HERRB–HelicopterElectricRegenerativeRotorBrake

PublishableSummary

Stateoftheart–BackgroundThe trend towards increased use of “more-electricsystems” in fixed-wing aircraft is motivated byimprovedoverallsystemefficiency,reducedfuelburnandlowermaintenancecosts.

Compared to large civil aircraft the ”more-electrification” of rotorcraft is less advanced. Ifapproached holistically, replacement of conventionalpower systems with electric equivalents has equalpotential for fuel savings and reduction inwhole lifecosts. However, for these benefits to be realisedsignificantarchitecturalchangesareneeded

To reduce development risk, the JTI Clean SkiesRotorcraft is employing a range of technology, that,together, would demonstrate the significant fuelsaving and performance benefits required foradoptioninfuturerotorcraft.

One potential route to improve the overall systemefficiency of an rotorcraft’s electrical system is torecoverenergy fromtheaircraft thatwouldnormallybelost.Inarotorcraft,asignificantamountofenergyis stored in the rotating blades of the main rotorsystem. In all current helicopters, this energy isdissipated through a conventional mechanical brakeasheatandhencewasted.

An electric machine can be used to provide acontrolledregenerativedecelerationofthemainrotor.Given that the braking function would only berequiredaftertheaircrafthaslanded,furtherbenefitsinclude:

• the unit could be used as an electric generatorduring normal flight, allowing the de-rating oftheexistingelectricalgenerators.

• provision of an electrically assisted startprocedure

• provision of an electrically assisted andcontrolled autorotation in the event of mainenginefailure

ObjectivesThemain project aim is the delivery ofmodel baseddesignframeworkandconstructionofafull-scale,all-electricbrakingsystemprototypeforthemainandtailrotorsofahelicopterincludingastaticbrake.

The final solution should be as weight competitivewith the systems that it intends to replaces aspossible. Hence, a critical element of the projectpresentedherewill be tominimise theweight of thesystem well below existing commercially availablemachines,exploitingtheverylowdutycyclerequiredfrom the application whilst maintaining the levels ofsafetyandreliabilityneededfromthistypeofaircraftsystem. For this to be viable, a thoroughunderstanding of the electrical and thermalbehavioural of the electric braking system must beformulated, resulting in the lowest mass systempossible.

Theobjectivesoftheprojectare:

1. Todetermineanddemonstratethesuitabilityofthe finally selected electrical brakingarchitecture as the ideal candidate for thedynamicbrakingapplicationbeingstudied

2. Todeliverthecapabilityofaccuratelymodellingthe electro-mechanical behaviour of a fullregenerative rotor braking system for theselectedtopologyunderarangeofrotorbrakingscenarios.

3. To evaluate the thermal and dynamicperformance of the supplied prototyperegenerativerotorbrakingsystem.

4. Todemonstrate thecapabilityandsuitabilityofthefinalsolutiontobeintegratedtoanexistingrotorcraft’s transmission with minimal designmodifications.

DescriptionofworkThe work in the HERRB project can be broadlycategorised into four main areas being covered byseventechnologyfocusedworkpackages:

WP0. DesignSpecificationWP1. RotorDecelerationSystemWP2. ElectricalPowerConditioningWP3. Rotor Stop and Hold Mechanism

andControlWP4. AircraftInstallationConfigurationWP5. SafetyandCertificationWP6. PrototypingandTest

andamanagementworkpackage:

WP7. Management

Figure1:IntegrationofWorkPackages

Theinter-dependenceofthetechnologyfocusedworkpackagescanbeseeninFigure1.

Specification(WP0)The call for proposal outlined the aim of the projectand gave an outline of aircraft level requirementsspecificforabrakingapplication.Inordertoallowfora meaningful comparison and to agree targetrequirements that meet the Technology ReadinessLevel (TRL) of the Call for Proposal (CfP) a detailedspecificationwas carried out based upon research ofthestateoftheartinthearea,currentstandards,focusgroups with project stakeholders and the relevantGreenRotorcraft team (GRC3) lead. ThePreliminaryDesign Specification (PDS) that resulted drove thedesign of each of the various systems in each of theprojectpackages(WPs).

The preliminary design specification encapsulatedrequirements for both the mechanical and electricalsystems of a potential solution to the problem ofelectricallyactuatedmainrotorbraking.

