BW 350 PSRU Failure, please share the word!

Discussion in 'Chevy' started by Andreas K, Aug 19, 2016.

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  1. Oct 18, 2016 #101

    Toobuilder

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    This is not a huge deal on this forum and I don't mean to bust your chops here Bill, but there is a very definite distinction between the "classic" SBC and the LS series. It's about as subtle as the distinction between the flathead Ford and the Y block that followed. Yes the old and new share some similarities, but the new was a groundbreaking, clean sheet design. I only bring this up because I know you are a stickler for correct terms and if you want to communicate properly with a gearhead, the terms "SBC" and "LS" set very different expectations for whatever conversation follows. Apples and oranges.

    To the more important point: Doesn't the overall length of the Lycoming main bearing (and resulting massive increase in surface area) contribute more to harnessing the gyroscopic precession of the prop than the overall diameter of the shaft? Or are we only concerned with the cantilevered section of the shaft outside the bearing?
     
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  2. Oct 19, 2016 #102

    TXFlyGuy

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    Why would you not operate at 3600rpm? This is a good range for power, and gives the best propeller tip speed (1.9-1 gear ratio) for max cruise speed.

    That 3600 figure is slightly over 50% of the engines rpm range. It could run at that rpm for hundreds and hundreds of hours. And I'm sure it has.
     
    Last edited: Oct 20, 2016
  3. Oct 20, 2016 #103

    Toobuilder

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    Because 3600 is really loafing that engine. Your situation is different because you are not looking for an all out performance powerplant. But if you were to really try to optimize the power to weight ratio of the combination, you'd want to lean on the engine a lot harder. Heck, even the RR Merlin turned 3400 or something close to that, and it has a huge stroke compared to the LS. Compare piston speeds between the two and the LS is hardly breaking a sweat.
     
  4. Oct 20, 2016 #104

    Monty

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    Yes. The long bearing lycoming/continental crank acts as two separate bearings to react out the prop precession loads. This isolates the adjacent crank throw. In a DD automotive engine the moment must be reacted between the last two crank bearings. This places the last crank throw in cyclic bending beyond what it normally sees. There is a history of failure from this in both VW and Corvair conversions.

    The diameter of the crank, though important, is not the deciding factor. A narrow fluid bearing is more simply supported than fixed. You need two of them to react a moment, unless it is very long, like an airplane crank snout. As Bill correctly pointed out, the material, processing, propeller inertia, and operation all contribute to the fatigue situation. Not a simple problem. Do your homework!
     
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  5. Oct 20, 2016 #105

    Monty

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    Piston speed....the all important variable. Like pitch line velocity, BMEP, and BSFC.....really tends to highlight the BS.
     
  6. Oct 20, 2016 #106

    Winginit

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    I think you may be talking about I guy I conversed with several years ago. There was a particular type of airplane, I'm thinking maybe a bi-plane, but some type of older historic design.There were several of them that had SBC direct drive engines in them and this fellow was building one just like his friends already had. Apparently they were all satisfied with the performance and reliability. I don't remember if they were using just an extension or a bearing supported direct drive. Suprisingly, the 500 Cadillac was actually a pretty light engine, especially if some aftermarket aluminum heads and intake manifold were used. If memory serves me, I think they were actually lighter than the Chevy, but not much. I believe I was suggesting he use a lighter LS engine and he said that in the particular airplane he was building that the weight and balance worked out better with the heavier engine. I think the original design of the airplane had used some type of heavy engine and that was why the Chevy was a good choice for a replacement.


    The Roger Fowler "Junkyard Dog" used an LS engine which was flown to Oshkosh and back from Memphis. Its crash was because of wing separation and had nothing to do with the engine setup.


    Junkyard Dog 001.jpg
     
    Last edited: Oct 20, 2016
  7. Oct 20, 2016 #107

    Toobuilder

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    The guy I talked to was Don Souser and he built a nearly full scale P6E replica and flew it around Corona airport for a few years. It had a crate ZZ3 SBC and just didn't quite "bring it". There was no external bearing. The prop was hanging right off the crank.
     
    Last edited: Oct 20, 2016
  8. Oct 20, 2016 #108

    TFF

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    Is that P6E the one at Oshkosh?
     
  9. Oct 24, 2016 #109

    wsimpso1

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    Rubber as the torsional spring element can work nicely. Like any spring, its purpose is to drive the first resonant frequency well below the firing frequency in operation, and ideally below firing frequency at idle. To do this and live, it has to have an excess of torque capacity above what the engine produces to take the full engine torque plus the firing oscillation. If you are safely above 1st resonant mode, say f/fn > 2, this can be reasonably small, say 1.25 times max engine torque. But if the rubber elements are not soft enough to drive fn below the operating range, you may never be able to make them strong enough. They must have low enough spring rate combined with enough engine and prop inertia to drive fn down to idle or lower.

