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DEVELOPMENT, MANUFACTURING, AND COMPONENT TESTING OF AN INDIVIDUAL BLADECONTROL SYSTEM FOR A UH-60 HELICOPTER ROTOR

Axel HaberZF Luftfahrttechnik GmbHKassel-Calden, Germany

Stephen A. JacklinNASA Ames Research Center

Moffett Field, California

Gary deSimoneSikorsky Aircraft Corporation

Stratford, Connecticut

Abstract

Within the framework of a cooperative research program joined by ZF Luftfahrttechnik GmbH (ZFL), NASA AmesResearch Center (ARC), and Sikorsky Aircraft Corporation (SAC) it was agreed to develop and manufacture a full-scale UH-60 Individual Blade Control (IBC) system to be installed in the LRTA and tested in the NASA NationalFull-Scale Aerodynamics Complex. The LRTA is NASA’s new Large Rotor Test Apparatus which was designed totest helicopter and tilt rotors in the 40- by 80-foot and 80- by 120-foot Wind Tunnels. The objectives of the UH-60IBC wind tunnel test program were to demonstrate the feasibility of the IBC system and to quantify the benefits to begained when applied to and operated with a UH-60 main rotor.

This paper describes the UH-60 IBC system engineering design, development, manufacturing, and component testingconducted by ZFL. The servo-hydraulic IBC actuators to be installed between the swashplate and blade pitch hornwere designed to withstand the control forces produced by a full-scale UH-60 rotor in a centrifugal field of up to 40g. They were designed to produce up to ±6.0 deg blade pitch motion at the 2/rev frequency, diminishing to ±1.6 degat the 7/rev frequency. A set of actuator test specimens was manufactured and fatigue tested with simulation of thecontrol forces and centrifugal load. Prior to the 80- by 120-foot wind tunnel testing, that was concluded by the end ofSeptember 2001, the complete IBC system was functionally tested (non-rotating and rotating, blades off). This paperdiscusses the IBC system integration issues as well as the test stand set-up used for actuator and system level testing.

Introduction

Active rotor control using full-scale blade root actuationdevices has proven successful in reducing vibration,noise, and other negative effects associated withhelicopter rotors operating in egdewise flow. The earlydeveloped Higher Harmonic Control (HHC) systemswhich actuate the swashplate of a conventionalhelicopter in the fixed frame are restricted to certainfrequencies of N/rev, (N-1)/rev, and (N+1)/rev, where Nis the number of rotor blades. Hence it follows, that fora four-bladed rotor the important 2/rev frequency cannot be generated by these fixed-frame HHC systems.This can only be achieved by rotor-mounted devicessuch as, for example, active pitch links, which replacethe rigid control rods by servo-hydraulic actuators.

____________________Presented at the American Helicopter SocietyAerodynamics, Acoustics, and Test and EvaluationTechnical Specialists Meeting, San Francisco, CA,January 23-25, 2002. Copyright © 2002 by theAmerican Helicopter Society International, Inc. Allrights reserved.

Each rotor blade can thus be controlled independentlyfrom the others in a certain pitch angle rangesuperimposed over the primary flight controls. Othercurrent Individual Blade Control approaches employ on-blade actuation devices like trailing edge flaps or tabs,active blade tips, or integral blade twist systems. Allthese concepts pose difficulties for the design andfabrication of the highly loaded main rotor blades, sothat most of them have not been implemented in fullscale, to date.

Under a previous cooperative U.S./German program,NASA tested an IBC system in the Ames 40- by 80-footWind Tunnel in 1993 and 1994. The testing wasperformed with a BO 105 rotor on the NASA/U.S.Army Rotor Test Apparatus using IBC actuatorsmanufactured by ZFL. The 1994 entry demonstratedsimultaneous reductions in vibratory loads and noise forsome IBC inputs, and modest improvements inperformance at high-speed flight conditions. As a resultof these findings, Sikorsky Aircraft, the U.S. Army,NASA Ames, and ZFL entered into a joint program toevaluate the potential of IBC to benefit the UH-60 BlackHawk helicopter. As a step towards reducing the risk

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and cost of flight testing, the IBC technology shouldfirst be evaluated on a Black Hawk rotor in the 80- by120-foot and 40- by 80-foot Wind Tunnels at the NASAAmes Research Center.

