Alfred Gessow Rotorcraft Center UNIVERSITY OF MARYLAND
Rotor Based Micro Air Vehicles: Rotor-Based Challenges & Opportunities Inderjit Chopra Alfred Gessow Professor & Director Alfred Gessow Rotorcraft Center (
[email protected]) Presentation at First US US-Asian Asian Demonstration & Assessment of Micro-Aerial & Unmanned Ground Vehicle Technology Agra, India, March 10-15, 2008
Micro Air Vehicles: Definition • Design Requirements • • • •
No dimension exceeds 15 cm (6 inch) Gross takeoff weight 100 grams Loiter time of 60 minutes Payload capacity of at least 20 grams
• Additional considerations • • • • • •
Minimum mechanical complexity Fully y autonomous ((out of sight g operations) p ) All weather operations Low production cost R id deployment Rapid d l t Low detection
Hover
Small
Novel
Future Vision of Microsystems
• Autonomous “mother” robotic microsystemscarrying hovering air vehicles, hopping robots, crawling robots and legged robots • Operator commands signals to mother ground robot to move quickly q y to the entrance of a cave • Communication between each robot as well as gateway node • Provides tactical situational awareness
MAV Applications • Military • Surveillance missions (over the hill and confined areas) • Infrared images of battlefields and urban areas (around the corner) • Mine detection in war zone
• Civil • • • • •
Biological/chemical agent detection Agriculture Monitoring C Communication i i Nodes/GPS N d /GPS Traffic monitoring (long endurance) Counter-drug Counter drug operations
Urban MAV Missions •Monitoring Traffic Flow •Surveillance Imagery
Existing MAVs UM GIANT [250g/15 min]
MicroSTAR [110g/25min]
Micro Commercial Rc Heli [350g / 15min]
Allied Aerospace iSTAR [1500g/15 min 9 in dia]
Weightt (g)
MICOR [135g/15 min] 1000
100
Objective Objective [100g/60min]
[100g/60min]
10
0 Microbat [10g/15 min]
10
20
30
40
Endurance (min)
50
60
Black Widow [80g/22min]
Index of Efficiency Figure of Merit FM =
Ideal Power required to hover Actual Power required to hover
Power Loading PL=
Thrust Produced Actual Power required
Power Loading (Thrust/Power)
Micro Air Vehicles: Key Challenges!! • Low Reynolds number aerodynamics: performance • Miniaturization: structural joints and friction • Micro Actuators and Sensors: high performance, light weight, large stroke • Sensitivity S iti it to t gustt • Out-of-sight control requirement • Large structural strains/material nonlinearities • Flying in constrained environment
Reynolds Number Effect Max Lift
Max Lift to Drag Ratio
Max Drag
Biological Lifting Mechanisms at Low Re Most Small Insects 100 < Re < 1000
•Delayed stall •Wake capture •Rotational circulation
Large Insects to Small Birds 1000 < Re < 15000
•Dynamic stall •Delayed stall •Wake W k capture t
Birds Re >15000
•Bound circulation •Quasi-Steady mechanisms
MICRO HOVERING AIR VEHICLES • Non Non-Hovering Hovering Vehicles: Fixed Fixed-wing wing based • Hovering Vehicles: Rotor Based •Single g main rotor ((with & without tail rotor)) • Ducted fan rotor • Co-axial rotor • Tiltrotor, tiltwing, quadrotor, hybrid systems • Revolutionary designs
• Hoveringg Vehicles: Flapping-Wing pp g g Based •Bird-flight based • Insect-flight based (Efficiency at small scale?)
• Hovering Vehicles: Reaction Based (power intensive)
Micro Hovering Air Vehicles: Rotor-Based
Coaxial Rotor MAV MICOR
(University of Maryland)
15 cm (6”) dia coaxial 22-bladed rotors
Swashplate controls only lower rotor
Weight 100 g, Payload ~10g Weight~100 10g 8% camber circular arc airfoils Re.75R ~20,000 Endurance ~ 10 minutes Fixed pitch, variable speed rotors (feedback on lower)
Coaxial Rotor MAV Development at UM Evolution of the MICOR MAV
1st Gen.
2nd Gen.
3ndd Gen.
2008
2000 • • • • •
1st Generation 100 g Weight Maximum Single g Rotor FM ~ 0.4 No Payload Capacity No Lateral Control - Unstable 3 Minute Hover Endurance
4ndd Gen.
