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The eigenvalue solutions <strong>of</strong> this equation yield the modal frequencies,ω , and modal deflection<br />

amplitudes, { q } . The free vibration mode shapes can be used to diagonalize the left-hand side <strong>of</strong><br />

Equation 2-4:<br />

[ { }{ } ... ] = {} φ<br />

q (2-6)<br />

1 2 q<br />

{} q []{} φ r<br />

= (2-7)<br />

This results in transf<strong>or</strong>ming the f<strong>or</strong>ced vibration equation into modal co<strong>or</strong>dinates:<br />

2<br />

1 2<br />

[ ω M + iωC<br />

+ K]{}<br />

r = ρV<br />

[ Q]{}<br />

r<br />

− (2-8)<br />

2<br />

The classical flutter equation is obtained by converting the damping term into “structural”<br />

damping:<br />

⎡ 2<br />

1 2 ⎤<br />

⎢<br />

− ω M + ( 1+<br />

ig)<br />

K − ρV<br />

Q {} = 0<br />

⎣<br />

2 ⎥<br />

r<br />

(2-9)<br />

⎦<br />

The Strouhal number defines the reduced frequency as:<br />

b<br />

k<br />

V<br />

ω<br />

= (2-10)<br />

Rearranging the classical flutter equation from Equation 2-9 into eigenvalue f<strong>or</strong>m gives:<br />

⎡ 2<br />

⎛ 1 1 b 1 −1<br />

⎞ +<br />

⎢ ⎜ + ρ M Q(<br />

k)<br />

−<br />

2<br />

2 2<br />

2<br />

⎢⎣<br />

2<br />

⎟<br />

⎝ ωi<br />

k ωi<br />

⎠ ω<br />

( 1 ig)<br />

⎤<br />

[]{} I r = 0<br />

Solving this equation yields the frequency-damping-velocity relationships:<br />

( 1+<br />

ig )<br />

⎥<br />

⎥⎦<br />

(2-11)<br />

λ f = λreal<br />

+ λimag<br />

=<br />

(2-12)<br />

2<br />

ω<br />

b<br />

V = ,<br />

k λ<br />

real<br />

λimag<br />

g = ,<br />

λ<br />

real<br />

1<br />

ω = (2-13)<br />

λ<br />

real<br />

22

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