Study of effects of sweep on the flutter of cantilever wings Page: 1 of 25
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STUDY OF EFFECTS OF SWEEP ON THE FLUTTER OF CANTILEVER WINGS'
By J. G. BARMBT, H. J. CUNNINGAM, a &nd I. E. GasRRIc
An experimental and analytical investigation of the flutter
of sweptback cantilever wings is reported. The experiments
.mployed groups of wings swept back by rotating and by
.*hearing. The angle of sweep ranged from 00 to 600 and
Mach numbers extended to approximately 0.85. A theoretical
analysis of the air forces on an oscillating swept wing of high
length-chord ratio is developed, and the approximations in-
herent in the assumptions are discussed. Comparison with
experiment indicates that the analysis developed in the present
report is satisfactory for giving the main effects of sweep, at
least for nearly uniform cantilever wings of high and moderate
length-chord ratios. A separation of the effects of finite span
and compressibility in their relation to sweep has not been made
eorperimentally but some combined effects are given. A dis-
cussion of some of the experimental and theoretical trends is
given with the aid of sereral tables and figures.
T'he present report is an outgrowth of the trend toward
the use of swept wings for high-speed flight and presents the
results of an analysis and of an accompanying exploratory
program of research in the Langley 4.5-foot flutter research
tunnel on swept cantilever wings. The material was
assembled in a memorandum form with a similar title in
1948. The chief purposes of the present report are to
provide a more detailed exposition of the analysis and to
make the main material more generally available.
Some previous experimental and analytical work on swept
wings is mentioned here. A preliminary experimental
investigation of the effect of sweep on flutter has been made
(reference 1) with a single, simple rigid wing mounted flexibly
on a base which could be rotated to various desired sweep
angles. This investigation was made at low 'Mach numbers
for two bending-torsion frequency ratios and at several
angles of sweepback. Another investigation (data un-
published) in which the density of the test medium was a
variable was conducted by D. Benun on the same type of
rigid, flexibly mounted wing at higher Mach numbers and at
sweep angles of 00 and 450. Other unpublished work on
swept ings exists, but a search of the available information
indicates a need for further systematic study.
The experimental work reported herein dealt with models
mounted as cantilevers at their roots. These cantilever
models differed from the rigid, flexibly mounted wings,
which had all bending and torsion flexibility concentrated
at the root, and thus were subject to different root effects.
In order to facilitate analysis the cantilever models were
uniform and untapered. The intent of the experimental
program was to establish trends and to indicate orders of
magnitude of the various effects of sweep on flutter rather
than to isolate precisely the separate effects.
The models were swept back in two basic manners-
shearing and rotating. For the case in which the wings
were swept back by shearing the cross sections parallel to
the air stream, the span and aspect ratio remained constant.
For the other case, a series of rectangular-plan-form wings
were mounted on a special base which could be rotated to
provide any desired angle of sweepback. This rotatory
base was also used to examine the critical speed of swept-
Tests were conducted also on special models that were of
the "rotated" type (sections normal to the leading edge
were the same at all sweep angles) with the difference that
the bases were alined parallel to the air stream. Two series
of such rotated models having different lengths were tested.
Inasmuch as the location of the center of gravity, the mass-
density ratio, and the Mach number have important effects
on the flutter characteristics of unswept wings, these param-
eters were varied for swept wings. In order to investigate
possible changes in flutter characteristics which might be
due to different flow over the tips, various tip shapes were
included in the experiments.
In an analysis of flutter, vibrational characteristics are
very significant; accordingly, vibration tests were made on
each model. A special study of the change in frequency
and mode shape with angle of sweep was made for a simple
aluminum-alloy beam and is reported in appendix A.
Theoretical analyses of the effect of sweep on flutter exist
only in brief or preliminary forms. In England in 1942,
W. J. Duncan estimated by certain dimensional consider-
ations the effect of sweep on the flutter speed of certain
specialized wing types. Among other British workers
whose names are mentioned in connection with problems
SSupersedes NACA TN 212I, "Study of Effect of Sweep on the Flutter of Cantilever Wings" by J. G. Barmbyr, H. . Cunningham, and L E. Garrck, 1950.
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Barmby, J. G.; Cunningham, H. J. & Garrick, I. E. Study of effects of sweep on the flutter of cantilever wings, report, September 9, 1948; (digital.library.unt.edu/ark:/67531/metadc60354/m1/1/: accessed October 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.