If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#71
|
|||
|
|||
Thanks for the generous words, Richard.
Keep in mind I'm not an expert in this field, but this process of thinking the issues through logically is also helping me to understand things better. I may well be wrong on some points, and I encourage those with more experience in this field to chip in and make corrections or contributions. Regards, Gordon. "Richard Lamb" wrote in message oups.com... 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 |
#72
|
|||
|
|||
Just one more post script about the Lycoming engine. It's interesting to
note that it is the prop that is the source of the excitations that set the crankshaft into resonance. The fact that the engine lacks counterweights (unbalanced) does not hurt it one bit -- except for the unpleasantness of the shaking. However, when you add the spring mass of that heavy prop, then that is enough to set the crank into resonance, but only at a certain rpm -- between 2000 rpm and 2300 rpm, approximately, and only if run there for some length of time to let the resonance build up. It's interesting also that the problem can be solved by two ways, either adding balance weights to the crankshafts, which some models of the same engine employ, or using a lighter prop, like the two-blade MT, which is STC'd without rpm restrictions. So we see plainly that the resonance problem is not just an issue of engine firing pulses, which is what a lot discussion seems to fixate upon. Regards, Gordon. "Gordon Arnaut" wrote in message ... 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)." |
#73
|
|||
|
|||
Gordon Arnaut wrote:
Just one more post script about the Lycoming engine. It's interesting to note that it is the prop that is the source of the excitations that set the crankshaft into resonance. Just like it is the wheels that push an engine around so that the car can move. -- 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)." |
#74
|
|||
|
|||
Gordon Arnaut wrote:
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. That should be and indication to you to shut up and learn from your elders. But let's back up a little and try to really understand this. I don't think my earlier explanation was completely satisfactory. Now your getting it. 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. So, is the goal here to talk gibberish about as many subjects as possible in the forlorn hope that there is an outside chance that you might be right about SOMETHING!! 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. Case in point. You obviously have no idea what you're talking about. You read a science book once in high school and now consider yourself a scholar. Here's a clue. Resonation requires TWO objects. You need something to vibrate, and something to cause the vibration. from: http://en.wikipedia.org/wiki/Resonate In physics, resonance is an increase in the oscillatory energy absorbed by a system when the frequency of the oscillations matches the system's natural frequency of vibration (its resonant frequency). 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 close, yet so far away. If you'd shut up and read what you wrote up to this point, you might get it. Dampeners do nothing to help the destruction of an engine that is running at its resonant frequency. They lower the harshness of the pulses, but the point of resonancy is that the energy from each pulse is stored in the system until the next pulse arrives. So you get 3sec run time vs 0.3sec? Big whup! -- 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)." |
#75
|
|||
|
|||
Ernest,
First off all, I described resonance perfectly accurately: You just pulled out an encyclopedia definition that worded it another way and said exactly what I said, that an object will resonate at its natural frequency, which causes its oscillations to increase in amplitude. Your saying that I am wrong is simply not true, and shows that you are simply acting out of spite -- as demonstrated by the bitter and combative tone of your message. Just for the record, my discussion of resonance in engines is light years beyond your silly encyclopedia blurb. I explained that in order to understand resonance, it is useful to think in terms of excitation forces -- power pulses, imbalance and outside spring mass systems, such as a propeller, for example -- and restraining forces, such as mass, stiffness and dampening. You are wrong on several fundamental points: first, a damper is a restraingin force and will do just that, it will dampen the oscillations resulting from resonation. It does this by clipping the harmonics, which are what cause the amplitutde of the oscillations to grow. A damper can be a spring or an elastomer tuned to the resonant frequency, or even a flywheel, which relies on inertia to dampen the oscillations. A sprag clutch which allows freedom of movement in one direction but not the other, also clips oscillations. All of these dampers work by counteracting the energy of the harmonics. Without the energy of the harmonics, the oscillations cannot increase. Point blank question: Are you are saying that if you bring an