

In certain embodiments, a bellows seal may be secured to the seal ring of the hub fairing and similarly connected fixably to an internal ring on the blade fairing. The bellows, typically formulated of a suitably flexible elastomer can then move in lead-lag, flapping, as well as pivoting motions. The bellows, in such a situation might be required to twist. However, suitable soft elastomeric materials and a suitable number of convolutions may provide for such an arrangement. In such an arrangement, the entire seal flexes, and yet is completely sealed at both ends, at the proximal end to the hub fairing and at the distal end to the blade fairing.

Regardless, the three degrees of freedom, although required to accommodate the relative motion between a rotor blade of a rotorcraft and the rotor hub thereof are not all the same size. For example, blade pitch control may vary from five to ninety degrees conceivably. More likely, blade pitch variation may typically only vary from about twenty to about forty-five degrees maximum. By contrast, flapping and lead-lag displacements of blade will typically be less than ten degrees.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 1, an aircraft10includes an airframe12or a fuselage12defining a cabin for carrying an operator, passengers, cargo, or the like. The airframe12may include one or more fixed wings14shaped as airfoils for providing lift to the aircraft. The wings14may be configured such that they provide sufficient lift to overcome the weight of the aircraft10only at comparatively high speeds inasmuch as the aircraft10is capable of vertical takeoff and landing (VTOL) and does not need lift from the fixed wings14at low speeds, e.g. below 50 mph or even 100 mph upon taking off.

In this manner, the wings14may be made smaller than those of fixed wing aircraft requiring a high velocity takeoff, which results in lower drag at higher velocities. In some embodiments the wings14provide sufficient lift to support at least 50 percent, preferably 90 percent, of the weight of the aircraft10at air speeds above 200 mph.

Control surfaces16may secure to one or both of the airframe12and wings14. For example a tail structure18may include one or more vertical stabilizers20and one or more rudders22. The rudders22may be adjustable as known in the art to control the yaw24of the aircraft10during flight. As known in the art, yaw24is defined as rotation about a vertical axis26of the aircraft10. In the illustrated embodiment, the rudders22may comprise hinged portions of the vertical stabilizers20.

The tail structure18may further include a horizontal stabilizer28and an elevator30. The elevator30may be adjustable as known in the art to alter the pitch32of the aircraft10. As known in the art, pitch32is defined as rotation in a plane containing the vertical axis26and a longitudinal axis34of the airframe of an aircraft10. In the illustrated embodiment, the elevator30is a hinged portion of the horizontal stabilizer28. In some embodiments, twin rudders22may be positioned at an angle relative to the vertical axis26and serve both to adjust the yaw24and pitch32of the aircraft10.

The control surfaces16may also include ailerons36on the wings14. As known in the art, ailerons36are used to control roll38of the airplane. As known in the art, roll38is defined as rotation about the longitudinal axis34of the aircraft10.

Lift during vertical takeoff and landing and for augmenting lift of the wings14during flight is provided by a rotor40comprising a number of individual blades42. The blades are mounted to a rotor hub44. The hub44is coupled to a mast46which couples the rotor hub44to the airframe12. The rotor40may be selectively powered by one or more engines48housed in the airframe12, or adjacent nacelles, and coupled to the rotor40. In some embodiments, jets50located at or near the tips of the blades42power the rotor40during takeoff, landing, hovering, or when the flight speed of the aircraft is insufficient to provide sufficient autorotation to develop needed lift.

Referring toFIG. 2, while still referring toFIG. 1, in the illustrated embodiment, the engines48may be embodied as jet engines48that provide thrust during flight of the aircraft. The jet engines48may additionally supply compressed air to the jets46by driving a bypass turbine62or auxiliary compressor. Air compressed by the bypass turbine62may be transmitted through ducts54to a plenum56in fluid communication with the ducts54.

The plenum56is in fluid communication with the mast46that is hollow or has another passage to provide for air conduction. A mast fairing58positioned around the mast46may provide one or both of an air channel and a low drag profile for the mast46. The mast46or mast fairing58is in fluid communication with the rotor hub44. The rotor hub44is in fluid communication with blade ducts60extending longitudinally through the blades42to feed the tip jets50.

Referring toFIGS. 3A-3C, rotation of the rotor40about its axis of rotation72occurs in a rotor disc70that is generally planar but may be contoured due to flexing of the blades42during flight. In general, the rotor disc70may be defined as a plane in which the tips of the blades42travel. Inasmuch as the blades42flap cyclically upward and downward due to changes in lift while advancing and retreating, the rotor disc70is angled with respect to the axis of rotation when viewed along the longitudinal axis34, as shown inFIG. 3A.

