Rotor blade pitch control

A mechanical independent blade control (MIBC) mechanism for controlling the pitch of each of the blades of a rotor blade system or a main rotor of a rotor aircraft independently of the other blades includes a plurality of actuators disposed in the fuselage below the hub of the rotor, each being operable to selectively control the pitch of an associated one of the blades independently of the other blades, and a plurality of mechanical linkages disposed within the annulus of the rotor mast, each coupled between a blade and an actuator and operable to transmit a force output by the actuator to a pitch horn fixed to an inner end of the associated blade. The mechanism enables the direction of pitch of each blade to be changed more than twice during one revolution of the rotor.

BACKGROUND

This invention relates to rotor blade systems and rotor aircraft, such as helicopters, in general, and in particular, to a mechanism that enables the pitch of each of the blades of the main rotor of the aircraft to be controlled individually and independently of the others.

As illustrated in the partial perspective view ofFIG. 1, most rotor aircraft, such as helicopters, include one or more power driven main rotors1that are equipped with a hub2rotatably supported on a rotor mast5and having a plurality of elongated blades3extending radially outward from it, each of which has an airfoil cross-section (not illustrated) and is coupled to the hub at the inner end thereof by three hinges4that enable the blade to rotate about three axes relative to the hub, i.e., to “flap” up and down, to “lag” forward and aft, and to “pitch” up and down and thereby change its angle of attack relative to the stream of air moving past it.

To move the aircraft vertically, the respective pitches of the rotating blades are all changed simultaneously, or “collectively,” which is effected by a “collective pitch lever” coupled to the blades through a rotating “swash plate” that is coupled to the blades by respective linkages.

On the other hand, horizontal movement of the aircraft is achieved by tilting the rotor such that the thrust of the rotor resolves into two components, a “lift” component that supports the weight of the aircraft, and a “horizontal thrust” component that propels the aircraft horizontally in the desired direction. This tilting of the rotor is effected by tilting the swash plate, which results in a “cyclic pitch control” of the blades, in which the pitch of each of the blades changes twice, i.e., one pitch cycle, per revolution of the hub. For example, to move the aircraft directly forward, the pitch, or angle of attack, of each blade is increased each time that blade passes over the tail of the aircraft, such that the lift developed by that blade is then temporarily greater than that of the other blades, and thereby results in a forward thrust component being applied to the aircraft by the rotor.

As a result of the foregoing method of operation and the effect of the relative speed of the aircraft moving through air, conventional rotor aircraft have a limited forward air speed, viz., about 180 knots (˜207 mph), due to the blade tip speed approaching the speed of sound on the advancing blade, and a stall condition occurring on the retreating blade. Additionally, when these limiting conditions are being approached, large vibrations begin to occur in the rotor, which causes component fatigue and increased pilot mental and physical work load. The vibrations cause the entire vehicle, including the pilot and aircraft cockpit controls, to shake and the aircraft displays to become blurred.

One effort to address the foregoing problem has been the development of so-called “tilt rotor” aircraft to provide enhanced helicopter lift capability, higher forward airspeed and reduced vibration. However, this approach adds wings, aero-surface controls, complex rotor conversion mechanisms, weight and cost.

In another approach, rotor aircraft designs have been developed implementing so-called “compound” systems that have both conventional rotor systems and additional forward propulsion systems. These compound designs typically also include additional lifting surfaces and aero-surface controls, which add significant complexity, weight and cost to the aircraft.

In a third approach, individual blade control (IBC) is used in conjunction with a lower rotor speed and “reverse rotor flow” technology. IBC systems enable the direction of pitch of each blade to be varied independently of the others and more than twice per revolution of the hub, as occurs in conventional rotor aircraft. EBC also enables the rotor system track and balance procedure to be implemented in software, thereby eliminating the time consuming process of manually adjusting the length of each pitch link. Typical approaches to IBC utilize either electrical motor actuators and slip rings, or hydraulic actuators, hydraulic swivels and electrical slip rings. Both approaches are complicated, add extensive installation congestion in the rotating section of the rotor system, and significantly reduce control reliability of the “flight critical” rotor system. The electric approach utilizes a screw mechanism that is susceptible to jamming and is dependent on slip ring technology, which is unreliable. Furthermore, lightening strike attachment to the rotor hub is a common occurrence and may completely eliminate all electrical control. The hydraulic approach is dependant on both electrical and hydraulic slip ring technology, neither of which is reliable. The lightening strike problem also exists with this design. External hydraulic leaks are centrifugally distributed onto numerous aircraft components, including the exterior body and windshield and require extensive clean up. The mass of the rotating power control actuators also introduces new stresses into the flight critical rotor system.

