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Thread: Torsional Vibration and PSRU Design

  1. #11
    Gordon Arnaut
    Guest Gordon Arnaut's Avatar

    Default Torsional Vibration and PSRU Design

    Charles,

    Thanks for that snippet from the Wilson book.

    Please note the part where he says how the "flexural properties of the
    propellor are key in determining how the SYSTEM (my emphasis) will respond
    to the excitations since the prop will resonate."

    Is this not exactly what I said about the prop beginning to resonate and
    then cuasing something else in the system to break?

    I have said all of this in my posts, with the exception of the part about
    prop excitations arising from aircraft manouevers, which is really part of
    the point about distubed flow.

    Thank you for confirming the correctness of my position. For the record now,
    I don't think there can be any serious question that the prop does not
    contribute a very real component to the excitations side of the equation.

    Regards,

    Gordon.

    "Charles Vincent" <xlch58@swbell.net> wrote in message
    news:tf00g.69031$Jd.37901@newssvr25.news.prodigy.n et...
    > Dan Horton wrote:
    >> Hello Charles,
    >> <<According to Ker Wilson, prop flutter has no real impact on
    >> torsional vibration. He could be wrong, but he devoted more than a
    >> half century to the subject. Blade passing frequency, however,
    >> apparently does come into play in some systems. So does whirl, but
    >> that isn't the internet topic of the year.>>
    >>
    >> Ahh, thank you, appreciate the confirmation.
    >>
    >> Lucky dog, wish I had my own copy. I have to beg my local
    >> librarian to get it from the UA library.
    >>
    >> Dan
    >>

    >
    > A quote:
    >
    > "In most practical cases coupled axial/flexural modes occur independently
    > of coupled torsional-flexural modes since there is usually no appreciable
    > coupling whereby component harmonics of the shaft torque are able to
    > excite symmetrical blade vibration."
    >
    > And to your earlier point:
    >
    > "In aero-engine/airscrew systems there are, in general, two series of
    > excitations. The airscrew is one source, of aerodynamic origin, arising
    > from the passage of the blades through a non-uniform airstream, or due to
    > the airstream entering the airscrew disc obliquely when the aircraft is
    > executing certain manouevres .... The other series originates from the
    > non uniform character of the engine torque."
    >
    > Hence the blade passing frequency. Still the flexural properties of the
    > propellor are key in determining how the system will respond to the
    > excitations since the prop will resonate.
    >
    > As far as modeling the propeller and determining its natural frequencies
    > (it has multiple as well) it appears to be a right bear. The shape is
    > complex and there are multiple modes of vibration and all of them have to
    > be adjusted for RPM because the stiffness varies with the centrifugal
    > force (the real kind). For an adjustable prop, the stiffness in the
    > plane of rotation changes with angle.
    >
    > Charles

  2. #12
    Charles Vincent
    Guest Charles Vincent's Avatar

    Default Torsional Vibration and PSRU Design

    > <<No joy there, the charts I have show the EA81 at over 200 ft-lbs at
    > 3000
    > rpm, climbing from there and not dropping below 200 through 6000rpm.>>
    >
    > Charles, better take a look at your units. The above would be
    > 114hp at 3000 and 228hp at 6000....a rather unusual EA81.
    >
    > Dan
    >

    Your right, it was late. I was reading the chart I had wrong. It showed
    torgue at the prop flsnge ( after reduction) plotted agsainst crankshaft
    RPM. I don't have a chart for the naked engine.

    Charles

  3. #13
    Charles Vincent
    Guest Charles Vincent's Avatar

    Default Torsional Vibration and PSRU Design

    Gordon, you can run down the rabbit hole of relativity with someone
    else. The fact is your statement that "it is the inertia produced by
    the centrifugal force," is wrong. Inertia is not the result of
    centrifugal force, it is the reverse. You could even argue semi
    successfully that centrifugal force was just another name for inertia,
    but you can't say inertia is produced by the centrifugal force and be
    correct.

