F lexible-wing MAVs F lexible-wing MAVs Dr. Peter Ifju, Bret Stanford Mechanical and Aerospace Engineering University of Florida
Special Thanks Special Thanks Students: James Davis Yongsheng Lian Bret Stanford Thomas Rambo Roberto Albertani Albert Lin Sponsors: Kyu-Ho Lee Brandon Evers Sewoong Jung Air Force Office of Scientific Research UF Faculty: Scott Ettinger AFRL at Eglin Air Force Base Mujahid Abdulrahim US Special Operations Command Don McArthur Rick Lind NASA Langley Research Center Warren Dixon Dan Claxton US Geological Survey Frank Boria Paul Hubner US Dept of Fisheries and Wildlife Wei Shyy Mike Sytsma Rafi Haftka Jos Coquyt Dragos Viieru David Jenkins Andy Kurdila Baron Johnson Mike Morton Carl Crane Warren Dixon James Clifton Franklin Percival Scott Bowman Mike Nechyba
Design Concept: Flexible, Thin, Design Concept: Flexible, Thin, Undercambered Wing Wing Undercambered Undercambered wing provides better aerodynamic characteristics at Reynolds No. below 100,000. Flexibility can be tuned for smoother flight in gusty wind conditions “adaptive washout”. We have built wings with improved longitudinal stability. Delayed/gentle stall has been documented Flexible wing can be morphed efficiently. Flexible wings can be folded for storage and deployed without assembly. Wing configuration can be engineered to be lightweight as well as durable
Benefits of the UF Designs Benefits of the UF Designs Durability Durability Gust Alleviation Gust Alleviation Storage Storage Morphing Stability, high lift Morphing Stability, high lift
Outline: Outline: • Introduction • Fabrication methodologies • Flight testing • Experimental program • In-situ deformation measurements • Structural model • Fluid structure interaction models • Model validation via deformation measurements • Optimization • Conclusions and future work
Custom MAV Design Software Custom MAV Design Software MAVLab: rapid wing generation • Span • Chord • Twist • Sweep • Airfoil geometry • Virtually any planform shape
CAD Model, Tool Path and Milling CAD Model, Tool Path and Milling
Finished Tooling and Finished Tooling and Composite Construction Composite Construction Prepreg unidirectional, woven carbon fiber Finished tool with layout pattern and Kevlar composite construction
Composite Construction Continued Composite Construction Continued Vacuum bagging Fuselage layup Component installation Assembly
Finished MAV in Less Than One Day Finished MAV in Less Than One Day • Latex rubber membrane material is applied • Fins are attached • Off to be flight tested
International Micro Air Vehicle International Micro Air Vehicle Surveillance Competition History Surveillance Competition History UF 4.5 in. (11.4 cm) record MLB 50 Maximum Dimension, cm MLB UF UF 25 UF BYU 15 UF KKU UF Smallest MAV to UF identify target at 600m 10 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Year
30 cm US European MAV Competition 30 cm US European MAV Competition
Three Wings were then Studied Three Wings were then Studied [ ± 45 3 ] [ ± 45 2 ] [0 2 ] Latex Skin Rigid Batten-Reinforced BR Perimeter-Reinforced PR • Composite wings constructed from carbon fiber composites, and latex rubber skin • All three wings have the same nominal shape: – AR = 1.25, root chord = 130 mm, wing span = 150 mm • Rigid wing: nominal aerodynamics • Batten-reinforced wing: adaptive washout • Perimeter-reinforced: adaptive inflation
Coefficient of Lift vs. Angle of Attack Coefficient of Lift vs. Angle of Attack • The low aspect ratio accounts for high stall angles • After stall, the lift of the perimeter reinforced wing is greater than The other wings before stall • The perimeter reinforced wing has higher C Lmax
Moment Coefficient vs vs Coefficient of Lift Coefficient of Lift Moment Coefficient • The perimeter reinforced wing has a higher negative slope • The rigid wing has the lowest • Static longitudinal stability of the perimeter reinforced wing is substantially higher than the rigid case with the batten reinforced wing intermediate
Wing Deformation Measurements Using Wing Deformation Measurements Using Visual Image Correlation Visual Image Correlation • The stereo-triangulation is achieved through twin synchronized cameras (35 mm lens, 1.3 mega pixels, 5-10 ms exposure times) each looking at a different angle • After a random speckling pattern is applied to the surface of the 3-D geometry in question, the VIC system digitally acquires the pattern, and tracks the deformation of each speckle Synchronized cameras Wind tunnel 250 Watt lamp Model
