Downstop load sensing system

Embodiments are directed to a tiltrotor aircraft having a wing, a proprotor pivotably mounted on the wing, and a downstop striker attached to the proprotor using a load pin, wherein the load pin is configured to generate an output signal representing a force between the proprotor and the wing. A downstop mounted on the wing is aligned to be in contact with the downstop striker when the proprotor is in a horizontal position. A conversion actuator moves the proprotor between a horizontal position and vertical position. A flight control computer is coupled to the output signal from the load pin and configured to control the conversion actuator, wherein the flight control computer is configured to cause the conversion actuator to increase the force if the force is less than a first selected preload value or to decrease the force if the force is greater than a second selected preload value.

BACKGROUND

Tiltrotor aircraft are hybrids between traditional helicopters and traditional propeller driven aircraft. Typical tiltrotor aircraft have fixed wings that terminate with convertible tiltrotor assemblies that house the engines and transmissions that drive the rotors. Tiltrotor aircraft are convertible from a helicopter mode, in which the tiltrotor aircraft can take-off, hover, and land like a helicopter, to an airplane mode, in which the tiltrotor aircraft can fly forward like a fixed-wing aircraft.

The design of tiltrotor aircraft poses unique problems not associated with either helicopters or propeller driven aircraft. In particular, certain loads, both static and dynamic, must be carried by the wings of tiltrotor aircraft that are not present in either helicopters or fixed wing aircraft. When the tiltrotor aircraft converts to the airplane mode, certain oscillatory vibration loads, such as longitudinal pitch loads and lateral yaw loads, are created by the rotors. Because of these unique airplane-mode loads, if a minimal structural stiffness is not maintained between the tiltrotor assembly and the wing, then the aircraft will become unstable. This minimal structural stiffness is based upon airplane-mode aircraft speed and related load factors.

Attempts have been made to measure and maintain a selected preload between the tiltrotor assembly and the wing while the tiltrotor aircraft is in the airplane mode, but none have adequately resolved the problem. For example, in some tiltrotor aircraft, the preload between the tiltrotor assembly and the wing is inferred using differential pressure transducers integral to the conversion actuator motor to determine the preload between the tiltrotor assembly and the wing. In these applications, the preload between the tiltrotor assembly and the wing can be set, but with only limited accuracy. In other tiltrotor assemblies, strain gauges used to provide a direct preload measurement. Such systems are undesirable in certain applications because the strain gauges are required to be bonded to the structure for installation, which reduces maintainability as the sensors are difficult to replace or calibrate. Accordingly, the problem of sensing and measuring the preload between a tiltrotor downstop assembly and a wing has not been adequately resolved.

SUMMARY

Embodiments are directed to the integration of a load cell or load pin into a downstop striker structure. This also for the direct measurement of the downstop preload force in a sensor that lends itself well to maintainability.

A tiltrotor aircraft comprises a proprotor pivotably mounted on a wing. The proprotor is configured to move between a vertical position and a horizontal position. In one embodiment, a downstop striker is mounted to a proprotor gearbox and a downstop is mounted on the wing. In another embodiment, the downstop striker is mounted on the wing and the downstop is mounted on the proprotor gearbox. In both configurations, the downstop striker is configured to contact the downstop when the proprotor is rotated to the horizontal position. The downstop striker may comprise a load pin, wherein the load pin is configured to generate an output signal representing a force between the proprotor and the wing when the downstop striker contacts the downstop. Alternatively, the downstop may be mounted on a load cell that is configured to generate an output signal representing the force between the proprotor and the wing. The tiltrotor aircraft further comprises a conversion actuator configured to move the proprotor between a horizontal position and vertical position, and a flight control computer coupled to the output signal from the load pin or load cell. The flight control computer is configured to control the conversion actuator, wherein the flight control computer implements a closed loop control system using the conversion actuator to apply force and set a desired preload value as measured by the load pin.

The conversion actuator is attached to the wing and is mechanically coupled to the proprotor. The downstop striker comprises a first end configured to be received in a downstop, and a mounting hole extending through the downstop striker, wherein the load pin is positioned within the mounting hole.

The tiltrotor aircraft may further comprise a downstop striker fitting attached to the proprotor gearbox. The downstop striker fitting comprises an open area adapted to receive the downstop striker. The open area is defined by a first surface having a first mounting hole and a second surface having a second mounting hole. The downstop striker comprises a third mounting hole extending through the downstop striker. The load pin comprises a first end, a second end, and a middle section. The first end is positioned within the first mounting hole, the second end is positioned in the second mounting hole, and the middle section is positioned within the third mounting hole.

