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#41
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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" wrote in message 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" wrote in message 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 |
#42
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Torsional Vibration and PSRU Design
Gordon Arnaut wrote:
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 |
#43
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Torsional Vibration and PSRU Design
Jim asks:
What was this rig-up on? What is it you do, Dan? It sounds interesting. So, do you have a marketable Suzuki flying? What association are you with the project? Scale JN-4C. In the real world I'm an automobile dealer, and it is not very interesting. However, it is a hell of a lot more profitable than getting in the PSRU business, so the only Suzuki you can buy from me comes with 4 tires and a horn. The JN-4C project had several goals. A friend wanted to proof custom software written to model torsional vibration in a complex aircraft drive system (a pusher with with long shafts, might I add). I wanted a new and improved PSRU. So, we modeled the old drive and then altered the model for optimum predicted results. Then I designed a drive to match the model inputs, built it, and ran it with telemetry to check the accuracy of the predictions. Along the way we developed a damper and tested it, ran two different props while I had the telemetry, played with strobing linear vibration, and a whole bunch of other stuff. End result was proven software and a pretty good PSRU, plus an education. Dan |
#44
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Torsional Vibration and PSRU Design
Really wonderful stuff you've been posting here Dan. Thank you very much for answering so many silly questions! George Graham RX-7 EZ with RD-1A Redrive. On 15 Apr 2006, Dan Horton wrote: Jim asks: What was this rig-up on? What is it you do, Dan? It sounds interesting. So, do you have a marketable Suzuki flying? What association are you with the project? Scale JN-4C. In the real world I'm an automobile dealer, and it is not very interesting. However, it is a hell of a lot more profitable than getting in the PSRU business, so the only Suzuki you can buy from me comes with 4 tires and a horn. The JN-4C project had several goals. A friend wanted to proof custom software written to model torsional vibration in a complex aircraft drive system (a pusher with with long shafts, might I add). I wanted a new and improved PSRU. So, we modeled the old drive and then altered the model for optimum predicted results. Then I designed a drive to match the model inputs, built it, and ran it with telemetry to check the accuracy of the predictions. Along the way we developed a damper and tested it, ran two different props while I had the telemetry, played with strobing linear vibration, and a whole bunch of other stuff. End result was proven software and a pretty good PSRU, plus an education. Dan George Graham RX-7 Powered Graham-EZ, N4449E Homepage http://bfn.org/~ca266 |
#45
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Torsional Vibration and PSRU Design
"Dan Horton" wrote End result was proven software and a pretty good PSRU, plus an education. Have you considered releasing some plans for the PSRU? How about your JN project? Are there pictures anywhere? Congratulations on your achievements. You should write a blog about it. I'm sure it would be an interesting read. -- Jim in NC |
#46
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Torsional Vibration and PSRU Design
Today's multi-v belts -- the kind used to drive engine accessories on newer
cars -- are highly efficient and can handle huge amounts of power, up to 1000 hp. Best of luck to ADK with his project. Regards, Gordon. "cavelamb" wrote in message Hmmmmm. If you are stating that a "serpentine belt", one that is a about 1 inch wide and is used in most current vehicles will transmit 1000HP you might need to get another very stiff drink. !!!!!!!!!!!!!!!!!!!!!!!!!!! G Ben |
#47
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Torsional Vibration and PSRU Design
In the real world I'm an automobile dealer, and it is
not very interesting. However, it is a hell of a lot more profitable than getting in the PSRU business, so the only Suzuki you can buy from me comes with 4 tires and a horn. The JN-4C project had several goals. A friend wanted to proof custom software written to model torsional vibration in a complex aircraft drive system (a pusher with with long shafts, might I add). I wanted a new and improved PSRU. So, we modeled the old drive and then altered the model for optimum predicted results. Then I designed a drive to match the model inputs, built it, and ran it with telemetry to check the accuracy of the predictions. Along the way we developed a damper and tested it, ran two different props while I had the telemetry, played with strobing linear vibration, and a whole bunch of other stuff. End result was proven software and a pretty good PSRU, plus an education. Dan /////////////////////////////////////////////// Ya know, I really like this guy. This is "experimental" aviation at its finest............................................ . Only in America !!!!!!!!!!!!!!!! Ben. |
#48
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Torsional Vibration and PSRU Design
What was this rig-up on?
What is it you do, Dan? It sounds interesting. It is refreshing to hear from someone who knows his stuff, unlike some other poster, as of late! g -- Jim in NC I can certainly second all of that--and add that I have deservedly stood corrected for blindly reciting a few things I heard of read on an earlier and vaguely related thread. Pete |
#49
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Torsional Vibration and PSRU Design
Dan Horton wrote:
Gordon says: It works because the springs have a preload of a certain force and will compress only when torsional oscillations reach a certain amplitude. Gordon, this is YOUR lucky day! You've bumped into the only guy on the net who has actually measured the spring rate of clutch disks. Gosh, Subaru, Chevy truck, Ford truck, Suzuki, a few others too! Let's look at the spring data for an EA81 2WD clutch disk. No trouble, got it right here on my hard drive. Everybody draw a plot, torque up the left side, degrees rotation across the bottom. Ready? Draw a straight line from 0-0 to 40 ft-lbs at 3.5 degrees, and from there, proceed straight to 162 ft-lbs at 6 degrees. At a tad past 6 degrees, the springs bottom and the spring rate becomes near infinite. Gordon, you got that? Please show us the "preload" that "will compress only when torsional oscillations reach a certain amplitude". Dan Gordon, if you preload the springs, what will protect your transmission when you rev the engine and pop the clutch? -- This is by far the hardest lesson about freedom. It goes against instinct, and morality, to just sit back and watch people make mistakes. We want to help them, which means control them and their decisions, but in doing so we actually hurt them (and ourselves)." |
#50
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Torsional Vibration and PSRU Design
Gordon Arnaut wrote:
Dan, I wasn't talking about bottoming the springs, just compressing them in order to introduce flexibility into the coupling and bring about a change in the critical frequency of the system as a whole. And this is exactly what happens. The springs do compress at a certain point when the twisting oscialltions overcome the spring pressure and rotational inertia. Up until that point the springs are not compressed and the coupling is in effect a solid coupling. As soon as the springs are compressed we have a flexible coupling that changes the critical frquencies of the system. Most clutches I've ever seen, you can easily twist the springs with no problem. Some will even rattle just slightly. Could you tell us what protects the transmission gears when someone revs the engine and pops the clutch? How could this component do it's job if it were already preloaded? -- This is by far the hardest lesson about freedom. It goes against instinct, and morality, to just sit back and watch people make mistakes. We want to help them, which means control them and their decisions, but in doing so we actually hurt them (and ourselves)." |
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