IBC system design

The full-scale IBC system for helicopter main rotorsdescribed in this paper was developed and manufacturedby ZF Luftfahrttechnik GmbH of Germany. The IBCequipment has been installed into NASA Ames‘ LargeRotor Test Apparatus (see Fig. 1) which carried aSikorsky UH-60 Black Hawk main rotor system. Thecurrent UH-60 blade pitch links were replaced by ZFL’sservo-hydraulic IBC actuators with a bandwidthsufficient to perform blade pitch motion control up to7/rev (30.1 Hz).

Fig. 1: LRTA in NASA Ames wind tunnel preparationarea

It was agreed to design IBC actuators mating with astandard UH-60 main rotor and being capable ofsuperimposing IBC angles of up to ±6.0 deg.Corresponding hardware has been laid out as anexperimental system for wind tunnel tests andfundamental active rotor control research.Development, design, and fabrication of the IBC systemcomponents was based on the considerable experienceof ZFL regarding IBC equipment production andhardware testing (Refs. [1] – [9]). Though utilized forwind tunnel tests only, all conceivable safety aspectshave been addressed during design and manufacture ofthe IBC hardware. With regard to the fabrication of thehydraulic system, particularly the actuator components,ZFL was able to refer to processes used in the CH-53GIBC flight test program carried out together with theGerman Army (Ref. [3]). The consideration of qualityassurance aspects during procurement and productionfollow examples of corresponding CH-53G methods thatyield to flightworthy hardware. The IBC electronics, onthe other hand, were based upon the BO 105 windtunnel test hard- and software used in the successful1993 and 1994 campaigns (Refs. [6], [7]).

Subassemblies

The mechanical layout of ZFL’s IBC system focuses onmodular design for easy installation into NASA ARC’sLRTA operating a UH-60 main rotor. The IBC systemis composed of the following main subassemblies:

• 4 blade root IBC pitch actuators,• hub-mounted hydraulic parts comprising

- hydraulic distribution collar,- hub adapter,- hydraulic pipes, hoses, and fittings,

• non-rotating LRTA-mounted parts comprising- hydraulic slipring (rotary transmission lead-

through),- hydraulic manifold including magnetic shutdown

valves,• electronic hard- and software.

Fig. 2 shows the general arrangement of the equipment.For clarity reasons a neat grouping is shown omitting allmechanical parts of the LRTA test stand excepting therotor shaft leadthrough.

IBC Actuator

HydraulicDistributionCollar

Hub Adapter

HydraulicManifold

HydraulicSlipring

ElectricalSliprings

Fig. 2: UH-60 IBC system for LRTA installation

ZFL has designed and fabricated the IBC actuators andthe control electronics. The adaptation hardwarerequired to interface the IBC system to the LRTA andthe UH-60 rotor has also been designed by ZFL.However, some parts and subassemblies like thehydraulic block and slipring have been manufactured by

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external suppliers. The general layout of the hydraulicblock including the realization of the shutdownfunctions and a specification for the manifold as well asfor the hydraulic slipring were defined by ZFL. Forthese important components ZFL has specifiedacceptance test procedures which had to be passedbefore the hardware could be approved. Most of thevendor parts as well as the materials used and semi-finished products have been purchased with testcertificates. Essential hydraulic parts and connections(e. g. swivel joints) to be installed in the rotating framewere chosen to meet aerospace standards.

Actuators

The blade root pitch actuators are the most sophisticatedtechnical components within the IBC system. Theirdesign is based on the considerable experience that ZFLhas gathered in several IBC wind tunnel and flight testcampaigns during the past fifteen years. However,regarding the capability in actuator stroke (controlauthority) and axial load performance the UH-60 IBCactuators represent a unique design. The desiredmaximum IBC amplitude was specified to ±6.0 deg,which led to a maximum necessary actuator stroke of 18mm (pp/2). The required design peak load resulted fromthe requirement to operate the IBC actuator in a pitchlink load envelope of –1,000 +/– 3,000 lbf. The loadrequirements correspond to common level flightconditions with additional load portions due to IBCsuperposition considered. IBC operation especially inthe wind tunnel should thus be possible for a wide rangeof the flight envelope.

