Thanks, "Caudett's husband" for the informative post. I have some comments on his findings, in [brackets below]. No answers, so just skip reading it if you like :-)
Gary Casey
Oshkosh/Vibration, or what I learned on my summer vacation:
First, all 550 Continental crankshaft separations and cracks have been
traced to prop strikes or likely prop strikes. We are back on flight status
with 15SC. [Were the crankshafts inspected after the strikes? If so, the inspection method might be suspect.]
Les Doud (Applications Engineer) of Hartzell, spoke at Jeff Edwards Forum
and spent 30+ minutes with me later discussing our Vibrations. The most
interesting point was the weight of a metal 3 blade prop causes it to act as
a gyroscope. [Of course. All props have more inertia than the engine itself, I think even the
composite ones] The engine and all moving parts can create a
harmonic more
easily as a result.[A "harmonic" is a vibration double or half in frequency as the base vibration. A harmonic of what? The torsional excitation frequency? An engine vibration frequency? Just saying "a harmonic" doesn't say anything.] Lighter weight composite props have much less gyro
effect and therefore are inherently smoother.[Well, I don't know if they are "inherently" smoother. They tend to exhibit resonant frequencies higher than heavier aluminum props, but is that good or bad? I don't know.] Our stiff composite airframes [Composite materials are somewhat LESS stiff than metal structures - carbon-based structures being stiffer than ons based on glass fibers. That's why composite airframes are typcially on the heavy side - to get the stiffness high enough. On my ES I can see the wings flex and I was never able to do that on any metal airplane.]
are more susceptible
than an aluminum one with "every rivet acting as a
damper".[I'm not sure I buy that. Sure, every joint in a metal plane is a source of damping, but then every fiber-to-epoxy joint in a composite is a source of damping. I don't know whether metal or composite planes absorb vibrations better, but I think composite planes have the edge here] I previously thought the main reason was composite blades had less
resonance than metal. [Everything has a "resonance", it's just a matter of the frequency and the damping inherent in the structure. Aluminum blades, being a homogenous structure, certainly have less damping that composite. They will probably have a higher resonant frequency. But then there is the question of the mode of vibration, but that's another discussion.]
After all the obvious sources have been checked and fixed, the best
diagnosis tool is a in flight Spectrum Analysis and Plot
with the top of the
line DynaVibe (tool cost $3000+). At least that is what the DynaVibe rep.
said in a forum and Les agreed. Mount the sensor tightly on something solid
like a spar cap, horizontal and parallel to the spar (not on anything like
the panel dust cover). [I certainly agree with that.] Determining increased first, second or third order
vibrations(crank rotation speed, twice crank speed, firing frequency] narrows down the source. Les thought in my case of unpredictable
vibration, the free iPhone ap "Vibration" could be accurate enough to find
the major shake. [That almost makes me want to go out and buy an iPhone! I think maybe I will]
Les advised checking prop orientation for parallel alignment of one of the
blades with #6 cylinder at Top Dead Center on compression. This is best for
smoothness. [I fail to see a connection between the prop orientation and
vibration, not that there couldn't be. In the case of a 4-cylinder engine and
2-blade prop I can see it.]
Paul Snyder, an Engineer with Lord Mounts, thinks he may be able to help.
He cited the Mooney Ovation with a vibration problem that was improved by a
single stiffer mount on the left front (probably airframe-specific]. Early Cirrus with 550's and 3 blade
Hartzell's vibrated, later versions have 6 mounts with 4 near the front to
better balance the static weight of the prop and engine[Yes, in a Continental with metal prop the CG is very close to the front mount, so it has to support all the engine weight]. He may have
suggestions when he knows the part number of the brown (silicone) mounts
supplied to me by Lancair.
Oshkosh is a excellent resource for all things aviation[ain't that the truth!]. You can button
hole the engineer/designer/owner and get a lot closer to answers. I was
impressed with their willingness to help us and the lack of defensiveness on
their
part.