Uncertainties in AircraftFlutter Prediction

School of Mechanical and Aerospace EngineeringOklahoma State University

Dynamics and Vibrations II

Charles R. O'NeillAndrew S. Arena Jr.

Presented by

Oklahoma City, OK28 February 2004

Aeroelastic Prediction Comparisons

Experimental Computational

Two Modern Methods

Real-time simulationImplementation IssuesPossible Destruction

Model Clarity / Run TimeVerification and Validation

Physical system characteristics cause uncertainties that affect both computational and experimental

aeroelastic predictions.

The objective of the BACTexperiments was to providebenchmark data for CFDvalidation. Unfortunately,the BACT has large flutter uncertainties.

Benchmark Active Control Technology

Stephans found that small CG shifts caused large shiftsin the flutter boundary. +1% CG -> 23% qf

Aerostructures Test Wing (ATW)NASA Dryden F-15 Testbed

Computational and Experimental Boundsdid not match......

The wing flutteredunexpectedly andwas lost.

AGARD 445.6The classic flutter testcase.

NASA Langley 1962

Swept tapered wing

Tested Mach # Range 0.499 -> 1.141

Popular validation andcalibration testcase forCFD applications.

Trivia: The Weakened Model #3 was created bydrilling regular holes in the basic mahoganywing to increase the wind-tunnel's testing range!

Computational Model: AGARD 445.6 Aeroelastic Wing

Two Mode Structural Model

First Bending Mode9.6 Hertz

First Torsional Mode38.2 Hertz

Root

Tip

V

The AGARD 445.6 testcase presentsan aeroelastic system typical of highperformance aircraft.

Structural Uncertainties

Damping Ratio

Stiffness

4%2%0%

+10%Nominal

-10%

0.4 0.6 0.8 1 1.2

0.5

1

1.5

2

0.4 0.6 0.8 1 1.2

0.5

1

1.5

2

Mach Number

Dyn

am

ic P

ress

ure

[p

si]

Transonic Pressure Shift

0 0.2 0.4 0.6 0.8 1

-0.5

-0.25

0

0.25

0.5

Cp

0.499

0.678 0.900

Mach Number

0 0.2 0.4 0.6 0.8 1

-0.5

-0.25

0

0.25

0.5

Cp

0.9601.072

1.141

Mach Number

0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2Mach

Center of Pressure 29% 40%

Below Critical Mach Number Above Critical Mach Number

Aeroelastic Structural DampingMach:

0.499

0.678

0.900

0.960

1.072

1.141

0 0.5 1 1.5 2

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

Dynamic P ressure [ps i]

Str

uct

ura

l Da

mp

ing

Ra

tio,

ζ

Non Observable -Probable Destruction

Observable Response

Aeroelastic Damping Bounds

0.4 0.6 0.8 1 1.2

0.5

1

1.5

2

Mach Number

Dyn

am

ic P

ress

ure

[p

si]

0.4 0.6 0.8 1 1.2

0.5

1B

ou

nd

ary

Fir

mn

ess

∆q =dq∗

dζ∆ζ

The expected error dueto a fuzzy stability boundary.

dζ

dq∗=dζ

dq· qf

The firmness of the stabilityboundary. Nondimensionalizethe damping ratio exit slope bythe flutter dynamic pressure.

∆ζ = 0.01

Perpendicular Crossing

Parallel Crossing

AGARD Aeroelastic UncertaintyAdd the uncertainties to estimate the actual uncertainty bound.

0.4 0.6 0.8 1 1.20

0.5

1

1.5

2

Mach Number

Dyn

am

ic P

ress

ure

[p

si]

48% 45%63%

109%

80%

60%

Percent Error

Parameters: ±10% Stiffness ±2% Damping Ratio ±1% Boundary Damping

AGARD Aeroelastic Uncertaintyfrom Published Literature

0.4 0.6 0.8 1 1.20

0.5

1

1.5

2

Mach Number

Dyn

am

ic P

ress

ure

[p

si]

ConclusionsUncertainties affect all testcases.

Uncertainties can vary over an experimental range.

Coupling experimental and computational aeroelastic analysis provides for improved understanding of the dominant system physics.

Uncertainty analysis for published aeroelastic experiments is not prevalent.

Perfect comparisons between experimental and computational predictions might not be realistic.