    Charles

    Gordon Arnaut wrote:
    > Charles,
    >
    > I guess the concept of relativity is just a "convenience" for doing the math
    > too right?
    >
    > Please do not descend any further into absurdities. Even Newton was well
    > aware of the relativity of motion long before Einstein came along, which is
    > why his law of inertias takes into account frames of reference.
    >
    > Please quit before you bury yourself in a sinkhole.
    >
    > Regards,
    >
    > Gordon.
    >
    >

  4. #14
    Gordon Arnaut
    Guest Gordon Arnaut's Avatar

    Default Torsional Vibration and PSRU Design

    Charles,

    What exactly are you saying? Once again, I find myself scratching my head
    trying to fathom your actual point. You are nothing if not a master of
    obfuscation.

    I said very plainly in my original post that the prop is a source of
    excitations, as is the cylinder firing of the engine, as well as imbalance
    in the system.

    I don't know who said what, but your post contained this in quotation marks:
    "In aero-engine/airscrew systems there are, in general, two series of
    excitations. The airscrew is one source..."

    So whoever said that, whether it is you or Wilson, it is quite plain and
    quite true. The Prop IS a source of excitations, whether they are of
    aerodynamic origin or whether they are due to resonance.

    If you disagree with this, please say so plainly, otherwise do not try to
    muddy the waters further -- it is only doing a disservice to the discussion.

    Regards,

    Gordon.

    "Charles Vincent" <xlch58@swbell.net> wrote in message
    news:b580g.69055$Jd.53962@newssvr25.news.prodigy.n et...
    > Actually Gordon, the words you quoted were my words not Wilson's. You
    > will notice there are no quotaion marks around them in my original post.
    > The text with the quotation marks is from Wilson. The excerpt was
    > actually confirming Dan's contention that the excitation source was
    > disturbed airflow, that it does not originate in the prop. Take the
    > disturbed airflow away and the natural hysterisis of the prop and rest of
    > the system will cause it to return to normal. So while excitations can
    > enter through the prop or they can enter through the crank, these two
    > components don't create the excitation, they react to them. There are
    > components in a redrive system that can originate excitations though,
    > which is why if you just want to fly it is easiest to pick a direct drive
    > wooden prop snd go fly. Not guaranteed, but much simpler.
    >
    > Charles
    >
    >
    >
    > Gordon Arnaut wrote:
    >> Charles,
    >>
    >> Thanks for that snippet from the Wilson book.
    >>
    >> Please note the part where he says how the "flexural properties of the
    >> propellor are key in determining how the SYSTEM (my emphasis) will
    >> respond to the excitations since the prop will resonate."
    >>
    >> Is this not exactly what I said about the prop beginning to resonate and
    >> then cuasing something else in the system to break?
    >>
    >> I have said all of this in my posts, with the exception of the part about
    >> prop excitations arising from aircraft manouevers, which is really part
    >> of the point about distubed flow.
    >>
    >> Thank you for confirming the correctness of my position. For the record
    >> now, I don't think there can be any serious question that the prop does
    >> not contribute a very real component to the excitations side of the
    >> equation.
    >>
    >> Regards,
    >>
    >> Gordon.
    >>
    >>
    >>
    >>
    >> "Charles Vincent" <xlch58@swbell.net> wrote in message
    >> news:tf00g.69031$Jd.37901@newssvr25.news.prodigy.n et...
    >>
    >>>Dan Horton wrote:
    >>>
    >>>>Hello Charles,
    >>>> <<According to Ker Wilson, prop flutter has no real impact on
    >>>>torsional vibration. He could be wrong, but he devoted more than a
    >>>>half century to the subject. Blade passing frequency, however,
    >>>>apparently does come into play in some systems. So does whirl, but
    >>>>that isn't the internet topic of the year.>>
    >>>>
    >>>> Ahh, thank you, appreciate the confirmation.
    >>>>
    >>>> Lucky dog, wish I had my own copy. I have to beg my local
    >>>>librarian to get it from the UA library.
    >>>>
    >>>>Dan
    >>>>
    >>>
    >>>A quote:
    >>>
    >>>"In most practical cases coupled axial/flexural modes occur independently
    >>>of coupled torsional-flexural modes since there is usually no appreciable
    >>>coupling whereby component harmonics of the shaft torque are able to
    >>>excite symmetrical blade vibration."
    >>>
    >>>And to your earlier point:
    >>>
    >>>"In aero-engine/airscrew systems there are, in general, two series of
    >>>excitations. The airscrew is one source, of aerodynamic origin, arising
    >>>from the passage of the blades through a non-uniform airstream, or due to
    >>>the airstream entering the airscrew disc obliquely when the aircraft is
    >>>executing certain manouevres .... The other series originates from the
    >>>non uniform character of the engine torque."
    >>>
    >>>Hence the blade passing frequency. Still the flexural properties of the
    >>>propellor are key in determining how the system will respond to the
    >>>excitations since the prop will resonate.
    >>>
    >>>As far as modeling the propeller and determining its natural frequencies
    >>>(it has multiple as well) it appears to be a right bear. The shape is
    >>>complex and there are multiple modes of vibration and all of them have to
    >>>be adjusted for RPM because the stiffness varies with the centrifugal
    >>>force (the real kind). For an adjustable prop, the stiffness in the
    >>>plane of rotation changes with angle.
    >>>
    >>>Charles