In another embodiment, an apparatus for maintaining a selected preload comprises an actuator for creating a force between a first member and a second member, wherein the first member and the second member are configured to pivot relative to each other. The force created by the actuator generates the selected preload between the members. A downstop striker is attached to the first member. A downstop is attached to the second member and is configured to receive the downstop striker. A load pin attaches the downstop striker to the first member. The load pin is configured to generate a signal representing the preload force. The signal may be a DC voltage, AC voltage, or digital signal, for example, and may be proportional to the force measured by the load pin. In another embodiment, the downstop is mounted on a load cell that generates the signal representing the preload force.

The apparatus may further comprise a microprocessor coupled to the load pin or load cell to interpret the signal. The microprocessor processes the signal and implements a closed loop control system using the conversion actuator to apply force and set a desired preload value as measured by the load pin. The microprocessor may be a flight control computer, and the actuator may be a conversion actuator for a tiltrotor aircraft.

The first member may be a proprotor, and the second member may be a wing on a tiltrotor aircraft. The actuator may be a conversion actuator for a tiltrotor aircraft.

Alternatively, the first and second members may comprise an airframe of a tiltrotor aircraft, and a tiltrotor assembly.

In a further embodiment, a method comprises providing an actuator for pivoting a tiltrotor assembly relative to a wing member and disposing a downstop assembly between the tiltrotor assembly and the wing member. The downstop assembly comprises a downstop and downstop striker. The downstop striker may be coupled to the tiltrotor assembly or the wing member using a load pin. Alternatively, the downstop may be coupled to the tiltrotor assembly or the wing member using a load cell. The load pin or load cell are electrically coupled to a microprocessor. The tiltrotor assembly is pivoted relative to the wing member with the actuator, which creates a force between the tilt rotor assembly and the at least one wing member. Pivoting the tiltrotor assembly may comprise, for example, moving the tiltrotor assembly between an airplane mode position and a helicopter mode position. The force in the downstop assembly is sensed using the load pin or load cell, which generates a corresponding signal that is received at the microprocessor. Dynamic loads generated during flight may increase or decrease the force.

The method may further comprise interpreting the signal with the microprocessor and sending a control signal from the microprocessor to the actuator in response to the signal.

The method may further comprise increasing the force with the actuator if the force is less than a first selected preload and decreasing the force with the actuator if the force is greater than a second selected preload.

DETAILED DESCRIPTION

FIGS.1A-1Billustrate perspective views of an example tiltrotor aircraft100configured for different flight modes. Tiltrotor aircraft100includes a fuselage101, a landing gear102, a wing103, a tail member104, a propulsion system105, and a propulsion system106. The fuselage101is the main body of the tiltrotor aircraft100, which may include a cabin (e.g., for crew, passengers, and/or cargo) and/or may house certain mechanical and electrical components for tiltrotor aircraft100. In the illustrated embodiment, tail member104may be used as a vertical and a horizontal stabilizer.

Propulsion system105includes a proprotor107that includes a plurality of rotor blades108. Propulsion system106includes a proprotor109that includes a plurality of rotor blades110. The position of proprotors107and109, as well as the pitch of rotor blades108and110, can be selectively controlled in order to provide flight capabilities (e.g., flight direction, thrust, and/or lift) for tiltrotor aircraft100.

The position of proprotors107and109are moveable between a helicopter mode position and an airplane mode position to provide different types of thrust for tiltrotor aircraft100.FIG.1Aillustrates tiltrotor aircraft100proprotors107and109in a helicopter mode position in which proprotors107and109are positioned substantially vertical to provide a lifting thrust.FIG.1Billustrates tiltrotor aircraft100in an airplane mode position in which proprotors107and109are positioned substantially horizontal to provide a forward thrust in which a lifting force is supplied by wing103. It should be appreciated that tiltrotor aircraft can be operated such that proprotors107and109can be selectively positioned between airplane mode and helicopter mode positions, which can be referred to as a “conversion mode.”

Features of propulsion system105are substantially symmetric to features of propulsion system106; therefore, for sake of efficiency certain features will be discussed only with regard to propulsion system105. However, one of ordinary skill in the art would fully appreciate an understanding of propulsion system106based upon embodiments described herein for propulsion system105.