With these boundary conditions, calculations regardinggeometry and hydraulics were carried out. Hydraulicflow rate and working piston area were sized to control aharmonic stroke signal with 18 mm amplitude (pp/2) at a

frequency of 2/rev under the specified load conditions.Table 1 gives an overview of the leading technical dataof the actuator. Figs. 3 and 4 show the result of theactuator layout and detail design activities.

Table 1: Technical data of the UH-60 IBC actuator

Parameter ValueNominal actuator length 637.5 +/– 19.0 mm

Actuator stroke(mechanical limit) ±19.5 mm

Rated IBC stroke(max. usable @ 2/rev) ±18.0 mm

Rated max. piston velocity(2/rev, full stroke) 0.97 m/s

Working piston area 13.93 cm2

Range of usable harmonics 2/rev – 7/revDesign peak load (IBC on) 17,800 N

Rated min. holding load(@ lock-out) 34,100 N

Mean hydraulic flow rate 55.0 l/minActuator weight (incl. rod ends) 13.8 kg

The actuator makes use of a working piston whichmoves inside the main cylinder housing in response tothe hydraulic pressure and flow control of a Moog servovalve. The piston extends and retracts, providing themotion or force commanded by the IBC controller.Actuator displacement is measured by two LVDTs(Linear Variable Differential Transducers); a full bridgestrain gage is used to measure the axial (pitch link) loadsduring operation.

As the actuators were designed to replace theconventional rigid control rods it is absolutely necessarythat any failure in the hydraulic or electronic circuitsdoes not lead to catastrophic or even critical rotor states.

STRAIN GAGE

SWASH PLATE ATTACHMENT

Fig. 3: Two-way view of the IBC actuator Fig. 4: Sectional view of the IBC actuator

SERVOVALVE

MAINCYLINDERHOUSING

SAFETYCYLINDERHOUSING

PRESSURETRANSDUCER

ELECTRICALCONNECTOR

HYDRAULICFITTING

FILLINGVALVE

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To a priori avoid such situations the actuators have beendesigned and equipped with an automatic fail-safe lock-out mechanism, that in case of emergency forces eachactuator working piston to immediately return into itsmid position. In that configuration the length of theactuator is equal to the conventional push rod geometryand remains fixed. For this purpose the actuatorsemploy a dual gas cylinder safety system making use oflock-out pistons which are retracted against a gas springby application of the 3,000 psi hydraulic systempressure (see Fig. 4, left safety cylinder). These pistonshave wedge-shaped ends that accurately fit intocorresponding openings machined into the workingpiston.

On loss of hydraulic pressure, both pistons automaticallycenter, driven by the pressurized gas volume alwayspresent during normal operation (see Fig. 4, right safetycylinder). Thus the safety philosophy is to generate animmediate pressure drop in the main hydraulic circuitwhen a failure or malfunction of the IBC system isdetected (fault detection algorithms and reconfigurationin the presence of detected faults are provided by thecontrol system). In this case the working piston isforced to immediately move into its mid position andremain there fixed as long as the hydraulic systempressure is down. Once the piston is locked, staticfriction ensures a rated holding load of more than 34 kN.It should be noted that only one of the two independentsafety pistons is sufficient to lock the actuator, in thissense the safety system is dual redundant.

Material selection and fabrication processes for theactuator component parts as well as foreign supplies andcontract services had been monitored by ZFL’s qualitymanagement and assurance departments to make surethat all relevant procedures met aerospace standards.The main and safety cylinder housings are made of high-quality wrought aluminum alloys, all the other partsarranged in the main load path (working and safetypistons, lower housing, tapped and threaded bushes) aremade of vacuum heat-treated steel. Crack detection ofeach machined metal part has been conducted accordingto MIL-STD-1949.

Fig. 5: Test specimen of the UH-60 IBC actuatorattached to a mounting plate

Surface coating and treatment had carefully been chosenas they determine wear of the sealing and guidingelements as well as fatigue strength of the loaded parts.

The elastomeric rod ends and jam nuts provided byNASA are attached to the actuator by means of tappedbushes. A photo of an assembled specimen for actuatorfatigue tests is shown in Fig. 5.

Three actuators were built and used for functional andfatigue testing only. For the wind tunnel test campaigns5 brand-new actuators were available, meaning thatthere was at least 1 back-up unit including the controllerhardware for the fully-equipped UH-60 baseline rotorwith 4 blades. Spares for most of the vendor parts andsome of the structural actuator components were alsoavailable.

