Progress in Aerodynamic and Aeroacoustic Integration of CROR ...

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RAeS Propulsion Conference 

„Progress Towards Open Rotor 

Propulsion Technology“ 

November 21 st , 2012 

Royal Aeronautical Society 

London, UK 

Progress in Aerodynamic and 

Aeroacoustic Integration of 

CROR Propulsion Systems 

Rinie Akkermans, Olaf Brodersen, Jan Delfs, 

Arne Stuermer & Jianping Yin 

DLR Institute of Aerodynamics & Flow Technology 

Braunschweig, Germany


Propeller & CROR @ DLR-AS 

- History of experimental & numerical analysis, 

design & testing of propellers & helicopters 

- CFD-based propeller analysis experience built up 

during the past 9 years 

- Cooperation with Airbus on Single Rotation 

Propeller-related topics 

- CESAR: Cost Effective Small AiRcraft (Q4/2006- 

Q1/2010) 

- Contribution to Piaggio led Task 

“Environmentally Friendly Propeller 

Propulsion” 

- Low-noise high efficiency propeller-airframe 

integration 

- CROR activities since 2007: Internal research, 

Airbus contracts (2008-present), Lufo-IV OPERO 

project with RR (2009-2012), EU-JTI SFWA 

activities (2008-present)


Recent Applications: 

Internal Research Activities on CROR Installation Effects 

Arne Stuermer & Dr. Jianping Yin 


Braunschweig, Germany


Research Geometry: 

Sizing, Nacelle and Pylon 

- Generic pusher CROR, sized for 150-seat AC: 

- TO-thrust Fx=88kN - Cruise thrust Fx=19kN @ design point 

M=0.75 @ h=35,000ft 

- 10-bladed D=4.2672m front rotor, 8-bladed 

aft rotor with 15% diameter crop 

- Generic pylon: 

- Symmetric airfoil, t/c=0.1 

- Untwisted for semi-installed case 

- x/D=0.15 distance to front rotor 

- Representative performance levels achieved: 

Cruise Performance of 10F2x8AC1 CROR 

M=0.75 @ h=35,000ft; J1=3.678, J2=4.203 Rotor 1 Rotor 2 Total 

Fx [N] 10,566 8,424 18,990 

η [%] 79.72 91.98 85.85 

- Previous systematic configuration studies reported in AIAApapers 

2008-5218, 2009-3134, 2010-4235


Aerodynamic Analysis: 

Installation Impact on Blades 

- Installation leads to non-uniform inflow for front rotor: 

- Pylon wake leads to localized increase in AoA 

- Aircraft incidence angle further distorts rotor inflow 

→Blade effective angle of attack variations 

- Overlapping impact of blade-blade interactions


The Case for Counter-Rotation of Installed Contra- 

Rotating Open Rotor Propulsion Systems 



Braunschweig, Germany 

From: AIAA2012-2785 

30th AIAA Applied Aerodynamics Conference 

June 25th, 2012 

New Orleans, LA


Introduction & Motivation: 

Non-Handed CRORs - Asymmetric Aircraft 

- Most likely engine-airframe integration scenario for empennage-mounted pusher 

CROR on notional T-tail aircraft configuration: 

- Same sense of CROR rotor rotation on both sides of the aircraft 

- Asymmetric aircraft and asymmetric aerodynamic and aeroacoustic CROR 

performance due to complex installation effects


Aerodynamic Analysis: Installation Impact on Blades 

- Aft blade shows full span 

unsteady loadings due to 

front rotor wake 

impingement 

- Pylon wake impact 

induced loading jump for 

both rotor blades 

- Aft blade shows tip vortex 

impingement on upward 

sweep: Front blades 

loading increase on 

downward sweep affects 

blade wake and tip vortex 

strength and trajectory 

UBP @ α=4º


Aerodynamic Analysis: 