Further, propulsion systems105and106are illustrated in the context of tiltrotor aircraft100; however, propulsion systems105and106can be implemented on other tiltrotor aircraft. For example, an alternative embodiment may include a quad tiltrotor that has an additional wing member aft of wing103and the additional wing member may have additional propulsion systems similar to propulsion systems105and106. In another embodiment, propulsion systems105and106can be used with an unmanned version of tiltrotor aircraft100. Further, propulsion systems105and106can be integrated into a variety of tiltrotor aircraft configurations.

Various engines, gearboxes, and drive shafts may be provided in various configurations to provide torque to proprotors107and109. For example, in at least one embodiment, propulsion system105may include an engine111within an engine nacelle112. Engine111is mechanically coupled to a proprotor gearbox (PRGB)113via a fixed gearbox114to provide torque to proprotor107to facilitate various flight capabilities. In at least one embodiment, engine nacelle112may include an inlet115, aerodynamic fairings, and exhaust, as well as other structures and systems to support and facilitate the operation of engine111.

Fixed gearbox114may include various gears, such as helical gears, in a gear train that are mechanically coupled to engine111and proprotor gearbox113(via other gears and/or gearboxes), as well as an interconnect drive shaft (ICDS)116. The interconnect drive shaft116may provide a torque path that enables a single engine to provide torque to both proprotors107and109in the event of a failure of the other engine.

FIGS.2A and2Bare partial perspective view diagrams illustrating example details associated with propulsion system105, in accordance with certain embodiments.FIG.2Aillustrates example details associated with aircraft100when the proprotor gearbox113is positioned in helicopter mode corresponding toFIG.1A, andFIG.2Billustrates example details associated with aircraft100when the proprotor gearbox113is positioned in aircraft mode corresponding toFIG.1B.

As discussed previously, propulsion system105includes engine111mechanically coupled to proprotor gearbox113via fixed gearbox114to provide various flight capabilities for tiltrotor aircraft100. Engine111and fixed gearbox114are not shown inFIGS.2A and2Bin order to illustrate other features of tiltrotor aircraft100.

Proprotor gearbox113is located above an upper skin201of a portion of wing103, while also being approximately centered between an inboard rib209and an outboard rib208. In at least one embodiment, proprotor gearbox113may be mounted above upper skin201of wing103using an outboard pillow block202, an outboard bearing assembly203, an inboard pillow block204, and an inboard bearing assembly205. Thus, proprotor gearbox113is structurally supported but rotatable about a conversion axis (generally indicated by dashed-line206) to allow the proprotor gearbox113to be rotated (generally indicated by arrows207) between helicopter mode (as shown inFIG.1AandFIG.2A) and airplane mode positions (as shown at least inFIG.1BandFIG.2B).

In at least one embodiment, outboard pillow block202may be structurally integrated with and/or otherwise structurally coupled to outboard rib208. In at least one embodiment, inboard pillow block204may be structurally integrated with and/or otherwise structurally coupled to inboard rib209. It is to be understood that the structural configuration for mounting proprotor gearbox113to wing103is implementation specific and that any combination of structural components may be used for such mounting depending on applications and/or implementations.

Propulsion system105may include other components including, but not limited to, a conversion actuator210(sometimes referred to as a pylon conversion actuator (PCA)), a downstop striker211, a downstop (or V-block or cradle assembly)212, and downstop striker fitting213. Propulsion system105may include other components as would be appreciated by one of ordinary skill in the art to facilitate flight capabilities for tiltrotor aircraft100; however, such components are not disclosed for sake of brevity only in order to discuss various features relating to downstop striker fitting213.

In accordance with some embodiments, downstop striker fitting213is not integrally formed into proprotor gearbox113; rather, fitting213may be mounted on or otherwise attached to proprotor gearbox113. Thus, downstop striker fitting213may be separate from any integrated casting features of proprotor gearbox113. In the example illustrated inFIGS.2A and2B, fitting213also provides features for mechanically coupling conversion actuator210to the fitting213; however, in other embodiments (e.g.,FIGS.4A and4B), the downstop striker fitting may be separate from the coupling conversion actuator. In at least one embodiment, downstop striker211may be mounted to fitting213and downstop212may be mounted to an upper surface of outboard rib208. In some embodiments, the arrangement of a downstop striker and downstop may be reversed. For example, in some embodiments, a downstop striker may be mounted to the wing and/or outboard rib208and a downstop or V-block may be mounted to or formed for fitting. This reversed arrangement offers an advantage of preventing debris collection in the downstop, which might prevent the downstop striker from seating correctly. In at least one embodiment, conversion actuator210may be a linear actuator, such as a telescoping ball screw; however, other conversion actuator types may be envisioned depending on applications and/or implementations.

