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Thread: Quill Shaft Design?

  1. #181
    Registered User Monty's Avatar
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    Re: Quill Shaft Design?

    The Prowler gearbox seems to be a stiff solution. It does not appear to have any quill at all. Astonishingly the reduction is bolted to the FRONT of the engine, thus eliminating the harmonic damper. The accessories are located on the normal PTO side of the crank. This makes the prop turn in the correct direction, but good grief! I can't see how there aren't problems, but seems to work!

    Using Billski's method of 2X the operating frequency would seem to favor the stiffer solution as long as the lash in the gearbox and the weight of the prop doesn't bring things down into the operating range. Fortunately most sane aircraft conversions with a gearbox keep the revs reasonable ~4000 rpm. So the primary frequency should be >2200 Hz for the gearbox case.

    In Toobuilders direct drive case, operation should be around 2500 rpm so the primary should be >332 Hz (I calculated this by multiplying 2xRPMx(4 firing pulses/rev)/60s/min) This isn't exact, you need to look at the firing order, I'm just doing ballpark here. IIRC the V8 is odd fire, so you will have even lower frequencies.

    YIKES! Direct drive results in low frequencies. If you are trying to push your resonance below idle.....I'm not sure how well that is going to work out.

    It would be interesting to "ring" the prop and see where the blade resonates. I used to have a spectrum analyzer program on my pda. Surely there is one for the iphone.

  2. #182
    Registered User rv6ejguy's Avatar
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    Re: Quill Shaft Design?

    Quote Originally Posted by wsimpso1 View Post
    When I tell you automotive crankshafts are a wet noodle, I guess I need to do two things: Give my credentials and tell you what I mean by that.

    First, I have spent 21 years doing transmission design work, most of it doing vibration control and isolation. And I have been doing it successfully, including predicting modes and killing problems.

    Second, I have had way too many people treat the crankshaft in their modeling like it is a rigid thing. It is certainly NOT rigid, and that was the point of my saying they are wet noodles. I will expound a bit. When folks treat the crankshaft like a rigid body and do not consider its contribution, you can predict the first order vibration mode of a system with a "stiff" reduction drive and when you build it find out that resonance occurs considerable lower... In general, it is considered good practice to design to put the crank 1st order resonance above 2x firing order at max rpm, but liberties have been taken on the topic. In any event, they are designed to those redlines, and then a little more than that. Now, if you do not need the engine to last very long, you can make it rev higher and let the 2x of firing drive some resonance. But if you boost the engine without boosting redline, now you can seriously raise power because you are staying below resonance.

    More later.

    Billski
    Nobody is questioning your credentials here but when you said feeling and hearing TV was "nonsense" I had to set the record straight. I've been flying and running different engines on test stands with propellers and gearboxes attached for a decade and you can easily identify TV at lower rpms without instrumentation.

    I don't think most people on this thread are assuming the crankshaft is absolutely rigid either. Your excellent posts here are helping all of us understand this topic better.

  3. #183
    Registered User rv6ejguy's Avatar
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    Re: Quill Shaft Design?

    Quote Originally Posted by Monty View Post
    The Prowler gearbox seems to be a stiff solution. It does not appear to have any quill at all. Astonishingly the reduction is bolted to the FRONT of the engine, thus eliminating the harmonic damper. The accessories are located on the normal PTO side of the crank. This makes the prop turn in the correct direction, but good grief! I can't see how there aren't problems, but seems to work!

    Using Billski's method of 2X the operating frequency would seem to favor the stiffer solution as long as the lash in the gearbox and the weight of the prop doesn't bring things down into the operating range. Fortunately most sane aircraft conversions with a gearbox keep the revs reasonable ~4000 rpm. So the primary frequency should be >2200 Hz for the gearbox case.

    In Toobuilders direct drive case, operation should be around 2500 rpm so the primary should be >332 Hz (I calculated this by multiplying 2xRPMx(4 firing pulses/rev)/60s/min) This isn't exact, you need to look at the firing order, I'm just doing ballpark here. IIRC the V8 is odd fire, so you will have even lower frequencies.

    YIKES! Direct drive results in low frequencies. If you are trying to push your resonance below idle.....I'm not sure how well that is going to work out.

    It would be interesting to "ring" the prop and see where the blade resonates. I used to have a spectrum analyzer program on my pda. Surely there is one for the iphone.
    The vast majority of of auto conversions with gearboxes actually operate above 4000 rpm in order to allow the engine to achieve power peak or near it. Few auto engines have PP down at or near 4000 so it does not make sense to put a gearbox on with the associated weight and complexity and not choose an appropriate gear ratio to allow the engine to spin up.

    When you say "odd fire V8", I am not understanding what you mean. Every V8 I've worked with has evenly spaced firing intervals.

  4. #184
    RJW
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    Re: Quill Shaft Design?