    One other topic about rubber springs. They usually have a rising spring rate. This means that as torque goes up, fn will rise too. In a car, that can be bad, but when you are turning a prop, the engine can only make torque that the prop can absorb, and that goes with rpm^2. So just make sure that f >2*fn at all speeds based upon the torque vs rpm relationship of the prop, and the system will isolate.

    While the damping of the rubber may seem useful, rubber elements that are cycling rapidly over large deflection ranges convert energy to heat, and they are poor at cooling. The damping will help you get through the resonance on starts and stops, but if you are doing much of it while operating, as others have noted, they will cook and fail.

    Billski
     
    Last edited: Oct 24, 2016
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  10. Oct 24, 2016 #110

    wsimpso1

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    Ross spotted and pointed out an important one for you guys. The system you have is the inertia of flywheel with a torsional spring and then the inertia of the prop. The other parts are pretty small actors. If the flywheel is really low inertia and the prop is really high inertia, you really have to get soft with that spring to drive fn down. But if you add some inertia to the flywheel (larger diameter is the lightest way, if you can, but a thickened rim is pretty weight efficient way to do it), and keep the same spring elements, your fn will drop, your ratio of f/fn will rise, and isolation just gets better when this happens. The transmitted vibration gets smaller, the springs or rubber elements move less on firing pulses, and the gear reactions to the housing drops. Everything gets smoother...

    The other way, taking weight out of the flywheel, ugh. If you only do it a little, the engine will run rough, your idle speed will have to be run higher than you might like, etc. Take the flywheel weight way down, and fn will get into your operating range, and you will break stuff. I worked on an airplane project where their test rig was all trashed up by broken shafts flailing around... We fixed the last design they had by increasing the diameter of the flywheel. Fn got near idle, engine smoothed out, idled better, and never broke another part. Reasons for not giving trouble when f=fn were the combination of:

    Torque at fn was tiny - prop torque is tiny at low rpm;
    There was an elasomeric coupling in the system - its rising rate springiness makes fn sort of spread out and less severe;
    The system had NO LASH in it and thus no impact loading.

    Billski
     
    Last edited: Oct 24, 2016
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  11. Oct 24, 2016 #111

    wsimpso1

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    It is possible that the prop could have issues, as well as other elements, and we should make sure that all of our resonant ranges are either at least above 2x max firing frequency or below 1/2 of min firing frequency. The PSRU and damper won't help with this if the prop's fn or other system resonant modes are in range of engine torsional vibe. We have heard of airplanes with red or yellow arcs on the tach, and that is what is being brought up here. This is why I point out that even when soft element is built in between the engine flywheel and the gearbox, the rest of the system still has to be stiff enough to keep the resonant modes well above max firing frequency

    Billski
     
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  12. Dec 2, 2016 #112

    BobbyZ

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    I did skip/skim the last 2-3 pages but I see no mention of anyone mentioning a viscous coupling to reduce TV.It's a pretty common solution these days and in my opinion they're great for isolating vibrations.Granted it would take some engineering to make work .But I know of a few manufacturers that might even have a off the shelf coupling designed to handle the loads required and the one is usually pretty helpful as far as engineering advice goes.Granted that would probably change as soon as you mention aircraft lol.

    Now they do require maintenance but truthfully its minor and I've seen the silicone fluid filled ones go thousands of hours behind industrial machines without a fluid change.

    I dont know what a CS prop needs to operate but you could feasibly use some parts from the Haldex catalog to control the prop and use a Haldex clutch.But realistically that is a stretch and would take some serious money and engineering to accomplish.

    Another option I've come across in the marine environment are rubber isolated couplings. They're quite simple,easy to design and I'd think they'd work a lot better then the sprung hubs mentioned earlier.Also if they fail,they can still provide a mechanical connection all but with a lot of racket and vibration.Although it could make the difference between landing in a populated neighborhood or reaching a runway.

    TV is nothing new as we all know and it seems like there should be a simple solution to deal with it.

    Another thing in regards to PSRU's I've noticed is most of the mounting options are less than ideal IMHO since they focus on the bell housing bolts and it allows more flex than is ideal.I honestly feel that this amplifies the TV problems we're having to deal with.
    Welding a aluminum block isnt rocket science these days. So why hasnt anyone looked into adding a few bosses closer to the center line of the crank to cut the flex?
    I'm by no means suggesting to use a welded boss as a replacement for the reinforced factory positions.But it would make a great supplement to tie things in closer to the center line.That alone would reduce the flex we see induced by adapter plates and it's be a lot closer to how the integral reductions have been designed.

    I know it would require relocating the starter but if you ask me the benefits far outweigh the the trouble involved in relocating the starter and re-balancing the motor to deal with the missing flexplate/flywheel.
     

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