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Alfred Gessow Rotorcraft CenterUniversity of Maryland
Performance and Rotor-Based Control of a Shrouded Rotor MAV
in Edgewise Flow
Vikram Hrishikeshavan
Graduate Research Assistant
Inderjit Chopra
Alfred Gessow Professor and Director
AHS International Specialists’ Meeting on Unmanned Rotorcraft and Network Centric Operations,Tempe, AZ, Jan 25-27, 2011
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MotivationPerimeter surveillance, building searches
Rotor based MAVs - Hover capability
- Maneuverability within confined spaces- Handle gust disturbances
Need to study vehicle performance in adverse flow conditions and enable sufficient contrallability
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Low Reynolds Number Effects
Main Vortex
Vortex Sheet
Rotor Plane
Wake Obstruction
MainVortex
Low Re aerodynamics limits rotor performance
Re = 10,000-100,000
Increased profile drag
Strong tip vortex
Increased tip losses
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Performance Improvement : Shrouded Rotor
•Reduces tip loss effects•Improves power loading•Protects rotor•Sturdy structure
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Shrouded Rotor Performance
0 50 100 150 200 250 3000
5
10
15
20
25
Thrust (g)
Mechanical Power (W)
No shroud
Shroud
More thrustfor samepower
122 mm unshrouded rotor(FM ~0.6)
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Shrouded Rotor Vehicles
ISTAR TiShrovCypher GTSpy
115 KgWeight
2 Kg 1.8 Kg 0.3 Kg
2.2 mRotor diameter
0.25 m
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TiShrov – Shrouded Rotor MAV
Shroud (45 g)Carbon /epoxy
Hingeless rotor(245 mm dia)
Circular camber, sharp LE carbon/epoxyDriven by 75 W brushless outrunner motor
Vanes for anti-torqueTwo deflectable flaps for yaw control
Battery3 cell 800mAH 20C LiPo
IMUComplimentary filter gyro and acc input
for pitch and roll attitude
Weight300 g
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Flight in Hover
Flybar Flybarless
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Adverse pitch up tendency
Response to Edgewise Flow
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Outline
• Performance in edgewise flow
• Strategies to improve controllability of shrouded rotor in edgewise flow
• Flight tests
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Shroud Design
lipr
Circular inlet
lipr
lip2r
Elliptic inlet
Elliptic inlet shroud – more hover efficient(Lakshminarayan,Baeder (2010)) - CFD
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Hover Performance Measurement
2% thickness (t/c)8-13% circular camber
Sharpened leading edge
2: 1 Taper @ 80%R
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Hover Performance
0 50 100 150 200 2500
2
4
6
8
10
12
Thrust (g)
Mec
hani
cal P
ower
(W)
Circular inletshrouded rotor
Elliptic inletshrouded rotor
10% improvement in power loading with elliptic inlet shroud
3%R tip clearance
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Performance in Edgewise Flow
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Edgewise Flow: Asymmetric Pressure Distribution
Hover, no gusts Hover, edgewise gust
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Edgewise Flow: Momentum Drag
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Thrust
Forces in Edgewise Flow
Drag
Pitching moment
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Measurement of Forces in Wind tunnel
1) Unshrouded (open) rotor
2) Circular inlet shrouded rotor
3) Elliptical inlet shrouded rotor
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Pitching Moment Measurement
Bearings
Open jet Wind tunnel
Drag load cell
Pitching momentmeasurement
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Drag MeasurementEdgewise flow
Drag load cell
Linear bearing
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Thrust: Open Rotor
1500 2000 2500 3000 35000
50
100
150
200
250
RPM
Thrust (g)
0 m/s
1500 2000 2500 3000 35000
50
100
150
200
250
RPM
Thrust (g)
0 m/s2 m/s 24 deg
collective
No change in thrust
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Thrust: Circular Inlet Shrouded Rotor
1500 2000 2500 3000 350050
100
150
200
250
RPM
Thrust (g)
0 m/s2 m/s
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Thrust: Elliptical Inlet Shrouded Rotor
1500 2000 2500 3000 350050
100
150
200
250
RPM
Thrust (g)
0 m/s2 m/s
Thrust for each rotor relatively unaffected by low edgewise flow
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Drag: Open Rotor
1500 2000 2500 3000 3500 40000
10
20
30
40
50
60
RPM
1 m/s
2 m/sDrag (g)
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Drag: Circular Inlet Shroud
1500 2000 2500 3000 3500 40000
10
20
30
40
50
60
RPM
1 m/s
1.3 m/s
1.7 m/s
2 m/sDrag (g)
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Drag: Elliptic Inlet Shroud
1500 2000 2500 3000 3500 400010
20
30
40
50
60
RPM
1 m/s
1.3 m/s
1.7 m/s
2 m/s