During operation, conversion actuator210may be actuated (e.g., via a flight control system) so as to selectively rotate proprotor gearbox113about conversion axis206to selectively position proprotor107in airplane mode and helicopter mode positions. Propulsion system105may be subjected to various aerodynamic and operational forces during operation such as thrust or torque loads, conversion actuator pre-load forces, aerodynamic shears, and so forth. Thrust or torque loads, for example, are forces produced by the rotation of proprotor107about a mast axis (generally indicated by dashed line214) that is collinear with the rotational center of proprotor107. In another example, when in airplane mode, conversion actuator210may provide a downward pre-load force that maintains the position of proprotor107in airplane mode.

In yet another example, operational failures and/or malfunctions of components may result in unintended forces being caused to propulsion system components. For example, conversion actuator210may malfunction in a ‘worst-case’ scenario and rather than applying nominal pre-load forces, which typically range between 2,000 pounds and 10,000 pounds of downward force, the conversion actuator may drive the downstop striker211into the downstop212at forces potentially order(s) of magnitude greater than nominal.

Thus, it is important to provide structural and mechanical support for components of propulsion system105to facilitate flight capabilities for tiltrotor aircraft100. Design of components that are used to provide structural and mechanical support for a propulsion system (e.g., propulsion system105) can implicate numerous considerations (e.g., performance considerations, manufacturing considerations, etc.) such as weight, failure, “worst-case” damage or wear rate, cost, and part count among others, and can be a challenging aspect of tiltrotor aircraft design.

Downstop striker fitting213may provide various structural and mechanical features for propulsion system105in accordance with various embodiments described herein. In at least one embodiment, fitting213may facilitate mechanically coupling conversion actuator210to proprotor gearbox113to facilitate conversion mode positioning of proprotor gearbox113in helicopter mode and aircraft mode positions. In other embodiments, downstop striker fitting213may facilitate downstop212/downstop striker211configurations in which the downstop212may be mounted to an upper surface of outboard rib208and downstop striker211may be mounted to fitting213. In at least one embodiment, the configuration may provide for aligning downstop striker211and downstop212along both forward-to-aft and inboard-to-outboard directions to allow downstop striker211to be driven into and held downstop212(e.g., at a given pre-load force) when the proprotor107is positioned in airplane mode.

In accordance with embodiments described herein, downstop striker fitting213may provide numerous technical advantages over other potential conversion actuator coupling configurations and over other potential downstop/downstop striker configurations. For tiltrotor aircraft100, conversion actuator210may be mounted to wing103at a location that is outboard of the proprotor gearbox mast axis214using a mounting assembly that is mechanically coupled to a spindle mount215. Conversion actuator210may be mechanically coupled to fitting213using a rod-end socket-style upper attachment element216that is secured to fitting213using at least one fastener. In at least one embodiment, a spherical bearing320may be seated in the upper attachment element216. As illustrated inFIG.2B, the open area220allows the upper attachment element216to freely rotate through the open bottom and aft sides of the fitting213when proprotor gearbox113is positioned in a helicopter mode position.

Spindle mount215may be mounted to outboard rib208and inboard rib209using various bearing assemblies (not shown) that allow the spindle mount215and conversion actuator210to rotate along forward-to-aft directions (generally indicated by arrows217) during operation. A lower mounting assembly may be mechanically coupled to spindle mount215using a forward bearing assembly218and an aft bearing assembly219that allow the conversion actuator210to tilt along inboard-to-outboard directions during operation.

In other embodiments, the downstop212/downstop striker211configuration using downstop striker fitting213may be replaced by other downstop/downstop striker configurations, such as the downstop striker400illustrated inFIGS.4A and4B. For example, some downstop/downstop striker configurations involve mounting a downstop along the forward side of the outboard rib208. At least one advantage of the configuration provided by fitting213provides for driving pre-load forces directly into the outboard rib208, which may not only eliminate cantilevered forces being driven along the forward side of the outboard rib but may also reduce part count and/or weight in comparison to other configurations.