In light of the foregoing problems, there is a long felt but as yet unsatisfied need in the field of rotor aircraft for a simpler, substantially more reliable, less expensive and lighter weight mechanism for providing individual blade control (IBC) for the rotor(s) of a rotor aircraft.

BRIEF SUMMARY

In accordance with the exemplary embodiments thereof described herein, the present invention provides a mechanical individual blade control (MIBC) mechanism for rotor blade systems and the main rotor(s) of a rotor aircraft, such as a helicopter, that affords a simpler, more reliable, higher harmonic individual blade control technique, enables a higher rotor aircraft speed to be achieved when used in conjunction with reverse flow rotor techniques (i.e., slowed rotor speed combined with multiple blade pitch changes), results in substantially reduced vibration at either high or low speeds while avoiding the overall design complexity of the prior art IBC mechanisms, and provides a technique for effecting automatic rotor system track and balance.

In one exemplary embodiment thereof, the novel MIBC mechanism comprises an elongated tubular rotor mast having a long axis, an upper end coupled to a hub of the rotor for conjoint rotation therewith, and a lower end coupled to and rotationally driven by an engine of the aircraft. A plurality of pitch tubes equal in number to the number of blades of the rotor are disposed coaxially within the rotor mast and fixed against conjoint rotation therewith. An actuator is coupled to a lower end of an associated one of each of the pitch tubes, each actuator being operable to selectably move the associated pitch tube up and down in the axial direction independently of the other pitch tubes. A pitch plate is disposed at an upper end of an associated one of each of the pitch tubes for conjoint rotation with the rotor mast. Each pitch plate has an inner periphery that is coupled to the upper end of the associated pitch tube for both conjoint axial movement therewith and rotational movement relative thereto, and an outer periphery that is coupled to an inner surface of the rotor mast for both conjoint rotation therewith and axial movement relative thereto. An elongated pitch link has a lower end fixed to an associated one of each of the pitch plates for conjoint axial and rotational movement therewith, and an upper end coupled to a pitch horn disposed at an inner end of an associated one of the blades of the rotor.

In another exemplary embodiment, a method for controlling the pitch of each of the blades of an aircraft rotor thrust system comprises coupling an upper end of an elongated annular rotor mast to a hub of the rotor system for conjoint rotation therewith, rotationally driving a lower end of the rotor mast, disposing a plurality of mechanical linkages within the annulus of the rotor mast, providing a plurality of actuators below the hub of the rotor, each actuator being operable to selectively control the pitch of an associated one of the blades independently of the other blades, coupling each of the linkages between an associated one of the blades and an associated one of the actuators, each linkage being operable to transmit a force output by the associated actuator to a pitch horn fixed to an inner end of the associated blade, and controlling the pitch of each blade of the rotor system independently of the other blades with the associated actuator.

In another exemplary embodiment, a rotor aircraft comprises a fuselage and a main rotor rotatably coupled to the fuselage. The rotor includes a rotatable hub that is rotationally driven by an engine, and a plurality of blades extending radially outward therefrom. Each of the blades has an inner end coupled to the hub for rotation of the blade about each of three mutually perpendicular axes relative to the hub. An upper end of an elongated annular rotor mast is coupled to the hub for conjoint rotation therewith, and a lower end of the mast is coupled to and rotationally driven by the engine. A plurality of actuators are disposed in the fuselage below the hub. Each actuator is operable to selectively control the pitch of an associated one of the blades independently of the other blades. A plurality of mechanical linkages are disposed within the annulus of the rotor mast. Each linkage is coupled between a pitch horn fixed to an inner end of an associated one of the blades and an associated one of the actuators and is operable to transmit a force output by the associated actuator to the associated blade to change its pitch independently of the other blades.

In one preferred embodiment, the pitch tubes are disposed concentrically within the rotor mast, and the outer periphery of each pitch plate is coupled to the inner surface of the rotor mast by a slider mechanism disposed at the outer periphery of the pitch plate and engaged in an axial slot or spline in the inner surface of the rotor mast for both conjoint rotation therewith and for relative sliding axial movement therein. The inner periphery of each pitch plate is coupled to the upper end of the associated pitch tube by a bearing coupled between the inner periphery of the pitch plate and the upper end of the pitch tube so as to enable both conjoint axial movement therewith and rotational movement relative thereto.

