Drive mechanisms for variable diameter rotor systems

A rotor hub assembly includes a rotor hub for supporting a telescoping rotor blade having an outboard section; and a drive mechanism associated with the telescoping rotor blade, the drive mechanism including: a motor; a spool driven by the motor; and a strap wound on the spool, the strap coupled to the outboard section of the telescoping rotor blade.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates generally to drive mechanisms for variable diameter rotor systems, and in particular to individual driven mechanisms for retraction/extension of rotor blades.

A tilt rotor or tilt wing aircraft typically employs a pair of rotor systems which are pivotable such that the rotors may assume a vertical or horizontal orientation. In a horizontal orientation (i.e., horizontal rotor plane), the aircraft is capable of hovering flight, while in a vertical orientation (i.e., vertical rotor plane), the aircraft is propelled in the same manner as conventional propeller-driven fixed-wing aircraft.

Variable Diameter Rotor (VDR) systems are known to provide distinct advantages. That is, when the plane of the rotor is oriented horizontally, the rotor diameter is enlarged for improved hovering efficiency and, when oriented vertically, the rotor diameter is reduced for improved propulsive efficiency.

Existing VDR drive mechanisms are described in U.S. Pat. No. 5,642,982, U.S. Pat. No. 5,636,969, U.S. Pat. No. 6,578,793, U.S. Pat. No. 4,142,697, U.S. Pat. No. 6,454,532 and U.S. Pat. No. 6,655,915, U.S. Pat. No. 6,030,177 and U.S. Pat. No. 6,019,578. The entire contents of these patents are incorporated herein by reference. While these VDR drive mechanisms are well suited for their intended purposes, and some employ the use of a multi-fiber strap, a need exists to reduce the amount of torsional fatigue in the strap. Also, there needs to be a method of monitoring strap elongation, so that a strap can be replaced on condition.

SUMMARY

According to one aspect of the invention, a rotor hub assembly includes a rotor hub for supporting a telescoping rotor blade having an outboard section; and a drive mechanism associated with the telescoping rotor blade, the drive mechanism including:

a motor; a spool driven by the motor; and a strap wound on the spool, the strap coupled to the outboard section of the telescoping rotor blade.

In another aspect of the invention, a drive mechanism for extension and retraction of a telescoping rotor blade having an outboard section, the drive mechanism comprising: a motor; a spool driven by the motor; and a strap wound on the spool, the strap for connection with an outboard section of the telescoping rotor blade.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a perspective view of a gimbal rotor hub assembly100having individual drive mechanisms for extension/retraction of rotor blades. Rotor hub assembly100includes a hub102for supporting four rotor blades, although any number of rotor blades may be employed. Hub102supports four blade arms104upon which are mounted drive mechanisms106. Drive mechanisms106are hydraulically driven, but may driven via other power sources (e.g., electricity, direct drive from aircraft transmission). Each drive mechanisms106drives a spool108(FIG. 3) upon which strap110is wound.

Strap110extends from spool108, through spindle114and into a rotor blade. A distal end of strap110is coupled to an outboard section of a variable diameter rotor blade. Strap110may be a multi-fiber strap, having a breaking strength and strain allowable that is well in excess of the blade centrifugal loads.

Activation of drive mechanism106retracts the outboard section of the rotor blade towards hub102. When commanded, the drive mechanisms106rotate the spools108(FIG. 3) to retract the blades in a prescribed amount of time. Each motor107(FIG. 3) is controlled by an individual, electro-hydraulic servo valve (EHSV). In order to provide precision control, each drive mechanism106is servo-controlled with a feedback from a rotary resolver or RVDT (Rotary Variable Differential Transformer) in the drive mechanism. The resolver tracks revolution count of spool108. This allows each drive mechanism106to be controlled separately to accommodate for differences across the drive mechanisms106(e.g., strap wear, strap elongation). To extend the blades, centrifugal force pulls the blades outwards, and the drive mechanism motors operate as pumps. The EHSV is used to throttle the pump power and limit extension speed. If electric motors are employed, a controller controls speed and direction of the motors rather than hydraulics.

The use of hydraulic hoses and swivels allows an individual strap110and spool108to be mounted on the blade arm and feather with the blade. This isolates each strap110to one blade, eliminating the high cycle torsion apparent in other designs. The use of servo-controlled hydraulic drive mechanisms106with rotary resolvers allows for precision control of blade position. Through the use of hydraulic hoses and swivels, the drive mechanisms106receive power from a central hydraulic system, which is located in either in the rotor head or on the airframe as described in further detail with reference toFIG. 7.

FIG. 2is a side view of the gimbal rotor hub assembly100ofFIG. 1.FIG. 3is a perspective view of a drive mechanism106in the gimbal rotor hub assembly ofFIG. 1. Drive mechanism106includes two motors107for controlling spool108. Evident inFIG. 3is a blade arm spindle114having lubricated centrifugal force bearings.

FIG. 4is a top view of a hingeless, flexbeam, rotor hub assembly200having individual drive mechanisms206in an alternate embodiment. Rotor hub202is a low virtual hinge offset, flexbeam design with four extendable blades retracted by a strap reel system powered by a dual-hydraulic system. Rotor hub202includes two stacked flexbeams204, but it is understood that any number of rotor blades may be employed. Drive mechanisms206are mounted on the outboard side of the spindle and are housed in fairings208. As described in further detail herein, the drive mechanisms206extend/retract the rotor blades through straps210. The drive mechanisms206are contained in the blade feathering system to minimize the warping strains in the strap induced by cyclic pitch and the large collective range required to convert between flight modes.

