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Denegri 23 provides a detailed description of the linear flutter analysis procedure for the F-
16 to be included herein. The doublet-lattice method aerodynamic model is composed of 13
panels, including the fuselage, inner wings, flaperons, outer wings, wingtip launchers, horizontal
tails, vertical tail, and rudder, that are subdivided into 616 discrete boxes. The underwing stores
are not modeled aerodynamically, and the only influence considered is their effect on the
structural mode shapes and frequencies. The wing panel is shown in Figure 2-2, and the
NASTRAN structural model is shown in Figure 2-3. The aircraft structure is derived from a
finite element model and represented by a lumped mass model. It is composed of 8532 degrees
of freedom representing the wing, fuselage, empennage, underwing stores, pylons, and
launchers. By considering only the modal deflections that influence the aerodynamic model, the
system reduces to 252 structural points.
The free-vibration analyses are conducted using the Lanczos method of eigenvalue
extraction. Aerodynamic influence coefficients are computed for a range of reduced frequencies,
Mach numbers, and sir densities. The aerodynamic panels are splined to the vibration modes
using the method of Harder and Desmarais. 25 The flutter equations are solved using the Laguerre
iteration method, 26 which is a variation of the classical k-method of flutter determinant solution.
All flexible modes up to 25 Hz, including all fundamental wing modes and several store modes,
are retained for the initial flutter analyses. Both symmetric and antisymmetric modes are
included in the analyses since a full-span aircraft flutter model is used. A modal deletion study is
then performed to isolate the primary modes in the predicted instability mechanism.
When interpreting the flutter analysis results, a critical point is considered to be the
velocity at which a modal stability curve crosses from stable, which requires negative structural
damping to produce neutral stability, to unstable, with positive damping. The analytical flutter
26