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Aerodynamic Simulation of Contra-Rotating Open Rotors.

Ignacio Gonzalez-Martino

To cite this version:

Ignacio Gonzalez-Martino. Development of Moderate-Cost Methodologies for the Aerodynamic Sim-

ulation of Contra-Rotating Open Rotors.. Fluid mechanics [physics.class-ph]. Université Pierre et

Marie Curie - Paris VI, 2014. English. �NNT : 2014PA066094�. �tel-01063434�

Sienes méaniques, aoustique, életronique et robotique

Onera / Département d'Aérodynamique Appliquée

Development of Moderate-Cost Methodologies

for the Aerodynami Simulation of

Contra-Rotating Open Rotors

Par Ignaio Gonzalez-Martino

Thèse de dotorat de Méanique des Fluides Numérique

Dirigée par Mihel Costes et Philippe Devinant

Présentéeetsoutenue publiquement leFebruary14, 2014

Devant lejuryomposéde :

Serge Huberson ENSAM (Paris) Rapporteur

Elie Rivoalen INSA deRouen Rapporteur

Georges Gerolymos UPMC(Paris) Président

Mihel Costes Onera (Meudon) Examinateur

Philippe Devinant InstitutPRISME (Orléans) Examinateur

Florian Blan AirbusOperations S.A.S.(Toulouse) Examinateur

Riardo Martinez-Botas ImperialCollege (London) Examinateur

BenoitRodriguez Onera (Meudon) Invité

Aknowledgments

Jetiensàremeriertouteslespersonnesquim'ontaompagnédeprèsoudeloin

dansette aventure de troisans,larendant ainsipossible.

Tout d'abord, mes enadrants à l'Onera, Mihel Costes et Benoit Rodriguez.

Votresupportauquotidien,votreonaneetvosenouragementsm'ont permisde

menerà bienettethèse! Enoremeripourtoutes vosreleturesqui ont permisà

e mémoire de prendre forme. Je n'oublie pasPhilippe Devinant mon o-direteur

poursonsuivi etpour ses onseils auours de mesreherhes.

Je tienségalementàremeriertrèshaleureusement lesmembresdujuryquiont

eulagentillessed'aepterdejugeretravail. EnpartiulierSergeHubersonetElie

Rivoalen pour avoir aordé de leur temps à laleture de e mémoire en tant que

rapporteurs.

Je remerie aussiFrédéri Barrois etFabienMagaud, mes enadrants à Airbus,

pour leur aueil au sein de l'équipe des Méthodes pour l'Aérodynamique, pour

l'intérêt portésurmes travauxde reherhe etpourleur soutien.

Je voudrais remerier toutes les personnesqui ont suiviette thèse,qui sesont

intéresséesàestravauxouquisimplement parleurjovialitéont rendulequotidien

agréableentantqueollègue. Ungrand meritoutpartiulieràtoutel'équipeH2T

et, bienévidemment, àtous lesdotorants del'Onera à Meudon.

Enn, jeremeriemafamilleetmesamispourleursoutienindéfetible. Votrein-

estimablesoutien,parfoisàdistane,m'apermisdetenirbonjusqu'aubout. Meri!

I amuntpuja i més amunt,

om auellsde brana enbrana;

d'aqueixa ova damunt

una altra veu de més blana.

Mes,ompuja de grat,

troba urtatota esala;

per un or enamorat

ada pasés un op d'ala.

Mn. Jaint Verdaguer,Canigó, CantVI

Nomenlature ix

Introdution 1

Literature Review 5

Propeller andOpenRotor assets&hallenges . . . 5

Researhon Propellers andOpen Rotors . . . 5

Key Assetsand Challengesof Open Rotors . . . 11

In-Plane Loads onPropellers andOpenRotors . . . 15

Impatof 1P Loadson Airraft . . . 15

Physial Mehanisms behind 1PLoads . . . 17

Methodsfor the Predition of AerodynamiPerformane . . . 21

AnalytialModels forPropellers . . . 22

NumerialSingularityMethods . . . 25

Potential, Euler andRANSMethods . . . 26

Eulerian/Lagrangian Coupling. . . 28

The Lifting-LineTheory . . . 31

TheOnsetof the Lifting-LineTheory. . . 40

Developing theLifting-LineTheory . . . 44

TheUnied Lifting-LineTheory . . . 45

NumerialAppliation of theUCLLTheory . . . 47

Methods and Tools 49 AvailableMethods for Single Propellers andCROR . . . 49

SeletedMethods for thePreliminaryDesignof Open Rotors . . . . 54

HOST. A Comprehensive Code for Aeromehanial Simulations . . . 55

An HeliopterOverall Simulation Tool . . . 56

TheBlade Modulefor Aero-Mehanial Simulations. . . 57

VortexWake ModelsinHOST: METAR andMESIR . . . 57

MINTWake Model . . . 59

InstallationEetsinHOST. . . 64

MainHypothesis oftheLifting-Line inHOST . . . 65

Computational Fluid DynamisSolver: elsA . . . 66

HOST/MESIR-elsA Coupling: RVAModule . . . 67

APIANSingle-Rotating Propeller . . . 68

1 HOST Assessment on APIAN Single Propeller Case 73

1.1 Parametri Study andBestPraties . . . 73

1.2 High-Speed Simulations Assessment with Wind Tunnel Data and CFDomputations . . . 78

1.2.1 Wind Tunnel Data . . . 78

1.2.2 elsA CFDComputations . . . 78

1.2.3 HOST-MINTSimulations . . . 80

1.2.4 Inidene Eet . . . 83

1.2.5 AdvaneRatioEet . . . 88

1.3 WayForward inHOSTSimulations . . . 91

2 A Physial Insight into Propeller 1P loads 93 2.1 Theodorsen'sTheory: A Two-Dimensional Analogy . . . 93

2.2 Quantifying theImpat ofAerodynami Mehanismson 1P Loads . 95 2.2.1 MainContributors on1P Load PhaseLag . . . 98

I Unsteady Airfoil Model 99 3 Analyzing an Unsteady Curved Lifting-Line Method (UCLL) 101 3.1 An UnsteadyCurved Lifting-Line Theory . . . 101

3.1.1 Theoretial Basis . . . 101

3.1.2 A NumerialImplementation oftheUCLLT . . . 105

3.2 Outer Domain. TheSingularPanel . . . 105

3.2.1 The DoubletFormulation . . . 106

3.2.2 The VortexFormulation . . . 111

3.2.3 Hess'Equivalene for Singular Panels. . . 112

3.2.4 Current Implementation intheAILE Code . . . 115

3.3 Inner Domain. Two-Dimensional Wake inLifting-Line Formulation . 116 3.4 Theoretial Analysisof theModel. . . 116

3.4.1 Inner Domain. AirfoilModel . . . 116

3.4.2 Outer Domain. TheSingular Panel . . . 118

3.4.3 Remarks on the Model . . . 118

4 DevelopingandImplementingan UCLL Method inHOST without Finite Part Integrals 121 4.1 The CompleteUnsteadyCurved Theory . . . 121

4.2 Implementation inHOST . . . 124

4.3 Conluding remarks . . . 125

5 UCLL Method Assessment onAPIAN Single Propeller Case 127 5.1 Impat ofInidene on High-Speed HOST-MINTSimulations . . . . 128

5.2 Impat ofAdvaneRatioon High-Speed HOST-MINTSimulations . 131

6 UCLL Method AssessmentonAI-PX7Counter-RotatingOpenRo-

tor Case 135

6.1 Test Case Desription . . . 135

6.2 Code-to-Code Assessment. elsA vs. HOST-MINT . . . 137

6.3 AerodynamiMehanisms behind 1PLoads. . . 143

6.4 Conluding Remarks . . . 147

II Code Coupling 151 7 Development of a Coupling Strategy between HOST-MINT and elsA Codes 153 7.1 Current Couplingbetween HOST/MESIRand elsA . . . 154

