Assuming a certain amount of artistic license, the reference article is
probably correct. I worked on another project with Prof Robt Scanlan
of Princeton Univ who was in charge of that Electra analysis.

As I recall he said that when an Electra was newly manufactured, the
whirl mode resonant frequency of the engine mount was about 5 Hz & the
flutter frequency of the wing was 3 Hz. After a period of service, the
engine mounts would crack (not that unusual in large aircraft) and
eventually the engine mount frequency would decay to the point that the
two frequencies would become so close that the engine motion would
couple into the wing flutter. Eventually the dynamic system became
divergent resulting in wing structural failure.

You must recognize however that the Electra whirl mode problem was and
is unrelated to the RPM restrictions with certain engine propeller
combinations. These restrictions address torsional resonance modes of
the propeller engine crankshaft combination - not whirl modes.

I understand the frequencies of concern are in the 220 Hz range (a
couple of half-steps below middle C on a piano) which is the sixth
harmonic of crankshaft rotation in a 4 cylinder engine. The resonant
mode shape is the back of the crank going to-and-fro while the prop
tips go fro-and-to assuming a theoretical observer riding on the
spinner (read that again carefully!). Changing the stiffness of the
crank (i, e, the solid core) changes the natural frequency of the
system enough to get it out of the operating harmonics range. Adding a
damper mass on the back of the crank is another way to reduce the
torsional vibration buildup and the resulting prop and crank stresses.


I could hear the 6th harmonic in my 172M at low cruise too even though
it was not placarded against it. There was a certain ringing harshness
in the noise, and any musical ear could recognize that component in the
cabin noise once it is pointed out. As a precaution I stayed away from
that RPM.

Frankly from my limited experience in vibration, I am surprised that
narrow a restriction is sufficient to prevent problems in the fleet.