Wind-tunnel vibration tests of dual-rotating propellers Page: 4 of 63
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blade is very small, the frequency for a maximum response
is a natural frequency of vibration; whereas, if the damp-
ing is appreciable, the frequency for maximum response is
a- somewhat less than the natural frequency. Centrifugal
9 force increases the natural frequencies for the various
-rt nodes and shifts the positions of all the nodes except the
I node enforced by the engine (reference 1). The nodes can
also be shifted by changing the blade angle of the propel-
ler, The shift of node positions can be several Inches.
EPgge-4.-.ihc aF_# - A propellor vibration may 'be
flatwise, edgewise, or torsional. A vibration is con-
sidered flatwise if the vibratory motions of the sections
of the bLede are primarily in a direction perpendicular to
the blade chords. For an edgewise vibration, the motions
of the blade sections near the shank are primarily along
the blade chords, while the motions near the tips are more
flatwise. Tho flatwise motion of the tips for an edgewise
resonance occurs because of the blade twist . Experiments
show that, if tho natural frequency for one of the higher
flatwiso nodes is near tle natural, frequency for the first
edgewise mode, the motions near the blade tip during edge-
wise resonance are so flatwise that it is difficult to tell
from the stress distribution along the blade whether tho
vibration is flatwise oF EdgowiseC (reference 2). The stress
distribation around the blade shank, however , shows which
vibration is occurring. The >lade motion for edgewise
resonance generally is sL.ch that the two maximum-stress
positions on the shank are in line with the leading edge
and the trailing oago at approximately three-fourths the
propellor radius. T'he maximum-stress positions for flat-
wise resonance are shifted 900 from the maximum-stress
positions for edgewise resonance,. For torsional vibration
of a blde, the blade twists periodically about a longi-
tudital axis within the blade, The frequency of this
vibration is relatively high, however, and only the funda-
mental mode is ever encountered. This viljration occurs
infrequently in comparison with the flatwise and edgewise
Stress di4 iz :bti4e-a-'1b-blade4- For flatwise vibration,
the maxiam- stress positions along the blade correspond to
the positions of maximum c/p where c is the perpendicular
distance of the extreme fiber from the neutral axis and p
is the racdius of curvature of the neutral axis. Far the
first mode, the minimum radius of curvature is somewhere near
the propeller hub (fig. 1); whereas, for the higher modes, the
positions of minimum radius of curvature are between the nodes,
For a given blade section, maximum c occurs at maximum blade
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Miller, Mason F. Wind-tunnel vibration tests of dual-rotating propellers, report, September 1943; (digital.library.unt.edu/ark:/67531/metadc62121/m1/4/: accessed May 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.