BW 350 PSRU Failure, please share the word!

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

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  1. Oct 1, 2016 #61

    rv6ejguy

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    I know of 4 failures of these boxes (one smaller 200Z model) and I may not have heard of them all. The high time one though has over 500 hours now on an LS1 I believe without reported issues.
     
  2. Oct 1, 2016 #62

    pictsidhe

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    Back to rubber 'dampers'. As has been pointed out, they're really just springs with some damping. There are reasons they are so popular: rubber has a much higher ratio of elastic energy to weight. They're cheap and easy to mass produce. I suspect that they're more compact too. Lastly, they have some intrinsic damping. Motorcycle cush drives are a good example. Many racers think they can do without. Their sprockets tend to destroy the wheel...
     
    Last edited: Oct 1, 2016
  3. Oct 1, 2016 #63

    AdrianS

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    We have used rubber "dampers" in engine dyno input shafts.
    They have their uses (mainly redicing rattle at idle), but I have seen a new one destroyed in under 15 minutes when one particular engine was held at revs where TV occured - it overheated and failed.
     
  4. Oct 1, 2016 #64

    rv6ejguy

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    I've done quite a bit of work on my drive and TV concerns including measurement and a math model to see where TV was occurring. It was found that increased flywheel inertia massively reduced the issues on 4 cylinder engines (mine and several others). This reduced the distress on the rubber dampers in the system. My first set went 360 hours, hoping the 2nd set lasts twice as long this time around. I've detailed my measurement and modifications in other threads here and on my website.

    Many people (including me) are/were fixated on lightweight flywheels. This is generally the wrong thing to do and complicates your task in moving high TV amplitudes outside your operating rpm range.
     
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  5. Oct 2, 2016 #65

    TXFlyGuy

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    Would 8 cylinder engines have better success as they have more even firing pulses?
     
  6. Oct 2, 2016 #66

    rv6ejguy

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    Since there are no torque reversals on V8s, I would say it's generally easier to lick TV issues compared to a four or six but the potential is still there. Most V8 drives have the the worst period down below 500 rpm (where humans can perceive anyway) however with so many drive types, flywheel and prop inertias mixed together, you could have a bad period anywhere. Human perception to vibration fades quickly above a firing frequency of about 15-20hz so only instrumentation can tell you what happening up in the flight operational ranges.
     
  7. Oct 2, 2016 #67

    Winginit

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    That statement applies to the engine only, correct ? The method used for adapting a propellor and the propellor itself will always be subject to its own possible harmonic issues, Correct ?
     
  8. Oct 2, 2016 #68

    rv6ejguy

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    No, this applies to the entire engine, coupler/damper, gearbox, propeller system- any part of which can be destroyed by TV.
     
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  9. Oct 3, 2016 #69

    Winginit

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    What I meant was that the V8 generally is more readily adapted to a direct drive propeller without incurring TV problems at operating rpms (2200-3400). Not impossible but less likely than a V8 that has had a PSRU coupled to it. With the PSRU, the V8 now becomes more TV possible and closer in comparison to the 6 or 4 cylinder engines. Is that correct, or am I still looking at it incorrectly ?
     
  10. Oct 3, 2016 #70

    rv6ejguy

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    I think there have been so many successful 4 cylinder DD conversions as long as lightweight props are used that your likelihood of success is pretty good regardless of the number of cylinders the engine uses. Going DD or with a drive is really influenced mainly by power to weight ratios required for a design and whether you need/want a C/S prop. A C/S prop generally does not play well with a DD auto engine without extensive mods to the crank and at least an extra front bearing support to take up the prop bending loads offered by the much heavier prop. Most auto DD conversions, almost universally, have lightweight wood or composite FP props installed.
     
    Last edited: Oct 3, 2016
  11. Oct 3, 2016 #71

    TXFlyGuy

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    I was guess that if you want real power (300 to 350hp), and performance, a CS prop with a PSRU is the only way to go.
     
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  12. Oct 3, 2016 #72

    BJC

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    And having a PSRU keeps a replica more authentic, since the Merlin had an integral PSRU.


    BJC
     
  13. Oct 3, 2016 #73

    Toobuilder

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    Thats debatable. 300hp direct drive is possible in theory using parts and engine management systems of today, but above that I agree the PSRU is essentially a requirement.

    That said, Ford has shown some impressive power (low end grunt) numbers out of a small displacement using high boost. I think that would require some fairly sophisticated engine management protocols to make sure a highly boosted engine would survive. Ross, feel free to chime in here.
     
  14. Oct 3, 2016 #74

    rv6ejguy

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    The Clarke brothers showed many years ago that turbocharging a DD engine worked very well to increase the power to weight ratios into a more reasonable zone for aircraft use. However when using mogas, this combination requires relatively low compression ratios and ideally, a well programmed EFI/EI system to work well with reliability. On 100LL, the detonation margins are higher and you could get along with higher CRs and less sophisticated engine controls perhaps.

