Why are PSRUs so hard to design?

[rv6ejguy]

Tracy used increased gear backlash to attenuate the worst TV periods as well. Not sure about flywheel mass. Two big factors which may not be applicable here.

Every engine, gearbox, prop combo will have different TV characteristics.

A properly selected Centaflex coupler is probably the most elegant solution but you'll need the numbers to pick the right one for this application.

[Lufthans]

And yet, any WW-II fighter with either a radial or a big V12 used a PSRU. And I don't believe I've ever seen a flywheel in one of these. They DID use heavy metal props though, which on our applications typically is a no-no.

Why does that work in those cases, and not in ours? The 9/12/18 cylinders on those engines making for a smoother engine to start with? Brutal force? More clever engineering? Or are these engines flawed too, with red areas in the rpm band and all that?

[rv6ejguy]

Originally Posted by Lufthans

And yet, any WW-II fighter with either a radial or a big V12 used a PSRU. And I don't believe I've ever seen a flywheel in one of these. They DID use heavy metal props though, which on our applications typically is a no-no.

Why does that work in those cases, and not in ours? The 9/12/18 cylinders on those engines making for a smoother engine to start with? Brutal force? More clever engineering? Or are these engines flawed too, with red areas in the rpm band and all that?

Multiple cylinders with no torque reversals (with much lower mean torque variation), clever engineering (quill shaft in the Merlin for instance), rolling pendulum tech on radials etc.

A 4 cylinder engine is a more challenging subject.

[jacoby]

Originally Posted by rv6ejguy

Multiple cylinders with no torque reversals (with much lower mean torque variation), clever engineering (quill shaft in the Merlin for instance), rolling pendulum tech on radials etc.

A 4 cylinder engine is a more challenging subject.

Speaking of the Merlin, here is a PDF on its TV: An Examination of the Torsional Vibration Characteristics of the Allison V-1710 and Rolls-Royce Merlin Aircraft Engines

By 1940, most of the high output radial aircraft engines were utilizing tuned pendulum dampers to minimize torsional vibrations in the crankshaft- propeller system. The situation in the in-line high output configuration was less consistent. While a number of earlier engines had adopted various types of dampers (mainly of the friction type) by 1940 all but one engine, the Allison V-1710, were without dampers of any kind. Rolls-Royce, Daimler-Benz and Junkers, all with liquid cooled V-12s and operating at comparably high outputs, were damper free. The Rolls-Royce Griffon, under development but not yet in service, never employed a damper.
This paper is an attempt to explain why the Allison engine was unique in this respect.

[rv6ejguy]

Excellent find. I'll pore over that when I have some time. Thanks for posting the link.

[jacoby]

Originally Posted by rv6ejguy

Excellent find. I'll pore over than when I have some time. Thanks for posting the link.

The author has a second white paper referenced from the Merlin paper that does an even deeper dive on TV (although without a PSRU): The Liberty Engine and Torsional Vibration

[Everwild]

https://www.motortrend.com/how-to/bl...aQ-pEpX37znOsM

Seems like an interesting possible base for a project.

[Lufthans]

Pity it's made of cast iron. 300 lbs for a long block is a lot of weight....

[Everwild]

For those who are interested in TV and how they addressed it in a Cosworth F1 engine. Pretty amazing bits of engineering.

https://www.youtube.com/watch?v=F39gjxxGHfY

[gasman]

Originally Posted by Everwild

For those who are interested in TV and how they addressed it in a Cosworth F1 engine. Pretty amazing bits of engineering.

https://www.youtube.com/watch?v=F39gjxxGHfY

Very good....