• • • • •
4th Generation Two bladed teetering rotors 135 g Single g rotor max FM ~ 0.65 Swashplate for cyclic control 20 minute hover endurance 25 g payload
MICOR Technology Transition DARPA Nano Air Vehicle (NAV) Program
MICOR
• Daedalus Flight Systems (DFS) subcontracting to the Draper Lab • Experience Gained d i MICOR during MONARC Development being put to work on the MONARC NAV • DFS involvelemt will aid in the rapid transition of technology to industry
MAV: Single Rotor & AntiAnti-Torque Vanes Stabilizer bar Main rotor
Motor
Anti-torque vanes Protective ring
MAV: Single Rotor & AntiAnti-Torque Vanes Main rotor - Two bladed teetering - Pitch flap coupling (δ3angle of -45°) -Servo paddles
Rotor diameter 27 cm
Rotor system y Motor
Swashplate Control
Vanes for anti-torque
- Longitudinal - Vertical - Lateral - Pitch - Roll
Yaw control surfaces
Servos
Battery pack
Vanes (feedback) - Anti-torque - Yaw control
Electronics 3 micro-servos Receiver, brushless motor controllers
MAV: Single Rotor & AntiAnti-Torque Vanes
Evolution of the Giant MAV
1st Gen. 2002 1st • • • •
Generation
27 cm diameter 310 gm gross weight Al i Aluminum construction t ti Basic RC Components
G 3ndd Gen.
2ndd Gen.
4ndd Gen. G 2008
4th Generation G ti • • • • •
25 cm rotor diameter 230 gm gross weight Carbon fiber construction Refined spider-type swashplate On-board stability augmentation
MAV: Single Rotor with Anti Anti--Torque Vanes Flight Testing
Rotor Hover Test Inverted rotor
Hall effect sensor
Thrust load cell
Measurement of Hover Performance: •Thrust •Torque •Rotational speed p
FM CT CP
Figure of Merit Torque Sensor
FM = Hover test stand
Ideal Power required to hover A t lP Actual Power required i d to t hover h
Blade Airfoil Variations Baseline Twisted
Tip-Taper
Planform-Taper
Planform-Taper
Camber Distribution
Planform Distribution
Sharpened Leading-Edge Airfoils 7.0% camber with LE camber
0.55 05 0.5
• Sharp leading-edge increases FM • Smaller rise in FM for cambered airfoil
Flat plate with sharpened LE 15º
0.4
FM
0.3
7.0% camber sharpened LE
7.0% camber
Flat plate
0.2 0.1 0
0
0.04
0.1
0.14
0.2
CTT/σ
Sharpened LE can improve airfoil performance
Blade Planform variations 0.65 Blade 1 1
Blade 2
σ e = 0.115 2 :1Taper @80%R
0.6
2
σ e = 0.094 2 :1Taper 1T @60%R
3
σ e = 0.137 Rectangular
4
σ e = 0.11
Figure of Me erit
Blade 4
0 55 0.55
0.5
Blade 3
0.45
0.4 0.1
0.12
0.14
0.16
0.18
0.2
0.22
CT/σ
Taper along with sharpened LE provides best rotor performance
0.24
Flow Visualization Strong tip vortices High induced velocities in tip region g Vortical shed wake obstruction increases DL and lowers FM
7% camber, 2.75% thickness with sharpened LE D=6” 2-bladed rotor, D=6 rotor 3600 RPM RPM, Re=36800 Rotor Plane
Main Vortex
Main Vortex VortexS heet
Wake Obstruction
Rotating-Wing MAV Performance Profile Eff t Effects
Induced Effects Better designs may come through careful aerodynamic optimization •Gains may not come through improvements in airfoils alone •Performance goals met through understanding of flow physics •Induced and profile effects have strongly interdependent effects •
Shrouded Sh d d Rotor R t S t System
Rotor Hover Efficiency Figure of Merit M: Hover Efficiency is defined in terms thrust production per unit input power For present designs: M is less than 0.6 Goal:Increase M over 0.8 Improvement of hover efficiency using duct around the rotor (plus safety protection of rotor)
Shrouded-Rotor Concept Key Design Parameters • Expansion ratio/Diffuser angle – Want this to be as large as possible for best performance
• Inlet lip p radius – Incoming flow forms a suction peak on the inlet lip; cause of thrust augmentation
• Blade tip clearance – Proximity of shroud wall reduces strength of blade tip vortices; reduces blade tip losses
Experiment: Model Configurations Electric motor
Test stand
Rotor by itself
Rotor with shroud attached