Impact on Front Blades 

- Spectral analysis of blade loading oscillations 

shows dominance of pylon wake impingement at 

rotor rotational frequency of f=n 

- Clear AoA-driven 1P-loading increases 

- Clear sense of rotation dependence of 1P 

loading amplitude 

- Front rotor DBP sense of rotation: 

Superposition of pylon wake impingement 

with AoA-induced loading rise increases 

overall amplitude of unsteady loading 

- Critical engine in terms of blade lifetimes 

- Rotor-rotor interaction: 

- Blade loading oscillations at even number 

higher harmonics of aft rotors BPF 

- Front rotor blades show only small impact of 

AoA increase


Low-Speed Aerodynamics: Rotor 1P-Loads 

- Non-uniform inflow leads to 1P-loads, an important consideration for structural design & 

handling qualities 

- CROR @ α=4º: Lift-dominated 1P-loads 

- UBP: 4.01% and 6.76% of front and aft rotor thrust respectively 

- DBP: 5.60% and 5.56% of front and aft rotor thrust respectively 

- Lateral and longitudinal trim required 

- Flight control system handling of dynamic asymmetric 1P-loads during maneuvers


Aeroacoustic Analysis: 

Tools & Approach 

- Noise radiation analysis using DLR FW-H 

Code APSIM (Acoustic Prediction System 

based on Integral Method) for farfield noise 

predictions 

- Permeable surface approach used based 

on nacelle Chimera data: 

- Analysis for farfield polar microphone 

arrays @ 10D from front rotor center 

- Flyover noise simulation using DLR 

HEMISPHERE-Code: 

- Simulation for twin-CROR powered 

aircraft flying at constant speed of 

M=0.2, constant altitude of h=150m 

and constant (CROR)-AoA of α=4º 

over rectangular area of 1200x500m 

- Comparison of co-rotating CROR (left 

DBP, right UBP) and counter-rotating 

UBP CROR installation 

Perturbation 

Nearfield 

CFD 

APSIM 

(Blade) Surface 

Pressure



Farfield Polar Directivities 

- Farfield polar directivities show CRORcharacteristic 

flat noise directivities 

- Rotor tones dominate near planes of 

rotation 

- Pylon wake impingement leads to front 

rotor tone increases in upstream arc 

- Strong interaction tone contributions in 

the up- and downstream arc 

- Higher (front rotor) blade loadings cause for 

most of the DBP cases noise penalty 

versus the UBP sense of front rotor rotation 

- UBP case shows higher upstream front 

rotor tone levels, possibly due to stronger 

pylon wake 

- Acoustically optimum sense of rotation 

of the (front) rotors with respect to the 

pylon (here: UBP) for community noise



Flyover Noise Analysis 

- Flyover noise contours show asymmetry for 

CROR co-rotation, with amplified secondary 

noise peaks in sideline direction vs. counterrotating 

CROR case 

- Effective Perceived Noise Level accounts for tone 

peak levels, annoyance as well as duration of 

exposure to levels near peak, which lead to 

strong weighting of interaction tones 

- Aft rotor tone fundamental has lowest 

frequency and directivities important primarily 

in plane of rotors 

- Front rotor tone levels have highest peak 

levels, higher frequency content but still 

relatively focused directivities 

- Interaction tones have high frequency 

content, relatively high peak levels and flat 

directivities (long exposure times)


Pylon Trailing Edge Blowing for the Control of CROR 

Unsteady Blade Loads 




From: 17. DGLR-Fach-Symposium der STAB 

November 9th-10th, 2010 

Berlin, Germany


Pylon Blowing: 

Rotor Inflow Distortion 

-Slot for pylon wake filling along blunt trailing 

edge 0.5


Pylon Blowing: 

Installation Impact on Blades 

- Spectral analysis of blade loading 

oscillations shows strong effect of pylon 

blowing: 