In another preferred exemplary embodiment, the aircraft includes a gearbox having an opening therethrough, with the lower end of the rotor mast extending into the gearbox and the mechanical linkages extending through the opening in the gearbox such that an upper end of each of the linkages is disposed above the gearbox and a lower end of each of the linkages is disposed below the gearbox, along with the actuators. This “hole-in-the box” arrangement provides a compact, protected mechanical control system design and locates the transmission closer to the rotating plane of the blades, thereby reducing rotor stresses, since the rotating plane of the blades can be located closer to the transmission wherein the rotor mast moments are reacted, and provides the power control elements in a stationary system that is easy to inspect and maintain. The robust annular rotor mast also serves to protect the flight critical linkages of the MIBC. The hole-in-the-box design also simplifies the pitch tube anti-rotation feature, as compared to a conventional “scissor” device.

The actuators of the MIBC mechanism can comprise either linear or rotary actuators, and further, may be either electromagnetic or hydraulic in operation. Additionally, for purposes of redundancy, a plurality of actuators may be coupled to a lower end of an associated one of the mechanical linkages, each of which is capable of selectably moving the associated linkage so as to effect pitch changes of the associated blade independently of the operation of the other actuators coupled to the linkage.

The use of the novel MIBC mechanism in combination with reverse rotor flow (slowed rotor speed, multiple blade pitch cycles per revolution), enables a more conventional rotor air-craft (i.e., a non-tilt rotor and no forward propulsion system) to 1) achieve a higher forward air-speed through use of a slowed rotor and reverse rotor flow techniques, 2) avoid the complications and weight of tilt rotor aircraft designs, 3) avoid the complication and weight of compound aircraft designs, and 4) provide higher harmonic controls to reduce vibration levels. The use of the novel MIBC in conjunction with conventional non-reverse flow rotor systems also provides: 1) a higher harmonic control (HHC) solution to reduce vibration, in that it enables the direction of pitch of each blade of the rotor to be changed more than twice, and indeed, as many times as desired, during a single revolution of the rotor hub, and 2) provides automatic rotor blade track and balance to reduce maintenance time.

A better understanding of the above and many other features and advantages of the novel MIBC mechanism of the invention may be obtained from a consideration of the detailed description of some exemplary embodiments thereof below, particularly if such consideration is made in conjunction with the appended drawings, wherein like reference numerals are used to identify like elements illustrated in one or more of the figures thereof.

DETAILED DESCRIPTION

FIG. 2is a partial cross-sectional view of a main rotor1of a rotor aircraft (not illustrated) incorporating an exemplary embodiment of a mechanical independent blade control (MIBC) mechanism10in accordance with the present invention, andFIG. 3is a cross-sectional view of the MIBC mechanism10ofFIG. 2, as seen along the section lines3-3taken inFIG. 2.

In accordance with the present invention, the MIBC10provides reliable higher harmonic individual blade control (IBC). The higher harmonic IBC solution, in turn, enables 1) a higher rotor aircraft speed when used in conjunction with reverse flow rotor techniques (i.e., slowed rotor speed, multiple blade pitch cycles per revolution) in a more conventional rotor aircraft design, 2) reduced vibration at high or low speeds in such aircraft designs while minimizing overall aircraft design complexity, thereby avoiding the complexity of tilt rotor aircraft designs and other compound aircraft designs that have addition of forward propulsion systems, and 3) automatic rotor blade track and balance.

As illustrated inFIGS. 2 and 3, the exemplary MIBC mechanism10comprises an elongated tubular rotor mast12having a long axis, an upper end coupled to the hub2of the rotor1for conjoint rotation therewith, and a lower end coupled to and rotationally driven by the engine (not illustrated) of the aircraft, typically a gas turbine or piston engine. As illustrated inFIG. 2, a plurality of pitch tubes14, which are equal in number to the number of blades3of the rotor, are disposed coaxially within the rotor mast and fixed against conjoint rotation therewith in the manner described in more detail below. In the particular exemplary embodiment ofFIGS. 2 and 3, the main rotor1includes3blades, but it should be understood that the MIBC mechanism10can be used with rotors having either a fewer or a greater number of blades.

