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#61
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"Ernest Christley" wrote You don't listen to or read the work of others, and you like to read your own writing way to much. It's called "The world according to Arnaut." Ain't it grand? chuckle -- Jim in NC |
#62
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Very impressive, Gordon.
Probably the most understandable description of harmonics and resonance, and how they can destroy stuff that I've ever seen here. Thanks Richard |
#63
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Gordon Arnaut wrote:
So more stiffness equals a lower resonating frequency. Do you want to think that one through again, Gordon? |
#64
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Thanks for pointing out my typo.
As stiffness increases so does the frequency at which it resonates, as we see on a taut guitar string, or drum, which increase in pitch as they are tightened. Regards, Gordon. "Bashir" wrote in message oups.com... Gordon Arnaut wrote: So more stiffness equals a lower resonating frequency. Do you want to think that one through again, Gordon? |
#65
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Jim,
Yes there is some confusion between a harmonic balancer and damper, but the device I'm referring to uses an elastomer tuned to a specific frequency to clip harmonic resonance. This has nothing to do with balance, which can be addressed in the ways you mentioned -- counterweights, balance shafts, etc. An engine can make a lot of vibration and shaking (up and down and side to side) and not have a problem with torsional resonance. The opposite is true too -- a very smooth engine can all of a sudden break its torque shaft from torsional resonance. That's why torsional resonance is known as the silent killer. Regards, Gordon. "Jim Carriere" wrote in message .. . Gordon Arnaut wrote: For example, most V-8 engines come with a harmonic balancer, even though they have four power pulses for each crankshaft rotation. That's because there is enough flex in the crankshaft that the crank can begin to resonate at some rpm within the operational range. Actually, V-8s have a harmonic balancer because they would otherwise have a first order imbalance. The physics explanation is pretty long and doesn't make a lot of sense anyway, but it's because the crankpins are 90 degrees apart (inline fours don't have this imbalance because the pins are in pairs 180 degrees apart, but they have second order imbalance instead... that is what a pair of balance shafts cures) and the mass on the ends of those crankpins (rods and pistons) flinging around are at different distances along the crankshaft. Also, the harmonic balancer on a V-8 is two weights, one on each end of the crankshaft. A lot of people don't realize there are two weights, not just the one on the front of the engine. Harmonic dampers are a different animal. They will smooth out power pulses on any engine configuration. Harmonic balancers have nothing to do with power pulses and everything to do with complicated vibration of large pieces of metal moving back and forth in different directions and different places. I think the terms balancer and damper are confused with each other because they look almost the same- a big part attached to the front of the crankshaft to make the engine smoother. |
#66
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He can be taught!! Who would have thought it!?
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#67
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"Bashir" wrote in message oups.com... He can be taught!! Who would have thought it!? Heaven forbid you should have written that what he said was crap! chuckle -- Jim in NC |
#68
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Ernest,
I have read Tracy Crook's piece on torsional resonance, even before you pointed to it. A couple of thoughts. First, Tracy has devised a good solid gearbox that has proven itself in service with a respectable number of flight hours. He is absolutely correct in pointing out that the crankshaft is a spring mass, as I have said earlier. So is the propeller, and the gear shaft of the transmission. Any complex piece of machinery is a combination of a number of spring masses, each with its own resonant characteristics. But let's back up a little and try to really understand this. I don't think my earlier explanation was completely satisfactory. The key thing to understand first is that any object will vibrate if force acts on it to displace it in some way. In astrophysics we know that the biggest objects in the universe vibrate, and even the universe itself vibrates -- and has left a trace of its vibrations as it expanded after the big bang. A guitar string vibrates if you displace it with a pick. An engine vibrates from power pulses. Even an electric motor vibrates from the power pulses of its magnets. But vibration is not resonation. Resonation is when an object vibrates at its particular resonant frequency, at which point the harmonics (which are vibrations that are integer multiples of the fundamental frequency; the second harmonic is twice the frequency of the fundamental; the third is trhree times, etc.) build on top of one another and lead to ever greater amplitude of the vibration (the string moves back and forth in an ever-wider arc until it breaks). We see this in a guitar string when it breaks unexpectedly while we are tuning it. As we were turning the tuning knob we just happen to hit the resonant frequency and the string suddenly hear an increae in volume (and a much richer, almost howling sound) and the vibration of the string gets visibly bigger unti it snaps. But if you just try to break that string by turning the knob tighter while keeping the string perfectly still, you would be surprised how much force it would