Referring toFIG. 3B, the angle74of the rotor disc70, relative to a flight direction76in the plane containing the longitudinal axis34and vertical axis26, is defined as the angle of attack74or rotor disk angle of attack74. For purposes of this application, flight direction76and air speed refer to the direction and speed, respectively, of the airframe12of the aircraft10relative to surrounding air. In autogyro systems, the angle of attack74of the rotor disc70is generally positive in order to achieve autorotation of the rotor40, which in turn generates lift.

Referring toFIG. 3C, the surfaces of the blades42, and particularly the chord of each blade42, define a pitch angle78, or blade angle of attack78, relative to the direction of movement80of the blades42. In general, a higher pitch angle78will result in more lift and higher drag on the blade up to the point where stalling occurs, at which point lift has declined below a value necessary to sustain flight. the pitch angle78of the blade42may be controlled by both cyclic and collective pitch control as known in the art of rotary wing aircraft design.

Referring toFIGS. 4-7, specifically, while continuing to refer generally toFIGS. 1-16, a system100for fairing a rotorcraft rotor in accordance with the invention may include a hub portion102or hub fairing102from which one or more blade portions104or blade root fairings104extend. The hub fairing102covers the rotor hub44of the aircraft10, while the blade portion104covers the area of the root of the blade42or rotary wing42closest to the hub44.

In general, a system100may include an aerodynamic hub portion102designed to minimize aerodynamic drag while passing through air. Typically, portions of the hub fairing102may behave substantially as if it were a static device, and not a rotating device spinning with the hub44of the aircraft10. Nevertheless, because the blades42extend from the hub44, a blade portion104of the fairing system100may extend from an asymmetric or not universally planar symmetric fairing system100.

Typically, the blade portion104may include a leading fairing106, as well as a trailing fairing108. The leading fairing106corresponds with the leading edge of the blade42, advancing into the surrounding air. The trailing fairing108corresponds to the trailing edge of the blade that is following through the air.

In the illustrated embodiment, the blade portion104of the fairing system100includes a cuff110. The cuff110has an internal surface that is substantially spherical.

Meanwhile, a central portion112of the blade portion104of the fairing system100may actually be a surface112fitted over a surface of the airfoil that is the blade42. On the other hand, the central portion112may indeed simply be a carefully fitted portion of the blade42that together with the leading106and trailing108fairing portions forms a single, smooth, aerodynamic surface.

In the illustrated embodiment, the blade42or wing42of a rotorcraft10may typically have a portion that extends inward through the fairing system100to connect to the hub44within the hub portion102of the fairing system100. Accordingly, the blade42or wing42may typically be formed as an airfoil shape. Nevertheless, in transitioning from what is substantially a circular cross-sectional geometry of a root passing into the hub portion102, from an airfoil shape of the blade42itself, a transition region exists. Since a circular cross section does not form a body of particularly low aerodynamic drag, the leading106and trailing108fairing portions may transition from the geometry of the root of the blade42to something of a more aerodynamic shape.

In general, the fairing system100of the rotorcraft10may establish a coordinate system. In general, a direction115represents a nominal vertical axis through the mast and hub44of the rotorcraft10. Thus, the direction115arepresents a vertical upward direction while the direction115brepresents a vertically downward direction, treating the axis of rotation26of the rotorcraft10as a nominal vertical axis. Accordingly, the vertical direction115extends along the axis26of rotation of the rotor40of the rotorcraft10.

Likewise, a longitudinal direction116along a blade42is represented as traversing from a center of rotation away from the hub44in a direction116a, and traversing opposite in a direction116b. Similarly, a lead-lag direction117represents a direction117aadvancing in the direction toward the leading edge or leading fairing106, while the direction117bpasses from the leading fairing106toward the trailing fairing108, generally.

A circumferential direction118represents the flapping direction of motion of the blade42, and thus the blade fairing104or the blade portion104of the fairing system100. Another direction118ais simply an upward or perhaps counterclockwise, circumferential direction with respect to an axis running along the length of a rotorcraft10, while the direction118brepresents the opposite, clockwise rotation about a longitudinal axis of the airframe12.

However, the circumferential direction118may apply to any rotor blade42at any point in its rotation about a central axis26. Accordingly, it is typical to speak of a leading edge as that leading edge extends orthogonally to the direction of motion of a rotorcraft10, sweeping in the direction of flight, with respect to the hub40. Nevertheless, the leading edge actually rotates through a full 360 degrees continually during flight.

A pivot direction119represents a circumferential rotation around the axis represented by the direction116. That is, for example, the direction118(e.g.118a,118b) is viewed within the plane formed by the directions116and115. Likewise, the circumferential direction119exists within the plane formed by the directions115and117.

The body120of the hub portion102represents the region that could have been formed as a completely smooth surface that appears aerodynamically static in a wind stream. For example, absent the shoulders122, projections122, or extensions122from the body120, the cross section of the body120could be completely circular when viewed in a plan view along the axis of rotation26. Nevertheless, the shoulders122or extensions122provide for the connection mechanisms and supporting actuators, lines, fluid handling, pitch-change horns and the like that operate to pivot the rotor blades42about their longitudinal116axes.