Hydraulic system

A hub adapter was designed to sit on top of the UH-60hub-mounted bifilar absorber (see Fig. 6) using a 15“diameter flange bolt circle for bifilar-to-adapter attachbolts.

HydraulicHose

Swivel Joint

HydraulicPipe

Top View

SectionalView

SwivelJoint

Fig. 6: Hub adapter with hydraulic pipes and hoses,swivel joints, and electrical connectors

The adapter has 4 supporting struts which merge in thecenter of the hub where 4 hydraulic standpipes end (2pressure and 2 return lines). These pipes wereconnected to the rotating shaft flange of the hydraulicslipring with screwed sockets welded on the lower endof each tube. The pipelines were routed through theLRTA transmission and rotor shaft up to the rotor head,where they were guided through and supported by plainbearing bushes installed in the hub adapter’s central diskarea. Each pipe was then linked to a flexible hose bymeans of a 90 deg elbow swivel joint. The hoses wereled out of the hub adapter in radial direction through 4

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outlet holes. They were attached to 4 brackets byanother set of ball bearing swivel joints so that thevertical loads due to frictional forces resulting out of amovement of the rotor hub relative to the pipes werenegligible. Between the brackets and the hydraulicdistribution collar again hydraulic pipes were installed(not shown in Figs. 2 and 6). The installation height ofthe hub adapter was 70 mm (distance between the bifilarflange and the upper hub adapter flange). On its topflange the identical bolt pattern as on the bottom wasused so that the base plate of NASA’s rotor-mounteddata acquisition system (RMDAS) unit could bemounted onto the hub adapter in the usual manner. Fourconnectors were integrated into the hub adapter asdepicted in Fig. 6. They interconnect the mast-guidedwire bundles with the cables coming from the actuatorsvia electronic boxes which were mounted to the bottomof the hydraulic distribution collar.

Bottom View

SectionalView

Fig. 7: Drawing of the hydraulic distribution collarwith gas accumulators, hydraulic fittings, andelectronic boxes

The hydraulic distribution collar (see Fig. 7) wasattached to the upper pressure plate of the rotor headusing 4 threaded pocket holes around an 11.68“diameter bolt circle (unequally spaced). The 4 hydraulicpipes that come down from the hub adapter (2 pressureand 2 return lines) were each connected to thedistribution collar by means of 90 deg elbow joints.Two joints were linked with the pressure oil duct, and 2

with the return oil duct located inside the distributioncollar.

To each hydraulic oil duct system 2 accumulators and 4hoses were connected. Four pairs of these 8 hoses ranoff the distribution collar from its bottom side directly tothe actuators (1 pressure and 1 return line per actuator).Thereby the hydraulic circuit to and from the hydraulicslipring was closed. For each actuator 1 electronic boxwas attached to the bottom side of the collar. Theseboxes contained amplifiers, inner-loop controllerelements and interface units for the IBC electronics.

A hydraulic rotary transmission leadthrough (hydraulicslipring) was used to connect the hub-mounted hydrauliccomponents with NASA’s wind tunnel facility hydraulicsupply. Hydraulic power in terms of oil pressure andfluid flow rate was fed into and returned out of therotating frame by special diaphragm gland cartridgesintegrated in the hydraulic slipring. The seal betweenstationary and rotating parts of these cartridge elementswas effected by sealing gaps which were kept to aspecified value by means of a hydrostatic controlmechanism that permitted slight wobble and eccentricrunning of the shaft. This hydrostatic control systemensured that the gap widths of up to 15 µm were keptconstant even at 3,000 psi of hydraulic pressure.Elastic, pressure dependent deformation was ruled outby a hydraulic compensation system. Fig. 8 shows somedetails of the hydraulic slipring design.

Fig. 8: Detail design of the hydraulic slipring

The hydraulic slipring rotor was attached to the hollowLRTA transmission output shaft by means of a mountingbar designed and fabricated by ZFL. An arrangement of2 electrical sliprings and a phototach was attached toand in-line with the hydraulic slipring shaft. The upperone of these electrical sliprings was directly mountedbeneath the hydraulic slipring using an adapterfabricated by ZFL to mate with the RMDAS slipringflange. The adapter additionally carried an electricalconnector holding device and a splash disk to protect theelectrical sliprings against hydraulic fluid in the eventthat a leakage occurs in the hydraulic slipring rotaryshaft seals. The hydraulic slipring shaft was fabricated

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with a cross-shaped center clearance (see Fig. 8) toallow some wire harnesses coming from the electricalsliprings to be led through the LRTA rotor shaftassembly. For easy installation and disassembly of theelectrical sliprings a plug-in connection for the wireharnesses was utilized.

