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MAE 589C - AIRWORTHINESS

IN AIRCRAFT DESIGN ANDCERTIFICATION

Class #4Dr. Stephen P. Cookspcook@ncsu.edu

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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