Wind-tunnel vibration tests of dual-rotating propellers Page: 3 of 63
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It was anticipated that dual- rotating propellers would
be subject to vibrations excited aerodynamically by blade
passages, Because reactionless types of Vibration for
certain dual- rotating propellers can occur with this exci-
tation, it was expected that some of the vibrations could
Although previous torcue- stand tests indicate that vi-
brations excited by blade passages are not serious, it was
decided to perform a series of tests with ground-adjustable
propellers in a wind tunnel where not only a high airspeed
can be attained but also the airspee&d can be varied to hold
the engine brake mean effective pressure constant over a
wide range of engine speed,
Vibration tests were conducted in the LMAL 36-foot hig4h-
speed tunnel, Six- and eight- bfade tual-rotating propellers
were operated as tractors, and seven- and eight- blade dual-
rotating propellers were operated as pushers; the pusher con-
dition was simulated by mounting a full- scale wing ahead of
the propellers, , easurements of propeller stress were made
for engine-speed ranges at low, intermediate, and high engine
powers, with a range of airspeeds up to 280 miles per hour,
The vibration staffs of HIamilton Standard Propellers
and the Curtiss- Wright Corporation Propeller Division ool-
laborated In conducting the tests and in analyzing the
GENERAL DISCUSS ON OF PROPELLER VIBRATIONS
Modes of vibration.- A propeller blade may vibrate in
any one of several modes; examples of mode shapes are given
in figure 1. The airplane engine enforces a node at the
propeller center, although the center mag actually have an
amplitude of displacement of the order of thousandths of an
inch, The amplitudes of the tips are of the order of tenths
of an inch, If tho node enforced by the engine is not counted,
the number of nodes for figure 1 is m - 1, where m is the
number of the mode, The vibratory frequency increases some-
what with the number of tha mode. The frequencies of the
various modes of a blade may be determined by vibrating the
blade with some driving oscillator, If the damping of the
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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/m1/3/: accessed December 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.