MODIFIED STRIP ANALYSIS METHOD FOR PREDICTING WING FLUTTER AT SUBSONIC TO
HYPERSONIC SPEEDS

( NASA Langley Research Center )

     A modified strip analysis has been developed for rapidly predicting
flutter of finite-span, swept or unswept wings at subsonic to hypersonic
speeds. The method employs distributions of aerodynamic parameters which
may be evaluated from any suitable linear or nonlinear steady-flow theory
or from measured steady-flow load distributions for the underformed wing.
The method has been shown to give good flutter results for a broad range of
wings at Mach number from 0 to as high as 15.3.

     The principles of the modified strip analysis may be summarized as fol-
lows: Variable section lift-curve slope and aerodynamic center are substi-
tuted respectively, for the two-dimensional incompressible-flow values of
2 pi and quarter chord which were employed by Barmby, Cunningham, and Gar-
rick. Spanwise distributions of these steady-flow section aerodynamic pa-
rameters, which are pertinent to the desired planform and Mach number, are
used. Appropriate values of Mach number-dependent circulation functions
are obtained from two-dimensional unsteady compressible-flow theory.

     Use of the modified strip analysis avoids the necessity of reevalu-
ating a number of loading parameters for each value of reduced frequency,
since only the modified circulation functions, and of course the reduced
frequency itself, vary with frequency. It is therefore practical to in-
clude in the digital computing program a very brief logical subroutine,
which automatically selects reduced-frequency values that converge on a
flutter solution. The problem of guessing suitable reduced-frequency val-
ues is thus eliminated, so that a large number of flutter points can be com-
pletely determined in a single brief run on the computing machine. If nec-
essary, it is also practical to perform the calculations manually.

     Flutter characteristics have been calculated by the modified strip
analysis and compared with results of other calculations and with experi-
ments for Mach numbers up to 15.3 and for wings with sweep angles from 0 de-
grees to 52.5 degrees, aspect ratios from 2.0 to 7.4, taper ratios from 0.2
to 1.0, and center-of-gravity positions between 34% chord and 59% chord.
These ranges probably cover the great majority of wings that are of practi-
cal interest with the exception of very low-aspect-ratio surfaces such as
delta wings and missile fins.

     This program has been implemented on the IBM 7094.

     COSMIC, and the COSMIC logo are registered trademarks of the National
Aeronautics and Space Administration. All other brands and product names
are the trademarks of their respective holders.



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