Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Hover Test of SNU Active Trailing Edge Flapfor Rotor Vibration Reduction
SangJoon ShinSeoul National University

Korea-Japan Joint Workshop on Rotorcraft
Yeosu, Republic of Korea

Session II Analysis & Test
February 10th 2023
13:00 – 13:20

Contents

I.

Introduction

II. Hover Test Preparation
III. Pre-test Prediction

IV. Conclusion

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Introduction❖ Rotor blade with a trailing-edge flap

➢ Rotor vibration and noise control by active flap blades▪

Active rotor – numerous studies have been done*, and recently by DLR, JAXA, NUAA, SNU, …
- Mechanism structural design, fluid dynamics, control, and wind-tunnel test

✓ Underlying structural dynamics and aerodynamic features yet to be extracted

FlapActuator▲ Trailing-edge flap blade concept▲ SMART rotor whirl stand test

▲ Sikorsky wind tunnel test

*Friedmann, P. P., “On-Blade Control of Rotor Vibration, Noise, and Performance: Just Around the Corner? The 33rd Alexander
Nikolsky Honorary Lecture,” American Helicopter Society 69th Annual Forum, Phoenix, AZ, May 2013.

3

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Introduction❖ SNUF (SNU Flap-blade) research objectives➢ Baseline multi-disciplinary investigation
Composite blade structures

Flap driving mechanism
Glassroving
Sandwich
spar

Cross-section optimization

Push
rod

Actuator

Guide

Hinge
mountActuatorrod

Actuator
hatchsupport

Flap
Flap hinge support

Flap angle
sensorMulti-body comprehensive rotor aeromechanics

Active rotor controls

Mid-fidelity aeromechanics analysis with full flap rotor

Closed-loop vibratory load control

4Introduction

Seoul National UniversityActive Aeroelasticity and Rotorcraft Lab.

❖ Hover test overview➢ Active flap blade aeromechanics, composite blade integrity correlation➢ Low-latency harmonic signal processing for higher harmonic control➢ Time-periodic and multi-blade nature w/ different collective➢ Hub load dynamics w/ different flap actuation modeHarmonic signal processing
Aeromechanics

N/rev

Composite blade analysis correlation

2N/rev

Time-periodic hub load

5

Contents

I.

Introduction

II. Hover Test Preparation
III. Pre-test Prediction

IV. Conclusion

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Test Preparation❖ Hover test stand fabrication➢ Seoul National University Rotor Test System (SNURTS)▪

Siheung Campus test center: 9X12 m, 240 kW electric power capacity▪

Collaboration w/ Chungnam National University: design/fabrication of the test stand▪

55kW AC motor direct drive, max. 2,000 RPM▪

6-axis balance: max. 2,000kgf thrust, 160kgf-m torque▲ SNURTS components▲ SNURTS control room

▲ SNURTS installed7

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Test Preparation❖ Test stand fundamental mode identification➢ Impact test and correlation▪

emergency

Test stand structural analysis by ANSYS- Contact normal stiffness adjusted to match test results▪

Major resonant speed identified: 300, 900RPM▪

ISO-1940: satisfactory vibration level @ 1,100~1,300 RPM

warning
2nd bending
excited▲ Test stand vibration level at 1,120 RPM

Concentrated
motor mass

1st bending
4.8 Hz

2nd bending
15 Hz▲ 3D FEM modal analysis correlation

1st torsion
22 Hz▲ Impact test result8Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Test Preparation❖ Test stand fundamental mode identification➢ Mode control by the stiffener▪

Bending and torsion modes of the stand may be adjusted▪

Bearing tower is structurally isolated with the test stand▪

k/rev : 1300, 2600, 3900, 5200 … RPM avoided

WHTW

H

TmmmmmmN

Baseline

Stiffener▲ Isolated bearing tower mode

Supportpillar 1stbending
RPM
(Hz)

Supportpillar 2ndbending
RPM
(Hz)

Bearing
tower
1staxial
RPM(Hz)Stand
1sttorsion
RPM (Hz)

Bearingtower
1stbending
RPM (Hz)

Stand2ndtorsion
RPM (Hz)295(4.92)924(15.4)

1,532(25.5)

1,320(22)4,022(67.0)

5,120(85.3)

20

60

103

336(5.60)

1,059(17.7)-

1,448(24.1)

-

-

20

60

106

362(6.04)

1,227(20.5)

1,538(25.6)

1,558(26.0)

4,084(68.1)

5,358(89.3)

20

120

10

6

383(6.39)

1,474(24.6)

1,538(25.6)

1,714(28.6)

4,154(69.2)

5,580(93.0)

20

150

10

6

388(6.47)

1,560
(26)

1,539(25.7)

1,776(29.6)

4,188(69.8)

