Despite rotational vibrations in crank shafts are a well-known and understood phaenomenon a proper calculation can only be done with the correct intake parameters. Unfortunally, non of the propeller companies can or will describe the dynamic behaviour of their propeller in regard to damping and elasticity. Of course, a crank shaft can and should be designed in a way that it will not depend on the damping of the prop. Unfortunally a hypothetical worst case of a complete stiff propeller with unlimited inertia will not necessarily result in the most critical results. There will always be one order of Eigen frequencies which will lay in the rpm range of the engine/propeller system and it would be nice to better understand the system as whole. Are there any (historic) data available of propeller elasticities and damping properties? The inertia will be quite easy to find. of course, in a variable speed propeller, the properties might change significantly with the blade angle and even with ageing of the prop bearings/blades.
Elasticity and damping properties of propellers
- Thread starter Nicknick
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Intresting, but I was thinking about air craft engines. I know, ship propellers offer some damping, but the extremly long drive shafts in some ships (fast ships with narrow stern and multi propellers) look indeed proplematic...
Some marine Diesel engines use crazy injection timings with different SOI for every cylinder to avoid resonances at critical rpm.
Some marine Diesel engines use crazy injection timings with different SOI for every cylinder to avoid resonances at critical rpm.
Aeroengineer1
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The critical frequencies and responses of propellers are indeed known by their manufacturers, but within usage on diesel engines, they tend to be the most problematic for propellers to survive. Of the successful aero diesel engines that I am aware of, this was a very non-trivial aspect of the development of the engine/propeller combination. There are ways to do this via manual deconstruction of a blade and then estimating the properties. Bearing stiffnesses under CF are likely the hardest to get right. Some basic ping testing could get you in the right direction. How you hold it while ping testing will play into it.
I"m in contact with a propeller supplier and he explained, why many propellers failed with Diesel engines and why his props will work. I can"t tell anything about it, but it sounded plausible.
Of course, there is a chance, that other factors might have been overlooked and give troubles too. Ping testing is a good approach, but I guess the centrifugal forces will have an hughe impact on the propeller stiffness. If the blade bearings (constant speed prop) are pre loaded they will get stiffer and the blades will surly be affected by gyroscopic stabilizing effects.
Of course, there is a chance, that other factors might have been overlooked and give troubles too. Ping testing is a good approach, but I guess the centrifugal forces will have an hughe impact on the propeller stiffness. If the blade bearings (constant speed prop) are pre loaded they will get stiffer and the blades will surly be affected by gyroscopic stabilizing effects.