mae589c class 4
TRANSCRIPT
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MAE 589C - AIRWORTHINESS
IN AIRCRAFT DESIGN ANDCERTIFICATION
Class #4Dr. Stephen P. [email protected]
Office: Broughton 3197Office Phone: 919-515-5260
Mobile: 240-538-6226
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Admin Next 3 Weeks
Tue, 02 Sep: MIL-HDBK-516 Chapter 5
No office hours today
Thu, 04 Sep: MIL-HDBK-516 Chapter 5/6
Tue, 09 Sep: NO CLASS Thu, 11 Sep: NO CLASS
Tue, 16 Sep: MIL-HDBK-516 Chapter 6
Thu, 18 Sep: HW #1 Discussion
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Structural Dynamics
Phenomena
Criteria/Standards
Verification Methods
Acknowledgements Dr. Thomas Strganac, Texas A&M
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Structural DynamicsCan Ruin Your Day
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Some Aeroelastic Phenomena Divergence permanent deformation due to
aerodynamic forces exceeding the elasticrestoring capability of the structure Control Surface Reversal Loss or opposite
response of a control surface due to elasticity of
structure Flutter oscillatory instability where one mode of
motion is driven to resonance by a secondmode.
Buzz/Buffet high frequency instabilities causeby unsteady separations, shocks, etc.
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Divergence Equations
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Flutter Equations
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1st Bending Mode 1st Torsional Mode
2nd Bending Mode 2nd Torsional Mode
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Structural Dynamics Criteria:Aeroelasticity
Criterion:
Verify that the air vehicle in all configurations(including stores) is free of flutter, divergence,and other aeroelastic instabilities
Standard: Margin of safety of 1.15 Ve on the limit speed
at both constant Mach number and constant
altitude Total damping, g, is not less than 0.03 for all
altitudes and flight speeds
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Criterion:
Verify that the aircraft is free of aeroelasticand aeroservoelastic phenomena due tosystems interactions
Standard:
Gain margin of 6dB and 60 deg of phasemargin for control system feedback loops
Structural Dynamics Criteria:Aeroservoelasticity
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Criterion:
Verify that the control surfaces and tabs contain eithersufficient static and dynamic mass balance, orsufficient bending, torsional and rotational rigidity, or acombination of these means, to prevent flutter, or
sustained limited amplitude instabilities for all flightconditions . . .
Standard:
Freeplay of surfaces and tabs less than 1 deg(becomes more stringent as control surface locationmoves outboard)
Structural Dynamics Criteria:Freeplay
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Criterion: Verify that the airframe structure withstands the aeroacoustic
loads and vibrations induced by aeroacoustic loads for the airvehicles specified service life and usage without cracking orfunctional impairment.
Standard:
All aeroacoustic loads sources associated with the air vehicleand its usage are identified.
Airframe designed with uncertainty factor of +3.5dB applied onthe predicted aeroacoustic sound pressure levels.
The airframe is designed for fatigue life such that a factor of 2.0is applied on the exposure time derived from the air vehiclespecified service life and usage.
Structural Dynamics Criteria:Aeroacoustics
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Criterion:
Verify that the structures, equipment, andequipment provisions in, adjacent to, orimmediately downstream of cavities open to
the airstream during flight are designed for theeffects of oscillatory air forces
Standard:
Aircraft must be surveyed for oscillatory flows;flow control devices may be added
Structural Dynamics Criteria:Cavity Flows
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Verification Methods Aeroelastic models
Consist of a model of the structure with a model ofunsteady aero loads
Solver that predicts frequency and damping behaviorfor different flight conditions and aircraftconfigurations
Large number of models available for both structuraldynamics and unsteady aerodynamics
Linear Computational
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Verification Methods Wind Tunnel
NASA Transonic Dynamics Tunnel is premierfacility
Wind tunnel models inertially scaled
Can use heavy gas to better match dynamicpressure
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Verification Methods:
Ground Vibration Testing
Aircraft structure is excited at various
frequencies and response recorded Resulting transfer function compared to
aeroelastic models
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Verification Methods:
Flutter Flight Testing
Very expensive
Configurations
Constant Mach flights
Constant Altitude flights
Flutter Excitation Control Unit
F/A-18E/F Clean Configuration:
132 flights 236 flight hours
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