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Integrated Computational and
Experimental Studies of
Flapping-wing Micro Air Vehicle
Aerodynamics
Kevin Knowles , Peter Wilkins, Salman Ansari, Rafal Zbikowski
Department of Aerospace, Power and Sensors
Cranfield University
Defence Academy of the UK
Shrivenham, England
3rd Int Symp on Integrating CFD and
Experiments in Aerodynamics,
Colorado Springs, 2007
Outline
• Introduction
• Flapping-Wing Problem
• Aerodynamic Model
• LEV stability
• Conclusions
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Micro Air Vehicles
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Defined as small flying vehicles with
 Size/Weight:
 Endurance:
150-230mm/50–100g
20–60min
Reasons for MAVs:
 Existing UAVs limited by large size
 Niche exists for MAVs – e.g. indoor flight,
low altitude, man-portable
Microgyro
MAV Essential (Desirable) Attributes:
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High efficiency
High manoeuvrability at low speeds
Vertical flight & hover capability
Sensor-carrying; autonomous
(Stealthy; durable)
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Microsensors
Why insect-like flapping?
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Insects are more manoeuvrable
Power requirement:
 Insect – 70 W/kg maximum
 Bird – 80 W/kg minimum
 Aeroplane – 150 W/kg
Speeds:
 Insects ~ 7mph
 Birds ~ 15mph
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Wing Kinematics – 1
• Flapping Motion
 sweeping
 heaving
 pitching
• Key Phases
 Translational
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
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downstroke
upstroke
Wing Kinematics – 1
• Flapping Motion
 sweeping
 heaving
 pitching
• Key Phases
 Translational
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
downstroke
upstroke
 Rotational
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
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stroke reversal
high angle of attack
Wing Kinematics – 2
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Mechanical Implementation
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Generic insect wing kinematics
Three important differences when compared
to conventional aircraft:
 wings stop and start during flight
 large wing-wake interactions
 high angle of attack (45° or more)
Complex kinematics:
 difficult to determine
 difficult to understand
 difficult to reproduce
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Key phenomena
 unsteady aerodynamics
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apparent mass
Wagner effect
returning wake
 leading-edge vortex
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[Photo: Prenel et al 1997]
Aerodynamics
Aerodynamic Modelling – 1
• Quasi-3D Model
• 2-D blade elements with
 attached flow
 separated flow
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leading-edge vortex
trailing-edge wake
+
• Convert to 3-D
 radial chords
centre of
rotation
Robofly wing
Knowles et al.
Aerodynamic Modelling – 1
• Quasi-3D Model
• 2-D blade elements with
 attached flow
 separated flow


^
~
leading-edge vortex
trailing-edge wake
~
^
• Convert to 3-D
 radial chords
 cylindrical cross-planes
 integrate along wing span
~
^
^
wing
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~
Aerodynamic Modelling – 2
• Model Summary
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6 DOF kinematics
circulation-based approach
inviscid model with viscosity introduced indirectly
numerical implementation by discrete vortex method
validated against experimental data
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Flow Visualisation Output
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Impulsively-started plate
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Validation of Model
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The leading-edge vortex (LEV)
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Insect wings operate at high angles of
attack (>45°), but no catastrophic stall
Instead, stable, lift-enhancing (~80%) LEV
created
Flapping wing MAVs (FMAVs) need to
retain stable LEV for efficiency
Why is the LEV stable? Is it due to a 3D
effect?
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2D flows at low Re
Re = 5
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Influence of Reynolds number
α = 45°
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2D flows
Re = 500, α = 45°
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Influence of Reynolds number
α = 45°
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Kelvin-Helmholtz instability
at Re > 1000
Re 500
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Re 5000
Secondary vortices
Re = 1000
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Re = 5000
2D LEV Stability
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For Re<25, vorticity is dissipated quickly and generated
slowly – the LEV cannot grow large enough to become
unstable
For Re>25, vorticity is generated quickly and dissipated
slowly – the LEV grows beyond a stable size
In order to stabilise the LEV, vorticity must be extracted
– spanwise flow is required for stability
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Structure of 3D LEV
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Stable 3D LEV
Re = 120
Re = 500
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Conclusions
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LEV is unstable for 2D flows except at very low Reynolds
numbers
Sweeping motion of 3D wing leads to conical LEV; leads
to spanwise flow which extracts vorticity from LEV core
and stabilises LEV.
3D LEV stable & lift-enhancing at high Reynolds numbers
(>10 000) despite occurrence of Kelvin-Helmholtz
instability.
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Questions?
Knowles et al.