7.1.1 HOST/MESIR-elsA CouplingStrategy . . . 154

7.1.2 MainAdvantages and ShortomingsoftheStrategy. . . 155

7.2 Implementation of a One-Way Coupling between elsA and HOST- MINTodes . . . 156

7.3 Assessment ofMESIRandMINTCouplingStrategies forSingle Pro- pellers . . . 157

7.3.1 Indued veloityelds . . . 159

7.3.2 Indued Veloities InterpolationTimeStep . . . 165

7.4 CouplingStrategy Assessment on APIANSingle PropellerCase . . . 168

7.4.1 Test Case Desription. . . 168

7.4.2 Blade Loading . . . 169

7.4.3 PropellerPerformane . . . 169

7.4.4 Blade Load Distribution . . . 170

7.4.5 Pressureon Blade Skin. . . 171

7.4.6 FlowAnalysis . . . 173

7.5 Indued Veloity Fields . . . 174

7.6 Conluding Remarks . . . 177

A Finite Part Integrals 183 A.1 Prinipal Value ofa SingularIntegral . . . 183

A.2 Hadamard's Finite PartIntegrals . . . 183

B Sweep Eet 187 C Alternative Form of the Singular Panel Integration 189 C.1 Development oftheSingularPanelIntegration . . . 189

C.1.1 TheCompleteParaboli Wake . . . 189

C.1.2 TheRegular Partof theParaboli Wake . . . 192

D Résumé en français 195

D.1 Introdution . . . 195

D.2 Un aperçudesméanismes àl'origine deseorts1P. . . 196

D.2.1 Lesméanismes physiquesà l'originedeseorts 1P . . . 196

D.2.2 Une méthode pour quantier les méanismes surleseorts 1P 197

D.3 Développement etimplémentation d'une méthode de ligne portante

ourbe etinstationnaire . . . 199

D.4 Évaluation de laméthode de ligne portante ourbe et instationnaire

dansleas d'héliesimple APIAN . . . 201

D.5 Évaluation de laméthode de ligne portante ourbe et instationnaire

dansleas del'Open Rotor AI-PX7 . . . 202

D.5.1 Évaluationde HOST-MINTdansun asd'open rotor . . . . 202

D.5.2 Un aperçudanslesméanismes deseorts 1P . . . 204

D.6 Développementd'uneméthodologiedeouplageentrelesodesHOST

etelsA . . . 206

D.7 Conlusions . . . 208

Bibliography 209

List of Figures 227

List of Tables 233

α

^{Propellerinidene (deg)}

α _∞

^{Freestreamangle ofattak(deg)}

α _ind

^{Indued angleof attak(deg)}

δx

^{Panel length(m)}

η _P

^{PropulsiveEieny,}

η = V _∞ T P Γ

^{Cirulation ontheblade (m}

² /

^s)

κ

^{Loal radius ofurvature (m),}

(1 + l ^′ (y) ² ) ^3/2 l ^′′ (y) Ω

^{Rotationalspeed (rpm)}

∂τ /∂ξ

^{Spanwise gradient of thethrust oeient,}

∂T

∂r

R ρ _∞ N ² (2R) ⁴ Φ

^{Veloity potential (m}

² .

^s

⁻¹

⁾

ψ

^{Azimuthalposition(deg)}

Ψ

1P

1Pload phaselag(deg),

arctan(F _Y /F _Z ) Ψ

M1P

1Pmoment phaselag (deg),

arctan(M _Y /M _Z ) ρ _∞

^{Freestreamdensity(kg.m}

⁻³

⁾

~v

^{Freestreamveloityvetor inthebody-xed refereneframe (m}

.

^s

⁻¹

⁾

~v _ind

^{Indued veloity(m}

/

^s)

ξ

^{Relativepropeller radius,}

r/R

C

Theodorsen's funtion,dependson reduedfrequeny

k c ₀

^{Maximumhord (m)}

C L

^{Airfoillift oeient}

C

1P

1Pload oeient,

q

F _Y ² + F _Z ² /(ρN ² D ⁴ ) C

M1P

1Pmoment oeient,

q

M _Y ² + M _Z ² /(ρN ² D ⁵ ) C

PW

Poweroeient,

P/(ρN ³ D ⁵ )

C

TH

Thrustoeient,

T /(ρN ² D ⁴ ) D

^{Propellerdiameter (m)}

f

^{Osillation frequenyof anairfoil (}

s ⁻¹

⁾

F _Y

^{Sidefore (N)}

F _Z

^{Vertial Fore(N)}

J

^{Propelleradvaneratio,}

V _∞ /(N.D)

k

^{Reduedfrequeny in}Theodorsen's theory,

2πf c/(2V _∞ ) l(y)

Quarter-hord line equation (m)

M _∞

^{FreestreamMah number}

N

^{Rotational speed (s}

⁻¹

⁾

n

^{Normalunit vetor}

P

^{Power (N.m.s}

⁻¹

⁾

R

^{Propellerradius (m)}

r

^{Radius (m)}

T

^Thrust(N)

U _∞

^{Freestreamveloity (m.s}

⁻¹

⁾

V _θ ^ind

Cirumferential induedveloity (m.s

−1

)

V _X ^ind

^{Axial induedveloity(m.s}

⁻¹

⁾

V _⊥

^{In-plane veloity(m.s}

⁻¹

⁾

V _X∞

^{Axial freestreamveloity(m.s}

⁻¹

⁾

w

^{Indued veloityintheairfoil planeand normal to thehord (m.s}

⁻¹

⁾

BPF Blade passingfrequeny (s

−1

)

CFD Computational FluidDynamis

CROR Contra-rotating open rotor

RANS Reynolds-averaged Navier-Stokesequations

UCLL Unsteady-urved lifting-line

In a ontext of oil risis, theU.S. Senate in 1975 direted NASA to look at every

potential fuel-saving onept that aviation tehnology ould produe. Within all

identied onepts, ontra-rotating open rotors (CROR) promised the highest po-

tential fuel saving for high-speed subsoni airraft [Hager 1988℄. Even if the open

rotor was the most hallenging onept, the signiant potential payo that ould

be obtained foredthe otherpartners to adhere to theprojetuntil theend of the

1980s. Bythattime,mainlybeauseofthedereaseinoilpriesandtheomplexity

of suhsystems,theinterest onthis engine tehnology diminished.

Today,theworldofommerialairraftisfaingasimilarontext. Totheriseof

oilpries, moreprevalentenvironmentalandnoise onernshave been addedinthe

lastyears. Thus, airraftandenginemanufaturers, aswell asresearhlaboratories

and aademia, are doing an important eort to push forward again the ontra-

rotatingopenrotoronept,stillpromisingamuhbetterpropulsiveeienythan

future turbofans. However, openrotor hallenges regardinginstallation onairraft,

noise andertiation remain.

Theresearhonontra-rotatingopenrotorsstartedatAirbusOperationsaround

ten years ago. During the last years, in ollaboration with a number of researh

enters, the ompany has launhed a number of projets aiming at developing the

numerial analysisapabilitiesadaptedtoeahdesignphaseoftheengineanditsin-

tegrationontheairraft. Inordertoidentifythebestnumerialmethodologyamong

allthedierentpossibilities,ithasbeenneessarytoidentifythemainrequirements

ateahofthe designphasesand ateahappliation ase. Thisrequirement identi-

ationhasbeen doneprogressively,leading tothedevelopment ofanumberoftools

whih, inthe end,where not ompletely adaptedfor theproblemto be addressed.

In partiular,the analysistools forpreliminary design phaseshave beenlearly

identied during these years. First, they should be able to provide an insight into

theglobal design spae ina relatively short elapse of time, and withlow CPUand

memory osts. Seond, this type of tools should be adaptable to easily address

an important number of topis that should be studied from the beginning of the

design. Third, inthe ase of propellers and open rotors, tools should also be able

to predit withenough auray themain ritial propellerparameters inorder to

point out the optimal engine designs, that an be used in the next design steps.

The ritialparameters inpropeller design aretheperformane (thrustand power)

andthe so-alled in-planeloads. Thesein-planeloads appearasaonsequene ofa

blade load disymmetry due to the eets of theengine installation on the airraft.