5,706(95.1)

20180

10

6392(6.53)

1,638(27.3)

1,540(25.7)

1,830(30.5)

4,218(70.3)

5,874(97.9)▲ Stiffener parametric examination

9

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Test Preparation❖ Rigid 4-blade SNUF rotor hub➢ Blade and grip structural integrity evaluation▪
▪
▪

Hingeless hub: grip structural analysis
Rotor load from analysis: 39kN → applied load factor 1.5: 58kN centrifugal load
Safety margin > 2▲ SNURTS rigid hub▲ VABS cross sectional modelingStress reduction▲ Grip M6-bolt reinforcement▲ Stress recovery▲ Strain recovery

10Test Preparation

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

❖ SNUF blade ground-test➢ Ground bench test (scheduled ~March 23’)▪

Strain gauge calibration by the tip displacement (flap, lag, torsion)- Tip displacement measure sensor for the test stand▪

Measurement and control program test▪

Actuator inner-loop position control test

▲ SNUF blade ground test

Laser displacement sensor▲ Ground bench test

▲ Fabricated grip and SNUF rotor
hub assembly11Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Test Preparation❖ SNURTS evaluation➢ Reference OLS rotor from NASA test*▪
▪
▪

2-m diameter, NACA0012, no twist, no taper
To evaluate the functionality of SNURTS
Validate against the present prediction▲ Present OLS hub▲ Blade modal test

▲ Present DYMORE configurationof the SNURTS-OLS rotor▲ Rotor hub modal test

▲ Fan plot from the present
hub test result*Floros, M. W., Gold, N. P., and Johnson. W., “An Exploratory Aerodynamic Limits Test with Analytical Correlation”, American Helicopter
Society 4th Decennial Specialists’ Conference, Jan 2004.

12

Test Preparation

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

❖ OLS rotor hover test
➢ Collective pitch sweep test▪

Test repeated 3 times and one open-door test▪

Sufficient ground height for OLS rotor▪

Functionality of SNURTS verified (run, collective, stop command)▲ Ground effect wake analysis→ No ground effect▲ Closed-door test (@ free run stop)13

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Test Preparation❖ OLS rotor hover test
➢ Collective pitch sweep test▪

98% ↑ reproducibility for the measured thrust and torque▪

Measured unrealistic figure of merit: 2012 KARI test as a reference▪

Present momentum theory inflow analysis gives good correlation to the reference test▪

Free-wake viscous influence should be carefully correlated▪

Balance designed for SNUF rotor 400 N-m rated torque→ torque resolution unmatched for max torque 40 N-m OLS rotor▲ Thrust-collective▲ Torque-collective▲ Figure of merit
14

Contents

I.

Introduction

II. Test Preparation
III. Pre-test Prediction

IV. Conclusion

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Pre-test Prediction

Seoul National UniversityActive Aeroelasticity and Rotorcraft Lab.

❖ Correlation against the comprehensive analysis➢ Multi-body dynamic analysis: DYMORE and CAMRAD-II▪

Incorporate SNURTS pitch link, servo actuators, shaft, trailing-edge flaps▪

Code-to-code comparison and evaluation▪

Time-marching free-wake analysis with the multiple-trailer wake▪

DYMORE-Simulink coupled simulation for the control▲ SNUF DYMORE configuration▲ SNUF CAMRAD-II configuration▲ Servo-pitch test and prediction▲ Free-wake analysis▲ DYMORE-Simulink coupled simulation
16

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Pre-test Prediction❖ Additional accuracy for the lifting-line theory➢ C81 table for an airfoil w/ a flap▪
▪
▪

Improved aerodynamic load prediction at the flap station
RANS (k-ω SST) by using FLUENT, matched y+=1 for all the cases
Advance ratio 𝜇 = 0 ~ 0.16 → Mach 0.3~0.5▲ CFD grid of NACA0015 15% flap

▲ Present CFD grid accuracy
comparison (NACA0012, no flap)▲ Present rotor operating condition

CM sign changed+δ:lift increase▲ Lift coefficient+δ:drag decrease
for aoa < 0+δ:drag increase
for aoa > 0▲ Drag coefficient+δ:momentdecrease▲ Moment coefficient

17Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Pre-test Prediction❖ Hover test prediction➢ Flap rotor test▪
▪
▪
▪
▪

Baseline rotor performance
Far wake 5 ages, 2˚ azimuth, 10˚ spatial step
Flap deflection (steady and harmonic)
Nonlinear blade twist achieved by the steady flap
deflection (2˚, 4˚, 6˚, 8˚, 10˚)
Positive flap deflection:
thrust and torque slightly increase, no FM variation▲ SNUF blade effective twist
due to the steady flap deflection▲ Blade loading vs collective▲ 𝑪𝑸 /𝝈 vs collective▲ Steady flap sweep F.M.
18Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Pre-test Prediction❖ Wind-tunnel test prediction➢ Isolated rotor in the forward flight, fixed collective
▪
▪
▪
▪
▪
▪

