<<...I was contemplating the DMF concept.>>
For the record (and I know you already realize this Mike), as applied here it is no longer a dual mass flywheel. The secondary mass has been removed. That leaves the primary inertia and a torsional soft element (the spring set). Placing a soft torsional stiffness at the engine output inertia is quite conventional. However, applying the guts of an automotive DMF to a PSRU is something I've not seen before, and has a number of interesting advantages, some mechanical, some vibrational. I suspect Jan had one of those wonderful "Ahhhh!" moments a few years back. Satisfying, eh Jan? <g>
<<As previously discussed, this should lower the resonant frequency of the system.>>
Actually "frequencies", plural. Theory tells us the number of natural frequencies in a torsional system is equal to the number of inertias minus one; the soft spring will move all the natural frequencies down the RPM scale. It is likely that we're only concerned with the lowest two in the context of system resonance, and a few higher ones in the context of propeller resonance.
The stiffness-inertia model would look something like this:
J's are inertia values, K's are stiffness values. The largest inertia is usually the propeller; in this case we know it to be 0.36889 slug-ft^2 if the user mounted Jan's recommended MT. The flywheel inertia is usually the next largest individual inertia, but in the case of a PSRU it will be about an order of magnitude less than the prop, in the area of 0.03. Yes, this is a generalization and should be considered as such. We don't know the actual inertia value for the Egg flywheel. K3 is the spring rate of the (non-)DMF.
The other inertia values (J) are the gearsets, each individual crankthrow, and J11, the harmonic balancer (another grossly misleading name) combined with the accessory inertias. The stiffness values (k) are whatever connects them together. This model treats the propeller as a single lump inertia, which is not entirely accurate.
The engineers here know all this stuff, but perhaps the above will help others as we proceed. Quick reminder; when any of the natural frequencies are matched by an exciting frequency, the system (or some part of the system) resonates.
<<However, the springs should also work as a low pass filter.....The springs in the DMF could act to filter out any engine-produced frequencies that would tend to excite the prop.>>
I think you're right Mike. I suspect a soft spring rate would indeed filter out an awful lot of the high exciting frequencies that would match the high natural frequencies of the prop blades. It would nicely explain the low values
rumored (hint, hint, Jan<g>) to have been recorded during the MT prop telemetry.
The big question regarding that prop telemetry is the strain values recorded near the root of the blades when the system resonates at the first and (perhaps) second natural frequency. F1 is probably below Jan's specified idle speed. Nobody knows where F2 lies, at least not right now. Yes, I hope Jan knows. Devil in the details, eh Mike?
<<I'm not very familiar with the MT propellers, but believe they're wood wrapped in composite. Such a design would be expected to have a fairly high resonant frequency.>>
May not be all that different compared to aluminum; not a huge difference in modulus. Here are some sample curves:
(post lunch edit: Whoops, brain f*rt! Forgot about the mass difference between wood/glass and aluminum; stiffness ain't the whole picture.)
I am pretty sure a wood/glass blade has a higher vibration decay loss factor, ie it would be more "dead" compared to aluminum.
<<I have not had the opportunity to compare the stiffness of the rubber type dampeners or isolators to the DMFs, but I would think they would be considerably stiffer than the DMFs spring rates.>>
A lot of the rubber couplers for diesel applications are quite stiff, but consider the Goetz coupler in a Rotax C or E box, or some in the Centaflex line from Lovejoy.
Great stuff guys, and thanks Jan.