    >>
    >>

  5. #15
    Gordon Arnaut
    Guest Gordon Arnaut's Avatar

    Default Torsional Vibration and PSRU Design

    Charles,

    I did in fact say that the inertia is produced by the centrifugal force,
    which is just as wrong as saying the centrifugal force is produced by the
    inertia.

    That is ridiculous, yet you cling to it. At least I have the good sense to
    go back and see that I did make an error in phrasing. I thought I had said
    that inertia is proportional to the centrifugal force, which is correct. The
    two are irrevokably linked.

    But one does not cause the other. The cause of both is the energy that
    produces the rotation to begin with and the centripetal force that keeps the
    rotating object from flinging off into space.

    If you have no centripetal force, you have no centrifugal force. You also
    have no rotational inertia.

    And there is nothing rabbit-holish about inertial reference frames. It
    applies fully to this discussion. Much of our understanding of modern
    physics has been built on the underpinnings of Newtonian relativity.
    Inertial reference frames are an important and very real concept, and we can
    thank Newton and Galileo before him for making us aware of the importance of
    those reference frames in understanding the physics of motion.

    Amazing how you can be so cavalier with your wording and then lecture me on
    the meaning of semantics.

    Regards,

    Gordon.

    "Charles Vincent" <xlch58@swbell.net> wrote in message
    news:ne80g.69056$Jd.36155@newssvr25.news.prodigy.n et...
    > Gordon, you can run down the rabbit hole of relativity with someone else.
    > The fact is your statement that "it is the inertia produced by the
    > centrifugal force," is wrong. Inertia is not the result of centrifugal
    > force, it is the reverse. You could even argue semi successfully that
    > centrifugal force was just another name for inertia, but you can't say
    > inertia is produced by the centrifugal force and be correct.
    >
    > Charles
    >
    > Gordon Arnaut wrote:
    >> Charles,
    >>
    >> I guess the concept of relativity is just a "convenience" for doing the
    >> math too right?
    >>
    >> Please do not descend any further into absurdities. Even Newton was well
    >> aware of the relativity of motion long before Einstein came along, which
    >> is why his law of inertias takes into account frames of reference.
    >>
    >> Please quit before you bury yourself in a sinkhole.
    >>
    >> Regards,
    >>
    >> Gordon.
    >>

  6. #16
    Dan Horton
    Guest Dan Horton's Avatar

    Default Torsional Vibration and PSRU Design

    <<Up until that point the springs are not compressed and the coupling
    is in effect a solid coupling.>>

    Good God, he still doesn't get it.

    Gordon, plot the supplied data and study at it carefully. If your
    assertion were true, the plot would not leave the Y axis until reaching
    some elevated torque value. Note that the real plot begins at 0-0.

