Tests of Aerodynamically Heated Multiweb Wing Structures in a Free Jet at Mach Number 2: Two Aluminum-Alloy Models of 20-Inch Chord With 0.064- and 0.081-Inch-Thick Skin Page: 12 of 40
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NACA M L55F3
occurred (at 226 cycles per second) had about 11 waves along the chord
with the maximum amplitudes at the trailing edge, a flag-waving action
involving chordwise bending of the airfoil section and referred to as
chordwise flutter; the distortions were somewhat similar to those shown
in figure 6 of reference 2. (This type of flutter apparently has been
little observed but is discussed to some extent in ref. 8.) Shortly
after the model began to flutter, a fatigue failure of the tip bulkhead
occurred in the form of tearing across the bulkhead at a section weakened
by rivet holes. The adjacent skins (both sides) began to tear and the
destruction continued as previously described. Since test conditions
ended at 10.8 seconds and the bulkhead failure occurred at 11.5 seconds,
all visible destruction actually took place during a period of decreasing
stagnation pressure; had the jet continued to run, the model undoubtedly
would have been completely destroyed. Motion pictures of the tests of
models MW-2 and MW-3 can be obtained on loan from NACA Headquarters,
Washington, D. C. (film entitled "Supersonic Jet Tests of Simplified Wing
Structures").
Although the preliminary results of reference 2 indicated that the
flutter of model MW-2 was induced by thermal buckling of the skin,-the
more extensive study of the motion pictures and the strain-gage records
reported herein reveals that it is impossible to state positively the
order in which these events occurred. The skin buckle in the rearmost
bays (each side) developed gradually (and may also have been developing
to a lesser extent in other bays), so that it is impossible to assign an
exact time of buckling; hence, the time given (10.0 seconds) is that when
an obvious buckle had developed. The wire strain-gage records reveal
more definitely that at 9.8 seconds, approximately the time of skin
buckling, the model began to flutter as already described.
Flutter associated with the aerodynamic heating of the models and
the resulting thermal stresses is felt to be a function of the reduced
stiffness of the structure brought about by a state of thermal stress
which is dependent upon the nonuniformity of the temperature distribution
but which is essentially independent of material property changes that
are functions of the temperature level. (See, for example; ref. 9.) The
only pertinent material property change that is expected to occur in the
test time is in the modulus of elasticity, which would amount to a maxi-
mum reduction of about 10 percent. No known accurate criterion exists
at present which will predict when and how structures such as these models
will flutter.
The failures of model MW-2 and its larger scale original model MW-1
were fundamentally similar in that flutter, either accompanied by or
closely preceded or followed by skin buckling, resulted in failure in
the vicinity of the trailing edge. The primary cause of failure in both
tests was aerodynamic heating. The thinner skin of model MW-2 caused its
skin to become hotter faster than the skin of model MW-1, and the hotter
skin and smaller webs of model MW-2 also resulted in higher interiorCONFIDENTIAL
CONFIDENTIAL
11
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Griffith, George E.; Miltonberger, Georgene H. & Rosecrans, Richard. Tests of Aerodynamically Heated Multiweb Wing Structures in a Free Jet at Mach Number 2: Two Aluminum-Alloy Models of 20-Inch Chord With 0.064- and 0.081-Inch-Thick Skin, report, August 9, 1955; (https://digital.library.unt.edu/ark:/67531/metadc61471/m1/12/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.