An actuator16is coupled to a lower end of an associated one of each of the pitch tubes14. Each of the actuators is operable to selectably move the associated pitch tube back and forth in the axial direction, i.e., up and down, within the rotor mast12independently of the other pitch tubes. A pitch plate18is disposed at an upper end of and generally perpendicular to an associated one of each of the pitch tubes14for conjoint rotation with the rotor mast12, and to that end, each of the pitch plates18has an inner periphery that is coupled to the upper end of the associated pitch tube for conjoint axial movement therewith and for rotational movement relative thereto, and one or more shoes, or sliders28, disposed on an outer periphery thereof that is coupled to an inner surface of the rotor mast12for conjoint rotation with, and for axial movement relative to, the rotor mast12.

The mechanism10further includes a plurality of elongated pitch links20, each having a lower end fixed to an associated one of the pitch plates18for conjoint axial and rotational movement therewith, and an upper end coupled to a pitch horn22disposed at an inner end of an associated one of each of the blades3of the rotor1.

In the particular exemplary embodiment ofFIGS. 2 and 3, each of the pitch tubes14includes an anti-rotation feature24, such as an axial slot or spline, disposed at a lower end portion thereof. A corresponding anti-rotation device26having one or both ends coupled to structure of the aircraft (not illustrated) extends through the anti-rotation feature of the innermost one of the pitch tubes14, and each of the other pitch tubes14includes a portion that extends through the anti-rotation feature24of the innermost pitch tube14such that each of the pitch tubes14is thereby fixed against conjoint rotation with the rotor mast12. The anti-rotation features serve to provide lateral support to the lower ends of the pitch tubes14, and additionally, serve to fix the pitch tubes against conjoint rotation with the rotor mast12. As will be appreciated by those of skill in the art, this lateral support and anti-rotation feature can be supplied by mechanisms other than the exemplary mechanisms described above and illustrated herein.

To effect conjoint rotation of the pitch plates18with the rotor mast12and still enable them to move axially within the annulus of the mast12, the outer periphery of each of the pitch plates18is coupled to the inner surface of the rotor mast12by one or more sliders28disposed at the outer periphery of the pitch plate18and engaged in a corresponding axial slot or spline30in the inner surface of the rotor mast12for relative axial sliding therein. As illustrated inFIG. 3, the inner periphery of each pitch plate18is coupled to the upper end of the associated pitch tube14by a bearing32that is coupled between the inner periphery of the pitch plate18and the upper end of the pitch tube14so as to enable both conjoint axial movement with, and rotational movement relative to, the associated pitch tube14. The bearings32are thus adapted to enable the associated pitch plate18to rotate relative to the associated pitch tube, while enabling axial forces to be transmitted from the pitch tubes14to the respective associated pitch plates18.

In the particular exemplary embodiment illustrated inFIG. 2, the aircraft includes a stationary gearbox34having an opening36extending vertically therethrough. The lower end of the rotor mast12extends into the gearbox34and is coupled therein to and rotationally driven by a rotor gear38coupled through a transmission (not illustrated) to the engine (not illustrated) of the aircraft. Additional sets of bearings32disposed between the rotor mast12and the gearbox34enable the rotor to rotate relative to the gearbox34. As illustrated inFIG. 2, the pitch tubes14extend through the opening in the gearbox such that the pitch plates18and the pitch links20are disposed above the actuators16, whereas, the lower ends of the pitch tubes14and the actuators16are disposed below the gearbox34in the fuselage of the aircraft. This arrangement, referred to herein as a “hole-in-the box” design, provides a mechanical control system that is compact and well protected, locates the transmission closer to the plane of rotation of the blades3, thereby reducing the stresses acting on the rotor1(since the rotating plane of the blades3can be located closer to the transmission wherein the rotor mast12moments are reacted), and disposes the power control elements in a stationary system that is easy to inspect and maintain, and simplifies the design of the pitch tube anti-rotation feature24. The robust annular rotor mast12of the mechanism10also serves to protect the flight critical linkages of the MIBC10.

The actuators16of the MIBC mechanism can comprise either linear or rotary actuators, and moreover, may be either electromagnetic or hydraulic in their operation. Additionally, for purposes of redundancy, a plurality of actuators may be coupled to a lower end of an associated one of each of the pitch tubes14, each of which is capable of selectably moving the associated pitch tube so as to effect the necessary pitch changes of the associated blade3independently of the operation of the other actuators coupled to that pitch tube.