object to resonance, there is nothing that damping can do to restrain the oscillations? I am asking again because that is exactly what you said. I just want to confirm because this is an elementary point of understanding resonance and if you don't understand this, then...well, I think we can draw our own conclusions... Also plain wrong is that resonance requires two objects. Resonance requres only one object and an excitation force acting on it. Period. An object is not a force. Also, I see now that your are unable to discuss politely --throwing around personal insults like confetti. Nice. Also, what right do you have to demand that I "shut up?" Why don't you shut up? Regards, Gordon. "Ernest Christley" wrote in message .com... Gordon Arnaut wrote: 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. That should be and indication to you to shut up and learn from your elders. But let's back up a little and try to really understand this. I don't think my earlier explanation was completely satisfactory. Now your getting it. 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. So, is the goal here to talk gibberish about as many subjects as possible in the forlorn hope that there is an outside chance that you might be right about SOMETHING!! 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. Case in point. You obviously have no idea what you're talking about. You read a science book once in high school and now consider yourself a scholar. Here's a clue. Resonation requires TWO objects. You need something to vibrate, and something to cause the vibration. from: http://en.wikipedia.org/wiki/Resonate In physics, resonance is an increase in the oscillatory energy absorbed by a system when the frequency of the oscillations matches the system's natural frequency of vibration (its resonant frequency). 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 close, yet so far away. If you'd shut up and read what you wrote up to this point, you might get it. Dampeners do nothing to help the destruction of an engine that is running at its resonant frequency. They lower the harshness of the pulses, but the point of resonancy is that the energy from each pulse is stored in the system until the next pulse arrives. So you get 3sec run time vs 0.3sec? Big whup! -- 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)." |
#76
|
|||
|
|||
Just one more note, about Ernest's talk of an engine "running at its
resonant frequency." Just where do we see engine's running at their resonant frequency? I have to wonder because if you are driving down the highway and holding a steady rpm -- perhaps with the aid of cruise control -- I think you will want to be sure not to run at this "resonant frequency." How laughable. If this were true there would be cars beside the highway every couple of miles or so. The fact is that engines are designed not to resonate. Designers do this by taking into account excitation forces and applying countering restraining forces. Mathematically the relationship is represented by : M a + D v + K x = Me ² e sin( t - ) For simplification, the above equation can be written as: Mass term + Damping term + Stiffness Term = Restraining Force The restraining forces, represented by the various terms in the equation, are what determines how a rotor behaves throughout its operating range. Any excitation force, such as imbalance, is always in equilibrium with the restraining forces of mass, damping, and stiffness. The amount of measured vibration, as a result of these combined forces, will depend upon the combined effect of all three terms in the equation. Regards, Gordon. "Gordon Arnaut" wrote in message ... Ernest, First off all, I described resonance perfectly accurately: You just pulled out an encyclopedia definition that worded it another way and said exactly what I said, that an object will resonate at its natural frequency, which causes its oscillations to increase in amplitude. Your saying that I am wrong is simply not true, and shows that you are simply acting out of spite -- as demonstrated by the bitter and combative tone of your message. Just for the record, my discussion of resonance in engines is light years beyond your silly encyclopedia blurb. I explained that in order to understand resonance, it is useful to think in terms of excitation forces -- power pulses, imbalance and outside spring mass systems, such as a propeller, for example -- and restraining forces, such as mass, stiffness and dampening. You are wrong on several fundamental points: first, a damper is a restraingin force and will do just that, it will dampen the oscillations resulting from resonation. It does this by clipping the harmonics, which are what cause the amplitutde of the oscillations to grow. A damper can be a spring or an elastomer tuned to the resonant frequency, or even a flywheel, which relies on inertia to dampen the oscillations. A sprag clutch which allows freedom of movement in one direction but not the other, also clips oscillations. All of these dampers work by counteracting the energy of the harmonics. Without the energy of the harmonics, the oscillations cannot increase. Point blank