ZFL’s rotor-mounted IBC hardware was linked withNASA’s hydraulic supply by means of a hydraulicmanifold (see Fig. 9) which contained the IBCemergency cut-off circuitry as well as 2 pressure (A andB) and 2 return ports (C and D) for hydraulic supply andreturn lines.

Fig. 9: Photo of the hydraulic block lying on its backside

The manifold mainly employed a couple of magneticand other valves that sat in or on a rectangular solidaluminum block. This block has been designed andmanufactured according to ZFL’s specification in orderto allow hydraulic fluid to flow from port A to B andback from D to C during standard IBC operation. The Aand C ports were to be connected with the LRTA’s mainhydraulic supply (A) and return (C) lines. Between theports B and D sat the hydraulic slipring and the rotor-mounted IBC system components, particularly the 4 IBCactuators as main power consumers. Fluid from theNASA supply was conditioned by redundant controlcircuits as described below. Beyond the vital safetyfunctions, the hydraulic manifold provided someadditional features like controlled pressure rise withadjustable fluid flow rate, pressure measurement in thesupply and return lines, and pulsation damping.

During normal IBC operation 2 main passage valveslocated between ports A and B are kept open while 2bypass valves are closed by hydraulic pressure built upin 3 independant control circuits. This pressure build-upwas maintained in the control circuits as long as thehydraulic system pressure (3,000 psi) provided by theNASA test facility was present on port A and each of 4magnetic safety lock-out valves were closed byapplication of 24 VDC from the main controls. In caseof a manually or automatically triggered shutdown the

electrical voltage over these safety lock-out valveswould be switched off so that they would mechanicallyopen (this would also occur when the control systemfails due to a complete loss of electrical power). In thiscase the pressure immediately would decrease in theopen control circuits which would lead to a mechanicalshutdown of the main passage valves and thus cut theIBC system off from the hydraulic supply. Fluid flowwould be interrupted at once which also would lead to apressure drop in the hub-mounted hydraulics.Simultaneously, the bypass valves of the manifoldwould open which would allow the hydraulic fluid topour out of the rotating system components into thereturn line. The cut-off circuitry provided at leasttwofold redundancy, because every essential part of theshutdown chain existed twice where both parts actedindependently of each other.

Electronic control system

The IBC electronic hardware mainly consisted of

• a main and monitoring VME bus computer systemfor digital and analog signal processing (2 units foreach system),

• the control console (operator-terminal, potentiometercontrol and emergency shutdown buttons),

• a hydraulic block valve control system,• 4 actuator controller boxes (1 for each actuator), and• the hub-mounted wire harnesses.

The control system virtually consisted of 2 physicallyand functionally partitioned channels within the samephysical package. Most of the main control andmonitoring electronics were installed in a 19“ rackenclosure which also contained a drawer for taking upthe control console hardware as shown in Fig. 10.

Operator-Terminal

Main System Controller

Monitoring System Controller

Main System Analog Circuits

Monitoring System AnalogCircuits

Pressure Controller and PowerSupply

Operator Controls

Slipring

Phototach

Hydraulic System

Power Supply MonitoringSignals

ActuatorController

Non-Rotating System Rotating System

Actuators

Electronic Rack

Fig. 10: Overview of the control system components

The main computer system was designed to control theactuators by generating the command signals for theservo valves and to perform the signal conditioning for 1LVDT set (1 set corresponds to 4 LVDTs, 1 peractuator). The monitoring system mainly performsmonitoring duties and signal conditioning for the 2nd setof LVDTs. Failures could be detected by both systems.They had equal (highest) priority to trigger anemergency shutdown. Thus a redundant stroke

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monitoring was realized using 2 LVDT sensors for eachactuator. Actuator travel versus command value wasmonitored by the two completely independent (main andmonitoring) systems, each of them including a computerfor FOURIER analysis and synthesis. When a certainstroke difference between commanded and actual valueat least of one of the LVDTs was detected or any othermalfunction occurred, hydraulic power was shut downwhich immediately disengaged the safety lock-outpistons and mechanically bolted the actuators in theirzero positions. This feature made uncontrolled actuatortravel impossible. A shutdown signal would also begenerated when the measured axial load in any one ofthe actuators reached a preset threshold value.

