Slowed Rotor Wind Tunnel Testing at UMD
Ben Berry Graduate Research Assistant Inderjit Chopra Alfred Gessow Professor
Alfred Gessow Rotorcraft Center University of Maryland
AHS Federal City Chapter Dinner Meeting February 19, 2014 Arlington, VA
The Push for Higher Speeds
Sikorsky/Boeing
AVX
U.S. DoD Future Vertical Lift (FVL) Joint Multi-Role Technology
Demonstrator (JMR TD) 230+ knots
2017 Flight Test
DARPA VTOL X-Plane 300 knots
2017 Flight Test
Karem Aircraft
Bell Helicopter
What are the Limitations?
Helicopter in Forward Flight
Advance Ratio = Forward Airspeed / Tip Speed µ = V/ΩR
Slowed-Rotor
Reverse flow region
Retreating Blade Tip, ΩR-V
µ = 0.5 µ = 1.0 50% RPM 100% RPM
Lift V
V Lift
Advancing Blade Tip, ΩR+V
Slowed-Rotor Compound Helicopters
Compound helicopters have: º Auxiliary thrust (propellers or jets) º Auxiliary lift (wings) º Both
DARPA Heliplane Carter Aviation SR/C Prototype
Obstacles to Implementation • Modeling tools not well-validated for this regime • Unknown impacts to vibrations, structures, linkages
State of Art for Data Collection Full-scale rotor tests: • PCA-2 1934 µ=0.72 • H-34 1966 µ=1.05 • UH-1 1968 µ=1.1 • UH-60A 2010 µ=1.0
º Comprehensive data set • All to µ=1.1 or less
Model rotor tests: • Meyer 1953 µ=1.0 • Jenkins 1965 µ=1.45 • Ewans 1973 µ=2.5 • Berry 2011 µ=1.2 • Limited data sets
UH-60A rotor in NFAC
Berry/Chopra 7
H-34 rotor
• Model rotor testing to advance ratios up to 2.5 º Generate unique, high-quality dataset
• Contribute to understanding of slowed-rotor
aeromechanics
Program Objectives
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EXPERIMENTAL SETUP
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Test Facility
• Glenn L. Martin Wind Tunnel (UMD), 7.75’ x 11’, 200 kts max • UMD rotor test stand, fully articulated Mach-scale hub
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Hub balance
Hydraulic motor
Swashplate actuators
Slipring
Blade grips
Rotor Description
• Composite rotors manufactured in-house º Rectangular carbon fiber spar º Carbon fiber skin with glass fiber finishing layer º Rohacell 31 foam core
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Number of blades 4 Radius (ft) 2.80 Chord (in) 3.1 Solidity 0.118 Lock No. 5.9 Airfoil section NACA 0012 100% RPM 2300 Tip Mach, hover 0.60 Tip Reynolds No. 1.1×106
Hinge offset 6.4%
Blade Strain Gage Placement
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0.76R: Torsion 3
0.49R: Flap Bending 2
0.41R: Torsion 2 0.37R: Lag Bending 1
0.26R: Flap Bending 1, Torsion 1
Embedded Flush to Surface
Prior to molding
Number of stations limited by slip ring
Pressure Sensor Integration
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3.10”
LQ-062 LL-072 LQ-080 LQ-062 LL-072
Slots milled into surface
(flush mount)
3D printed nose-piece, built-in pressure ports
LQ-062 LL-072
Kulite models numbers
Rotor Tracking
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PERFORMANCE RESULTS: COLLECTIVE SWEEPS
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Collective Sweep
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30% RPM, Shaft tilt = 0°
Increasing collective pitch decreases rotor thrust??
µ=0.25 30 kts
µ=0.58 70 kts
µ=0.41 50 kts
µ=0.83 100 kts
µ=1.04 125 kts
Thrust
High µ Reverse Flow Effect on Trim
1. Positive Increment to collective is made
2. Negative longitudinal cyclic is applied
3. Additional negative longitudinal cyclic
Vresultant
Retreating side Advancing side
Net result: positive collective leads to negative thrust
Lift
VIBRATORY HUB LOADS: DYNAMIC CALIBRATION OF ROTOR HUB BALANCE
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Dynamic Calibration of Hub Balance
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Rotor Hub
Load Cell Shaker
Rotor Hub Balance
• Sine sweep signals were applied to shaker • Applied load (input): Shaker load cell • Response (output): Hub balance components
0 50 100 150 200 250 300 350 4000123456
0 50 100 150 200 250 300 350 400-180-90
090
180
Dynamic Calibration: Vertical Force
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Frequency, Hz
Phase, deg
Gain (out/in)
Freq of interest (4/rev)
46.7 Hz
Hub Normal Force response to Applied Normal Force
0 100 200 300 40001234
0 100 200 300 400-180-90
090
180
Dynamic Calibration: Axial Force
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Frequency, Hz
Phase, deg
Gain (out/in)
(4/rev)
46.7 Hz
Hub Axial Force response to Applied Axial Force
RESULTS: BLADE PRESSURES
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Advance ratio sweep
• Higher harmonic variations increase with advance ratio
• Unexplained pressure variation near 350° azimuth
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θ0=0°αs=2°
x/c=0.013
49% radial station
µ=0
µ=1.41
µ=1.41
µ=0
SUMMARY
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Summary
• Future rotorcraft will have to be capable of efficient flight above 200 kts
• Slowed-rotors are necessary to achieve those speeds
• Very little test data currently exists on slowed-rotor behavior at those conditions
• Goal of UMD program is to address that deficiency
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Acknowledgements
• Supported by the Vertical Lift Research Center of Excellence (VLRCOE), through the NRTC
º Mahendra Bhagwat serving as Program Manager and Technical Agent, grant number W911W6-11-2-0012
• Helpful advice regarding experimentation/manufacturing: º Tom Norman (NASA Ames) º Anubhav Datta (AFDD) º Mark Potsdam (AFDD) º Joon Lim (AFDD) º Johannes Riemenschneider (DLR)
• Donation of composite materials º Karl Bernetich (Boeing Philadelphia) º John Vertel (UTC Aerospace Systems)
• Fellow students for help with the testing º Graham Bowen-Davies, Anand Saxena, Xing Wang, Alexis Boulegue,
Bastien Luz 26