In the illustrated embodiment, each of the shoulders122or extensions122may itself be seen as an extension122from the body120by extending away from the central shape. Thus, as many blades42as extend from the hub44of the rotor40may benefit from a shoulder122or extension122formed therearound.

The spherical closure130or the spherical surface130is formed about a center of rotation132that exists hypothetically at some distance away, and may, but need not, exist at the center of the hub44and at the spatial center of the fairing system100. This center of rotation132is the center with respect to which each of the directions115,116,117,118,119, is defined. The center132of rotation, as illustrated inFIGS. 5 and 6, is that theoretical center about which pivoting of the blades42occurs, flapping of a blade42occurs, and lead-lag distortion occurs. Again, lead-lag movement may be distortion, some degree of freedom of motion, or both.

In general, referring toFIG. 6, while continuing to refer generally toFIGS. 1-16, a blade42may include a wall134. In the illustrated embodiment, the wall134represents a wall134of the fairing104or blade portion104of the fairing system100. Through the central portion of the root of each blade42passes a spar box cavity136. That is, a spar, formed as a box or closed cross section will extend as a structural support member from the hub44out and along the blade42.

Accordingly, a spar box cavity136is formed by, or to receive, the spar required to mechanically connect the hub44to the rotor blade42for support and for sustaining each of the forces imposed by flapping, lead-lag motion, and pivoting for blade pitch control. Typically, near an inside corner138of the spar box cavity136, or spaced away therefrom, a shear web142may be added to stiffen the blade42. The spar box cavity136and the wall134of the blade portion of the fairing system100may be further augmented by ribbed connectors144to provide connection, stiffness, or both. The shear web142may extend continuously or by attachment, to continue the shear web142within the root portion to a blade portion146thereof. Meanwhile, the flight surfaces139may actually be formed proximate the root of each blade42by a combination of a portion of the blade in the central portion112, with the leading106and trailing108fairing portions or blade portions104ahead, and behind thereof, respectively.

In the illustrated embodiment, a seal fitting148receives a seal150. The seal150extends from the opening in the extension122or shoulder122of the body120, and seals against the internal surface of the cuff110of the blade portion104of the fairing system100. Therefore, the seal150may touch the spherical inner surface of the cuff110of the blade portion104of the fairing system100, or may connect substantially fixedly, with sufficient flexibility between the mating spherical surfaces110,130in order to permit three degrees of freedom of motion of the cuff110and its connected blade portion104of the fairing system100with respect to the hub portion102of the fairing system100. Accordingly, an inner, spherical surface of the cuff110pivots, and moves in three degrees of freedom with respect to the spherical closure surface130of the shoulder122of the body120.

Referring toFIGS. 8-15, while continuing to refer toFIGS. 1-15, in one embodiment, the body120may be formed of an upper portion124separable from a lower portion126. The line of demarcation along which the upper portion124and lower portion126may be connected may be characterized as a parting line128. The parting line128may be formed to have a snap, seal, interface, substrate, or the like effecting securement of the upper portion124to the lower portion126. In an alternative embodiment, some type of aerodynamic surface matching may occur proximate the parting line128in order that the upper portion124and lower126meet smoothly, thus avoiding excessive aerodynamic drag.

Nevertheless, in certain embodiments, the upper portion124, the lower portion126, or both of the body120may be secured by fasteners directly to some portion of the hub44with respect to which no relative motion is required. It is also preferable that the upper124and lower portion126be separable at a parting line128in order that the fairing system100may be removed from the hub44without requiring complete dismantling of the rotor40.

For example, removing a blade42from the hub44may be a substantially labor intensive process. Removing a segment of the fairing system100formed of a smooth composite polymeric or fiber-reinforced polymeric material would be substantially simpler and more straightforward.

In certain embodiments, the lower portion126may also be formed in multiple pieces. No substantial advantage is gained by forming the upper portion124in more than a single piece. Structural integrity, simplicity of manufacture, minimizing inventory, and providing for suitable strength, wear, durability, and the like may better be served by forming the upper portion124of the body120as a single monolithic, individually molded piece.

By contrast, removing the lower portion126requires either removing the rotor40from the rotorcraft10or separating the portions of the lower portion126in order to remove them from around the mast46of the rotorcraft10.

In the illustrated embodiment, a surface130or closure surface130formed on a shoulder122or extension122of the hub portion102of the fairing system100may be formed in a spherical shape. The spherical end130or closure130thereby provides a surface that can tolerate three degrees of motion with respect thereto, namely a flapping motion in the flapping direction118, a lead-lag motion in that direction117, as well as a pivoting motion in the pivoting direction119about the axis116extending longitudinally along each blade42.