Theimportaneoftheseloadsontheglobalairraftdesignimposesthatpreliminary

design tools should orretly aount for these eets of installation. In addition,

the mean propeller moments and load distribution on theblade are preious data

for the strutural design of the blade and the pith hange mehanism. Moreover,

issues and to estimate vibrations. Finally, the auray of thepreditions mustbe

suientto establishthe maintendenies orsensitivityoftheseritialparameters

on a given modiation onan enginedesign.

This Ph.D. thesis starts withan extended Literature Review hapter. First, a

review of the researh on propellers and open rotors and an explanation of their

main assets and hallenges of these engine tehnologies are presented. Seond, the

aerodynamimehanismsattheoriginofin-planeloadsareidentied,anditsimpat

on airraft design is evoked. Third, an historial reviewof the simulation method-

ologies ispresented. Last, a speialattentionis paidto thelifting-linetheory,from

its onset to the lastdevelopmentsinthe90's.

After,intheMethods andTools hapter, themainavailablemethodologies areriti-

allyanalyzedtohelpthe readerbetterunderstandtheurrentontextofnumerial

methodologies for the simulationof open rotors. Here, inthis introdution, only a

rapid overviewofthemost representative onesisprovided.

Among the dierent methodologies developed speially for the aerodynami

simulationofopenrotors,therstandmoresimpleoneswerethesteadysingularity

methods. These lassial approahes, like the lifting-line or the the lifting-surfae

methods,solveonlytheinvisidproblemand,ingeneral,theysimplifythegeometry

of theblade. However, they have proven to preditopen rotor performanes with

a relatively fair auray and at very low omputational osts, of the order of the

seonds. Moreover, as the solution of the problem is obtained by superposing a

series of singularities distributed in the spae, these methods provide a preious

tool to obtain a better insight into the mehanisms behind a ertainaerodynami

phenomenon.

Unfortunately,the steadyorquasi-steadyapproahesinthemajorityoftheseodes

prevent themfrom preditingthe open rotorin-plane loads withenough auray.

During the last deades, due to the inrease in the omputational apabilities,

numerous CFD odes solving the Reynolds-Averaged Navier-Stokes (RANS) equa-

tions have been developed. Their suess to reprodue the main key aerodynami

phenomena of interestfor the ompanies have leadto its hegemony intheworld of

numerial simulations foraeronautis.

Anumber ofmethods existsto apply these RANSsolversto thease ofontra-

rotating open-rotors. The rst and more simple methodology isalled themixing-

plane. It solvesthe steadyRANSequations ina one-blade-per-rowdomain andthe

information between the rows is averaged azimuthally so as to be able to reah a

onverged steadystate.

This steadyapproah has theadvantage to aount for visosity and ompressibil-

ity eets and it is a powerful tool for the predition of open rotor performane.

However,duetoitssteadyapproah,themixing-planeannottakeintoaountthe

eets of installation of the engine on the airraft, and therefore it annot predit

thein-plane loads.

ThemoreomplexRANSmethodology onsistsofonsidering thefullomputa-

tionaldomain andofsolvingtheunsteadyRANSequations. Thisapproahenables

ingandhenethein-planeloads. Unfortunately,urrent omputationalapabilities

arenot adaptedto usethis approah inpreliminarydesign phases.

In the last years, a numberof frequeny-based CFD methods have been applied

to thesimulationofontra-rotating open rotors. Originally developed for turboma-

hinery, these methods take the advantage of seeking for a periodially-dominated

solution, whih is the ase for the major part of onsidered open rotor ases. The

periodiity hypothesis avoids the omputation of long transitive solutions, hene

reduing the global omputational osts. Though theoretially very interesting ap-

proahes, these methods are still not mature and rst results on open rotors have

shownsigniant onvergene problemsto be addressed.

Within theAirbus group, other business units have developed their simulation

toolsfortheaerodynamidesignoftheirproduts. Inpartiular,AirbusHeliopters

Frane 1

, hasdeveloped a omprehensive odefor theaeromehanial simulation of

rotorraft. The struture of the ode is highly modular, enabling the integration

of very dierent models into a single omputation. In partiular, theaerodynami

modelouplesamodulebasedonthe blade-element method,andanunsteadywake

modelbased onthelifting-line theory. Thisallowfor unsteadytime-marhed simu-

lations. Besides,the eetsof installationan betakeninto aount inHOST,and

thereforein-plane loadsmay be orretlypredited.

Afteradetailedstudyofthedierentsimulationtoolsforopenrotors,presented

in this introdution as a rapid overview, it has been highlighted that none of the

available tools fulll the main requirements for preliminarydesign phases. On the

ontrary, anexternal omprehensive ode, HOST,has been identied asa possible

andidate to ll thisgap inthe available methodologies.

Therefore,themainmotivationofthepresentPh.D.thesisistheneedtodevelop

and assess reliable moderate-ost methodologies for the aerodynami simulationof

openrotorsthat areadaptedto theneedsof Airbus'designers.

Todoso, arstassessmentofthe HOSTodeontheAPIANpropellerase,an

advanedsingle-propellergeometry,hasbeenperformedinChapter1. Thepropeller

performane,thein-planeloads,andthebladeloaddistributionhavebeenanalyzed

and ompared to experimental data and CFDunsteadyRANSomputations when

possible. A partiular attention has been paid to theeets of the rotating-speed

and the propeller inidene. Moreover, based on a series of parametri studies, a

numberof best praties inpropeller simulationsould be established.

In a seond step, an original method to analyze the impat of a number of

aerodynami mehanisms on the in-plane loads has been proposed in Chapter 2.

This method uses the linearity hypothesis on whih all singularity methods are

based to deompose thedierent ontributions to the unsteadyaerodynami loads

oftheblade. Withthis,themajorontributorstothepropellerin-planeloadsould

beidentied,togetherwith themain limitsof thepresent modeling oftheproblem.

Based onthesetwomainresults,theontribution ofeahtermandthelimitsof

themodels,the next two hapters have been devoted to theimprovement of oneof

1

thesemodels,ontributingtothein-planeloads: theunsteadyairfoilmodel. First,in

Chapter3thenumerialimplementationofGuermondandSellier'sunsteadyurved

lifting-linetheory[Guermond 1991℄proposedbyMuller [Muller 2007℄hasbeen rit-

ially analyzed. Aspeialattention hasbeenpaidto theintegrationofthesingular

integrals by Finite Parts in the alulation of indued veloities [Hadamard1932℄.

Themain shortomings ofthis numerial approah have been putforward.

From the limits of the model studied in theprevious hapter, a new numerial

implementation of Guermond and Sellier's theory has been developed and imple-

mentedintheHOSTode. Hene,Chapter4hasbeen devotedtothedevelopment,

the theoretial analysis, and the implementation of an unsteady airfoil modelthat

enables,ononeside,toorretthequasi-steadyairfoildatatoinlude theloadsdue

to airowunsteadiness and,on theotherside,to aount for theloalblade sweep

and urvature. Again,the mainassets andshortomingsoftheimplementedmodel

have been put forward.

Chapter5fousesontheassessmentofthisnewunsteadyairfoilmodelinHOST

whenappliedtotheAPIANase,asingle-rotatingpropeller. Thesameomputation

onditions andomparisonsperformedinChapter1arehereusedtoassessthenew

implemented model.

The assessment of the HOSTode done inChapters 1 and 5 for the ase of a

single-rotating propeller has been extended in Chapter 6 to the AI-PX7 ase, an

Airbus'generiontra-rotatingopenrotorgeometry. Themodelstoaount forthe

eets of installation and the unsteadiness in the airfoil inow have been assessed

byomparingHOSTsimulationresultstomoreomplexandaurateCFDuRANS

simulations. Theopenrotorperformane,thein-planeloads,andthebladeunsteady

loadings have been againanalyzedand ompared.

Finally,the method to analyze 1P loads developed in Chapter 2 hasbeen adapted

and applied to the ase of a ontra-rotating open rotor. This method has been

developed to provide abetterinsightinto theaerodynamimehanismsontrolling

propeller andopen rotorin-plane loads.

