Yes, I agree. That said, rather than "distributed mass and distributed spring", perhaps it would be better to stick with the classic presentation, a series of individual inertia values connected by stiffness values. Again, the goal is the promotion of understanding with a non-FEA audience, not absolute accuracy of result.
Modeling a distributed spring-mass as a string of springs and masses is the classic way of doing it and exactly what I had in mind. Trouble is that this scheme is all still pretty involved for a non-professional in that field. I remain highly skeptical that amateurs will do any of this math...
Bill, I've gotten pretty good results modeling one piece wood props as a single accurate inertia determined with bifilar suspension, or at most two inertias, hub and blades.
I don't doubt it. Wooden props tend to be pretty stiff and thus transmit firing vibe all the way out.
When I say "pretty good results" I mean subsequent live run results vs prediction.
I am interested? Prediction of vibe amplitude? Eigen shapes and frequencies?
I understand you have huge experience with automotive drivetrains, but have you modeled, built, and tested a PSRU?
What I have huge experience with is isolating engine vibe from downstream components. When you spec the spring rate range properly, all sorts of sins do not need to be chased later.
As a matter of fact, I have worked on an engine-drive-prop problem for an airplane, solved their problem, and demonstrated that they had nothing else in range vibration-wise.
The folks working on it are a pretty smart bunch, they had a couple race car guys involved, and some of their schemes had run for two minutes before coming apart, throwing pieces for hundreds of feet, their test rig was all scarred up and repaired from having broken shafts and prop blades flailing about, etc. They had racks full of broken pieces from schemes they tried that had failed. I modeled their scheme, proposed solutions, we implemented one, and it worked on both eliminating broken parts and obtaining a smooth idle. Not only did the modeling point the way to the solution, it also allowed us to clear the operating range of other vibe modes, and resulted in over 200 hours of successful test runs. Then we rented a Rotec (I am fluent in their use and have used them at three different companies now) and cleared the operating range with all cylinders and with cylinders not firing... So yes, I know something about the topic of modeling and measuring vibration. I have also seen the results of folks who have tried lesser methods and messed up test stands, dyno cells, and prototype vehicles.
I have talked about this in the past, and I do not expect non-professionals to run analyses of these topics even for educational reasons. There are a couple approaches, they are somewhat non-intuitive and mathematically dense, and requires good programming to get right, even at a simplified level. The other way is to model the system on CAD, transfer it to FEA, and turn loose the Eigen analysis tools. Both require a lot more than a passing interest on the part of the analyst to get anything meaningful out of the process. These are paths that I do not expect we will get from our interested amateurs...
Concepts I have been trying to get out to folks. Doing the analysis? Nah...
Billski