Regarding the hardware, each actuator was equippedwith a controller board that was manufactured in SMD(Surface Mounted Device) technology. The controllerboard was contained in a separate electronic box (1 foreach actuator). It performed the conditioning of thestrain gage and the 2 LVDT signals. The power supplyfor both LVDT signal conditioners (36 VDC) wasprovided independently for redundancy purposes. Ananalog controller with P characteristic controlled theservo valve (inner loop). Finally, each actuatoremployed 2 gas pressure sensors to continuouslymonitor the status of both independent safety systems.All signals were transmitted as 4 to 20 mA currents.The pressure sensor signal receiver with 8 current signalinput channels was associated to the monitoring system.In every channel the current signals were transformedinto voltage signals (0 to 10 V) which were transmittedto the VME bus computer of the monitoring system.The actual pressure values were displayed on theoperator-terminal. If 1 pressure value fell below acertain threshold this would be indicated by a colorchange (from green to red) of the respective display.

Actuator 1RMDAS Cat. A+B Monitoring

System

Slipring Connector

Controller 4

Controller 3

Controller 2

ActuatorController 1

X1.1

Xp1.1

Xp2.1

X2.1

X5.1

X5.2

X5.3

X5.4

X4.2

X4.3

X4.4

XR1

X6.3

X6.2

X6.1

X3.1

X4.1

X6.4

Flex CouplingX5.5

Fig. 11: Electronic hardware components in the rotatingframe (cabling and connector layout)

Fig. 11 outlines the electronic components that werelocated in the rotating system. It also sketches the wiresand connectors purchased or manufactured by ZFL.While ZFL was responsible for the fabrication of therotor-mounted wire harnesses, NASA made the cablesbetween LRTA (electrical slip rings, hydraulic block)

and control room (IBC control rack). For connectingthe IBC lines from the hydraulic block and from thestationary side of the electrical sliprings to the IBC rackZFL had supplied NASA with mating plugs as well asdetails of their pin assignments.

From each actuator 3 wire harnesses ran to theircorresponding controller boxes. These bunchescontained the cabling for the servo valve, 2 LVDTsensors, the strain gage, and 2 gas pressure sensors ofevery actuator. Four additional harnesses (1 peractuator each combining the lines of the previous 3cables) were installed between the 4 controller boxesand the hub adapter (Fig. 6 shows the connectors wherethese cables end). Inside the hub adapter these IBCcables (altogether 4 × 16 = 64 lines plus 1 shield) werebrought together with 35 NASA analog signal line wiresto 1 single wire harness. This harness together with 2supplementary 100 line wire bundles (reserved forNASA analog signal transmission) and one RMDAScable (NASA digital signal transmission) was routedthrough the LRTA rotor shaft and through the hydraulicslipring to meet with 4 plug receptacles in an electricalconnector holding device. NASA wired these plugreceptacles to the control lines from the rotating side ofthe electrical sliprings to mate with the 100 pinconnectors wired to the 4 rotor shaft traversing cables.Details of the pin assignments had been sent to NASAtogether with the connector parts. ZFL manufactured alltheir wire harnesses (most of them with connectors) andconducted continuity checks as well as voltage andinsulation tests (testing voltage: 500 V).

Blade 2

Blade 3

Blade 4

Blade 1

BladeDynamicsActuatorP

Inner Loop:Actuator Stroke Control

xactualxlead ϑactual

Acomϕcom

Aactual, ϕactual

Compen-sation

Intermediate Loop(1 for Each Blade):Amplitude and PhaseCompensation of theActuator Stroke FOURIER

Analysis(FFT)

ψref

∆x

ShutdownLogic Circuit

FOURIERSynthesis

(IFT)

xcom

ψref

deg→

mm

FOURIERSynthesis

(IFT)

deg→

mm

Aleadϕlead ϑlead

ψref

mm→

deg

Fig. 12: Control system architecture (inner andintermediate loops)