Rotor inside shroud, not connected to shroud h d
Experiment: Thrust Ratio vs. Total Power Thrust Coefficient, CT
Power I Increase lip li radius: di Increase I thrust th t Decrease tip clearance: Increase thrust
Thrust Ratio, Ttotal / Tfree
Power Diffuser angle: Thrust increases with small angle Decrease in net thrust
Pressure Distribution – Edgewise Flow • Pressure distributions asymmetric • Increased suction on windward side, decreased suction on leeward side => Nose-up pitch moment • Over-pressures on lateral surfaces facing into flow g => Drag • Net increase in suction in axial direction => Increase in thrust
Shrouded vs. Open: Thrust, Power •
•
Shrouded rotor thrust ~50% greater in hover &edgewise fl flow, lower l in i axial i l flow fl Shrouded rotor power 20—50% lower
Shrouded vs. Open: Pitching Moment • Pitching moment up greater in to 10 times g edgewise flow
Major Issues: Weight, Resonance with shroud frequencies, Pitch moment control
Shrouded Rotor MAV concept Rotor Shroud
Vanes
Shroud made lightweight and stiff hingeless rotor (1) reduce rotor plane movement (2) enhanced maneuverability
Shrouded Rotor MAV integration
+
Center Body
Battery and Sensor attachment area
MAV Performance improvements 25
Mechanical Pow wer (W)
20
No shroud 15
Shroud
10
5
0
0
50
100
150
200
250
Thrust (g)
Hover efficiency and payload capability increased
300
Shrouded rotor MAV attitude control HingelessRotor
Hiller bar Optimum p phasing
Hingeless Rotor
Reduced rotor plane movements. Keeps blade tip clearance variations to a minimum Enhanced control power that can be utilized during gust response
Hiller Bar Blade pitching axis
Swashplate input – Hiller bar flaps – main rotor pitches
Teetering rod with aerodynamic paddles Transfers cyclic to rotor and provides gyroscopic i stability. t bilit Eliminates pitch/roll cross coupling with suitable phasing
Rotary Wing Micro Air Vehicles: Unconventional Configurations
Cyclocopter A set of blades rotate around an axis of rotation parallel to blades; pitch varies periodically p p y once per p revolution to produce p thrust in desired direction
Advantages: • High maneuverability: Instantaneous change of thrust vector • All airfoils operate at maximum efficiency Blades
Major Concerns: • Complex wake interaction • Complexity C l it off pitch it h change mechanism
Rotation
Axis of rotation Direction of flight
Pivot point
Cyclocopter MAV Design • 2 contra-rotating 3-bladed rotors • Blade span & diameter = 6”; Chord = 1” • Passive blade pitching; Blade pitching amplitude = 40° • Required Thrust = 150 gm/rotor; Design RPM = 2000 • Operating chord Reynolds number = 22000 • Power Loading (Thrust/Power) = 6 grams/watt (10 lbs/hp) at take off thrust
Appears to be a viable concept for low Reynolds number operation
Component
Weight (g)
% Total
Electronics
22
7.7
Cyclo – MAV
Li-Po Battery
40
13.9
Weight
Motors
39
13.6
Servos
12.5
4.4
Rotors
155
54
Structure
18.5
6.4
Total
287
100
Breakdown
Parametric Studies • • • •
40 40° 45°
Res sultant Thrust ((grams)
150
35° 30° 25°
RPM
Design Point
Ae erodynamic Pow wer Loading (grams/w watt)
Key Results At 2000 RPM and 40° pitching amplitude the rotor with NACA 0010 blades produced thrust required to hover produced better Reverse NACA blades p power loading (thrust /power) that normal NACA at lower pitching amplitudes Blades did not stall even at 45° pitching amplitude Bl d flexibility Blade fl ibilit deteriorated d t i t d the th rotor t performance significantly
Reverse NACA at 25° Pitch
RPM
Digital Particle Image Velocimetry (DPIV) Results Tip vortices from either side of the cyclocopter blade, wake age = 30° to 75°
DPIV measurements captured the two tip vortices which convectdownwards
Wake age = 30°
Wake age = 45°
Wake age = 60°
Wake age = 75°
Digital Particle Image Velocimetry (DPIV) Results Skewed Wake Structure • • • •