Therefore, this rst part of theThesis aims at validating a omprehensive tool

for theaerodynamidesign ofpropellers andopenrotors. Moreover,thistoolopens

thedoor to takle multiple propeller andopen rotorsimulations of industrialinter-

est: whirl utter simulations, blade pith default studies, aeroelasti simulations,

aeroaousti preliminarystudies, et.

Based on the limits in the hypothesis of the lifting-line methodology, an ex-

ploratory study has been performed to assess a possible future methodology in

order to better apture the blade-vortex interation phenomenon, and the three-

dimensionalompressibilityandvisouseetslosetothebladewall. Hene,Chap-

ter7exposesthepartialimplementation andapreliminaryassessmentofaninnova-

tive Eulerian/Lagrangian oupling between HOST and a CFD uRANSsimulation

using theelsA ode.

Thethesisnisheswithaonlusionandanumberoffutureworksthatmightbe

interesting toexploreasawaytoontinuewiththedevelopmentof thesemoderate-

Propeller and Open Rotor Assets & Challenges

Researh on Propellers and Open Rotors

On Deember the 17th 1903 the rst manned, sustained and self-ontrolled ight

beame reality when theWright brothers' airraft tooko at Kitty Hawk for a 12

seonds ight. Although being rudimentary, Wright's airraft was propelled by a

woodenpropellerwithasurprisingpeakeieny around

70%

^{. WilburWright was}

therstpersontoreognizethatapropellerisnothingmorethanarotatingtwisted

wing, that onverts shaftpower into propulsive thrust [Anderson 2001℄. Sine that

momentandforthenext45years,propellers remainedthesoleavailablepropulsion

systemfor airraft.

During therst deadesofthe20thentury,theaviationdevelopedat anaston-

ishingratemainlyinEuropeandtheUS.Asanaturalonsequene,theresearhon

wing aerodynamis andpropulsive systemsourished duringthose years.

Propeller Researh at NASA. Fromits ineption aviation hasbeen driven by

theonsistentdesiretoyfaster,farther,andhigher. Thisnaturalevolutionbrought

early to light theloss ineieny of propellers when inreasing thespeed of ight

[Sullivan1977℄. Asaonsequene ofhigheradvaningand rotational veloities,the

relative veloity nearblade tips ame lose to Mah 1. Thisinreased signiantly

the propeller wave drag and led to an important eieny losses. As a way to

overometheselimits, dierentblade geometrieswere tested between 1927 and the

mid-1950's at NACA(the NASApredeessor). Variablepith-angle systems inthe

1930's,highlyloadedandsweptbladetipsinthe1940's,andthinairfoilsinthe1950's

werethe three main tested innovations. First,the blade pith-angle is modied in

orderto adapttoeah ightondition, determined byaight speed andarequired

thrust. Seond,sweptbladetipsmodifythe aerodynamibehaviorofthebladeand

reduetheintensityofshokwaves. Finally,thinairfoilsarebetteradaptedforhigh

subsonionditions.

The propeller tehnology advanedsteadily until the1950's. By that time, im-

portant progresses on turbojet tehnology were ahieved, presenting tremendous

speedadvantageswithrespetto propellers. Moreover,propellersatthattimewere

unableto overomeompressibilitylossesathighsubsonionditionswithadequate

strutural reliability. Thus, by end of the 1950's, the interest on propellers waned

andtheaviationindustryplungedintothedevelopmentofturbojetengines. Bythat

time, even if it was known that propellers had a muh more important propulsive

eieny than turbojet engines even for relatively highMah numbers (

M ∼ 0.6

^),

Figure 1: Propeller andopen rotor eienyasestimatedbyJeraki [Jeraki 1981℄

The Advaned Turboprop Program in the U.S. The Middle East oil em-

bargo in 1973 made perspetives hange again. Fuel pries tripled and disrupted

airline servie. Fuel osts,whihsofar had been asmall portion of operatingosts,

started to aount for almost half of an airline's budget. This pushed industries

andgovernementstoput fueleieny asoneofthemainresearhprioritiesandto

lookfor all possible fuel-eient solutions for airraft and engines. Six main teh-

nologial projets were launhed inthe United States in1976: three were airframe

related (omposite materials, pratial ative ontrols, and laminar ow ontrol)

andtheotherthreewerepropulsionrelated. Whilethetworstaimedat improving

existing turbojet engines, the latter and most hallenging projet was to develop

advanedturboprops.

Similar to turbofans, a gas-turbine ore is used in turboprop and open rotor

engines. The energy of fuel ombustion in the inner ore is used to drive one or

two large-diameter fanswhih propel an important amount of ool air. The ratio

betweentheoolairowandtheairowpassingthroughtheengineoreisommonly

known as the Bypass Ratio (BPR). As the ool air is not onned by the naelle,

turbopropsandopenrotorspresenta muh higherBPRthan turbofans. Therefore,

it inreases onsiderably thepropulsive eieny

η

P

of this kind of engines, whih

is the main asset of propeller-driven engines (see Fig. 1) ompared to turbofans

(

η

^P

≃ 0.6

^).

The Advaned Turboprop (ATP) program was oially launhed in 1978

[Hager1988℄. Sine the beginning oftheprogram, two typesof ongurations were

tested: thesingle-rotatingpropellers,onsistingofonerowofblades;andtheontra-

rotating propellers,onsisting of two rows of blades rotating inopposite diretions.

Although the advaned turboprop onept promised thehighest potential fuel sav-

ings,atleast

30%

^{,itpresentedseveralhallenging pointsoming fromtheavailable}

tehnologies and the market. The tehnial ones were the ruise performane, the

Figure2: Windtunnel testsduring the ATPprogram [Hager 1988℄

The main market issue was the ustomers' aeptane of propellers, that may be

seenasa stepbakwards ompared toturbojetengines.

The ATPprojet wasstrutured to solve tehnial problemsthrough odedevelop-

ment and sale-model tests before ground and ight testing of large-sale systems.

Until 1986, several wind tunnel tests were onduted on single-rotating propellers.

Meanwhile, in 1986, the rst ontra-rotating propeller test rig was nished and

tested. Finally,groundandighttestswereondutedbyNASAandseveralompa-

nies(GeneralEletri,Pratt&Whitney-Allison,HamiltonStandard,et.) between

1986 and 1988 [Harris 1987℄.

The ATP program ended in1987. Asthe energyrisis passed inthe1980'sand

thefuelpriesdereased, therewasnolonger afavorableratio ofosttoimplement

turboproptehnologyversussavingsinfueleieny. Therefore,insteadofdevelop-

inganallnewairraftwithhigh-speedontra-rotatingpropellers,industriesfoused

againon traditional andheaperturbofan-poweredairraft.

The Propeller and Open Rotor researh at Onera. The researh on ad-

vaned propellers started at Onera, the Frenh researh enter in aerospae, by

end-1978, after a request from Aérospatiale ompany and theFrenh Government.

The CHARME researh program on high-speed propellers was oially launhed

more than three years later, in the beginning of 1982. This program was arried

out by the ollaboration of Aérospatiale and Ratier-Figea ompanies, and Onera

researh enter. Several alulation methods with dierent levels of auraywere

implemented and wind tunnel ampaigns were onduted (see Fig. 3), leading to

the denition of a series of advanedsingle-rotating propellers (HT1, HT2 & HT3

propellers) [Bousquet1985,Bousquet1986℄.

Moreover,numerialstudieswereperformedtodeneHTC5ontra-rotatingpro-

pellerblades. Thiswasthe resultof aollaboration between Aérospatiale ompany

and Onera in the CHARME advaned propeller integration program. The main

goal was to develop the AS100 airraft, a 100-seat short- to medium-haul airraft

projetonduted by Aérospatiale inthe1980's. The CHARMEprogram washow-

Figure3: HT3minimum-bodyongurationinOnera S1MAtransoniwind tunnel

faility

theATPNASAprogram: fuelpriesdereasedandthedevelopmentofahigh-speed

propeller-poweredairraft wasno longer eonomiallyinteresting.