Downstop striker fitting213may be mounted or otherwise attached to an attachment structure221that is integrated into proprotor gearbox113using, at least in part, a first inboard attachment portion222(FIG.3) and a second inboard attachment portion223that may be integrally formed for the inboard side224of fitting213. It is to be understood that the structural configuration attachment structure221is implementation specific and that any structure and/or structural components may be provided for an attachment structure of proprotor gearbox to facilitate mounting a conversion actuator/downstop striker fitting thereto depending on applications and/or implementations.

Accordingly, fitting213may provide various structural and/or mechanical features integrated together into a unitary component, which may provide numerous technical advantages over other conversion actuator mechanical coupling configurations and downstop/downstop striker configurations. Other advantages that may be provided by downstop striker211are discussed hereinbelow.

FIG.3is a simplified side, cross-sectional view diagram (from a forward perspective) illustrating details associated with fitting213. The cross-section ofFIG.3is cut along a line as generally indicated by the lines labeled “3” inFIG.2B. The enclosed top side301and the enclosed inner side302aof the outboard connection portion303may meet and be structurally integrated together with a diagonal structural element304at an internal structural element305of the fitting213. Spherical bearing320of the upper attachment element216is attached to the outboard connection portion303of fitting213. Diagonal structural element304may extend between the internal structural element305and the cornered structural element306.

Downstop striker211may be mounted to fitting213by mounting the striker211within the cavity307of fitting213and securing the striker211within the cavity307using at least two fasteners308,309that each extend through corresponding holes (not labeled) provided in the forward side312and the aft side313of fitting213. An upper fastener308may be inserted through the mounting slot310and a lower fastener309may be through the mounting hole311to secure the striker211within the cavity307of the fitting213. In one embodiment, pin309may be a load pin as described herein. Load pin309would then be capable of directly measuring the downstop preload force. It will be understood that the position of the forward side312(FIG.2B), aft side313, and outer enclosed side302bare implementation specific and may be adjusted based on the dimensions of the striker211and/or the dimensions of the fitting213depending on various applications and/or implementations.

Diagonal structural element304may include an opening314through which the striker211extends within the cavity307of the fitting213. Downstop striker211may have a first end315and a second end316and may be mounted to fitting213in a vertical orientation in which the first end315represents the end of the striker211that is to be received by and in contact with downstop212when proprotor gearbox113is in the airplane mode position. The second end316of striker211may include a mounting slot310in which the slot extends through the striker211. Striker211may also include a mounting hole311that extends through striker211between the first end315and the second end316. The mounting hole311may be positioned to be closer to the bottom side317of the fitting213when the striker211is mounted within the fitting213. It is to be understood that the position of the mounting slot310and the mounting hole311for the striker211are implementation specific and may be adjusted based on the dimensions of the striker211and/or the dimensions of the fitting213depending on various applications and/or implementations.

FIGS.4A and4Bdepict an alternative embodiment of a tiltrotor downstop striker fitting400. A downstop striker fitting400includes a base member401configured to pivotally and slidingly receive an angled, tunable striker arm402. Base member401is preferably made of aluminum but may be made of any other sufficiently rigid material. Base member401includes a plurality of mounting apertures403for mounting to pivoting proprotor component, such as gearbox113. Striker arm402is generally L-shaped having a post portion404and a leg portion405. Striker arm402is preferably made of titanium but may be made of other materials for which the mechanical properties, in particular bending stiffness, may be adjusted, or “tuned,” by altering the geometrical dimensions of striker arm402.

Post portion404and leg portion405of striker arm402intersect at a generally cylindrical corner portion406. Corner portion406includes a cylindrical channel407that passes transversely through corner portion406along an axis408. Bushings424are coupled to the interior of channel407to reduce friction during movement of striker arm402. Leg portion405extends away from corner portion406and terminates at a forked end409having an upper fork409aand a generally parallel lower fork409b.

Base member401includes a plurality of tabs410aand410b. Tabs410aand410bare generally parallel and extend perpendicularly outward from base member401. Tabs410aand410binclude apertures411aand411b, respectively, passing therethrough. Apertures411aand411bare aligned along an axis412. A slip bushing413is received by apertures411aand411band channel407. Slip bushing413is preferably an anti-friction bushing having a Teflon coating. Slip bushing413is held in place between tabs410aand410b, preferably by retaining washers. A pivot pin414passes along axis412through channel407, bushings424, slip bushing413, and apertures411a,411b, and is releasably received by a fastener415. In this manner, an anti-friction pivot Joint A (FIG.4B) is created, about which post portion404and leg portion405pivot. In one embodiment, pin414may be a load pin as described herein. Load pin414would then be capable of directly measuring the downstop preload force.