Key Results DPIV measurements captured the wake contraction Extremely high induced velocities were measured in the wake Skewed wake structure was observed Showed the presence of a leading edge vortex Wake Contraction
Cyclocopter-blade
Leading Edge Vortex
Cyclocopter MAV Future Work • Development of a flight capable cyclocopterMAV – Develop control schemes for blade pitch control, thrust vector t control t l – Systems integration
• Perform flight tests to understand the flight characteristics
Flapping Rotor System Goal is to improve micro rotor performance by flapping blades at a high frequency (say 4/rev) with a small amplitude
Flapping Rotor 6/rev Flapping No Flapping
No Flapping
6/rev Flapping
6/rev Flapping
At a collective of 30 deg, rotor RPM 660, 6/rev excitation -Thrust increase by 15% -Torque reduction of 30% - Figure of merit increase by 30% Challenge: An efficient way to apply small amplitude high frequency flapping motion
No Flapping
Rotor-Based MAV: Challenges Near-Term Research (Years 1-3) Understand low Reynolds aerodynamics (wakes and flow separation) Improve figure of merit (over 0 0.7) 7) FEM/CFD comprehensive analysis and validation
Mid-Term Research (Years 3-5) Develop swashplateless concept (low friction flextures) Identify efficient concepts Smart actuation to increase figure of merit (about 0 0.8) 8)
Far-Term Research (Years 6 and beyond) Multifunctional structure with distributed actuators and sensors Optimize a revolutionary concept (minimize weight, increase performance)
Hovering MAV Goals Item
Current Level Expected Level
Endurance
100 g
20 g
Range
50 m
10 km
Control
Vertical
All DOF
Primary
Swashplate
Swashplateless
Power
Battery
Micro-engine
Autonomous
None
Vision-based
MAV Challenges and Technical Barriers • Aerodynamics y at low Reynolds y numbers • Unsteady aerodynamics and wakes modeling • Flow separation control (passive or active, synthetic jets, blowing, etc)
• Large strain comprehensive analyses • Wings/blades morphing • Multifunctional wings/blades
• Stability and Navigation • • • •
Flight stability in gusty environment Autonomous flight (out-of-sight) I Insect-inspired ti i d navigational i ti l strategies t t i Collision avoidance including swarms
• System Integration • Revolutionary concepts • Electronics & computer miniaturization • Flight testing and validation
Gross We G eight (kg)
• Structures and Materials (ultralight)
Micro Air Vehicles ((Less than 6”))
F-18
Sender
10-4
• High performance batteries and fuel cells • Fundamental issues related to micro engines • Evaluation of existing mini IC engines
10 0-3 10-2 10-1 1 1101 102 103 104 105 106
• Efficient power/propulsion at small scale
103
104
105
106
Reynolds Number
107
108
Flight Inspired by Nature at Low Re Propulsion and Power - efficient batteries - micro engines - thermal efficiency - energy gy Storage g
Sensing and Navigation - miniature electronics - Insect based guidance
Lightweight Wing Structures - active shape deformation - wing morphing
Low Reynolds Number - delayed Stall - flow separation control - wake capture Biomimetic Kinematics - actuation (thorax) - efficiency - frictionless
Maneuvering Capability - distributed control surfaces - gust prone
Acknowledgements Graduate Students Mat Tarascio Beerinder Singht Jason Pereira Ben Hein Eric Parsons Beverly Beasley Jaye Falls Nitin Gupta Peter Copp Brandon Fitchett Moble Benedict A. Abhishek
Sponsor ARO (Gary Anderson/Tom Doligalski)
Faculty Colleagues Norm Wereley (Aero) Darryll Pines (Aero) Jayant Sirohi (Aero) Paul Samuel (Aero) Anubhav Datta (Aero) Gordon Leishman (Aero) Roberto Celi (Aero) Jim Baeder (Aero) Ben Shapiro (Aero) Ell Etki Ella Etkins (Aero) (A ) Chirs Cadou (Aero) Marat Tishchenko (Aero) Bala Balachandran (Mech) Elisbeth Smela (Mech) Satinder Gupta (Mech) Rama Chellappa (EE) Reza Ghodesi ((EE)) Shivjumar (NCA&T) Srini Srinivasan (ANU)
