Torsional vibration

Discussion in 'Firewall Forward / Props / Fuel system' started by LBarron, Feb 19, 2007.

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  1. Feb 19, 2007 #1

    LBarron

    LBarron

    LBarron

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    Hello all,

    I've been doing a bit of reading about torsional vibration and one common theme is interesting - it appears that the aviation community is by far and away the most concerned about this issue. There are plenty of other "systems" that use long drive shafts - automobiles, boats, jet skis, machinery - but they all seem to be satisfied with a quick and easy flywheel or damper/absorber solution (and a generic solution at that - nothing that targets the precise frequency where the most damage would occur) and there doesn't seem to be too much of an uproar about shaft failures. Cars aren't stranded by the side of the road, aluminum jet ski shafts don't bust after a few hours, boats cruise around with long propshafts quite nicely. Granted, falling off your jet ski isn't going to hurt as much as falling out of the sky in your homebuilt, but the concern does seem quite lopsided.

    So, does anyone know why (yes, I understand the safety issue) it strikes terror into aviation folks but is just one more thing to be addressed by everyone else?

    :ponder:
    Leland
     
  2. Feb 19, 2007 #2

    RonL

    RonL

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    Hi!
    Glad you made this comment, i have always wondered why one larger, high torque engine, mounted at the center of all points of rotation, and driving a hydraulic, or mechanical drive system, handleing two, four, or more props, would not be a good way of powering a plane.
    If i remember right i have a book, that covers the aircorbra, a plane with the engine behind the pilot, and a long driveshaft up to the front, and driving thru a reduction gearbox to the prop.

    Thanks for the question.

    RonL
     
  3. Feb 20, 2007 #3

    orion

    orion

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    The Aircobra had a pretty massive damper and still had a couple of shaft failures. Considering the pilot sat on the shaft, catastrophic failure could be quite unnerving.

    As far as the differences are concerned, yes, in-flight safety is of paramount interest but the emphasis being put on the analysis of the drivetrain is not so much due to a "terror" issue but more so due to the understanding that the physics of an air prop application are significantly different than those of a car or boat. While it is often feasible to incorporate a more generic damping device, in aircraft application you not only have to worry about the primary excitable frequency (the natural frequency of the system) but also of its various harmonics. As such, it is better to know where the problem lies than trying to take a guess. Furthermore, it is also important to take into account the approach to the problem. The two common methods used are of course damping, and changing the systems excitable frequency. Usually we try for the combination of the two but really, either by itself can work.

    As far as why other shaft driven applications don't fail, it's usually because the system already has some damping associated with it so the problem does not tend to manifest itself as easily or dangerously as in systems that have no inherent ability to dampen out or absorb the impulses. A car engine driving a tire or a boat engine driving a prop are systems where the damping mechanism already exists. In a car, the transmission usually represents a system that has a bit of inherent damping ability (due to gear slop, tranny oil, clutch or torque converter, etc.), as do the tires. Tests by the SAE and others indicate that the tires alone pretty much take care of the problem due to their extreme low natural frequency and the high damping characteristics of the rubber.

    In the case of a boat, the drive system and the relatively small diameter prop have a very low inertia thus feedback is unlikely. Furthermore, the prop is running in water, which too has excellent damping characteristics.

    The combination of airplane engine and prop are unique since air is not a good damping medium and the prop has not only a substantial inertia but also an inherent spring characteristic, which is the principal energy/force generator behind the feedback issue. As such, the shafting is not so much the problem (since in most cases we're generally talking about relatively short shafts and short shafted reduction drives) - it is more so the whole assembly playing together.

    And regarding hydraulics, regardless of design or combination of elements, at the torque and rpm's you need to make this work, the drive system would be quite inefficient. To compare, in a gear or silent chain configuration, the system generally loses about one to three percent of the transmitted energy per mesh. This loss is evidenced as heat, which is absorbed and carried away by the lubricating oil. In the hydraulic system, this loss can be well over thirty percent. The losses are a function of line friction (thus long hydraulic lines with high pressures would be very detrimental), and pump and motor friction. This heat then has to be ducted somewhere so even for moderate power levels, the installation would require some pretty sizeable heat exchangers to prevent the system from cooking itself into oblivion. Also keep in mind that most hydralic motors turn at reltively low rpms. So here you'd need a special drive or a gear box to increse the prop shaft speed. Given the weight of these componets, plus the hydraulic fluid and the heat exchangers, your installed power to weight ratio would make a Lyc look like a miracle engine.
     