    Dan

  7. #17
    Dan Horton
    Guest Dan Horton's Avatar

    Default Torsional Vibration and PSRU Design

    Bill writes:
    <<small torque sensors that can be clamped to a rotating shaft that
    transmit their data wirelessly>>

    Yes, for torsion the classic setup is a wheatstone bridge strain
    guage array feeding a telemetry transmitter. I shopped new digital
    transmitters a few years ago but didn't buy, hoping for better prices
    and bandwidth later. We'll see. In 99/early 2000 we used a wheatstone
    bridge and a borrowed analog transmitter from Wireless Data. The
    bridge was still on the propshaft at S&F that year. I was there all
    week and nobody ever asked why the propshaft was wrapped in strapping
    tape <g>.

    Dan

  8. #18
    Gordon Arnaut
    Guest Gordon Arnaut's Avatar

    Default Torsional Vibration and PSRU Design

    Dan,

    You're right about one thing. I don't get what your objection is.

    Are you saying the springs immediately begin to compress at the first sign
    of torque? Hence the plot beginning at 0-0?

    I don't see how this is possible unless the springs were installed without
    any preload at all. My understanding is that the springs in a clutch disk
    are under preload, so the torque has to rise to a certain level before they
    will compress. Until that point it is a solid coupling.

    If the springs had no preload, it would never be a solid coupling. It would
    contantly be compressing and decompressing. How could that kind of clutch
    even be usable in a car? It would be lurching all the time.

    Also glad you mentioned the rubber torsional coupling brought up by the
    poster in reference to the Alfa driveshaft.

    The Rotax boxes you mention use this for the same reason the Ross box uses
    the clutch disk with the springs. But as I was trying to point out, the
    springs are not used to actually isolate the vibrations but to introduce
    variable stiffness into the system.

    Regards,

    Gordon.

    "Dan Horton" <danhorton@elmore........> wrote in message
    news:1145119785.166084.238210@u72g2000cwu......... o...........
    > <<Up until that point the springs are not compressed and the coupling
    > is in effect a solid coupling.>>
    >
    > Good God, he still doesn't get it.
    >
    > Gordon, plot the supplied data and study at it carefully. If your
    > assertion were true, the plot would not leave the Y axis until reaching
    > some elevated torque value. Note that the real plot begins at 0-0.
    >
    > Dan
    >

  9. #19
    Charles Vincent
    Guest Charles Vincent's Avatar

    Default Torsional Vibration and PSRU Design

    My original remarks and the quote was in response to the following
    statement you made:

    Gordon Arnaut wrote: "Yet another source of excitation in airplanes is
    the spring effect of the prop, where the blade tips whipsaw back and
    forth as they undergo acceleration and deceleration due to the torque
    spikes of cylinder firing."

    I had assumed that you were refering to the potential of the blades to
    flex parallel to the axis of the hub due to variations of blade loading
    created by the periodic variations of torque. I made that assumption
    since you were describing the prop as the source of the excitation and
    yet still made specific mention of the engine torque variations. I had
    once thought axial loading and flexure might feed back into the system
    at one time myself, but Ker states that it does not( except in
    extraodrinary cases), which I took note of in my studies -- thus the
    quote. If you were not refering to axial vibrations, then your
    statement is silly since you identify the prop ( or its spring effect)
    as the source of excitation and yet you end the statement with the
    actual excitation source ( torque spikes ). Alter the frequency of the
    "torque spikes" and the system returns to normal.

    Dan is the one that brought up blade interactions with disturbed
    airflow. And I quoted the passage regarding issues of a non axial
    airstream (which is not the same as disturbed airflow).

    Even via he prop, Wilson is relatively proscriptive:

    Wilson wrote: "The airscrew is one source, of aerodynamic origin,
    arising from the passage of the blades through a non-uniform airstream,
    or due to the airstream entering the airscrew disc obliquely when the
    aircraft is executing certain manouevres ...."

    The phrase "of aerodynamic origin" when not cropped out is the
    significant part. You may want to claim the prop as a source since it
    is complicit in the translation of the vibrations, but the same could be
    said of the crank, so whats the point? In any event you state:

    Gordon Arnaut wrote: "Yet another source of excitation in airplanes is
    the spring effect of the prop"

    The Wilson quote mentions nothing of spring effectof the prop as a
    source of excitations and in fact if you think about it you will see (
    or at least most people would) that the vibrations "of aerodynamic
    origin" do not require any springiness of the prop blade to impinge upon
    the system.