The actuators together with their correspondingcontroller boxes were installed in the rotor head with theelectronic boxes mounted on the bottom side of thehydraulic distribution collar. They contained the innerloop controllers for actuator stroke control, which hadbeen designed to optimize each actuator’s dynamicresponse in the time domain. Fig. 12 gives an overviewof ZFL‘s control system architecture comprising theinner and an additional intermediate loop which bothconstituted the core element or “open-loop“ controllevel of the IBC system. The intermediate loop wasused to adjust the input IBC amplitudes and phaseangles in an adaptive manner to compensate for any

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remaining amplitude and phase errors of the actuators.This loop was set up in the frequency domain and basedon fast/inverse FOURIER transform (FFT/IFT).FOURIER analysis of the actuator displacement wasperformed to determine if those coefficients agreed withthe commanded motion coefficients. The error signalswere fed back to drive the adaptive compensation blockuntil the measured harmonics matched the commanded.Separate feedback loops were provided for eachactuator, thus it was guaranteed that all actuatorsperformed equally though phase shifted according totheir individual rotor azimuth. A third, outer loop couldbe added to this cascade architecture and thereby“closed-loop“ control capability could be realized veryeasily. The outer loop generated command values forthe IBC amplitudes and phase angles according to thepreset values (open-loop) or derived from suited statevariables (closed-loop). The range of usable harmonicswent in integral multiples of 1 rotor revolution from2/rev up to 7/rev.

The electronic rack needed to be connected with thehydraulic manifold in order to control the IBChydraulics. ZFL provided a wiring diagram for therequired cable harness which was fabricated by NASA.Fitting plugs for the connection of the wire harness withrack and manifold were also provided by ZFL.

IBC system installation

The IBC hardware was installed into the LRTA insummer of 2000. These activities took place in theAmes wind tunnel preparation area, where all theassembly work and preliminary testing could beperformed with the necessary support.

Fig. 13: UH-60 main rotor head with IBC actuators

The hydraulic and mechanical subsystem installationaccording to Fig. 2 could be set up without majorinterface problems, even though all the component partsof the IBC system had never been assembled before.This was due to a constant communication processestablished early between the involved organizations,that allowed a smooth and effective design work. Apreliminary and a critical design review were conducted

in 1998 in order to present the current design status andto clarify interface problems. The IBC hardwareinstallation concluded with the successful connection ofthe hydraulic system with the control electronicscomponents. Final non-rotating and rotating tests donein the preparation area without blades delivered theproof that ZFL’s IBC system worked as planned. InOctober 2000, NASA began the installation of theLRTA into the 80- by 120-foot test section with the IBCsystem integrated into the rotor hub as shown in Fig. 13.

Testing

IBC actuator testing

Comprehensive functional and fatigue testing wasperformed using a set of IBC actuator test specimens.These tests were conducted in ZFL’s test stand center onspecial rigs. Fig. 14 gives an idea of the set-up for thefunctional tests, where the test specimen was installedhorizontally between a rigid frame and a hinged beamthat allowed the actuator’s working piston to moveinside the actuator housing.

Fig. 14: Functional test of the UH-60 IBC actuator

Centrifugal load on the structure was simulated by amechanical spring mounted between the frame and theactuator pulling it down in vertical direction. Two typesof hinged beams existed for the test rig, which alloweddifferent types of axial load excitation (against a staticmass or a mechanical spring) to simulate blade loads.The functional test campaign was comprised by staticleak checks at 4,500 psi (1.5 times the operatinghydraulic pressure), static axial load tests at ±50,000 N,and extensive dynamic actuator testing. The dynamicproperties and open-loop control characteristics of theIBC actuator were determined, showing that thespecified control authority could be realized.Particularly, the safety lock-out mechanism wasextensively examined, as it is a vital device for thesystem safety. A 200 hours run with sequences of allusable harmonics concluded the functional tests, whereno mechanical damage on the guiding and sealingelements occured, and no hydraulic leakage could bedetected.

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Actuator fatigue testing was performed on another testrig, as depicted in Fig. 15. The test specimen washorizontally installed in a rigid frame unable to moveand a centrifugal load was simulated by suspendingweights to the actuator main housing. A force controlcycle controlled the servo valve of the actuator, whichgenerated alternating axial forces of ±26,000 N atfrequencies of 13 to 18 Hz. The dynamic axial loads,produced by a hydraulic pressure of 3,000 +/– 580 psi,acted on the test specimen and were taken up by the stifftest rig structure. Axial forces were monitored andmeasured by the actuator strain gage and an additionalload cell installed in the main load path of the testspecimen. Thereby, a functional test of the actuatorstrain gage could be conducted, too.