Base member401includes a second plurality of tabs417aand417b. Tabs417aand417bare generally parallel and extend perpendicularly outward from base member401. Tabs417aand417binclude apertures418aand418b, respectively, passing therethrough. Apertures418aand418bare aligned along an axis419. A retainer pin420is received through apertures418aand418b. Retainer pin420has a pair of flat recessed portions421aand421bdisposed axially on opposing sides of retainer pin420. It is preferred that at least recessed portions421aand421bof retainer pin420are coated with an anti-friction material, such as Teflon. Retainer pin420is free to rotate within tabs417aand417babout axis419. Flat recessed portions421aand421bare configured to slidingly receive forks409aand409b, thereby forming a sliding and pivoting Joint B (FIG.4B). Because forks409aand409bare allowed to slide relative to retainer pin420, recessed portions421aand421ballow leg portion405of striker arm402to pivot about axis412. However, leg portion405has sufficient stiffness to prevent forks409aand409bfrom translating enough relative to tabs417aand417bsuch that forks409aand409brelease from retainer pin420. In other words, the sliding connection of forks409aand409bwith retainer pin420allows striker arm402to pivot about axis412and pivot pin414(i.e., Joint A).

As shown inFIG.4B, striker arm402passes from Joint A to Joint B along a slot422in base member401. Slot422allows leg portion405of striker arm402to remain in a generally horizontal position and flex or bend in a vertical plane without restriction. Slot422is configured to accommodate variations in the vertical thickness of leg portion405. In addition, slot422allows downstop striker fitting400to maintain an overall low vertical height or profile. Although the terms “vertical” and “horizontal” are used herein, it should be understood that these terms are used only for ease of explanation and are not intended to be limiting as to the directions in which the present invention functions.

With downstop striker fitting400configured and assembled in this manner, oscillatory vibration loads, such as pitch loads and yaw loads, represented by the lateral loads and vertical loads indicated by arrows inFIG.4B, generated by tiltrotor assemblies105,106while in the airplane mode are transferred from tip portion423of post portion404to leg portion405and forks409aand409b. It should be understood that the lateral loads and vertical loads represented inFIG.4Binclude dynamic loads generated during flight, such as when tiltrotor aircraft100goes into a dive or pulls up abruptly. Because post portion404is short, providing the low-height feature of the present invention, post portion404does not bend sufficiently to absorb or isolate the vertical and lateral loads. Thus, the vertical and lateral loads are transferred to leg portion405by post portion404. As leg portion405bends, the vertical and lateral loads generated by tilt rotor assemblies105and106are isolated and absorbed, thereby preventing the vertical and lateral loads from being transferred to wings103. Thus, wings103do not require additional structural support to absorb or dampen the oscillatory vibration loads.

In some embodiments, downstop striker fitting400may be used in place of downstop striker fitting213. In such a configuration, tip portion423of striker arm402will engage downstop212or other V-block structure, when propulsion systems105and106are rotated forward to the aircraft mode configuration.

FIG.5is a cross-sectional view diagram illustrating details associated with tiltrotor downstop striker fitting400ofFIG.4B. The cross-section ofFIG.5is cut along a line as generally indicated by the lines labeled “5” inFIG.4B. Certain features ofFIGS.4A and4B, such as bushing413and washers, are not included in order to simplify the drawing. In one embodiment, pin414is a load pin that is fitted with internal strain gauges, which allows load pin414to the measured load at Joint A and to produce a proportional signal. The outer surface501of load pin414has two circular grooves502and an axial bore503. Inside the central bore503and adjacent to the external grooves502, force-measuring strain gauges504are mounted. Strain gauges504measure the force being applied to load pin414. This force is represented by an electrical signal that is transmitted by load pin414on wire505. Grooves502on the exterior circumference501of load pin414define the area506between the measured forces.

When operating in airplane mode, tip portion423of post404contacts a downstop or V-block, such as downstop212. Post404applies force on the middle section506of load pin414, while the fixed supports at tabs410aand410bapply pressure in the opposite direction. This is the force being measured by strain gauges504inside load pin414. These preload force measurements are transmitted as an electrical signal via cable505to a flight control computer507, sensor module, or other aircraft system. In other embodiments, the preload force measurements may be transmitted wirelessly from load pin414to flight control computer507or other systems. Flight control computer507may be, for example, a microprocessor-based device that executes software instructions to control and monitor the flight controls and aircraft systems, such as a conversion actuator.