Rotor Deceleration System (WP1) andElectricalPowerConditioning(WP2)WP1&2focusedonthedesignandoptimisationofthedynamic (or regenerative) aspect of the rotor brake.Detailed and coupled modelling of the thermal andelectromagnetic and electric machine, powerelectronicsbasedconverteranddrovethefinaldesigntowards a minimum installed mass of the entiresystemratherthanonacomponentwisebasis.

Figure2:ElectricMachineThermalModel

Figure 3: Transient Heatsink Model ShowingTransientTemperatureVariationinHeatsink

ResultsGeneratedDuring the course of the project the followingscientific/technologicalresultsweregenerated:

• Integratedmachinedesignforusedinamediumsizedrotorcraftwithoutsupplementarycooling

• Development and calibration of a full thermalmodel for the finally selected topology ofelectricmachine

• Validatedoptimisationprocess forbalancingofDC andAC copper losses in the design processofafixedspeedelectricmachine.

• Process for early stage (pre-prototype)calibration of complex stator assemblies inthermalmodels

• Design and test of a magnetically activemechanical wedge for secure, vibrationresistant coil retention in concentrated woundopenslotmachines.

• Temperature dependent system level modelsfor fast simulation of converter/heatsinkthermal behaviour including machine andcontrollerbehaviours.

• Validatedswitchingperiodbasedcalculationoflosseswithpotentialforreal-timeestimationoflosses

RotorStopandHoldMechanismandControl(WP3)WP3 considered the design of the electric static(orholding) brake from the concept stage rightthroughto embodiment and final prototype.Unlike thedynamicbrake,topologicalconstraintsexistedonthisaspectofthedesignresultinginalargesolutionspacethatneededbemethodicallyandobjectivelysearchedresultinginanintegratedandoptimumfinaloutcome. A morphological approach was used to deviseconcepts that could potentially satisfy therequirements based its individual sub-functions.Thesewerethenratedandrankedobjectivelyyieldedthe final design. A new modelling framework andgeneration of new data was created/measured tosupportthedesignoptimisationtoensurealow-massfinaldesign.

A final optimisation phase was conducted thatdemonstrated that without functionally altering themethodofoperationandhence finalperformance,anaircraftsystemcouldberealisedatamassmuchlowerthanthatoftheinitialprototype.

ResultsGeneratedDuring the course of the project the followingscientific/technologicalresultsweregenerated:

• Morphologicalapproachtoasystemdesignforastaticelectrichelicopterbrake

• Validatedgeneraliseddesignframeworkforthefinallyselectedstatic,electricalbrakingsystem

• Static and quasi-static co-efficient of frictiondata for key materials used in friction-basedbrakingsystems

• Validate and optimised solution to the staticbraking requirement for a medium sizehelicopter

Figure4:MorphologicalChart

Aircraft Installation Configuration (WP4),SafetyandCertification(WP5)WP4consideredthedesignsproposedinWP1,2&3and based jupon analysis or optimisation of thedesigns,ensuredthattheymetascloselyaspossibleatthe prototype stage requirements for a modernrotorcraftintermsofpackaging,integration,reliabilityandsafety.

A full analysis of the machine integrity (static anddynamic)wasundertakenanddemonstrated that theassemblywas both functional and designed to safetyfactors appropriate to the function of the brake andthe TRL being pursued. The principle excitingfrequencies were identified and the implications oftightintegrationoftherotorbrakehighlighted.

A system-levelmodelwas constructed that took low-(orcomponent-)leveldataasitsinputandabstractedit to the detail required to allow simulations of longtimedurationstobeundertakenallowinglossanalysisundervariousscenariostobereviewed.

Figure5:FullHERRBsystemsandinteractions

Variouscasestudieswerecompared in termsof timetoachieveacompletebrakingcycle,energyrecoveredon land, total predicted conversion efficiency. Thetoolchain also permitted the ability to conduct aninitialstudyintothepotentialfortheelectricmachineand converter (electric dynamic brake) to act as arotor acceleration device prior to the rotorcraftenginesbeingengaged forasimultaneousengineandmainrotorstart.

As well an an integration study, a reliability and fullfailuremodesandeffectsanalysiswasundertakenforthe prototype solutions proposed within the HERRBproject based upon their state of development at thecriticaldesignreview(CDR).

Thestudydemonstratedseveral cases forall severityratings in which the reliability of the developedsystems fell outside of the target ranges for thatseverity rating highlighting those system that wouldrequire furtherattentionpriortobeingconsideredtobesufficientlyfaulttoleranttoenterservice.