    Quote Originally Posted by Autodidact View Post
    Have a look at this animation:

    http://www.vibratechtvd.com/images-n..._peakstart.gif

    I thought that the location of the peak stresses was very interesting.
    It looks like the webs are the most flexible part.
    Very cool animation! Of course we think of the main journals as twisting but I havenít visualized what happens to the crank throws when this happens. Excellent! Thanks! I wonder how hard it would be to estimate the contribution of the crank arms to torsional rigidity using our dumb old analytical methods? Assume the journals are rigid and analyze the arms as twisting boxes to find their spring rates? Hmm...maybe this would be something where it would be wiser to simply measure a crank.

    Rob

  5. #185
    Registered User Monty's Avatar
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    Re: Quill Shaft Design?

    Quote Originally Posted by rv6ejguy View Post
    Nobody is questioning your credentials here but when you said feeling and hearing TV was "nonsense" I had to set the record straight. I've been flying and running different engines on test stands with propellers and gearboxes attached for a decade and you can easily identify TV at lower rpms without instrumentation.

    I don't think most people on this thread are assuming the crankshaft is absolutely rigid either. Your excellent posts here are helping all of us understand this topic better.
    How do you separate torsional modes from all the others by ear? Under power? Across the operating range?

  6. #186
    Registered User BBerson's Avatar
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    Re: Quill Shaft Design?

    The animation was cool. Maybe the cranks arms need to be larger. Some cranks do have full circle arms for strength , balance and central flywheel effect.

    What about welding discs to the stock crank arms to make it stiffer? ( instead of quill shaft)

  7. #187
    Registered User wsimpso1's Avatar
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    Re: Quill Shaft Design?

    I usually write first for the OP, then for others reading and participating in it. I shall make an exception for general educational purposes.

    Our modeling in the auto industry of the engine includes the crank, rods, pistons, cams and cam drive, front end accessories, drive system all the way to the ground. Skip anything and important modes get missed. For powertrain vibe, this is all that matters. Note, we are not talking about crankcases and trans cases and engine mounts. We are calculating the what goes on with rotating parts...

    As far as feeling torsional vibration of the engine and drive mechanism, anyone who believes he/she is feeling torsional vibration is actually feeling something else. If you feel or see a buzz in the airplane someplace, you have a load path from the powerteam cases (engine crankcase and/or gearbox) to the engine mounts to the rest of the airframe and you have the powerteam making a vibration that is fairly well transmitted through this path. If you get the forcing function in resonance with something in the airframe, it can amplify into a huge buzz - capable of breaking things.

    We have been talking about the rotating part system vibrating in resonance with the forcing functions and that may or may not make big enough reactions to the airframe to feel anything. There are generally gaps in the frequency spectrum between resonant modes. If the resonance of the rotating parts occurs at a frequency that is between resonant modes of the airframe, you can feel NOTHING while some segments of the rotating parts are tearing themselves apart. Here is why - the only torsional connections between the rotating motions and the airframe are the gas pressure between piston and cylinder head, and any reduction drive reactions. The OP is not working with a reduction drive - so all we have is gas pressure pushing on the heads and pistons and the prop pulling on the air. Just because someone else is feeling a vibe does not mean it is internal parts in torsional vibration - it is most likely a firing vibe in resonance with other things in the airframe. The OP can not feel TV, and any implication that he can determine if his system is OK because he can not feel anything is "nonsense", and has been known longer than I have been alive.

    More later with the rest of the questions.

    Billski
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  8. #188
    Registered User Toobuilder's Avatar
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    Re: Quill Shaft Design?

    Quote Originally Posted by Monty View Post
    ...Have you ever considered replacing the metal blades with wood/composite? I know they are little slower, but it might make your life a lot easier, and eliminate the need for anything exotic...
    Frankly, yes, it is certainly a consideration for my personal project. However, if this concept is to be a "viable" alternative to the 540 Lycoming, then it should be able to swing a standard propeller. Not that I intend to market this thing by any means, but it would be nice to know what is possible in case someone copies the concept (if it works, that is).


    Quote Originally Posted by Monty View Post
    ...A quill does not have to be a tiny spindly thing. The main gearbox quill on the DB60X engines was a hollow shaft that fits around the crank snout. The gear fits over the top of it. It was driven from the front of the crank and was only as long as the gear itself. It drove the rear of the gear using splines...
    ...and this is where I have perhaps inappropriately titled this thread. By common definition, a quill shaft passes through another - one driving the other. While that is true in this case, I also have a torsion action, which is a common, but not exclusive characteristic of the quill. And it's really the torsion action that is under discussion here.

  9. #189
    Registered User rv6ejguy's Avatar
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    Re: Quill Shaft Design?

    When the engine/ prop/ gearbox is attached to a metal test stand or an airframe, high amplitude, low frequency TV will usually set up a harmonic in that structure and this is what you can feel and usually see and even sometimes hear in the case of torque reversals on gears. Ever listen to PSRU gears hammer at F1 or F2? Slow a Rotax down to 600-800 rpm and you'll see. TV not present here? I think not.