Drag (g)
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Drag: Comparison
Shrouded rotor drag-speed slope is much higher than open rotor
0 0.5 1 1.5 2 2.50
10
20
30
40
50
60
Wind speed (m/s)
3300 RPM
Elliptic inlet shroud
Circular inlet shroud
Open rotor
Drag (g)
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lipr
lipr
Elliptic inlet has more projected surface area in direction of flow
Leads to more drag
Projected Surface Area
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Pitching Moment
1500 2000 2500 3000 3500 40000
200
400
600
800
1000
1200
RPM
Pitching moment (g-cm)
Open rotor
2 m/s1 m/s
1500 2000 2500 3000 35000
200
400
600
800
1000
1200
RPM
Pitching moment (g-cm)
1 m/s
1.3 m/s
1.7 m/s
2 m/s
Circular inlet shroud
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Pitching Moment
0 1000 2000 3000 40000
200
400
600
800
1000
1200
RPM
Pitching Moment (g-cm)
1 m/s
1.3 m/s
1.7 m/s
2 m/s
Elliptic inlet shroud
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Pitching Moment: Comparison (3300 RPM)
0 0.5 1 1.5 2 2.50
200
400
600
800
1000
Wind speed(m/s)
Pitching moment (g-cm)
Elliptic inlet shroud
Open rotor
300-400% higher pitching moment than open rotor
Circular inlet shroud
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Alleviate Pitching Moment
Vents
Flaps
- Increased weight (flap actuators)
- Structural integrity of shroud reduced
- Complex control in free flight
Use rotor with swashplate control
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Rotor control authority in edgewise flow
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Swashplate transfers cyclic(10 deg max)
Rotor generates nose-down moment
Hingeless, flybarless rotorForce response ~ 45 deg delay
Edgewise flow
Rotor Control
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Control Authority: No Edgewise Flow
2000 2500 3000 3500 40000
500
1000
1500
2000
2500
RPM
Total Control Moment (g-cm)
180
220
280
Controlauthority dropsat higher collectives
Open Rotor
Total controlmoment (g-cm)
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Control Authority: No Edgewise Flow
2000 2500 3000 3500 40000
500
1000
1500
2000
2500
RPM
Total Control Moment (g-cm)
2000 2500 3000 3500 40000
500
1000
1500
2000
2500
RPM
Total Control Moment (g-cm)
Circular inlet shroud Elliptic inlet shroud
180240
300
180
240
300
Control authority increases quadratically with RPMHigher operating RPM -> Higher control authority
Total control moment (g-cm) Total control moment (g-cm)
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Comparison: No Edgewise Flow
2000 2500 3000 3500 40000
500
1000
1500
2000
2500
RPM
Total Control Moment (g-cm)
Elliptic inlet shroudCircular inletshroud
Open rotor
80-100 %
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CFD study on MAV shrouded rotor (Lakshminarayan, Baeder 2010)
- Shroud thrust contribution increases as rotor collective increases- Most of shroud thrust at instant rotor passes a given azimuth
Rotor bladelocation
Ratio of shroud thrust to total thrust
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Shrouded Rotor Control Authority
+ve cyclic -ve cyclic
Control Authority =Mrotor + MShroud
Shroud augments rotor control authority
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Control Authority in Edgewise Flow
+ve cyclic -ve cyclic
M2 (Nose-down moment)
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Control Authority in Edgewise Flow (2 m/s)
Collective Unshrouded Circular lipshroud
Elliptic lipshroud
(M2) (M2) (M2)20 0 -8% -5%22 0 -8% 024 +8% -19% +3%26 - -19% +3%
Operate rotor at lower collective to minimize control authority reduction
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• Elliptic inlet shrouded rotor: Increase in adverse pitching moment not
equivalent to increase in control authority – more sensitive to edgewise flow
Only circular inlet shroud considered in following sections
What is the gust tolerance?
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Open-jetwind tunnel
Pitot tube
Gimbal
Gust Disturbance Response
PID attitude control IMU
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10 15 20 25 30 35 40 45-0.5
0
0.5
10 15 20 25 30 35 40 450
0.2
0.4
Pitch (rad)
Gust ~ 2 m/s
Gust Disturbance Response
Can the tolerance be increased ?
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Increase Control Authority
L2L1
1) Increase cyclic pitch ()2) Increase differential lift for a given cyclic pitch ()
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Cyclic Pitch Magnitude
100 150
old new
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2000 2500 3000 3500 4000500
1000
1500
2000
2500
3000
RPM
Control Moment (g-cm)
Old 100
New 150
Cyclic Pitch Magnitude
Shrouded rotor
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Differential Lift
L
Increase chord of the airfoil
More control moment - Increase L for a given
c
Does it depend on solidity and/or planform?