Inparallel and within a moretransnational ontext, airframe manufaturers in

ollaboration with researh enters and aademia launhed in the late 1980's four

suessive European-funded researh programs on high-speed propellers: GEMINI,

GEMINIII, SNAAP,and APIAN.

Thesefour programswere devotedto thenumerial and experimental investigation

oftheaoustiandtheaerodynamieetsofinstallation onahigh-speedpropeller

for a representative regional airraftarhiteture.

TheGEMINIpilot program(1990-1992)fousedondesigningandprovingthefeasi-

bilityofagivenregionalairraftongurationfortherestoftheprograms. Between

1993 and1996, two programswerelaunhed inparallel: GEMINIII, devotedtothe

analysis of propeller aerodynamis when installed on theairraft (see Fig. 4), and

SNAAP, devoted to the isolated propeller aeroaoustis. Between 1996 and 2000,

the APIANprogram, basedon the experieneand tools developed previously, ana-

lyzed theaeroaoustis of the installedpropeller.

TheAPIANprogram inluded several wind tunnel ampaigns, rst ona minimum-

bodypropelleronguration and theninstalledon an airraftrepresentative geom-

etry (see Fig. 5). A partiular eort was done to better understand the inidene

and installation eets on propeller performane, as well as the predition of the

propeller aoustifootprint.

The knowledge on high-speed propellers aquired during these projets has

helped in the design and prodution of propeller-powered airraft in Europe in

the last years, like for example the Airbus Generi Transport Airraft A400M

[Malard2005℄.

TheOpenRotorRenaissane. Today,theworldofommerialairraftisfaing

aontextsimilartothe1970's. Totheriseofoilpries,moreprevalentenvironmental

Figure4: GeminiIIfull-airraftmodelatOneraS1MAtransoniwindtunnelfaility

Figure5: APIANFull-airraft modelinthelow-speedDNW-LLFwindtunnelfail-

ity

manufaturers, as well as researh laboratories, are doing an important eort to

pushforwardagaintheContra-RotatingOpenRotoronept,stillpromisingamuh

better propulsive eieny than futureturbofans.

In2000agroupofpersonalitiesfromthekeyaeronautialstakeholdersinEurope

published a report with their vision for 2020 of ommerial aviation in Europe.

This group of personalities established also the AdvisoryCounil for Aeronautial

Researh in Europe (ACARE), to develop and maintain the Strategi Researh

Agenda to ahieve the Vision 2020. In its rst doument, the ACARE proposed

the following environmental goals for 2020 to the aeronautial industry inEurope:

(1) Total engagement by the industry in the task of studying and minimising the

industry's impat on the global environment. (2) A redution in pereived noise to

one half of urrent average levels. (3) Eliminatenoise nuisane outside the airport

boundary by day and night by quieter airraft, better land planning anduse around

airports and systemati use of noise redution proedures. (4) A

50%

^{ut in CO2}

emissions per passenger kilometre (whih means a

50%

^{ut in fuel onsumption in}

the new airraft of 2020) andan

80%

^{ut in nitrogen oxide emissions. [Adv2001℄}

Thesegoalshavebeenassumedbystakeholdersinthreemainpointsinludedinthe

ACAREStrategi Researhand Innovation Agenda(SRA)[Adv 2004℄:

•

^{ReduingfuelonsumptionandCO}

₂

^by

50%

^with

20%

^{redutionomingfrom}

theengine alone,

•

^{Reduingthe pereivedexternalnoiseby}

50%

^{,with6dBperoperationforthe}

engine alone,

•

^{Reduing NO}

_X

^by

80%

^,with

60

^to

80%

^{oming fromthe enginealone.}

The SRA has provided a roadmap outlining the strategi orientations to be taken

if Europe wants to develop a more sustainable aviation setor. As a onsequene,

important researh programsat Europeanand nationallevelshave appeared inthe

last years: EU ollaborative researh inAeronautis and Air Transport, theClean

Sky Joint Tehnology Initiative (JTI), the SESAR Joint Undertaking, and many

national researh projets oming frompubli or privateestablishements.

In 2012, the ACARE members have reonsidered the Vision 2020 and extended it

to a newhorizon towards 2050.

The Clean Sky JTI researh program, launhed for the 2008-2017 period, is

devoted to the development of breakthrough tehnologies to signiantly inrease

the environmental performane of air transport. The program aims at delivering

demonstratorsinallsegmentsofivilairtransportgroupedintosixIntegratedTeh-

nologial Demonstrators or tehnologial areas: the SMART Fixed Wing Airraft

(SFWA),theGreenRegionalAirraft(GRA),theGreen Rotorraft(GRC),theSus-

tainableand GreenEngines (SAGE),theSystemsforGreen Operation (SGO), and

theEo-Design (ECO).

Theresearh onopenrotorshasbeen signiantly enouraged withintheClean

and universities areworkingtogetherto delivertherst European CROR-powered

ighttestdemonstratorby2020. Duringlastyears,industriesandresearhlaborato-

rieshaveusedseveral existinganalysisapabilitiesanddevelopednewonesthatare

speiforopenrotors. Itisworthmentioningthatdierentwindtunneltestshave

beenonduted bythe European manufaturersRolls-Roys,Snema,and Airbus.

In parallel, similarprograms have been launhed byNASAin theUS:the Sub-

soni Fixed Wing (SFW) program and the Environmentally Responsible Airraft

(ERA) projet [Suder2012℄. With theollaboration of aeronautial manufaturers

anduniversities,NASAfousesondevelopinganddemonstratingintegrated system

tehnologies up to mature readiness levels by 2020. Similar to European ACARE

goals,theERAprojetaimsatsimultaneouslyreduefuelburn(by

50%

^),emissions

(

−75%

^NO

_X

^{inlanding andtake-oyle, and}

−55%

^{atruise), and noise (}

−42

^dB

relative to FAAStage 4referene) infutureairraft.

With the large amount of experiene that NASA aquired during the 1980's and

theATPprogram,severalanalysisapabilities havebeenreused inthelastyearsto

updatethe researh onopen rotors. Aerodynamiand aeroaoustiperformaneof

isolated and installed open rotors have been measured at low- and high-speed on-

ditions. Moreover, a number of blade geometries have been developed and tested

to optimize the aousti signature of open rotors [VanZante2012℄. Reently, dif-

ferent onventional and unonventional airframe arhitetures have been tested for

aoustishielding [Czeh2013℄.

Key Assets and Challenges of Open Rotors

Main Assets. Three main assets of ontra-rotating propellers with respet to

turbofan engines havebeen identied:

•

^{Higher bypass ratio: inreasing the ratio between the propelled ool air (un-}

ombustedair)overtheairgoinginto theengine ore(airinvolvedinombus-

tion) inreases signiantly the engine propulsive eieny. Open rotors are

likelyto oerbypassratiosabove30,whereasturbofansareexpetedtoreah

bypassratiosof

14

^to

18

^{. Thus,openrotorshavebetterpropulsiveeienies}

even at Mah numbers around 0.8 and are expeted to oer signiant fuel

savingswithrespetto turbofans.

Comparedto a1998-referene, airraftpoweredwithUltra HighBypassratio

(UHB)turbofanarelikelytogivea

27%

^{fuelgain,whereasopen}rotor-powered airraftpromise a

36%

^{fuelgain [VanZante2013℄.}

•

^{Swirlreovery: thehighlyloaded,} single-rotating propellershavean eieny loss of

6

^to

8%

^{due to residual swirl [Hager 1988℄. Most of this loss an be}

reovered with a well-designed ontra-rotating propeller. Moreover, as the

rear rotorfaes a swirledairowfrom the front rotor, its eieny isgreater.

Indeed, undera ontra-rotating swirl, a smallerblade pith is needed for the

same blade inidene and loading (see Fig. 6). For a given blade loading, a

o-axis omponent, whih is linked to the torque. Inreasing the thrust and

reduingthetorqueimpliesinreasingthepropulsiveeieny

η

P^RR

oftherear

rotor.