    And I didn't even bother to comment on this gem of yours:

    Gordon Arnaut wrote: "My original point was that if you have a flywheel
    with enough inertia, it will be an effective restraining force to act
    against excitations that would otherwise produce vibration."

    Which is just plain wrong. In the industry I work in there are machines
    with moments of inertia measured in tons and they are still subject to
    torsional vibration issues. Upping the moment of inertia just alters
    the resonant frequency ranges. Without damping of some sort, excitation
    in the critical range will still drive the system into resonance. If
    you meant to say "if you have a flywheel with enough inertia, it will
    lower the resonant frequency of the system to a range conducive to safe
    operation" that would be a different thing.

    Charles

    Gordon Arnaut wrote:
    > Charles,
    >
    > What exactly are you saying? Once again, I find myself scratching my head
    > trying to fathom your actual point. You are nothing if not a master of
    > obfuscation.
    >
    > I said very plainly in my original post that the prop is a source of
    > excitations, as is the cylinder firing of the engine, as well as imbalance
    > in the system.
    >
    > I don't know who said what, but your post contained this in quotation marks:
    > "In aero-engine/airscrew systems there are, in general, two series of
    > excitations. The airscrew is one source..."
    >
    > So whoever said that, whether it is you or Wilson, it is quite plain and
    > quite true. The Prop IS a source of excitations, whether they are of
    > aerodynamic origin or whether they are due to resonance.
    >
    > If you disagree with this, please say so plainly, otherwise do not try to
    > muddy the waters further -- it is only doing a disservice to the discussion.
    >
    > Regards,
    >
    > Gordon.
    >
    >
    >
    >
    >
    >
    >
    >
    > "Charles Vincent" <xlch58@swbell.net> wrote in message
    > news:b580g.69055$Jd.53962@newssvr25.news.prodigy.n et...
    >
    >>Actually Gordon, the words you quoted were my words not Wilson's. You
    >>will notice there are no quotaion marks around them in my original post.
    >>The text with the quotation marks is from Wilson. The excerpt was
    >>actually confirming Dan's contention that the excitation source was
    >>disturbed airflow, that it does not originate in the prop. Take the
    >>disturbed airflow away and the natural hysterisis of the prop and rest of
    >>the system will cause it to return to normal. So while excitations can
    >>enter through the prop or they can enter through the crank, these two
    >>components don't create the excitation, they react to them. There are
    >>components in a redrive system that can originate excitations though,
    >>which is why if you just want to fly it is easiest to pick a direct drive
    >>wooden prop snd go fly. Not guaranteed, but much simpler.
    >>
    >>Charles
    >>
    >>
    >>
    >>Gordon Arnaut wrote:
    >>
    >>>Charles,
    >>>
    >>>Thanks for that snippet from the Wilson book.
    >>>
    >>>Please note the part where he says how the "flexural properties of the
    >>>propellor are key in determining how the SYSTEM (my emphasis) will
    >>>respond to the excitations since the prop will resonate."
    >>>
    >>>Is this not exactly what I said about the prop beginning to resonate and
    >>>then cuasing something else in the system to break?
    >>>
    >>>I have said all of this in my posts, with the exception of the part about
    >>>prop excitations arising from aircraft manouevers, which is really part
    >>>of the point about distubed flow.
    >>>
    >>>Thank you for confirming the correctness of my position. For the record
    >>>now, I don't think there can be any serious question that the prop does
    >>>not contribute a very real component to the excitations side of the
    >>>equation.
    >>>
    >>>Regards,
    >>>
    >>>Gordon.
    >>>
    >>>
    >>>
    >>>
    >>>"Charles Vincent" <xlch58@swbell.net> wrote in message
    >>>news:tf00g.69031$Jd.37901@newssvr25.news.prodig y.net...
    >>>
    >>>
    >>>>Dan Horton wrote:
    >>>>
    >>>>
    >>>>>Hello Charles,
    >>>>> <<According to Ker Wilson, prop flutter has no real impact on
    >>>>>torsional vibration. He could be wrong, but he devoted more than a
    >>>>>half century to the subject. Blade passing frequency, however,
    >>>>>apparently does come into play in some systems. So does whirl, but
    >>>>>that isn't the internet topic of the year.>>
    >>>>>
    >>>>> Ahh, thank you, appreciate the confirmation.
    >>>>>
    >>>>> Lucky dog, wish I had my own copy. I have to beg my local
    >>>>>librarian to get it from the UA library.
    >>>>>
    >>>>>Dan
    >>>>>
    >>>>
    >>>>A quote:
    >>>>
    >>>>"In most practical cases coupled axial/flexural modes occur independently
    >>>>of coupled torsional-flexural modes since there is usually no appreciable
    >>>>coupling whereby component harmonics of the shaft torque are able to
    >>>>excite symmetrical blade vibration."
    >>>>
    >>>>And to your earlier point:
    >>>>
    >>>>"In aero-engine/airscrew systems there are, in general, two series of
    >>>>excitations. The airscrew is one source, of aerodynamic origin, arising
    >>>
    >>>>from the passage of the blades through a non-uniform airstream, or due to
    >>>
    >>>>the airstream entering the airscrew disc obliquely when the aircraft is
    >>>>executing certain manouevres .... The other series originates from the
    >>>>non uniform character of the engine torque."
    >>>>
    >>>>Hence the blade passing frequency. Still the flexural properties of the
    >>>>propellor are key in determining how the system will respond to the
    >>>>excitations since the prop will resonate.
    >>>>
    >>>>As far as modeling the propeller and determining its natural frequencies
    >>>>(it has multiple as well) it appears to be a right bear. The shape is
    >>>>complex and there are multiple modes of vibration and all of them have to
    >>>>be adjusted for RPM because the stiffness varies with the centrifugal
    >>>>force (the real kind). For an adjustable prop, the stiffness in the
    >>>>plane of rotation changes with angle.
    >>>>
    >>>>Charles
    >>>
    >>>

