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More Thoughts on PSRUs

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rv6ejguy

Well-Known Member
Supporting Member
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Jun 26, 2012
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6,150
Location
Calgary, Alberta, Canada
Time to redirect the current PSRU discussion from the DD V8 thread to a place with a more appropriate and searchable title.

A few observations:

A spring is NOT a damper so a sprung center clutch disc with steel springs is a useless addition in a PSRU if you want to reduce TV.

A toothed belt does not absorb TV either. Many people believe this as fact.

Jack Kane of EPI is a very, very smart fellow. He has a concise grasp of almost every topic related to automotive conversions for aircraft however IMO, despite in depth analysis of other PSRU designs, he fails to consider the reality of successes in the real world of these other drive designs which have been flying for decades by the hundreds- I am thinking RAF and RFI belt drives in particular, which number well over 1000, having something like 300,000+ flight hours on them by now with very few failures or problems. We also have the Robinson and Geschwender Hy-Vo chain drives which are well proven and reliable over many years. In other words, there is more than one way to skin a cat and we should be open to the advantages and disadvantages of each design.

Jack is worth listening to. You will learn a lot, however I believe he is not right about everything and may have become somewhat myopic in "the best way" to design a drive. We know there are plenty of successful drives with both helical gears and bath oiled gears which work just fine like the billions of automotive gearboxes and differentials in service over the years.

I think the important thing is that Jack and other MEs who understand engines and TV well, open our minds to all the possible pitfalls in engineering terms which we may have never considered when designing or shopping for a drive. That in itself is very valuable IMO. He certainly narrows down failure modes on certain drive designs which ring totally true from my experiences in the field.

Of course we have seen successful PSRU designs using all the known and common methods of reduction. Conversely we have seen many which blew up pretty quickly, usually because of poor design or more likely, ignoring the realities of TV.

Jack sets the bar high for PSRUs. He basically wants an almost zero maintenance gearbox which will last in excess of 2000 hours. A lofty goal for experimental use but something expected for the certified aircraft world. In the experimental world, most people would not have a problem replacing a belt every 500 hours which is 5-10 years of flying for the vast majority of owners. We also know cost is a big consideration for many so $20K PSRUs are far out of the question for most users.

In the end, whether a drive is designed using the best formal engineering practices or LAR (looks about right) engineering, the proof is in the pudding- only many hundreds or thousands of hours of trouble free, real world operation truly validates any design.

Let the discussion fly...
 
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