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MAE 3241: AERODYNAMICS AND FLIGHT MECHANICS
Examples of Dihedral, Anhedral, Adverse Yaw and Leading Edge Extensions April 15, 2011 Mechanical and Aerospace Engineering Department Florida Institute of Technology D. R. Kirk
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DIHEDRAL Dihedral, d, is upward angle from horizontal in a fixed-wing aircraft from root to tip, as viewed from directly in front of aircraft Purpose of positive dihedral is to confer stability in roll axis d
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ZERO DIHEDRAL, STEADY LEVEL FLIGHT
W
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ZERO DIHEDRAL ROLL L Fy Vside W
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POSITIVE DIHEDRAL, STEADY LEVEL FLIGHT
W
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POSITIVE DIHEDRAL ROLL
Wing is at positive angle of attack relative to incoming wind L Fy Vside W
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SUMMARY
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SUMMARY
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ANHEDRAL Anhedral required to keep aircraft from becoming too stable
Significant anhedral often seen on aircraft with a high mounted wing High mounted wing is above center of mass which confers roll stability due to pendulum effect, so additional dihedral is not required
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DIHEDRAL ( BOEING 737) and ANHEDRAL (BAE 146)
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C-5 GALAXY EXAMPLE
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POLYHEDRAL McDonnell Douglas F-4 Phantom II is one such example, unique among fighters for having dihedral wingtips Added after prototype flight testing (original prototype of F-4 had a flat wing) showed need to correct some unanticipated roll instability - angling the wingtips, which were already designed to fold up for carrier operations, was more practical solution than re-engineering entire wing
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EXAMPLE: ADVERSE YAW
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EXAMPLE: F-18 VORTEX FENCES
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WHY LEADING EDGE EXTENSIONS?
Idea is generate vortex that interacts with rest of wing Vortex speeds up flow of air over a wing and allow a plane to reach a higher angle of attack than it would be able to otherwise Also causes drag
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CFD EXAMPLE OF LEADING EDGE EXTENSIONS
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CFD EXAMPLE OF LEADING EDGE EXTENSIONS
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VORTEX FROM LEADING EDGE EXTENSIONS
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VORTEX FROM LEADING EDGE EXTENSIONS
Vortex Burst
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SUMMARY OF FIX Solution was to implemented LEX fence
Fence is essentially another vortex generator itself Fence creates a second unsteady vortex that interacts with main vortex created by leading edge extension Interaction strengthens rotation of main vortex so that vortex bursting is eliminated near vertical tail Repair was so effective that it not only solved tail buffet problem but fence also allows F-18 to reach even higher angles of attack
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ADDITIONAL FIX
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