    Do I think strain gauging the system is superior to direct human observation? Absolutely but it does not change the fact that the human body is quite sensitive in the head, neck and shoulders to frequencies in the 20-30Hz range and eyes in the 60-90Hz range, 90-500Hz to the touch. Sit in a structure like an aircraft with an engine experiencing high amplitude TV at these frequencies and you will know it is there. Above these frequencies, probably not. On the Subaru fours we have direct evidence of TV at low rpms buy observing rubber coupling physical distress and very quick heating of the coupler when operating in this rpm range.
    Last edited by rv6ejguy; May 8th, 2013 at 09:23 PM.

  10. #190
    RJW
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    Re: Quill Shaft Design?

    Quote Originally Posted by Monty View Post
    The Prowler gearbox seems to be a stiff solution. It does not appear to have any quill at all. Astonishingly the reduction is bolted to the FRONT of the engine, thus eliminating the harmonic damper. The accessories are located on the normal PTO side of the crank. This makes the prop turn in the correct direction, but good grief! I can't see how there aren't problems, but seems to work!
    You are seeing things rightly. It doesnít work. There have been problems. Some are still trying to use this setup and are still having problems. This is another drive that was put together with little understanding of how a system like this behaves as far as I can tell.

    Rob

  11. #191
    Registered User PTAirco's Avatar
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    Re: Quill Shaft Design?

    Quote Originally Posted by wsimpso1 View Post
    When I tell you automotive crankshafts are a wet noodle, I guess I need to do two things: Give my credentials and tell you what I mean by that.

    i
    One of the most useful things I was ever taught by an old aeronautical engineer from the 30s and 40s was this: "Picture everything as if it was made of rubber, if you want to understand how a part behaves under load." I already had that notion, but it was a great way to put it into words. There is nothing rigid in aviation, whether in airframes or engines. Everything bends, deflects and twists no matter how solid it may appear.
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    "Aeronautical engineering is highly educated guessing, worked out to five decimal places. Fred Lindsley, Airspeed."

  12. #192
    Registered User rv6ejguy's Avatar
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    Re: Quill Shaft Design?

    Quote Originally Posted by Monty View Post
    How do you separate torsional modes from all the others by ear? Under power? Across the operating range?
    On my present installation, I've done bifilar suspension tests of the major masses- crankshaft, flywheel, modified flywheel, propeller etc. Next step is to calculate the MOIs of the gearbox parts and stiffness of each element which I will be working on shortly. I just completed getting everything back together and doing initial running. So far, after doubling flywheel inertia, I've seen a massive reduction in F1 and F2 amplitude. Hopefully the modeling will be able to show where F3 is and how high the amplitude is. I was not able to strain gauge this particular gearbox due to the dimensions of it. The first installation with light flywheel had 358 hours on it. Nothing broke but I was never happy with the rather serious F1 and F2 characteristics.

    I think Toobuilder could also do these simple tests since he has an engine and maybe the prop and flywheel already. DD setups are way easier to model as there are less parts, no backlash unpredictabilities and everything is rotating in the same direction at the same rpm.
    Last edited by rv6ejguy; May 8th, 2013 at 09:56 PM.

  13. #193
    Registered User wsimpso1's Avatar
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    Re: Quill Shaft Design?

    The OP is talking about a direct drive - the only periodic input to the cases and then the airframe is the gas pressures on the heads. This is zero rotating part vibration path to the airframe. This is fundamentally different from a geared system where you do have reactions from a torsional vibration of the rotating parts to the cases to the airframe.

    Strain gage the case? You will only know what is transmitted to the case, and have to infer everything else. Better to put a Rotec on the rotating parts and actually know what is happening by direct measurement.

    Next, yes, you can have lash in the gear train open and close, causing gear rattle. Anytime your firing oscillations are larger than the elastic deformations of the crank to gears, you will have rattle. The problem with rattle is that it is impact, which can excite all orders, just like hammer tests. This actually should set your minimum engine speed...

    Human sensing? 4th order idle is 40 Hz, 4800 rpm 8th order is 640 Hz. While frequencies vary by 16 times, amplitude varies by 256 times. You want to trust human sensing to find all of that?

    Billski

  14. #194
    Registered User Kristoffon's Avatar
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    Re: Quill Shaft Design?

    Quote Originally Posted by wsimpso1 View Post
    This is zero rotating part vibration path to the airframe.
    Wrong. Wrong. Wrong. Wrong.

    Let me introduce you to Newton's third law. Engine tries to turn propeller. As a result, it also tries to turn the engine mounts, which try to flip the airframe over, which leads to vibration than might be felt (or not) by the occupants.
    Building: Osprey-II

  15. #195
    Registered User Autodidact's Avatar
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    Re: Quill Shaft Design?

    Billski, try not to get too upset by some of the comments here.
    "Milk cures wing dope poisoning."

    ó Flying and Glider Manual

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