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Home> Maintaining
your Continental and Lycoming engine
Prevention of Engine Mount Failures in Low Powered
Aircraft 
This article was published by:
Department of Commerce
Civil Aeronautics Administration
September 29, 1942
Prepared by: Aircraft Engineering Division
I have slightly edited the article for brevity - John
Schwaner
The number or reports of engine mount failures recently received
indicates that such failures are becoming chronic for several models of light
aircraft powered by 4-cylinder opposed type engines and incorporating the
platform type engine mount...
The reported failures are u8sually cracks found in or near
welded joints and are principally due to recurrent resonate vibration. As you
know, most engines have "rough" spots at certain RPM values at which
the engine will vibrate somewhat more than usual. The vibration responsible for
fatigue cracks occurring in these mounts is due to the engine vibrating about
its thrust axis, and is produced by the firing impulse frequency from the engine
(2 times crankshaft RPM). It is believed that the steps listed below2 will
result in safer operation and will reduce the occurrence of cracks in the engine
mounts.
Vibration can be greatly reduce by avoiding engine operation at
"resonant" speeds (RPM's). Therefore, it is recommended that the
following brief checks be performed:
1. Run the engine while the airplane is on the ground, starting
with the lowest idling speed used, (which should be no lower than 550 RPM)
gradually increasing engine speed to practically full throttle and noting any
particularly rough speed encountered. Care should be taken not to overheat the
engine.
2. Note this speed, go above it, and check it also while
decreasing RPM.
3. If this critical or rough speed is in the normal flight
range, check it also in flight.
4. This rough speed may then be indicated on or near the glass
face of the tachometer in order to remind pilots that continued operation within
100 RPM of this speed is to be avoided.
Roughness due to propeller unbalance will usually persist over a
wide speed range and become worse only at higher speeds. Resonant vibration on
the other hand is characterized by the vibratory option of the engine or of some
other component of the engine installation attaining a maximum amplitude while
in resonance and then diminishing as engine speed is either increased or
decreased
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