As illustrated inFIGS. 2 and 3, the pitch plates18are disposed one above the other, and accordingly, lower ones of the pitch links20are arranged to extend through openings40in upper ones of the pitch plates to enable a free axial movement thereof. In the particular exemplary embodiment ofFIGS. 2 and 3, the rotor1incorporates 3 blades, and accordingly, the lowermost pitch link extends upward from the lowermost pitch plate and through corresponding openings40in the middle and uppermost pitch plates, and the middle pitch link extends upward from the middle pitch plate through another corresponding opening40in the uppermost pitch plate.

Referring toFIG. 2, in operation, the stationary, centrally located power actuator16moves up and down. Vertical forces are transmitted into the associated, non-rotating central pitch tube14, bearing32, and then into the rotating pitch plate18. As the pitch plate is moved up and down by the pitch tube14, it transfers force into the associated blade3via the associated pitch link20, which is coupled to the associated pitch horn22. (SinceFIG. 2is a cross-sectional view, only two pitch links20and pitch horns22are seen therein, as a third pitch link20, pitch horn22and associated blade extending outward from the page are not visible.) The pitch horn22converts the linear motion of the pitch link into rotational pitch movement of the associated blade3, which is rotatably coupled to the hub2of the rotor1, about the long axis of the blade.

As discussed above, the innermost pitch tube14has an anti-rotation feature24, such as a slot or spline, that laterally supports the pitch tube and reacts the frictional torque developed by the rotating bearings32. Each of the other pitch tubes also incorporates an anti-rotation feature that engages the innermost pitch tube. The anti-rotation feature of the innermost pitch tube also provides lower lateral support for the other pitch tubes. The rotating sliders28on the outer race of each pitch plate provide upper lateral support to the pitch tubes. The sliders28are engaged in corresponding slots or splines30in the inner surface of the rotor mast12. This arrangement enables the relative positions of the respective upper and lower ends of the pitch links20to be maintained synchronously.

As will be appreciated from the foregoing description, the novel MIBC mechanism10utilizes an independent power actuator16and an affiliated mechanical control path, or linkage, to control each blade3of the rotor independently of the others. These mechanical linkages include independent pitch plates14(instead of a single swashplate) to individually control the pitch of each rotor blade. This approach enables the lower reliability and redundant power control components to be kept in the stationary (i.e., non-rotating) system, and enables the use of only high reliability mechanical components to transmit control forces into the rotating rotor system, thereby providing a higher degree of reliability of control of the rotary system. The power control actuators16move the bearing-supported pitch tubes14up and down within the stationary system. The pitch tubes14, in turn, move the bearing-supported pitch plates18up and down, which enables the control forces to be reliably transmitted into the rotating system. The rotating outer race of each pitch plate thus controls the pitch link20and its associated blade3independently of the other mechanical linkages.

The use of the novel MIBC mechanism10in combination with a slowed rotor and reverse rotor flow (multiple blade pitch cycles per revolution), enables a conventional rotor aircraft (i.e., one with a non-tilt rotor and no forward propulsion system) to achieve a higher forward air-speed, avoid the complications and weight of tilt rotor and compound aircraft designs, and provides higher harmonic controls to reduce vibration levels. The MIBC also provides a higher harmonic control (HHC) and automatic rotor system track and balance solutions for use on conventional, i.e., non-reverse flow rotor systems that reduces vibration at the upper speed limit and reduces maintenance time. Thus, while conventional rotor blade control is limited to a single pitch cycle, i.e., one up and down cycle, per revolution of the hub, due to the use of a single swashplate design, the MIBC mechanism10of the present invention provides independent higher frequency control for each blade, e.g., two or even more pitch cycles per revolution.

The MIBC mechanism10thus provides the benefits of IBC while avoiding the problems with electrical and hydraulic IBC systems, minimizes the total number of components, provides the necessary rotor system reliability needed for helicopter “flight critical” rotor control and offers excellent maintainability characteristics.

By now, those of skill in this art will appreciate that many modifications, substitutions and variations can be made in and to the MIBC mechanism of the present invention without departing from its spirit and scope. In light of this, the scope of the present invention should not be limited to that of the particular embodiments illustrated and described herein, as they are only exemplary in nature, but instead, should be fully commensurate with that of the claims appended hereafter and their functional equivalents.