take to snap that string in tension. On thicker strings, like on a bass, you would not have enough strength in your hand to do it, despite the help from the mechanical advantage of the gear knob. It's the same thing with a crankshaft, except that the vibration is a twisting back and forth of the shaft, rather than a swinging side to side like on a string or a tuning fork. That crankshaft is going to be vibrating with every power pulse because each power pulse exterts a force on the lever arm of the crankpin which causes a twisting of the shaft. And in the split second after the power pulse subsides, the shaft will swing back twisting back beyond neutral -- just like a guitar string when you displace it swings to both sides of its neutral axis as it vibrates. So the crankshaft will be flexing and vibrating at all times when the engine is running. Is this bad? No. This has nothing to do with resonance. Yet this is where all the confusion comes in. An earlier poster pointed out that V-8 engines use balancers in order to smooth out imbalances and lessen vibration and shaking. This is desirable because we would all rather have an engine or any piece of equipment that vibrates less not more. But this does nothing to address the problem of resonance. So what causes the crankshaft to get to the point where it starts resonating? Well, just like our guitar string it needs to be displaced with enough force and in the right way -- force applied at just the right number of times per second, or its frequency. Once this is accomplished, the crankshaft will begin to resonate. It will literally ring like a bell, with the harmonic notes all coming out and all of them joining together to cause the amplitude of the vibration to intensify (the crank will begin to twist back and forth in a wider and wider arc, just like the guitar string going berserk.) At some point, the crank cannot twist anymore and it will break. Now if we were concerned about avoiding this situation how would we proceed? Well, we know that an object's resonant frequency is related to its mass. If we make a bigger crankshaft it will resonate at a lower frequency, hoepfully below the actual operating rpm of the engine. But this isn't alway possible. Another approach is to make that shaft stiffer, which will actually increase the shaft's resonant frequency, but also has a much more important benefit. It now takes much more force to displace it, or bend it from its neutral axis. Think of a very thick bass guitar string that is tightened as tight as you can get it. Now try to pluck that string. You can't get it to vibrate, because you don't have enough force in your finger to displace it far enough from its neutral axis that it will vibrate. So if you can make the crankshaft stiffer, it will take more force to cause vibration, perhaps more force than the power pulses of the engine will produce and then resonance can never set in. So the engine manufacturers do take this into consideration and design crankshafts that will not fail from resonance. So what's the problem? Well the problem is when we go to attach something to the engine. And engine isn't much good unless it is hooked up to something and doing something useful -- like driving a propeller. And that's where things get complicated, because now we are adding another spring mass to the system. We now have two possible problems, either the engine can set the propeller into resonance, or the propeller can set the crankshaft into resonance. And we haven't even got to the gearbox yet. Just the prop and engine is enough of a problem that each engine and prop must be tested and certified as a combination. You can't just bolt on any certified prop any certified engine. This is also why we see homebuilt aircraft breaking crankshafts or props. So now when we add a gearbox too, we have multiplied the possible scenarios that can go wrong. Both the crankshaft and prop are vibrating springs, with the gearbox in the middle. What is required is a design approach similar to the powertrain approach used in auto industry. However, very few people in the homebuilding community are trained in the nuances of this particular discipline (including me). So we have people of various technical abilities trying to tackle this problem in a bootstrap, eyball engineering kind of way. For example, Crook talks in his article about how springs in a clutch plate do not work satisfactorily becuase they would be tuned for only one frequency, while the engine oeprates over a wide range. Yes, this is true if you are concerned about elimintaing the harshness of everyday vibration. But if you want to stop resonance, then all you need to do is exactly that -- tune the damping device for that one frequency. Here again we see the issue of harshness and vibration clouding the issue of resonance. In any case, it is not an easy problem and the automakers have a lot fo very thoruoghly trained people working out drivetrain issues for every new combination of engine and drivetrain -- and they don't even have a prop at the other end. Regards, Gordon. ] "Ernest Christley" wrote in message .com... Gordon Arnaut wrote: Ernest, You are right that springs and elastomers do not technically dampen kinetic energy, they simply store and relase it at a later time (how much later depends on the frequency at which it is tuned). However, both springs and elastomers can achieve our objective of clipping of destructive harmonics if they are tuned to the resonant frequency of the object that we want to protect from resonance. You don't listen to or read the work of others, and you like to read your own writing way to much. http://rotaryaviation.com/PSRU%20Zen%20Part%202.html -- 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)." |
#69
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Just a quick additional note to clarify my point about stiffness.