    I believe many people are just as afraid of turbocharging aircraft engines as installing PSRUs though...
     
  15. Oct 3, 2016 #75

    RJW

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    A general observation:

    Without an understanding of vibration with forcing and damping the probability of producing a successful conversion is greatly reduced. It doesn’t matter if the conversion includes a PSRU or is direct drive. “Successful” conversions produced without this understanding are a combination of overbuilt parts and luck.

    An understanding of vibration can be had by taking the first semester of a freshman year of physics and then supplementing this information with more in-depth study usually taught in an upper division physics course on mechanics. The freshman course will give basic physics concepts and an introduction to vibration. The advanced physics material covers coupled oscillators with forcing and damping.

    On top of this, training in the mechanics of materials and the fundamentals of machine design are essential.

    Add to the above a reasonable amount of experience working with machines that function in an environment where vibration is a primary design problem and you have the bare minimum necessary to come up with a rational design.

    Here is a good example of only some of the concepts that must be mastered:

    https://www.youtube.com/watch?v=I5ioTNXEUgs

    There are more lectures that cover forcing, damping, etc.

    Rob
     
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  16. Oct 4, 2016 #76

    BBerson

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    Propane is about 120 octane and $1.69 a gallon here. Can be delivered to the airport for a slight higher price.
    The tanks are heavier if kept under pressure, but getting it to -44F° and no pressure tank might work for some missions.
     
  17. Oct 11, 2016 #77

    wsimpso1

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    The more cylinders, the more firing pulses per engine turn. A 4 cylinder at 1200 rpm is firing at 40 Hz, while a V8 at 1200 rpm is 80 Hz.

    That makes it easier to design an isolation system that is soft enough. A "soft" system means a compliant element is inserted somewhere between engine flywheel and the prop. This deliberately drives the natural frequency of the system low. In cars, where we can make real and transmit real power at low rpm, we shoot for resonance at one-half of the min rpm. In an airplane project, well, the torque we can make goes with the square of rpm, so we can only make very little power down low and high power only at high rpm. With a prop, you can probably cheat and plan for resonance close to min engine speed. I was able to on an airplane project I worked on in 2010.
     
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  18. Oct 11, 2016 #78

    rv6ejguy

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    Torque of a typical SI engine doesn't vary as the square of the rpm. Not sure what you're trying to say here?

    Both auto and aircraft stock type atmo engines typically have peak torque at around 1/2 to 3/4 max engine rpm and max power near redline rpm.
     
  19. Oct 11, 2016 #79

    wsimpso1

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    In cars and trucks and motorcycles, where we mechanically connect the engine through tires to the ground, we can set any engine speed and throttle angle and apply the resulting engine torque up to the WOT curve to the powertrain.

    We are not dealing with that sort of system when we turn a prop.

    When your engine only has a prop to absorb the power, you have a different situation. With a fixed pitch prop (of usable size) and a given atmospheric density and airspeed, torque that can be carried by the prop is a function of rotation speed squared. If you try to apply more torque than is given by that curve, the engine accelerates to the new speed with great rapidity, and generally will come nowhere close to full torque for any rpm until the steady state speed is approached during the accel (even considering I*alpha torque).

    Put a constant speed prop (of usable size) on the same engine, and yes, you will have a band of RPM where you can make one torque, but at any one prop pitch, the prop will have still have a minimum speed where it can absorb WOT torque. Want the prop to turn slower than that, you have to reduce engine output, and it then responds along a torque proportional to rotational speed squared curve.

    Yes, you can shift the curve around some by changing the ambient inflow velocity, the atmospheric density, and the pitch of the blades, but the basics remain, with biggest actor still being rpm squared. The end result is still that an engine and prop combination that works in flight can only make small torque at idle speeds. Try to increase torque and it rapidly accelerates...

    The reason for my pointing this out is that you may not need to drive the resonance frequency all of the way to one-half of firing frequency at idle speed IF power is small AND IF some vibration absorption exists. I would never hope for this to be acceptable in a road vehicle with traction drive. In a propellor driven vehicle, I have seen it work with the resonance somewhat above idle speed. Yeah, the system in question was somewhat overbuilt (for other reasons). It did run for hours right at the worst spot for resonant vibration (measured) with small vibrational amplitudes that did not get anywhere near fatigue strengths of the components. Could we have driven the resonance below idle speed? Yeah, but the airplane would have weighed more to do that, and this worked.

    Billski
     
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  20. Oct 11, 2016 #80

    rv6ejguy

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    Got it, you were referring to propeller power absorbtion vs. rpm which is a squared function.

    I believe that this is usually expressed as hp though, not torque in propeller design since hp already encompasses rpm as rate of work. Perhaps Jan could comment on this when he designs a prop?
     

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