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National Advisory Committee for Aeronautics, Research Memorandum - Flutter Investigation of Two Thin, Low Aspect Ratio Swept, Solid, Metal Wings in the Transonic Range by Use of Free Falling Body

Two thin, solid, metal wings were tested for flutter by means of
the free— falling body technique to a Mach number of 1.23. The wings
were untapered, swept M50, and had a semispan aspect ratio of 1.2
This test was performed to explore the. possibility of a bending t
of flutter which was indicated by a simplified flutter analysis based
on two—dimensional, unsteady, compresSible—flow theory. Although the
calculations indicated the possibility of flutter, none was obtained.

For the Mach number range under _consideration the calculated flutter—
speed coefficients showed a marked decrease in value with increase in
Mach number and around Mach number 1.0 were so conservative that they
may be considered pessimistic. It appears that the simplified analysis
is too conservative for the type of wing configuration tested and that
a less approximate analysis, possibly including finite—span effects, is
needed to reduce the margin between theory and experiment.

Recent trends in the design of interceptor type of aircraft have
led to studies of the flutter characteristics of thin, low-aspect—ratio,
swept wings at high altitudes. Because of .the lack of theory_ for finite
span and for the transonic range, calculations for these studies have
been based on two-dimensional compressible- flow considerations. This
simplified analysis indicates that such wings will develop a bending
type of flutter in the transonic speed range, particularly at high
altitudes. As a preliminary experimental investigation of this type
of flutter, two untapered‘h50 sweptback wings-of low aspect ratio have
been tested by the free-fall technique. In order to provide data for
a range of mass-density ratios, one of the wings was made of magnesium
with a section thickness ratio of h percent_ and the other was made of
steel with a section thickness ratio of 3 percent.

The present paper gives the results obtained from this experiment
and a comparison of the experimental results with a series of calcu-
lations based on two- -dimensional, unsteady, compressible- flow theory.
The normal flow Mach numbers for which calculations were made ranged
from O to 1.0.