Wind Tunnel VIC Tests Procedure Wind Tunnel VIC Tests Procedure
VIC Results: BR Wing Out- -Of Of- -Plane Plane VIC Results: BR Wing Out Displacements Displacements Deformation patterns here imply that the wind load subjects the leading edge to torsion Primary region of deformation: battens are forced to bend upwards due to wind loading Wing fixed here: Non-zero displacement implies a small rigid body rotation of entire model 12 ° AOA, Wind Speed = 13 m/s
VIC Results: PR Wing Out- -Of Of- -Plane Plane VIC Results: PR Wing Out Displacements Displacements The carbon fiber perimeter exhibits substantial bending The primary region of deformation occurs as the membrane billows upwards due to the aerodynamic forces Wing fixed here: Non-zero displacement implies a small rigid body rotation of entire 12 ° AOA, Wind Speed = 13 m/s model
MAV Structural Modeling MAV Structural Modeling • Accurate finite element wing modeling can provide insight into the complicated fluid-structure interaction over a flexible MAV • In keeping with the composite nature of the wing, three different elements are used: shells to model the carbon fiber weave (red), beams to model the battens (green), and membranes to model the latex skin (blue)
Static MAV Model Validation Static MAV Model Validation • Visual image correlation is an ideal tool for finite element validation • Static model validation was conducted by hanging small weights from the wing, and comparing numerical and experimental displacement fields Experimental (VIC) Numerical (FEA) Out-of-plane displacements caused by a 7 g load at the tip of the outer left batten (MAV clamped at trailing edge)
Fluid Structure Interaction Model Fluid Structure Interaction Model High fidelity finite element analysis (FEA) structural model Define rigid wing geometry With nonlinear membrane properties Conduct CFD on rigid wing Apply aero loads from CFD to FEA Navier Stokes based computational fluid dynamics Deformed shape analyzed by CFD (CFD) model with master/slave perturbation techniques for remeshing Apply new aero loads to FEA Stop when wing geometry converges
Fluid Structure Interaction Model Fluid Structure Interaction Model Convergence Convergence
Comparing BR Model and Experiment Comparing BR Model and Experiment Out-of-plane displacement Chord-wise strain
Comparing BR Model and Experiment Comparing BR Model and Experiment Span-wise strain Shear strain
Comparing PR Model and Experiment Comparing PR Model and Experiment Out-of-plane displacement Chord-wise strain
Comparing PR Model and Experiment Comparing PR Model and Experiment Span-wise strain Shear strain
Pressure, Streamlines and Deformation Pressure, Streamlines and Deformation Rigid Batten Perimeter 0AOA, top Rigid Batten Perimeter 0AOA, bottom
Pressure, Streamlines and Deformation Pressure, Streamlines and Deformation Rigid Batten Perimeter 15AOA, top Rigid Batten Perimeter 15AOA, bottom
Comparing BR Model and Experiment Comparing BR Model and Experiment
Pressure, Streamlines and Deformation Pressure, Streamlines and Deformation
PR Membrane Pretension vs. Deformation PR Membrane Pretension vs. Deformation
BR Membrane Pretension vs. Deformation BR Membrane Pretension vs. Deformation
PR Pretension vs. Performance PR Pretension vs. Performance
BR Membrane Pretension vs. Deformation BR Membrane Pretension vs. Deformation
Conclusions and Future Work Conclusions and Future Work • The design space can be greatly increased by employing flexibility • Flight tests and wind tunnel tests have shown appreciable gains in some flight parameters with both the batten reinforced and perimeter reinforced membrane wing • Advanced structural deformation measurement techniques provide high fidelity information that can give insight into the mechanisms that lead to enhanced flight performance • Fluid structure interaction models can give insight into how to improve specific flight characteristics • However no flexible wing design is the best at everything • Topological optimization is currently being used for determining better ways to reinforce the wing for specific objective functions • Future work to validate the fluid structure interaction model by experimentally characterizing the flow field is desired.
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