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Blade Planforms
(a) Original planform
(b) Rectangular 0 1096.e 0 1322.e
0 1494.e 0 1784.e (c) Taper 140% chord (d) Rectangular 140% chord
Measure ΔThrustΔCollective instead of ΔL
Δθ
Tested with circular inlet shroud
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CT vs Collective Circular inlet shrouded rotor
0 10 20 30 400
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
Collective (deg)
CT
Original taper
140% taper
Rectangular140 % Rectangular
Stall
Expect higher L for a given for higher solidity rotor
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Blade planform: Control Authority
2000 2500 3000 3500 40000
500
1000
1500
2000
2500
3000
3500
4000
RPM
Control Moment (g-cm)
Original taper
Rectangular
140 % Rectangular
Caution: Higher rotor should have lower collective for same RPM and thrust
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Performance
0 10 20 30 40 500
2
4
6
8
10
12
14
16
Collective (deg)
Pow
er lo
adin
g (g
/W)
Original taper140% taper
Rectangular140 % Rectangular
Planform chosen based on performance at lower collective
Thrust : 300 g
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Control Authority v/s Adverse Pitching Moment
0 100 200 300 400 5000
500
1000
1500
2000
2500
Thrust (g)
Moment (g-cm)
140
180
220
Adverse pitching momentat 2m/s edgewise flow
140
180 220
Nose-down control moment
Control margin gust tolerance can be increased
Rectangular planform rotor
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Wind tunnel – attitude response
Gust tolerance increased to about 3 m/s
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Flight Tests
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VEHICLE
CAMERAS
TRACKINGMARKERS
VICON for attitude and position feedback control in gusts
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Vicon flight tests
Station keeping hover Gust input from board
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Experimental Set-up(MRI, Joppa, MD)
Flow straightener
CEP
X
Y
Measure the circular error probable as gust velocity increases
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CEP
Greater error in position hold as gust velocity increases
0 0.5 1 1.5 2 2.5 3 3.50
500
1000
1500
2000
2500
3000
Wind speed (m/s)
Circ
ular
err
or p
roba
ble
(mm
)
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Conclusions
• MAV shroud should be designed for high hover performance and low adverse pitching moment simultaneously
• Shrouded rotor - 30% higher power loading - 80% higher control authority (hingeless
rotor,swashplate) - can accept higher cyclic pitch range (stall
delay) - 300% higher adverse pitching moment • Adverse pitching moment of shrouded rotor not a
function of operating RPM or rotor collective
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Conclusions (2)
• Control authority varies quadratically with RPM, (linearly with operating thrust)
• Improve control authority - Increase cyclic pitch range - High operating RPM, high rotor solidity and low
collective • Gust tolerance increased from 2 m/s to 3 m/s with
suggested changes (increased pitch and rectangular planform)
Shrouded rotor MAV viable platform for low gust environments
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Future work
• Optimize rotor conditions to increase gust tolerance (incorporate collective control in swashplate)
• Onboard controller to increase controller bandwidth
• Inner and outer loop gain tuning (reduce overcorrection to disturbances)
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Acknowledgements
This research was supported by the Army’s MAST CTA Center for Microsystem Mechanics with Dr. Joseph Mait (ARL) and Mr. Chris Kroninger(ARL-VTD) as Technical Monitors. The author wishes to thank Jay Baldwin from MRI, Joppa and Greg Gremillion, UMD for assistance with VICON tests. The author also wishes to thank Dr. Paul Samuel for technical advice.
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0 10 20 30 400
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
Collective (deg)
CT
OriginalRectangularTaper 140% chordRectangular 140% chord
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0 10 20 30 400
0.1
0.2
0.3
0.4
0.5
Collective (deg)
CT/
e
OriginalRectangularTaper 140% chordRectangular 140% chord
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Avionics and Telemetry
Pulse code
IMU records attitude change
Control box
generated
Servos respond
Bluetooth
2.4 GHzRadio signal
1
3
4
Pulse code
IMU records attitude change
Control box
generated
Servos respond
Bluetooth
4
22
1
3Pulse code
IMU records attitude change
Control box
generated
Servos respond
Bluetooth
Pulse code
IMU records attitude change
Control box
generated
Servos respond
Bluetooth
1
3
4
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Sensor
1 tri-axis accelerometer3 rate gyros2 magnetometers
Communication: Bluetooth
Resolution: 10 Bit ADCSampling frequency: 200Hz
Input voltage: 5 V
Weight: 29 gInertial Measurement Unit
(IMU)
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Latency and Bandwidth
BWdelay
0.37ω =τ
Closed loop bandwidth(Tischler 1989)
3.7 rad/s bandwidth sufficient for handling vehicle dynamics
ms
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Inner Loop Control
Complementary filter to fuse gyro and accelerometer outputs – accurate attitude estimates
p – Roll rateq – Pitch rater – Yaw rateu – x velv – y velw – z vel
– Roll ang – Pitch ang – Yaw ang
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Features:•LPC2138 ARM7 Processor•Freescale MMA7260Q triple-axis accelerometer, •2 Invensense IDG500 500 degree/second dual-axis gyros (total of 4 axis)•Honeywell HMC1052L and HMC1051Z magnetic sensors•Roving Networks Bluetooth Module•Tri-color status LED
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2.8 m/s Edgewise Flow
10 15 20 25 30 35
0
0.2
0.4
10 15 20 25 30 35-1
-0.5
0
0.5
Time (s
Pitch(rad)
Control input(non dim)
Unable to return to hover attitude
Time (s)