Using integrated load pin414provides accurate and direct measurements of preload forces without impacting the configuration of the downstop striker fitting. Additionally, the incorporation of a replaceable load pin414simplifies the maintenance of the preload force sensors. Load pin414may also be used in the downstop striker fitting213as illustrated inFIG.3, wherein fastener309in downstop striker211may be replaced by load pin414or similar device.

The electrical signal output from load pin414may be a DC voltage, AC voltage, or digital signal, for example, that is sent to flight control computer507. The signal is processed in flight control computer507and compared to a pre-determined acceptable range, which is selected, for example, to ensure adequate preload between proprotors107and109and wing103to maintain aircraft stability. Flight control computer507also monitors the signal from load pin414to ensure that the structure is not overloaded. If the signal from load pin414indicates that the load between the downstop striker (e.g.,211or404) and the downstop or V-block (e.g.,212) is below a first pre-determined value required to maintain aircraft stability, then flight control computer507may send a warning signal to the pilot and an appropriate signal to the conversion actuator210to increase the force exerted by the downstop striker on the downstop. Likewise, if the signal from load pin414indicates that the preload between the downstop striker and the downstop is above a second pre-determined value necessary to maintain aircraft stability, then flight control computer507sends a warning signal to the pilot and an appropriate signal to conversion actuator210to decrease the force exerted by the downstop striker on the downstop. The first and second pre-determined values may be the same value or may be different values (e.g., for hysteresis).

FIG.6depicts an alternate configuration for a downstop assembly. A proprotor gearbox601is rotated between a horizontal position and a vertical position relative to wing602by an actuator603. A downstop striker604is mounted on wing602. A downstop605is mounted on the proprotor gearbox601and is configured to impact downstop striker604when proprotor gearbox601is rotated forward to the horizontal position. Downstop striker604is part of a downstop assembly606, which includes a load pin607.

Load pin607measures forces applied on downstop striker604by downstop605. Forces between downstop striker604and downstop605are created by actuator603when proprotor gearbox601is rotated to the horizontal position. In one embodiment, load pin414(FIGS.4A and5) may be used as load pin607and may send a signal representing the measured force to a flight control computer, which determines if the desired preload has been applied by actuator603.

FIGS.2A and2Billustrate a configuration in which the downstop211is mounted on the rotating proprotor gearbox and the downstop212is mounted on the wing, which is fixed relative to the proprotor gearbox.FIG.6illustrates an alternative configuration in which the downstop605is mounted on the rotating proprotor gearbox and the downstop striker is mounted on the wing. It will be understood that other variations in the positioning of the downstop and the downstop striker are also possible. For example, the position of the downstop and the downstop striker may be positioned directly in line with the rotating proprotor gearbox or may be offset inward or outward from the proprotor gearbox centerline.

In further embodiments, instead of providing a load pin in the downstop striker assembly, a load cell may be incorporated into the downstop.FIG.7depicts a downstop701. Downstop701comprises a channel702that is adapted to receive a downstop striker, such as striker211(FIG.2A) or striker604(FIG.6). Downstop701is mounted on load cell703, which has a middle section704between end portions705and706. Grooves707allow middle section704to flex relative to end portions705and706. End portions705and706are mounted on pads708, which provide space for middle section704to deflect slightly toward surface709when a downstop striker contacts downstop701. Surface709may be, for example, a proprotor gearbox or an aircraft wing depending on how the downstop assembly is configured.

Internal strain gauges710are positioned within a central bore711of load cell703. Strain gauges710measure forces being applied to load cell703. The forces are represented by an electrical signal that is transmitted by load cell703to flight control computer712over wire713. Alternatively, the signals from load cell703may be transmitted wirelessly. The signal output from load cell703may be a DC voltage, AC voltage, or digital signal.

When operating in airplane mode, a downstop striker contacts downstop701and applies force on the middle section704of load cell703. Pads708on surface709apply pressure in the opposite direction. This force is measured by strain gauges710inside load cell703. The flight control computer712, a sensor module, or other aircraft system compares the forces from load cell703to the desired preload force and adjusts the proprotor actuator as appropriate to maintain the desired preload.