Ωr V ∞

W β

x y

b

z

Propellerplane

θ T

F _Ω R ~

(a)WithoutFRswirl

~v

^FRind

Ωr

Front

Rotor

Ωr V ∞

~v

^FRind

W β

x y

b

z

Propellerplane

θ − ∆θ

R ~ T + ∆T

F _Ω − ∆F _Ω

(b)WithFRswirl

~v

ind^FR

Figure6: Eet ofthefront rotor induedswirl on rearrotor eieny

•

^{Redued tip diameter:} ontra-rotating propellers an oer around twie the thrust of a single-rotating propellerfor a given diameter. Thisis partiularly

interesting for the integration on largeairraft.

At theend ofthe ATPprogram,fuelgainsof

25

^to

30%

^{ompared to turbofans}

of that time were estimated by NASA engineers. Today, although these benets

would not be so drasti due to the onstant progress on turbofans, the open rotor

tehnology is still the most radial step-hange in ommerial airplane propulsion

withthesuient readiness levelfor anentry inserviein2020 or 2025.

Open Rotor Challenges. However, ontra-rotating propellers still present sev-

eral ompetitive market and tehnologial hallenges that should be addressed in

theyearsto ome [Butterworth-Hayes 2010℄.

•

Competitive tehnologies. The eieny of urrent engines improves at an average of

1%

^{a year, whih means that lassial turbofan engines available}

in 2025 are likely to be

11%

^{more eient than today's prodution without}

any major tehnology risk. Meanwhile, the Pratt & Whitney geared turbo-

fan PW1000G ould provide around

17 − 19%

^{fuel eieny gain by 2020}

[Norris2007℄, whereas the CFM International diret drive LEAP ould pro-

videaround

15%

^{gainby2015-2017 [Dron 2008℄. Inparallel withopen rotors,}

UHB turbofans are being studied. They provide a very eient and quieter

solution, but theypresent somemajordrawbaks: theweight anddragpenal-

ties, and the issues linked to installation on airraft due to the size of the

•

^{Inreased ight times. An open rotor powered airraft is likely to have a}

ruisingspeed

5

^to

10%

^{slowerthanaturbofanpoweredairraft. Thislimitin}

ruiseveloityomesfromtheimportant lossesofeieny andaerodynami

instabilitiesourwhenthebladetipsreahsupersoniveloities. Evenifthis

limitinruisespeedanbeanimportantdrawbakformedium-andlong-haul

airraft, itbeomesnegligible for short-haulights, where theruisephase is

shorter, i.e. aninrease of

5

^to

10

^{min .}

•

^{Regulatoryissues. Theenginelayoutandthebladeontainmentaretwoissues}

tobeinvestigatedfortheairworthiness ertiationofopenrotorpoweredair-

raft.

For the ertiation of a turbofan engine,it must be demonstrated that one

released fanblade an be safelyontained inwithin theengine's fan ase. In

the aseofanopenrotor,where thenaelleand thefan aseareabsent,rules

foronventionalpropellersareappliedanddesignersmustprovethattheprob-

abilityof bladereleaseisextremely remote,i.e. under

1 × 10 ⁻⁸

^yinghours.

For light weighted highly-loaded open rotor blades, this requirement is areal

hallenge.

Airraftmanufaturersmustalsoensurethatritialairraftsystemsandpas-

sengers are suiently proteted in the ase of blade release. To fulll this

requirement, itmaybeneessaryto repositionertain airraftsystemsor the

useofshielding.

•

Aessibility and maintenane osts. Mounting the engines under the wing oersa easy aess for on-wingheks and maintenane. However, what was

possibleuntilnowforturbofanengines,itisnotlikelytobewiththeopenrotor,

mainly due to the more important engine diameter. New engine positioning

ouldhaveasigniantimpatonmaintenanetimesandost. Moreover,the

reliabilityofsomeomplexomponentssuhasthegearboxorthebladepith

ontrol systems must be assessed to make open rotors a desirable hoie for

airlines.

•

^{Publi pereption. In the 1980's passengers onsidered propeller powered air-}

raftasoutmoded,noisyand slow. Today,theenvironmental onerns of the

traveling publi wouldmake theopenrotor easierto beaepted.

•

^{Eets of} installation. The airframe-engine interation is more important in an open rotor powered airraftthan ina turbofan powered airraft, asinthe

rstase theengine isnot isolatedfromtheairrafteet byanaelle. Thus,

thepylon,thefuselageandthewingan impatsigniantlytheperformane

of the open rotor by introduing a non-homogeneous inow in its propeller

planes [Borst 1981,Blok1984℄. Some engine manufaturersbelieve that the

installation impat and extraweight ofopen rotorswill oset their fuelburn

benet [Barrie 2007℄.

•

^{Pith hange mehanism. Asopen rotorblades seelarge hanges ininletow}

dependingon theight point,reliable pithhange mehanisms shouldbede-

veloped inorderto ensure aeptable propulsive eienies. This mehanism

should enablethe bladeto sweep allthepithangles tooverreverse,ground

idle, taxi,take-o,limb,ruise, approah, andfeather.

•

^{Strutural design and vibrations. In order to be eient, advanedpropeller}

blades must be light, highly-loaded and present very omplex geometries in

terms of sweep, twist, and airfoil thikness. This makes theblade strutural

design an important hallenge. The struture mustfulll three main require-

ments: withstandwithforeignobjetdamage,befreeofhigh-speedandstati

stall utter, and ope with fatigue issues due to fored vibrations in all the

ight domain.

Moreover, thevibrationstransmittedbytheopenrotorengineontheairframe

anbeimportantandleadtostruturalfatigue. Theseaspetswerestudiedby

Donelsonetal. [Donelson1988℄intestampaigns. Inordertoavoidresonane

minor modiationsandspeimaterialswereusedintheairframestruture.

Theydemonstrated thatinthatasevibration levelswereequivalentto those

of aturbofan poweredairraft. However, thosemodiationsledto a penalty

inthe airframe mass, whih annotbe negletedinfuturestudies.

•

Integration in theairframe. Theintegration oftheengine withintheairframe beomes a ritial issue beause of the propeller diameter. Several dierent

airraft layouts are still under investigation, as all of them present impor-

tant advantages but also drawbaks. Due to the installation eets, a non-

homogeneousairowisinduedintheplanesofrotationofthepropellers,and

thus the diretion of the resultant fores and moments is modied. Conse-

quently,togetherwiththethrustandthetorque,in-planeforesandmoments

appear. These omponents an reah important values and annot be ne-

gleted in the predition of the airraft stability and handling qualities. For

example, these loads and moments are a dimensioning fator of the vertial

and the horizontaltail planes of theairraftfor thepusher onguration.

Flight tests, wind tunnel tests and numerial simulations onduted in the

1980's [Vernon1987, Donelson1988℄ bring to light the impat of the open

rotor engine at low- and high-speeds for a pusher onguration. These am-

paigns showed howtheengine installation systems,theforesthey developed

and theirutterlimits, played animportantrole intheoverallairraftperfor-

mane and operability.

•

^{Aoustis. Probably, aoustis isthe major issue to be addressed to obtain a}

suientlymatureopenrotortehnology[Peake 2012℄. Animportant amount

of aousti data was olleted during the dierent ight tests in the 1980's.

Analysisputforward thatthenoise generatedbyan isolatedopen rotorould

beverysensitivetoanimportantnumberofparameters: thenumberofblades,

thedistanebetweenrotors,thespeedofrotation,thepropellerdiameters,et.

Moreover, the noise generated by an open rotor has shown to be very sensi-

tive to its installation on theairraft [Blok1984℄. A numbernumerial sim-

ulations have been onduted reently aiming at studying the main aousti

souresbothofisolatedandinstalledopenrotors[Stuermer 2009,Peters 2010,

Boisard2012℄: the bladetipnoise dueto transonispeeds, theinterationbe-

tween the wake and vortexfromthe front rotorand therear rotor blade, the

pylon-rotorinterationintheaseofapusheronguration,andthepropeller-

wing interation in the ase of a puller onguration. All these phenomena

indue aertainnoise levelboth inthepassengers'abin and onthefar eld.

