Wind-tunnel vibration tests of dual-rotating propellers Page: 26 of 63
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25
radius from thrust- axis level and the horizontal cable was
relatively close t o the dual- rotating propeller,
The same engine- excited vibrations appeared for test
5 as for the first four tests; the vibratory stresses of
frequency 4*N were somewhat larger for the rear blades
The engine speeds for the resonance at the frequency 4jN
were again lower for the front propeller,
9a.at J fksrm as nb cniifwo ft-blale
~alzJn ste n 1B a3E_: ara.- No vibrations having the blade-"
passage frequency of 8n were detected. for test 6 (figs. 46
to 49) . Also, no important vibrations occurred which can
be attributed to the presence of the wing. A vibration of
frequency 2n, which has been attributed to the wake of the
horizontal cable, was not found.
Engine- excited vibrations occurred as for the first
five tests; the 44N component of vibratory stress was
greater for the rear propeller. The engine speeds for the
important; vibration of 4,NT were lower for the front pro-
peller by from 50 to 100 rpm.
"e st-s i a la t e ___2a:1L m-w-riu- .sag 414!2
an&.conP alanr=t a~La Cnnt~aEaas4az4 -p- et- For
test 7, tho blade-passage frequencies are 8n and 6n for
tho front propeller and for tho rear propeller, respec&.1
tively. Vibrations at these frequencies appeared for the
respective propellers (figs, 50 and. 51). The flexural
vibrations at these frequencies were not serious. Dia-
phragm stresses of frequency En were as high, however,
as -14,000 pounds per square inch for the front blades
(fig, 50). Inasmuch as diaphragm stress-s of the same
frequency occurred at about the same engine speed for the
rear blades (fig. 51), it is expected that much of the ex-
citation for vibration of the front blades at a frequency
of 8n was caused by the presence of the wing.
The engine- excited vibratory stresses were quite high.
The frequency 42N appears because of torsioaal oscillation
of the propeller shaft; the frequency 4-N occurs because
13
of the reverse propeller-shaft whirl at a frequency of
3SN; the frequency of 41iN occurs because of the forward
propeller-shaft whirl at a frequency of 5jT.
Test8s sirmulatedpusher condition* two four-blae
Curtiss hollow steel propellers.- The blade-passage fre-
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Miller, Mason F. Wind-tunnel vibration tests of dual-rotating propellers, report, September 1943; (https://digital.library.unt.edu/ark:/67531/metadc62121/m1/26/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.