Wind-tunnel speed 30 m/s (𝜇 = 0.162), shaft angle 𝛼𝑠 = −6°
Steady collective mode flap deflection sweep (2˚, 4˚, 6˚, 8˚, 10˚)
Thrust increased most when 0˚ → 2˚Control authorities: rolling moment > pitching moment
Flapped section pitching moment drives the entire blade to the opposite AOA
A soft in-plane blade responds in a ‘control reversal’ mode

→ Additional control phase lead or lag will be induced+δ:lift increase+δ:momentdecrease(b) Rolling and pitching moment▲ Propulsive loads control authorities▲ Sectional lift

▲ Sectional pitching moment
19

ContentsI.

Introduction

II. Test Preparation
III. Pre-test Prediction

IV. Conclusion

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Conclusion

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

❖ Conclusion and test schedule➢ Conclusion▪

SNUF blade design: blade structural design, test and analysis▪

SNURTS preparation: 3m-diameter 75HP class Mach-scaled rotor test stand➢ Test schedule and future works▪

Hover test schedule: April ~ May 2023▪

SNUF baseline rotor test (passive flap)-RPM: 1,100 ~ 1,300 RPM-Collective sweep: 2°~12°▪

1- active flap actuation test▪

4- active flap actuation test▪

Future work: 5-hole fast response probe for wake measurement▪

Wind-tunnel tests on ROKAFA21

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Thank you22Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Appendix❖ Blade preliminary structural test*➢ Tensile test▪

Root and flap component safety margin > 2➢ Blade modal test▪

Blade No. 1 has 20% lower torsional frequency▪

Blade Nos. 2~5: within 10% frequency difference▲ Blade modal test

Root load

Flap load▲ Tensile test

▲ Root and flap component tensile load

▲ MAC plot for Blade #1*Im, B. U., Lee, C. B., and Shin, S. J., “Experimental Evaluation on a Mach-scaled SNUF Blade for Active Vibration Control,”45th European Rotorcraft Forum, Poland, 201923

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Appendix❖ Flap rotor blade dynamics measurement➢ Bench test▪
▪
▪

Flap deflection: measured by the potentiometer and calibrated by the digital protractor
DAQ: 0 ~ 5 V → 0 ~ 100 V by amplifier
Linearization of the flap driving mechanism

∗𝑉𝑐𝑚𝑑
=

▲ 2V square wave input▲ Flap deflection calibration

𝑉𝑠𝑎𝑡 − 𝑉𝑡ℎ
𝑉− 𝑉𝑑𝑒𝑎𝑑 + 𝑉𝑡ℎ
𝑉𝑠𝑎𝑡 − 𝑉𝑑𝑒𝑎𝑑 𝑐𝑚𝑑▲ Linearization of the flap driving voltage▲ 2V step input responses▲ Bouc-Wen hysteresis modeling24Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Appendix❖ Flap rotor blade dynamics measurement➢ Bench test▪

3rd order flap deflection/command voltage transfer function identified▲ Transfer function identificationin time-domain▲ 1 Hz▲ 60Hz▲ Frequency response▲ 90Hz25

Seoul National University
Active Aeroelasticity and Rotorcraft Lab.

Appendix❖ IBC control design and simulation➢ Controller design▪

Sub-optimal LQR/LQE-observer baseline*▪

LTI system identification using the dedicatedN, N ± 1/rev input by the discrete Fourierfilter**▪

Output regulation:
1∞

1∞

𝐽 = ‫׬‬0 𝑦 𝑇 𝑄𝑦 + 𝑢𝑇 𝑅𝑢 𝑑𝑡 = ‫׬‬0 𝑥 𝑇 𝐶 𝑇 𝑄𝐶𝑥 + 𝑢𝑇 𝑅𝑢𝑑𝑡
22▲ Flap rotor system identification▲ LQG rotor vibration reduction control design▲ Closed-loop simulation▲ Relative stability analysis*Im, B., Lee, C., Kee, Y., and Shin, S.J., “Investigation of Linear Higher Harmonic Control Algorithm for Rotorcraft Vibration Reduction”,
Journal of Dynamic Systems Measurement and Control, Vol. 143, (1), 2021, pp. 011008-1 - 011008-12.
**Im, B., Kang, S., Kong, G., Park, S., Cho, H., and Shin, S. J., “Improved Higher Harmonic Control Analysis for HART-II Rotor”, The Vertical
Flight Society's 77th Annual Forum & Technology Display, May 2021

26