Stiffness is a restraining force that acts against an excitation force -- as we see in the taut guitar string. Mass and damping are also restraining forces. So it is not just that it takes more force to displace a stiff object enough to set it to vibrating -- it is technically correct to think of stiffness as a restraining force. Also, since we are talking about excitation versus restraint it should be noted that most discussions of torsional resonance fixate on the engine power pulses as a source of exciation. This is quite erroneous because as soon as we add a propeller we have an object with a very large moment arm and hence inertial mass that can -- and does -- produce very powerful excitation. The gearbox itself can also be a source of excitation because it too has inertia and mass, although far less than a propeller. Another source of excitation is imbalance. We see this in the non-counterweighted Lycoming engine in which this imbalance creates enough of an excitation that, when combined with the excitation of a propeller can set the crank into resonance, resulting in a broken crank. It should be added that no piston engine is perfectly balanced. That's why even a V-8 needs a balancer. On an opposed engine, the opppoosing cylinders do not balance each other perfectly either because they are not operating on the same plane. The result is a rocking couple and second order imbalance. Apparently this is where the rotary has a big advantage because it can be brought into perfect dynamic balance. But the important thng to remember is that imbalance is just one of the excitation forces that can contribute to resonance -- although not usually in a big way. Also, it is useful to remember that if you add a gearbox to the engine, the concern will be with excitations coming from both the engine and prop and setting a gear shaft into resonance. This is why redrives have such a dismal record. Regards, Gordon. "Gordon Arnaut" wrote in message ... Ernest, I have read Tracy Crook's piece on torsional resonance, even before you pointed to it. A couple of thoughts. First, Tracy has devised a good solid gearbox that has proven itself in service with a respectable number of flight hours. He is absolutely correct in pointing out that the crankshaft is a spring mass, as I have said earlier. So is the propeller, and the gear shaft of the transmission. Any complex piece of machinery is a combination of a number of spring masses, each with its own resonant characteristics. But let's back up a little and try to really understand this. I don't think my earlier explanation was completely satisfactory. The key thing to understand first is that any object will vibrate if force acts on it to displace it in some way. In astrophysics we know that the biggest objects in the universe vibrate, and even the universe itself vibrates -- and has left a trace of its vibrations as it expanded after the big bang. A guitar string vibrates if you displace it with a pick. An engine vibrates from power pulses. Even an electric motor vibrates from the power pulses of its magnets. But vibration is not resonation. Resonation is when an object vibrates at its particular resonant frequency, at which point the harmonics (which are vibrations that are integer multiples of the fundamental frequency; the second harmonic is twice the frequency of the fundamental; the third is trhree times, etc.) build on top of one another and lead to ever greater amplitude of the vibration (the string moves back and forth in an ever-wider arc until it breaks). We see this in a guitar string when it breaks unexpectedly while we are tuning it. As we were turning the tuning knob we just happen to hit the resonant frequency and the string suddenly hear an increae in volume (and a much richer, almost howling sound) and the vibration of the string gets visibly bigger unti it snaps. But if you just try to break that string by turning the knob tighter while keeping the string perfectly still, you would be surprised how much force it would take to snap that string in tension. On thicker strings, like on a bass, you would not have enough strength in your hand to do it, despite the help from the mechanical advantage of the gear knob. It's the same thing with a crankshaft, except that the vibration is a twisting back and forth of the shaft, rather than a swinging side to side like on a string or a tuning fork. That crankshaft is going to be vibrating with every power pulse because each power pulse exterts a force on the lever arm of the crankpin which causes a twisting of the shaft. And in the split second after the power pulse subsides, the shaft will swing back twisting back beyond neutral -- just like a guitar string when you displace it swings to both sides of its neutral axis as it vibrates. So the crankshaft will be flexing and vibrating at all times when the engine is running. Is this bad? No. This has nothing to do with resonance. Yet this is where all the confusion comes in. An earlier poster pointed