- 1P-loading remains, so marginal impact 

on fluctuation amplitudes @ f=n 

- All higher harmonic oscillations thereof 

reduced significantly for front rotor 

- Exception: Interaction frequency with aft 

rotor blade @ f=2*B A, 4*B A,… 

- Similar differences seen for aft rotor, but 

some additional higher frequency 

oscillation induced


Pylon Blowing - Aeroacoustic Analysis 

Tone Contributions 

- CROR-characteristic flat noise directivities 

- Rotor tones dominant near planes of rotation 

- Interaction tones dominate up & downstream 

- Pylon-wake impact: 

- Strong increases in front rotor tone 

levels, especially upstream 

- Moderate aft rotor tone level increases 

- Little impact on interaction tones 

- Blowing impact: 

- Notable reductions in front rotor tone 

levels, some aft rotor tone reductions 

- Benefits of pylon blowing: 

- Blade fatigue and vibration benefits 

- Small overall noise reductions but 

significant amelioration of front rotor 

tone levels, benefitting overall noise 

characteristics (“annoyance”, EPNdB)

Accurate Noise Emission Predictions for Non-Periodic 

Contra-Rotating Open Rotor Cases 

Rinie Akkermans, Jan Delfs 



From: AIAA2012-2262 

18th AIAA/CEAS Aeroacoustics Conference 

Colorado Springs, CO, USA


Non-Periodic CROR Noise Analysis: 

A Challenge for CFD-CAA 

- Unique problems for the numerical analysis of CROR 

configurations featuring unequal rotor rotational speeds: 

- Periodic pressure time-histories readily obtainable 

for equal rotation speed CROR cases 

- Typical rotor rotational speed ratios for nonperiodic 

CRORs lead to obtainment of true signal 

periodicity for unfeasibly long uRANS simulation 

times 

- Simple DFT/windowing of non-periodic uRANS data a 

possible source of error in results 

- DLR-AS Technical Acoustics Department development 

of correction method in DLR-AS FWH-Code APSIM+: 

- Relevant frequencies are known a-priori 

- Simultaneous least-square minimization approach: 

n F=n A 

n F≠n A


CAA Analysis of Non-Periodic CROR Noise: 

Reconstruction of Artificial Signals 

DFT without correction With correction 

- Application of correction approach to artificial signal allows for reconstruction of 

original time history with an error approaching machine accuracy 

p preconstructed 

 

2. 

510 

13



Application to WTT Results: Impact on Spectra 

Rotor plane, near field Upstream, near field 

- Utilizing the developed method for the numerical analysis of a CROR WTT shows 

significant corrections of individual tones are necessary versus a straight DFT 

analysis



Application to WTT Results: Impact on Directivities 

Without correction: With correction: 

WTT, isolated CROR WTT, installed CROR CAA, isolated CROR CAA, installed CROR 

- Use of uncorrected DFT yields CAA directivity results with large discrepencies 

towards WTT data 

- Corrections greatly improve correclation between numerics and experiment 

- Installed case: large influence of corrections on CAA-predictions 

- Conclusion: DLR-AS FWH-Code APSIM+ applicable to the predicition of arbitrary 

CROR configuration noise emissions


Summary & Conclusions 

- Broad scope of CFD/CAA investigations have 

improved understanding of CROR-airframe 

installation effects 

- Further extension of multi-disciplinary analysis 

under way to include blade aeroelasticity 

- Robust validation of all components in the 

process chain slated to be completed in the 

frame of Airbus-led Clean Sky JTI SFWA 

project


Progress in Aerodynamic and Aeroacoustic Integration of 

CROR Propulsion Systems 

Rinie Akkermans, Olaf Brodersen, Jan Delfs, 




Thank you for your attention! 

Questions? 

RAeS Propulsion Conference 

„Progress Towards Open Rotor 

Propulsion Technology“ 

November 21st, 2012 

Royal Aeronautical Society 

London, UK

Progress in Aerodynamic and Aeroacoustic Integration of CROR ...

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