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