I assume there is some story out there about some analytical study or specific test case or field failure that the original poster is trying to track down. I would like to hear the story too. Maybe something like Kevin Eldridge's prop failure at Reno? I don't have the answer to the original question but I believe Hartzell props and 10:1 pistons do not automatically equal an unsafe application based on compression and harmonics.
Bob Axsom
My opinion is that if you have an EI with precise timing that there might be more of a possibility of a prop resonance failure than when using the somewhat scattered timing of a magneto.
Not doubting this, but not sure I understand - why is the timing of a magneto less precise? Don't the points open at the same point of every revolution?
Unfortunately, the points don't always open at the same precise interval due to the changes that take place on the points metal buid-up, the cam is not precise, the cam-follower can have some bounce, and you have a gear train with its slop and variations. I have heard that typically you can expect 1? to 3? variation from cylinder to cylinder. Haven't you ever noticed that with a timing light on a car that the timing mark will bounce around a little bit? It only takes a small variation to reduce the power pulses into the prop resonance cycle to keep the resonance from building up destructively. A crank-triggered EI, especially with capacitive discharge, will always make the power pulses fall at very precise intervals. I say especially CD since the typical delay from trigger to the high voltage pulse is usually just 3-4 microseconds, whereas in an inductive EI or magneto the voltage rate-of-rise can be subject to losses on the plug insulator due to carbon or lead buildup.
Even with crank triggered CD ignition I am sure there are lots of other variations that contribute to variations in torsional vibration. Things like lean mixture misfire, differences in cylinder by cylinder mixture strength and so on. These things affect cycle by cycle variations. Ambient temperature, humidity, altitude etc. affect day by day (or seasonal) variations. Torsional vibration results from the batch charge processing of our multi cylinder engines. Even gas turbines have blade pass frequencies in the main shafts and gearwheel tooth frequencies in the accessory drive sections. You can't get away from them!