    >

  10. #20
    Dan Horton
    Guest Dan Horton's Avatar

    Default Torsional Vibration and PSRU Design

    <<Are you saying the springs immediately begin to compress at the first
    sign of torque? Hence the plot beginning at 0-0?>>

    Very good Gordon. That is indeed what a 0-0 datapoint indicates.

    <<I don't see how this is possible unless the springs were installed
    without any preload at all.>>

    You find out all kinds of interesting things when you actually
    measure and think. Steady reliance on Google searches results in a lot
    of GIGO <g>

    <<My understanding is that the springs in a clutch disk are under
    preload, so the torque has to rise to a certain level before they will
    compress.>>

    Some clutch disks are indeed that way. No need to start at 0-0
    when designing a clutch for an engine that makes, say, 150 or 200
    ft-lbs of torque at idle. They only need to be soft enough to set
    system F1 well below idle speed.

    <<If the springs had no preload, it would never be a solid coupling.>>

    Very good Gordon! And even if they do have preload, as described
    above, it still isn't intended to be a solid coupling when in
    operation.

    Again consider the lessons found in the Subaru clutch. The range
    of torque capacity is 0 to 162 ft-lbs. A late 1980's EA81 was rated
    73hp @4800 and 94 ft-lbs torque @ 2400. Don't know about idle speed
    torque (anybody have a chart?), but let's guess 40 ft-lbs. So, we have
    40 ft-lbs as we ease away from a stop, 94 ft-lbs in economy cruise, and
    80 ft-lbs when pushing hard. Read carefully Gordon. All these numbers
    are well within the range of 0 to 162. Actually they are all within
    the single 1547 ft-lbs/rad spring rate found between 3.5 degrees and 6
    degrees. Clearly engine torque has the springs in play at all times.

    <<It would contantly be compressing and decompressing.>>

    Give that man a gold star! Yes Gordon, overall angular
    displacement of the clutch center varies with throttle position. At
    the hertz level, angular displacement oscillates at the exciting
    frequency.

    <<How could that kind of clutch even be usable in a car? It would be
    lurching all the time.>>

    Think again.

    Dan

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