In exemplary embodiments, the strap210terminates at the blade inboard end at spool212, passes through the non-extending torque tube214to a wide pulley240in the blade and then back inboard to the outboard tip of the torque tube214.FIGS. 8 and 9depict an extended and retracted rotor blade, respectively. Shown inFIGS. 8 and 9are the strap210and pulley240. In this manner, the strap210and motors216only have to sustain half of the blade centrifugal force.

Stops are used to fix the outboard location of the outboard section of the rotor blade. An outboard stop serves to physically limit travel of the outboard section of the rotor blade in when extended as shown inFIG. 8. Inboard locks250(FIG. 7) are used to secure the outboard section of the rotor blade when retracted. By using physical stops and locks, strap210is only under load during the transition of the outboard section of the rotor blade. Further, motors216are only used during the transition of the outboard section of the rotor blade. Centrifugal force holds the outboard section of the rotor blade on the outboard stops when extended. Locks250hold the outboard section of the rotor blade when retracted. The brake assembly on each motor216is used to ground a motor that has lost its power supply during transit and the brake is used to hold the spool212when the hydraulic supply is shut off.

Mounting the drive mechanisms206in the feathering system, has additional maintenance benefits. Unlike some previous configurations, the entire drive/blade/tube assembly can be removed as a single unit by removing the fasteners in a spindle and disconnecting the hydraulic hoses and data lines. Making the telescoping blade assembly a line replaceable unit allows for straightforward blade replacement. A damaged or malfunctioning unit can be sent back to depot to get repaired and refurbished without having to disassemble the strap, spools, motors, bearings, blade, and tube on the aircraft.

FIG. 5is a side view of the hingeless rotor hub assembly ofFIG. 4. Shown inFIG. 5are flexible hydraulic hoses218coupled to the drive mechanism206. Centrifugal force bearings220are positioned in the spindles214.

FIG. 6is a perspective view of a drive mechanism206in the hingeless rotor hub assembly ofFIG. 4. The drive mechanism206includes two over-center variable displacement, hydraulic motors216. The motors216are oriented perpendicular to one another, so they will fit within the strap system fairing208. Motors216are hydraulic, but may be electric. The EHSV on each motor is used to control the speed of spool212hydraulically when extending the rotor blades. If electric motors are used, a controller would be used to control the electric motor speed. The EHSV on each motor216is again activated when in braking mode (i.e., rotor blade extension) to allow the motor216to act as a pump and return fluid to a source. A hydraulic power supply shutoff solenoid and brake assembly222on each motor output shaft is used to fix the position of the motor216once the desired blade position is reached.

Differential gearing226combines the output force from the motors216and transfers that force to a cross shaft. A rotary actuator228is coupled to the cross shaft and includes a compound, planetary gear set that reduces the high speed motor output into a low speed rotary actuator output. A resolver (e.g., a quad rotary variable differential transformer) determines position of the cross shaft and thus the position of spool212. The speed of each motor216is controlled by its own Electro Hydraulic Servo Valve (EHSV). The EHSV receives feedback from the resolver and varies motor speed to keep the four blades extending or retracting in synchronization.

FIG. 7is a perspective view of the hingeless rotor hub assembly ofFIG. 4. Located in the mast inFIG. 7is a coupling assembly230that transfers hydraulic power and electrical power from a source in the stationary airframe to the rotating rotor system. Hydraulic lines are used to connect the coupling assembly to a manifold that distributes fluid to each drive mechanism206. The hydraulic power supply may include dual pressure sources, providing two pressure hoses and two return hoses to each drive mechanism.FIG. 7shows the hydraulic return and pressure lines218. Swivel fittings and flexible hoses are used to allow for blade flapping and pitching motion without hose rupture. The hydraulic power source could also be mounted on the rotor head and be electrically powered through a larger coupling assembly from a source on the airframe.

In operation, when blade retraction is desired, hydraulic power is provided to motors216to turn spools212and reel in straps210. This draws the outboard sections of the rotor blades towards the rotor hub202. A controller monitors the resolvers associated with each drive mechanism206and provides feedback to EHSV to maintain synchronized retraction of the blades. When rotor blade extension is desired, the controller activates the shutoff and brake solenoid on each motor216causing the motors to operate as pumps driven by the centrifugal force of the blade. The blade extension rate is governed by the controller monitoring the resolvers and adjusting the fluid flow through the pump with the EHSV.

A blade retract lock250is employed to offload the strap during airplane mode. Fittings are installed on the inboard end of the blade spar, and a spring loaded locking device250is attached to the torque tube. The use of retract locks250to hold the outboard section of the rotor blade offloads the strap and increases the life of the strap. The locks250can be released by using the drive mechanism to pull the blade inboard just enough to release the latch.

Embodiments place the motor and strap reel in the feathering frame to reduce strap twisting, which improves the life of the strap. Additionally, real time data from the rotary resolver is used to synchronize the blades. By tracking the number of resolver revolutions during the extend/retract cycles, a controller monitors how the strap elongates over time as they wear. This is possible because there are hard defined hard stops at the fully retracted and fully extended position.

Besides monitoring the elongation of the straps, by comparing that elongation data to ground test data on the straps, it can be determined when the straps should be retired and replaced with fresh ones. Similarly, if there is a large change in the number of revolutions required to extend/retract the blade it can be deduced that the strap has been damaged in some way and requires immediate attention. An elongated strap will have reduced thickness requiring more spool revolutions will be required to make the full stroke. This rotary resolver information is fed into the aircraft's health and usage monitoring system (HUMS) and permits the system to have data about the aircraft, not need recurring inspections, and can optimize the strap life all to minimize the maintenance requirements of the VDR system.