Absorbent materials suh as liners may redue this noise, but imply an in-

rease in weight of the fuselage and the engine. Community noise levels are

more important in open rotor engines than in turbofan beause the naelle

onnesan important partof the propagated noise.

Several keydesignparameters havebeenstudied for thenoiseredutionofin-

stalledopenrotorsnamely: thediskloading,theaft rotorlipping,theinter-

rotorspaing [Khalid 2013℄,the leading-edge vortexontrol,or pylon blowing

[Czeh 2013℄, et. Reent full-airraftnoise models basedon open rotorwind

tunneldatapreditaeptablelevelsofnoise toaomplishtheFAR36 Stage

IVnoise regulationswitha

12.6

^{EPNdB margin[Hendriks 2013℄. However,it}

isalsonotedthattoahieve theselevelsofnoise unonventional arhitetures

maybeneessary,likeforexampleablendedwing-bodyarhiteturewithopen

rotorsmountedon therearpart ofthefuselage[Suder2012, Czeh 2013℄.

In-Plane Loads on Propellers and Open Rotors

The fores and moments developed by an open rotor an impat in a signiant

way the aerodynami behavior of the whole airraft during ight. Thus, it is im-

portantto understandtheir physial mehanisms andpreditthemauratelyfrom

the rst design steps. This setion is devoted to the omprehension of in-plane

loads generated by single-rotating and ontra-rotating propellers. As itwill be de-

tailed hereafter, these loads appear as a onsequene of non-homogeneous inows,

produed bytheairraft inideneor theinuene ofthe airframeelements on the

engine,i.e. pylon,fuselage, wing,et.

Impat of 1P Loads on Airraft

The balane of fores and moments in an open rotor powered airraft depend sig-

niantly on thethrust, torque,1P loadsand 1Pmomentsgenerated bytheengine

(see Fig. 7). Moreover, theslipstream eet due to thepropellers an alsohave an

impat on the aerodynami behavior of the airraft. Thrust and 1P loads impat

diretly the drag balane. Depending on the airraft attitude with respet to the

freestream,theontributionof1Ploadsanbepositiveornegative. Thevertial 1P

load and the side 1P moment impat the longitudinal stability of the airraft and

Figure 7: Longitudinal and lateral stability of an open rotor powered airraft

[Barth 2012℄

respet tothe angle of attak andthe angle of yaw impatthelongitudinaland side

stability;theyontribute tothe dimensioningof thevertialand thehorizontaltail

planes; and thus they have an impat on the massand on thefrition drag of the

airraft. Finally,theslipstreameet impatsthebehaviorofthedierent surfaes.

Inthe aseofreversethrust,for example,theimportant owperturbationan lead

toapartialortotallossoftheeetivenessofthevertialtailplane. Theslipstream

foresandmomentsanmodifytherequireddimensionofdierentairraftelements

and thus their massandtheir resulting frition drag.

Barth [Barth 2012℄ onduted a number of numerial studies on an open rotor

poweredairraftinpusheronguration. Itwasshownthatinthisongurationthe

engineshaveapositiveimpatonthestatiandthedynamistabilityoftheairraft.

eet on the overalldrag. In this onguration, Barth estimateda signiant drag

redution, thatwouldlead to aninteresting redution oftheblokfuel burn.

Physial Mehanisms behind 1P Loads

In thegeneral ase of a non-axisymmetri inow, eah blade experienes unsteady

loadsalongarotation. Asaresult, whenaddingtheontributionofeahblade, the

netfore may be no longer normal to the propeller plane. Therefore, theresulting

forean bedeomposedinto a loadnormal to thepropellerplane, thethrust,and

anin-plane load. Thesein-planeloads areusually alled1P loads,asthey appear

asaonsequeneofa1/revosillationoftheinowonditionsinthebladeframe. As

ithasbeennotedbefore, theyonstitute adimensioning fatorintermsof stiness

anddampingoftheengineinstallationsystemsandintheairrafthandlingqualities.

Thus,itisimportanttounderstandtheiraerodynamimehanismsandpreditthem

auratelyinsimulations.

1PLoadModulusorVertialFore. Averyommonaseofnon-homogeneous

inow is the propeller in inidene. Very early it was realized that a propeller in

thatonditions develops signiant in-planeloads [Clark 1913,Lanhester 1917℄.

For the simple ase of an isolated propeller, the inidene implies a periodi

variationoftheinowangleandrelativeveloityseenbyeahblade. Astheveloity

andangleofattakofthedownward moving bladearemoreimportant thanfor the

upwardmoving blade, therstgenerates moreliftand drag. Projeting theseloads

for all blades on the propeller plane results in a net fore in the diretion of the

inidene, i.e. a vertial positive fore if the angle of inidene of the propeller is

positive(see Fig.8).

This loal inidene seen by the propeller an appear due to the freestream

veloity, but also to the installation eets. For example, even for the so-alled

minimum-body propeller onguration, the spinner and the wind tunnel rig an

have a non-negligible eet on the propeller aerodynamis when it is plaed in

inidene.

However, this purely geometrial explanation of 1P loads aounts only for

thepreseneofavertial load, butannotexplain themeasured andpreditedside

omponent. Indeed,inwind-tunneltestmeasurements, aswell asinseveralsortsof

simulations, a non-zero sidefore appears.

1P Load Phase Lag or Side Fore. To give an insight on the side fore aero-

dynami mehanisms, a rotating refereneframe is onsidered, following one blade

alonga yle. Asan example,we onsider asinglepropeller CFDsimulation using

theelsA CFDode [Cambier 2008℄. It an thenbe notied thatthemaximumand

minimum aerodynami loads of the blades are not respetively plaed at

90 ^◦

^and

270 ^◦

^{,but shiftedby aertainazimuth angle, asshownin Fig.9(a).}

Figure9(b)showsthebladeloadsintherotatingframeprojetedonthepropeller

y

z ψ

F Z 1P

V ∞ sin α Ωr

V ∞ sin α sin ψ V ∞ cos α

W θ

x

y − z

plane

β

Downwardmovingblade

Ωr V ∞ sin α sin ψ V ∞ cos α

W θ

x

y − z

plane

β

Upwardmovingblade

Figure8: Geometrial explanation of1P loads ofa propellerunderinidene

α

(a)Bladesetionalloadsandoordinates.

Azimuth angle [ ]

B la d e l o a d s [ N ]

0 90 180 270 360

-100 -50 0 50 100 150

Rotating frame

Reference blade

Amplitude

Phase lag

(b)In-planebladeloadsintherotatingframe.

Figure 9: Blade irulation and in-plane loads along a ylefor a propellerin ini-

dene. elsA simulation.

a ertainamplitudeand phaselag. Addingthesefores inthexedrefereneframe

we obtain the propeller 1P loads. Thus, a link an be established between blade

loads' amplitude and phase lag, and propeller 1P loads' amplitude and phase lag,

respetively.

Therefore, to understand the physis behind 1P loads, the analysis is going to be

done, not by the lassial deomposition into a vertial and a side fore, but by

deomposing 1Ploads into modulus and phaselag withrespetto the

0 ^◦

^azimuth.

While 1P load modulus an be explained by the simple geometrial approah

desribedbefore,1Ploadphaselagisamoreomplexphenomenon. Severalpossible

explanationshavebeen putforwardinliterature,takingasstartingpointthisphase

lag between geometrial inidene andaerodynamiinidene:

1. Distanebetweenurrentblade andneighborwakes. Downwardmoving blades

Dueto the inlination of thewake, the bladeat

ψ = 0 ^◦

^{is loserto the wake}

shedbythe downwardmoving bladethanthebladeat

ψ = 180 ^◦

^{. BytheBiot-}

Savart'slaw, weandeduethatveloities induedon therstwillbegreater

than on the latter,and sotheblade at

ψ = 0 ^◦

^{will generate lesslift and drag}

than the one at

ψ = 180 ^◦

^. Integrating all blades, it an be onluded that a net side fore will appear in the diretion of the downward moving blade

[Ortun 2012℄.