question: Are you are saying that if you bring an object to resonance, there is nothing that damping can do to restrain the oscillations? I am asking again because that is exactly what you said. I just want to confirm because this is an elementary point of understanding resonance and if you don't understand this, then...well, I think we can draw our own conclusions... Also plain wrong is that resonance requires two objects. Resonance requres only one object and an excitation force acting on it. Period. An object is not a force. Also, I see now that your are unable to discuss politely --throwing around personal insults like confetti. Nice. Also, what right do you have to demand that I "shut up?" Why don't you shut up? Regards, Gordon. "Ernest Christley" wrote in message .com... Gordon Arnaut wrote: 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. That should be and indication to you to shut up and learn from your elders. But let's back up a little and try to really understand this. I don't think my earlier explanation was completely satisfactory. Now your getting it. 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. So, is the goal here to talk gibberish about as many subjects as possible in the forlorn hope that there is an outside chance that you might be right about SOMETHING!! 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. Case in point. You obviously have no idea what you're talking about. You read a science book once in high school and now consider yourself a scholar. Here's a clue. Resonation requires TWO objects. You need something to vibrate, and something to cause the vibration. from: http://en.wikipedia.org/wiki/Resonate In physics, resonance is an increase in the oscillatory energy absorbed by a system when the frequency of the oscillations matches the system's natural frequency of vibration (its resonant frequency). 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 close, yet so far away. If you'd shut up and read what you wrote up to this point, you might get it. Dampeners do nothing to help the destruction of an engine that is running at its resonant frequency. They lower the harshness of the pulses, but the point of resonancy is that the energy from each pulse is stored in the system until the next pulse arrives. So you get 3sec run time vs 0.3sec? Big whup! -- 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)." |
#77
|
|||
|
|||
And just one more thing.
I said previously that auto designers take resonance into account and design powertrain systems not to resonate. They do this by applying restraining forces as I mentioned in my previous message. The key thing for us in the amateur-built aircraft community is that we are putting together a powertrain piecemel. We take an auto engine, a prop, a gearbox and put them all together. Sometimes this can result in resonation of one or more pieces in the system. What I have been doing is just trying to think through the problem and apply known facts about resonance and known design principles. Regards, Gordon. "Gordon Arnaut" wrote in message ... Just one more note, about Ernest's talk of an engine "running at its resonant frequency." Just where do we see engine's running at their resonant frequency? I have to wonder because if you are driving down the highway and holding a steady rpm -- perhaps with the aid of cruise control -- I think you will want to be sure not to run at this "resonant frequency." How laughable. If this were true there would be cars beside the highway every couple of miles or so. The fact is that engines are designed not to resonate. Designers do this by taking into account excitation forces and applying countering restraining forces. Mathematically the relationship is represented by : M a + D v + K x = Me ² e sin( t - ) For simplification, the above equation can be written as: Mass term + Damping term + Stiffness Term = Restraining Force The restraining forces, represented by the various terms in the equation, are what determines how a rotor behaves throughout its operating range. Any excitation force, such as imbalance, is always in equilibrium with the restraining forces of mass, damping, and stiffness. The amount of measured vibration, as a result of these combined forces, will depend upon the combined effect of all three terms in the equation. Regards, Gordon. "Gordon Arnaut" wrote in message ... Ernest, First off all, I described resonance perfectly accurately: You just pulled out an encyclopedia definition that worded it another way and said exactly what I said, that an object will resonate at its natural frequency, which causes its oscillations to increase in amplitude. Your saying that I am wrong is simply not true, and shows that you are simply acting out of spite -- as demonstrated by the bitter and combative tone of your message. Just for the record, my discussion of resonance in engines is light years beyond your silly encyclopedia blurb. I explained that in order to understand resonance, it is useful to think in terms of excitation forces -- power pulses, imbalance and outside spring mass systems, such as a propeller, for example -- and restraining forces, such