Fig. 15: Fatigue test of the UH-60 IBC actuator

A test rig extension according to Fig. 16 was developedin order to allow actuator fatigue testing with 2specimens simultaneously. The test loads weregenerated with one (active) actuator while the other(passive) one was hydraulically unpressurized.However, the passive test specimen was in safety lock-out configuration with the safety lock-out pistonsengaged (see Fig. 4). As this actuator was charged withthe full test loads by the active test specimen via thelever shown in Fig. 16, fatigue testing of the lock-outsystem components could be performed as well.

In the course of the fatigue test campaign some actuatorcomponent modifications were implemented. The finaldesign version which was manufactured for the windtunnel tests had a proven fatigue endurance limit of±7,300 N for 108 load cycles and a rated static limit of48,250 N.

Fig. 16: Extension of the fatigue test rig

Hydraulic system component testing

For all vital rotating components comprehensive stressanalyses had been conducted. Essential parts of the hub-mounted hydraulics as well as electrical connectors metaerospace standards. The hydraulic manifold and thehydraulic slipring were manufactured by externalsuppliers based on ZFL’s layouts and specifications.These components were tested according to acceptancetest procedures defined by ZFL.

The acceptance tests comprised leak and pressurechecks, pulsation tests, hydraulic flow measurements,and additionally shutdown time determination for thehydraulic manifold, and internal leakage measurementsfor the hydraulic slipring. All parts and componentscould be qualified for the application in the UH-60 IBCwind tunnel test system.

Check-out test of the IBC system

Prior to the wind tunnel runs, which commenced in July2001, the whole IBC hardware was tested on the systemand subsystem levels in the 80- by 120-foot test section.Procedures for a pre-run check-out and a 50 hours checkwere derived from a failure mode and effects analysis(FMEA) prepared by ZFL and Sikorsky during the IBCsystem engineering design process.

The pre-run check-out test should demonstrate systemlevel operativeness and was to be performed prior toeach wind tunnel run. Essentially, the status of theelectronic control hardware was determined. A checklist was provided, that included every necessary checkpoint to be executed. Similar check lists were providedfor the 50 hours check, which basically tested themechanical and hydraulic system components in order todiscover hidden failures. This test was to be conductedafter 50 hours of IBC operation.

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Further non-rotating and rotating tests on the systemlevel were conducted with the following objectives:

• check-out of the cabling, sensors, and gages,• test of the data recording system,• determination of the mechanical actuator stroke

limits,• determination of the control performance:

- system damping and phase delay for eachactuator and harmonic,

- maximum IBC amplitudes for each harmonic,• triggering of manual and automatic shutdowns,• measurement of the shutdown time.

Fig. 17: LRTA with the UH-60 IBC system in the 80-by 120-foot Wind Tunnel

The rotating functional tests with the complete IBCsystem have successfully been accomplished and haveshown that the specifications could be achieved. Theseactivities concluded the preparatory installation work inApril and May 2001 (see Fig. 17). The operatability ofthe control system as well as the shutdown functionscould finally be shown.

Concluding remarks

The development, manufacturing, and componenttesting of an IBC system for a full-scale UH-60helicopter rotor was described. Servo-hydraulic IBCactuators were designed by ZFL to replace the standardUH-60 pitch links realizing a blade root actuationsystem with one actuator per blade. The actuators weredesigned to produce up to ±6.0 deg blade pitch motionat the 2/rev frequency, diminishing to ±1.6 deg at the7/rev frequency.

The IBC system was successfully integrated into theLRTA and prepared for UH-60 IBC wind tunnel tests inthe 80- by 120-foot test section of NASA‘s NationalFull-Scale Aerodynamics Complex. Subsequent to theactivities described in this paper two IBC wind tunneltest campaigns were conducted in July/August and

September of 2001.

Acknowledgments

The ZFL portion of the research related to this reporthas received partial funding from the German FederalMinistry of Education and Research (BMBF) as well asthe Federal Ministry of Economics and Technology(BMWi) under reference nos. 20H9701 and 20H9903.The authors are to be held responsible for the contentsof this publication.

References

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