2. Unsteady blade motion andnear wake eet. Asit hasbeen explained before,

when thepropelleris plaedat a given inidene, the airowveloity seenby

the blade and its geometrial angle of attak hange during a yle at 1/rev.

The aerodynami behavior of a lifting surfae is not the same whether it is

under steady or unsteady onditions: a hysteresis yle appears for periodi

movements of this kind (Fig. 10). These dierenes between steady and un-

steadyaerodynamibehaviorsomefromtwo sides: rst,therelativeveloity

variations onthe blade and,seond, theeet of the unsteadyvortiity shed

inthe wake.

By analogy, we an extend this explanation to the ase of rotating blades.

Regardless oftheindued veloities,eah point of thebladewill experienea

dierentveloityvetor,notonlyduetorotation,butalsoduetothefatthat

theyarenotplaedatthesameazimuth,beauseofpith,sweepanddihedral

angles. Thisfat isequivalent to the pithingmotion ofan airfoil, where the

inident veloityvariesalongthehord.

As the downward moving blade is put in inidene, the wake beomes more

loaded in irulation. This irulation indues a veloity on theblade whih

tends to diminish the eetive (or aerodynami) inidene. This inrease in

indued veloities delays themaximum loading of theblade. For theupward

moving blade, the same explanation is valid to explain the phase lag in the

minimum bladeloading.

3. Dynami stall. It is awell-known but diultto predit phenomenonin heli-

opter aerodynamis [Leishman 1989℄. The dynami stall appears ommonly

at high-speed forward ight onditions. In these ases, while the advaning

blademight beundertransoni speedsat lowangles ofattak, theretreating

blade sees very low veloities at high angles of attak. This an indue the

stallof important partsof the rotor blade.

Propellers are more likely to experiene another type of dynami stall, more

similar to the one experiened indelta wings [Jarrah 1989℄(see Fig. 11). In

the aseof a propeller, theo-axisinow isalwaysrelatively small ompared

with theaxial omponent. Thismeans that relative veloity variations along

a yle arenot very important. However, asblades areswept for high-speed

onditions and highlyloaded, the stall an appear for thedownward moving

blade at high angles of attak. This stall, whih takes the form of a leading

edgevortex,an shift the maximumloadand thus indue aertain phaselag

Figure 10: Comparison of theoryand experiment for the lift and pithing moment

oeientsinfullyattahedowunderosillatoryplungeforingonditionsat

M = 0.4

^{(in[Leishman2006℄)}

Figure11: Unsteadylift of adelta wing (in[Jarrah1989℄)

4. Compressibility and evolution of shok wave eet. Experimental and

numerial studies on heliopter rotors in forward ight [Caradonna 1978,

Chattot1980℄ an explain this phenomenon. Analogously to what happens

on the advaning blade of the rotor, where the development of shok waves

is shifted with respet to relative Mah number, a propeller in high-speed

onditions might developalso thiskind of hysteresisyle.

5. Blade aeroelastiity [Srivastava 1990℄. There are two main ontributions to

bladedeformations: theinertialeetsandtheairloads. Thesetwophenomena

tendmainlyto untwist theblade. However,while inertial eetsareonstant

along ayle, airloads on a propellerin inideneare unsteady. Thus, asthe

deformation of the blade is not instantaneous with respet to the loads, a

ertainphaselagissuseptible toappearbetween themaximumloadandthe

maximumbladedeformation. Thismight have animpaton thephaselagof

the propellerloads.

Other possible mehanisms may exist,suh aslaminar-turbulent transition, but to

the author's knowledge, they have not been investigated and they are far beyond

theobjetives ofthepresent study.

Methods for the Predition of Aerodynami Performane

Animportantnumberofmethodshavebeendeveloped duringmorethanahundred

years for the predition of propeller performane and 1P loads, presenting a wide

dispersionintermsof type ofapproah,alulation osts, auray, anddomain of

appliation.

The researhon propellers beingdiretlylinked toits useinindustrialontexts, we

an distinguish two main periods in the development of propeller predition tools.

Therst periodgoesfromthe beginning ofaviation tothe1950's,withtheonsetof

turbojet tehnology and the derease in interest on propellers. The seond period

oftime goesfromthe1970's,withtheMiddle-Eastoilrisis andtheinreaseinfuel

osts,to today.

During the rst half of the entury, propeller tehnologies developed steadily

basedmainly on numerous experimentaltests. Numerial tools played a seondary

role, giving some orders of magnitudefor design phases or preparing experimental

ampaigns. A number of methods were developed for eah spei appliation,

yielding to a wide sope of approahes. Two main topis are of interest here: on

oneside,thepredition ofpropellerperformane,andon theotherside,theairraft

stabilityand aeroelasti issues dueto propellers.

Therstgroupoftools,fousedonestablishingtheoptimumbladeloaddistribution

fortheidealaseofanisolatedpropellerunderhomogeneousairows. Onthatases,

only the steadypartof the problemwasonsidered.

Ontheotherhand,theresearhonhandlingqualitiesandaeroelastiityofpropeller-

onditions. Empirialmodelsorsimulationsoftheunsteadybladeloadswereneeded,

whihwere limitedby the available alulation apabilities.

The renaissaneof propellers inthe 1970's was mainly motivated byfuel osts.

If the researh in the rst 50 years was pushed forward mainly by experimental

apabilities, in this seond period it has evolved rapidly to an equilibrium and

evenanhegemonyofnumerial toolsoverexperimentaldata. Comprehensive multi-

disiplinary tools are more and more ommon enabling to treat all the dierent

domains of study in a single problem. Still, it is ompulsory that researhers and

engineers in the 21st entury deide whih of the multiple available methods will

present the optimal auray-ost ratioin theonsidered problem.

Analytial Models for Propellers

Propeller performane predition. First methods were mainly based on ex-

perimental data olleted from marineor aeronautial propellers. Thanksto these

data, semi-empirialexpressionsouldbeformulated veryearlyinthe20thentury.

These expressions often gave a theoretial optimum that helped in the design of

more eient propellers.

Inthemarinepropellerdomain,therstandsimplestmethodistheaxialmomen-

tum theoryproposedbyRankine [Rankine 1865℄. Thistheorymodels thepropeller

byadiskofpressuredisontinuityinaninompressible invisidow. Thepropeller

indues only an axial veloity(i.e. no swirl), and itis alulated byappliation of

themassonservation law. Finally,thepropellerperformaneis obtained fromthe

momentum onservationlawintheaxialdiretion.

A rst improvement to Rankine's theory was introdued by Froude [Froude 1878℄:

thegeneralmomentumtheory,takingtheeetsofslipstreamrotationintoaount.

Only in reent studies, the momentum theory has been extended to onsider om-

pressibility eets [Vogeley1951℄, arbitrary radial load distribution [Conway 1995℄

and highly-loaded onditions [Conway 1998℄.

These simple approahes are good enough to predit the overall performane of

single propellers, but they fail at prediting the details of the airow around the

propeller. This omes from the fat that these theories do not onsider essential

elements inthe problem, suhasthenite numberof blades orthe visouseets.

Aseondgroupofmethodsstartedwiththebladeelement theory,alsoproposed

byFroude[Froude1889℄. Thistheorydeomposesthebladeintonite-spansetions

along thepith axis. Eah setion is modeled by its loal airfoil data at a ertain

inidene and veloity. Thus, thepropeller performane is obtained from theinte-

gration of the loads at eahblade station.

The major drawbak of this method was identied by Glauert, as it does not on-

sider theslipstreameetontheinident veloity. Thus,Glauertproposedatheory

oupling the blade element and themomentum theories [Glauert 1922℄. The wake

modelintrodues the global eet of the wake, but doesnot onsider radialveloi-

ties due to the slipstream ontration. Moreover, they arealways limitedto lightly