as mass, stiffness and dampening. You are wrong on several fundamental points: first, a damper is a restraingin force and will do just that, it will dampen the oscillations resulting from resonation. It does this by clipping the harmonics, which are what cause the amplitutde of the oscillations to grow. A damper can be a spring or an elastomer tuned to the resonant frequency, or even a flywheel, which relies on inertia to dampen the oscillations. A sprag clutch which allows freedom of movement in one direction but not the other, also clips oscillations. All of these dampers work by counteracting the energy of the harmonics. Without the energy of the harmonics, the oscillations cannot increase. Point blank question: Are you are saying that if you bring an object to resonance, there is nothing that damping can do to restrain the oscillations? I am asking again because that is exactly what you said. I just want to confirm because this is an elementary point of understanding resonance and if you don't understand this, then...well, I think we can draw our own conclusions... Also plain wrong is that resonance requires two objects. Resonance requres only one object and an excitation force acting on it. Period. An object is not a force. Also, I see now that your are unable to discuss politely --throwing around personal insults like confetti. Nice. Also, what right do you have to demand that I "shut up?" Why don't you shut up? Regards, Gordon. "Ernest Christley" wrote in message .com... Gordon Arnaut wrote: 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. That should be and indication to you to shut up and learn from your elders. But let's back up a little and try to really understand this. I don't think my earlier explanation was completely satisfactory. Now your getting it. 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. So, is the goal here to talk gibberish about as many subjects as possible in the forlorn hope that there is an outside chance that you might be right about SOMETHING!! 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. Case in point. You obviously have no idea what you're talking about. You read a science book once in high school and now consider yourself a scholar. Here's a clue. Resonation requires TWO objects. You need something to vibrate, and something to cause the vibration. from: http://en.wikipedia.org/wiki/Resonate In physics, resonance is an increase in the oscillatory energy absorbed by a system when the frequency of the oscillations matches the system's natural frequency of vibration (its resonant frequency). 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 close, yet so far away. If you'd shut up and read what you wrote up to this point, you might get it. Dampeners do nothing to help the destruction of an engine that is running at its resonant frequency. They lower the harshness of the pulses, but the point of resonancy is that the energy from each pulse is stored in the system until the next pulse arrives. So you get 3sec run time vs 0.3sec? Big whup! -- 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)." |
#78
|
|||
|
|||
Just one more point about dampers:
Ernest said: "Dampeners do nothing to help the destruction of an engine that is running at its resonant frequency. They lower the harshness of the pulses, but the point of resonancy is that the energy from each pulse is stored in the system until the next pulse arrives. So you get 3sec run time vs 0.3sec? Big whup!" Here are the facts: dampers do not just give you an extra few seconds of protection. They nip resonance in the bud very effectively. Yes elastomer or spring-type dampers do delay the release of energy -- both a spring or a piece of rubber store and relase energy at a later time, usually a split second, as I mentioned earlier in this discussion. However, the significance of this is not that it merely delays the onset of resonance -- and the destruction of the engine -- by three seconds, as Ernest would have us believe. The signficance is that by absorbing, then releasing the energy from resonation a split second later, the timing of the resonation is disrupted and the object cannot continue to resonate. It stops resonation in its tracks. Here's how it works: the first harmonic that appears with resonation is dampened by the spring or rubber, or sprag clutch or whatever, so that subsequent harmonics cannot be set into vibration, so the oscillation does not build. The destructive chain is broken by disrupting the timing of the first harmonic. If you look at a harmonic damper of a car engine, you will see an inner metal disk and an outer metal ring, and a strip of rubber in between. That rubber is tuned to precisely disrupt -- or clip -- harmonics at the crankshaft's resonant frequency. (This is a function of the density of the rubber compound). And this is why when you have your harmonic damper overhauled, it will come with a new strip of rubber, replacing the old strip which has hardened and aged and no longer absorbs energy at the intended frequency. Springs in a clutch plate can do the same thing if they are tuned at the same frequency as the rubber compound. A