out that V-8 engines use balancers in order to smooth out imbalances and lessen vibration and shaking. This is desirable because we would all rather have an engine or any piece of equipment that vibrates less not more. But this does nothing to address the problem of resonance. So what causes the crankshaft to get to the point where it starts resonating? Well, just like our guitar string it needs to be displaced with enough force and in the right way -- force applied at just the right number of times per second, or its frequency. Once this is accomplished, the crankshaft will begin to resonate. It will literally ring like a bell, with the harmonic notes all coming out and all of them joining together to cause the amplitude of the vibration to intensify (the crank will begin to twist back and forth in a wider and wider arc, just like the guitar string going berserk.) At some point, the crank cannot twist anymore and it will break. Now if we were concerned about avoiding this situation how would we proceed? Well, we know that an object's resonant frequency is related to its mass. If we make a bigger crankshaft it will resonate at a lower frequency, hoepfully below the actual operating rpm of the engine. But this isn't alway possible. Another approach is to make that shaft stiffer, which will actually increase the shaft's resonant frequency, but also has a much more important benefit. It now takes much more force to displace it, or bend it from its neutral axis. Think of a very thick bass guitar string that is tightened as tight as you can get it. Now try to pluck that string. You can't get it to vibrate, because you don't have enough force in your finger to displace it far enough from its neutral axis that it will vibrate. So if you can make the crankshaft stiffer, it will take more force to cause vibration, perhaps more force than the power pulses of the engine will produce and then resonance can never set in. So the engine manufacturers do take this into consideration and design crankshafts that will not fail from resonance. So what's the problem? Well the problem is when we go to attach something to the engine. And engine isn't much good unless it is hooked up to something and doing something useful -- like driving a propeller. And that's where things get complicated, because now we are adding another spring mass to the system. We now have two possible problems, either the engine can set the propeller into resonance, or the propeller can set the crankshaft into resonance. And we haven't even got to the gearbox yet. Just the prop and engine is enough of a problem that each engine and prop must be tested and certified as a combination. You can't just bolt on any certified prop any certified engine. This is also why we see homebuilt aircraft breaking crankshafts or props. So now when we add a gearbox too, we have multiplied the possible scenarios that can go wrong. Both the crankshaft and prop are vibrating springs, with the gearbox in the middle. What is required is a design approach similar to the powertrain approach used in auto industry. However, very few people in the homebuilding community are trained in the nuances of this particular discipline (including me). So we have people of various technical abilities trying to tackle this problem in a bootstrap, eyball engineering kind of way. For example, Crook talks in his article about how springs in a clutch plate do not work satisfactorily becuase they would be tuned for only one frequency, while the engine oeprates over a wide range. Yes, this is true if you are concerned about elimintaing the harshness of everyday vibration. But if you want to stop resonance, then all you need to do is exactly that -- tune the damping device for that one frequency. Here again we see the issue of harshness and vibration clouding the issue of resonance. In any case, it is not an easy problem and the automakers have a lot fo very thoruoghly trained people working out drivetrain issues for every new combination of engine and drivetrain -- and they don't even have a prop at the other end. Regards, Gordon. ] "Ernest Christley" wrote in message .com... Gordon Arnaut wrote: Ernest, You are right that springs and elastomers do not technically dampen kinetic energy, they simply store and relase it at a later time (how much later depends on the frequency at which it is tuned). However, both springs and elastomers can achieve our objective of clipping of destructive harmonics if they are tuned to the resonant frequency of the object that we want to protect from resonance. You don't listen to or read the work of others, and you like to read your own writing way to much. http://rotaryaviation.com/PSRU%20Zen%20Part%202.html -- 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)." |
#70
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Well, MY last post on this subject was accused of being pornography...
Some people have NO sense of humor Richard |
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