    Last edited: Feb 20, 2007
  4. Feb 20, 2007 #4

    LBarron

    LBarron

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    So, a possible solution might be:

    engine >> damper >> drive shaft >> damper >> prop

    There will be some inefficiencies in the system, but everything will be somewhat isolated from everything else. And if one were to use a carbon fiber shaft, the shaft vibration concern is reduced even more. Ditto for using a small diameter prop, which is what I'm interested in.

    Hmmm....

    Thanks Orion.
    Leland
     
  5. Feb 20, 2007 #5

    Midniteoyl

    Midniteoyl

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    Remember all those broken cranks?
     
  6. Feb 20, 2007 #6

    orion

    orion

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    You only need one damper between the engine and the shaft. Remember that the purpose of this mechanism is not only to dampen any potential torsional feedback but to also change the natural frequency of the system away from the natural frequency of the impulses. This is not something to eyeball - knowledgeable analysis is a must.

    A graphite shaft and a small prop will result in a high natural frequency - something you need to avoid if you're powering the system with an engine that will turn a relatively high rpm. Remember harmonic frequencies!!!
     
  7. Feb 20, 2007 #7

    CAB

    CAB

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    One word- weight.

    CAB
    Bearhawk#862
     
  8. Feb 21, 2007 #8

    LBarron

    LBarron

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    Dang! I was going to use a high revving rotary too!
    :D

    Just kidding. And I appreciate your comments. I'll get the expert advice/analysis when I get closer to choosing specific parts.

    Cheers,
    Leland
     
  9. Feb 21, 2007 #9

    plncraze

    plncraze

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    In Contact magazine and in places on the internet you will find an article by Don Hessenauer (I hope I spelled that right) that has some quick formulas about figuring out torsional vibration. It is on the internet. just google torsional vibration and you will run into it. Also Orion's website, Oriontechnologies.net, has two articles on reduction drives which can start you out. Emphasis on start!
     
  10. Feb 21, 2007 #10

    LBarron

    LBarron

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    Hi Plncraze,

    I've read that article, thanks. Several times actually. Just never made the connection to the damping effects of tires and water. But, I did make a note not to stand next to the driveshaft if I ever get to test my idea!

    Just wondering.... why isn't designing an airplane ever included with "rocket science and brain surgery".... [​IMG]

    LB
     
  11. Feb 21, 2007 #11

    orion

    orion

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    I've been told that it depends on the type of airplane and is strongly a function of what you really want the airplane to do and how far you want to get into the nitty-gritty of the design process. Although I haven't done brain surgery (ick!!!) I have worked on numerous aspects of rocket science and yes, if your requirements are complex, designing the airplane can get as involved.

    Regarding engines, personally I think rotaries are great engines and about as perfect for airplanes as you can get (as far as the installation envelope is concerned). Unfortunately though, one of the key harmonic frequencies of rotaries happens to occur at just about 4,500 rpm - at that rpm there is little or no warning of failure: One second it's running fine and a split second later, silence and mess. The torsional coupling then has to not only be able to absorb and dissipate the energies involved but also (and preferably) change the natural frequency of the entire system well away from those excitable regions. This can be done in two ways.

    The first approach is as in the Powersport engine where the system is designed to be quite beefy and more importantly, very, very stiff. As such, the natural frequency of the system is quite high and thus the impulses have no affect.

    The second approach is commonly evidenced by the use of some type of spring assembly and/or visco-elastic donuts. Personally I do not like the spring idea since that does not incorporate any dampening capabilities but in either case, the components shift the natural frequency of the system to very low levels so again, the impulses have no ability to excite the feedback behavior.
     
  12. Feb 22, 2007 #12

    Peter V

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  13. Feb 23, 2007 #13

    PTAirco

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    Thompson Coupling

    Guys who come up with these kind of things restore my faith in humanity. Elegance in engineering has to be the hardest thing to achieve - any fool can come up with a complex mechanism that more or less works. This is a perfect example of such elegance.
     
  14. Feb 24, 2007 #14

    Norman

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    Donald P. Hessenaur
     

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