sprag clutch is another approach, so is a flywheel (which dampens out the energy from the harmonic by the force of its spinning inertial mass). The Eggenfellner gearbox uses uses this approach and has the best record so far. Ernest I am amazed that you don't even know this much, yet you have the brass to hurl unprovoked personal insults. Regards, Gordon. "Gordon Arnaut" wrote in message ... And just one more thing. I said previously that auto designers take resonance into account and design powertrain systems not to resonate. They do this by applying restraining forces as I mentioned in my previous message. The key thing for us in the amateur-built aircraft community is that we are putting together a powertrain piecemel. We take an auto engine, a prop, a gearbox and put them all together. Sometimes this can result in resonation of one or more pieces in the system. What I have been doing is just trying to think through the problem and apply known facts about resonance and known design principles. Regards, Gordon. "Gordon Arnaut" wrote in message ... Just one more note, about Ernest's talk of an engine "running at its resonant frequency." Just where do we see engine's running at their resonant frequency? I have to wonder because if you are driving down the highway and holding a steady rpm -- perhaps with the aid of cruise control -- I think you will want to be sure not to run at this "resonant frequency." How laughable. If this were true there would be cars beside the highway every couple of miles or so. The fact is that engines are designed not to resonate. Designers do this by taking into account excitation forces and applying countering restraining forces. Mathematically the relationship is represented by : M a + D v + K x = Me ² e sin( t - ) For simplification, the above equation can be written as: Mass term + Damping term + Stiffness Term = Restraining Force The restraining forces, represented by the various terms in the equation, are what determines how a rotor behaves throughout its operating range. Any excitation force, such as imbalance, is always in equilibrium with the restraining forces of mass, damping, and stiffness. The amount of measured vibration, as a result of these combined forces, will depend upon the combined effect of all three terms in the equation. Regards, Gordon. "Gordon Arnaut" wrote in message ... Ernest, First off all, I described resonance perfectly accurately: You just pulled out an encyclopedia definition that worded it another way and said exactly what I said, that an object will resonate at its natural frequency, which causes its oscillations to increase in amplitude. Your saying that I am wrong is simply not true, and shows that you are simply acting out of spite -- as demonstrated by the bitter and combative tone of your message. Just for the record, my discussion of resonance in engines is light years beyond your silly encyclopedia blurb. I explained that in order to understand resonance, it is useful to think in terms of excitation forces -- power pulses, imbalance and outside spring mass systems, such as a propeller, for example -- and restraining forces, such as mass, stiffness and dampening. You are wrong on several fundamental points: first, a damper is a restraingin force and will do just that, it will dampen the oscillations resulting from resonation. It does this by clipping the harmonics, which are what cause the amplitutde of the oscillations to grow. A damper can be a spring or an elastomer tuned to the resonant frequency, or even a flywheel, which relies on inertia to dampen the oscillations. A sprag clutch which allows freedom of movement in one direction but not the other, also clips oscillations. All of these dampers work by counteracting the energy of the harmonics. Without the energy of the harmonics, the oscillations cannot increase. Point blank question: Are you are saying that if you bring an object to resonance, there is nothing that damping can do to restrain the oscillations? I am asking again because that is exactly what you said. I just want to confirm because this is an elementary point of understanding resonance and if you don't understand this, then...well, I think we can draw our own conclusions... Also plain wrong is that resonance requires two objects. Resonance requres only one object and an excitation force acting on it. Period. An object is not a force. Also, I see now that your are unable to discuss politely --throwing around personal insults like confetti. Nice. Also, what right do you have to demand that I "shut up?" Why don't you shut up? Regards, Gordon. "Ernest Christley" wrote in message .com... Gordon Arnaut wrote: 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. That should be and indication to you to shut up and learn from your elders. But let's back up a little and try to really understand this. I don't think my earlier explanation was completely satisfactory. Now your getting it. 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. So, is the goal here to talk gibberish about as many subjects as possible in the forlorn hope that there is an outside chance that you might be right about SOMETHING!! 