ResultsGeneratedDuring the course of the project the followingscientific/technologicalresultsweregenerated:

• Design modelling framework for assessing thecompatibilityof therotorbrakeprototypewithanexistingairframe

• Full system non-linear level coupled model ofthe chosen electric drivetrain derived directlyfromcomponentlevelanalysis

• Applicationofsystemmodelforoptimisationofprototypesubsystemsbaseduponsystem-wideconstraints.

• Generation of reliability data and associatedfault assessments for full permanent magnetelectricdrivesystemsofsignificantpower.

• Fault tree analysis of a braking system for ahelicopter considering all possible interactionsbetween various subsystems and harshenvironment experienced during rotorcraftflight.

PrototypingandTest(WP6)Atestrigwasconstructedthatcouldoperateovertheoperatingrangesoftheprototypedequipment.

Each of the three systems were prototyped basedupon the status of the designs at the Critical DesignReview.

Figure6:EDBSDynamometerDesign

Each of the systems was tested in order to bothcharacterise that system and provide validation datafor the modelling tools developed. Test resultsdemonstratedthatthefinalmachineprototypehadanelectromechanical performance very close to thatpredicted.

Generatorratingsweremeasuredbaseduponneedingenough thermalheadroom toaccommodate theextrameasured transient temperature rise required for asingle optimised braking cycle. The results agreedwellwith thepredictionsmade in the thermalmodelattheCriticalDesignReview.

Predicted generator continuous ratings weremeasuredatlowervaluesthanpredictedattheCriticalDesignReview. Calibration of the thermalmodellinghighlighted the primary factors responsible for thedifferences between predicted and measuredbehaviour.

The power converter measurements demonstratedresults commensurate with expectations for a suchconverter,aspredictedbythemodellingactivity.Thisconfirmed the converter to be an appropriate matchfortheelectricmachine.

ResultsGeneratedDuring the course of the project the followingscientific/technologicalresultsweregenerated:

• Manufactureandcharacterisationofafull-scaleintegratedelectricmachineprototypeforuseonthemaingearboxofamedium-sizedaircraft

• Validationofthestructureofthethermalmodeland the results output from the finallycalibrated thermal model for the electricmachine

• Validationofareal-timecapablelosspredictingmechanism for a IGBT-based power electronicconverter which has the potential not only toinform design but allow continuousoptimisationofdriveoperationinreal-time.

Timeline&MainMilestonesM04(Jan‘12) DetailedSpecificationAgreed

M10(Jul’12) Modelling tools finalised and tradestudiesforproposedarchitecturescompleted

M15(Dec’12) PreliminaryDesignReview

M23(Aug’13) CriticalDesignReview

M40(Nov’14) Systemprototypeconstructed

M42(March’14) System characterised andbenefitsdemonstrated.

EnvironmentalbenefitsThe developed system is capable of capturing andallows re-use of a significant amount energy on therotorcraft that is current dissipated each time theaircraftisstopped.

The suitability of the developed system to replaceexisting equipment on the aircraft, reducing totalsystem mass and thereby improving overall fuelefficiencyoftheaircrafthasbeenshown.

MaturityofworksperformedThe work resulted in a TRL 5 prototype system inwhich the architecture, subsystems, components themajorityof thepackaging,wouldberepresentativeofthose which could be installed in an aircraftdemonstrating the potential for flight worthyoperation.

Project Summary Acronym : HERRB

Name of proposal: Helicopter Electrical Regenerative Rotor Brake

Technical domain: Aircraft Electrical Drives

Involved ITD Green Rotorcraft

Grant Agreement:

Instrument: Clean Sky

Total Cost: 698330€

Clean Sky contribution: 523748€

Call: JTI-CS-2011-01-GRC-03-007

Starting date: Oct 2011

Ending date: March 2015

Duration: 42 months

Coordinator contact details: Dr. David Drury

* Room 5.03, Merchant Venturers Building

Woodland Road, Bristol, BS8 1UB

United Kingdom

( +44 (0) 117 954 5390

d.drury@bristol.ac.uk

Project Officer: Andrzej B. Podsadowski

mailto:Andrzej.Podsadowski@cleansky.eu

Participating members UNIVERSITY OF BRISTOL UK

MOTOR DESIGN LIMITED UK

ADAPTED SOLUTIONS GmbH DE