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. Case in point. You obviously have no idea what you're talking about. You read a science book once in high school and now consider yourself a scholar. Here's a clue. Resonation requires TWO objects. You need something to vibrate, and something to cause the vibration. from: http://en.wikipedia.org/wiki/Resonate In physics, resonance is an increase in the oscillatory energy absorbed by a system when the frequency of the oscillations matches the system's natural frequency of vibration (its resonant frequency). 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 close, yet so far away. If you'd shut up and read what you wrote up to this point, you might get it. Dampeners do nothing to help the destruction of an engine that is running at its resonant frequency. They lower the harshness of the pulses, but the point of resonancy is that the energy from each pulse is stored in the system until the next pulse arrives. So you get 3sec run time vs 0.3sec? Big whup! -- 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)." |
#79
|
|||
|
|||
"Ernest Christley" wrote So, is the goal here to talk gibberish about as many subjects as possible in the forlorn hope that there is an outside chance that you might be right about SOMETHING!! gordon likes to see himself type and post. He is so totally clues, it is hard to know where to start. My solution? Just let himself talk to himself. It is solution I wish everyone here would adopt, and after a while with nobody playing with him, he might go away. -- Jim in NC |
#80
|
|||
|
|||
Bashir,
Actually, I spoke too quickly when I conceded a mistake. Tautness and stiffness are two different things. A taut string will vibrate at a higher frequency than a loose string, but we have not changed its inherent stiffness or elasticity (e). If you increase the stiffness (decrease the elasticity) of an object, you will decrease its resonant frequency, as I first stated. The resonant frequency of a system is symbolized by "w n" and pronounced "Omega-sub-n". An object's mass and elasticity determines its resonant frequency, and is expressed mathematically as: wn = ?(k/m) K is the value for elasticity, while m is the value for mass. So we see that lower elasticity (greater stiffness) results in a lower frequency of resonation. So making a crankshaft stiffer does decrease the rpm at which it will resonate. It also increases the value of restraining force acting against excitaiton. So the benefits are cumulative. We can see a real-world example of this in V-8 engines which would not last very long without a harmonic damper, even though they have much smoother torque pulses than a 4-cylinder. The reason is that the crankshaft has to be much longer and thereby less stiff -- or more elastic. On most four-cylinder engines, dampers are not needed because the short, stout crank actually resonates at a frequency below the oeprating range. Hence resonance will never be encountered. It's useful at this point to back up and define what resonant frequency of an object -- or system -- really means. Stated most simply it is the frequency at the object or system will vibrate if it is excited by a single pulse. The actual torsional resonance of an engine can be calculated if you know the torsional rate of the crankshaft (which is its spring value) and its mass moment of inertia, which is a function of crank stroke and weight, number of journals, dimensions of the flywheel, torsional absorber, accessories. So now we know a little about resonance and how it affects a crankshaft. But what happens when we attach a propeller or gearbox-propeller combination to that engine? Well, now we are dealing with not just an object but a system. And this system has its own torsional resonance frequency, which is different from that of the single object itself, like the crankshaft. A key concept here is tranmissibility, which is the ratio between the amplitude of the excitation torque, and the amplitude of the output torque. In simple terms, this means that the gearbox and propeller can be subjected to vibratory forces many times higher than the torque peaks produced by the engine. Here is where damping comes in. But even with damping there will be some amplification of vibratory forces transmitted from the gearbox to the gearbox and prop. There is some good reading at this website, with specific info on how torsional resonance is dealt with in designing aircraft PSRU systems: http://www.epi-eng.com/BAS-VibBasics.htm Regards, Gordon. "Bashir" wrote in message oups.com... He can be taught!! Who would have thought it!? |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Diesel aircraft engines and are the light jets pushing out the twins? | Dude | Owning | 5 | October 7th 04 03:14 AM |
The light bulb | Greasy Rider | Military Aviation | 6 | March 2nd 04 12:07 PM |
Light Twins - Again - Why is the insurance so high? | Doodybutch | Owning | 7 | February 11th 04 08:13 PM |
Light Twins. How soft??? | Montblack | Owning | 19 | December 3rd 03 10:38 PM |
Light Twins. How soft??? | Montblack | Piloting | 19 | December 3rd 03 10:38 PM |