Torsional Vibration and Resonance - Basic Theory and Issues

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Lendo

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P-99, A good find, I love it. Will it ever be any good for Aircraft, who knows.
George
 

karmarepair

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I realize it's difficult, dangerous even, to make general statements in this area, but are "Giubo" AKA "Rotaflex" couplings IN GENERAL torsionally soft enough for this decoupling function?
Now for getting the resonance to low rpm, ideally we want the first resonance order signficantly below idle. That is what we do in cars. And we do this by putting a soft spring between the flywheel mass and the transmission input, so the geartrain is isolated from the firing pulses. Put resonance at least a half-octave below idle and you will have largely isolated the engine from everything downstream of the spring. The spring must be able to carry all of the engine torque plus the oscillation of the crank times the spring rate, which is the torque ripple that is left after the spring. And the spring rate is chosen to drive the resonance safely below idle. If the resonance is up in the operating range (even at the low end), any lash in the geartrain, splines, etc, will be exercised and be noisy, even if not destructive, and you have to go through the resonance when you shove the throttle up - some systems can get hung at the resonance point. Put the resonance down below idle and you go through it once as the engine comes up to idle after cranking/firing, and again when you turn it off. You already see this in the engine mount system going through resonance on start and on stop. The rotating parts are nicely handled this way too.

So, the system has to be designed to have a suitably soft element to get the resonance below idle. If the soft element is not soft enough, the resonance only gets moved around in the operating range... Steel springs, rubber springs, etc will all work if you can get low enough spring rate and high enough torque capacity. A number of PSRU out there have rubber donuts or other elements that seem to work pretty well. Some rubber spring designs actually become rising rate springs: At low torque (and low rpm), the spring is soft and keeps the resonance below idle; as torque comes up also rpm goes up, and the spring gets stiffer, but the resonance is still safely below operating speed. This obviously has to be DESIGNED or LABORIOUSLY DEVELOPED. Tracey Crook did it. If you want a 160 hp Mazda rotary scheme see if Pat Panzera will work with you a little. Size things improperly and you will either destroy the rubber elements or have resonance in the operating range or both. Oh, and then there is that pesky thing that can happen on a fast descent - while the power is pulled back, the torque is high (but in the opposite direction to when power is being made), but the windmilling prop is running along at half to two-thirds of cruise speed - you can get into resonance if the spring rates are a tad too high.

<snipped - RRY>

Billski
 

pictsidhe

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I realize it's difficult, dangerous even, to make general statements in this area, but are "Giubo" AKA "Rotaflex" couplings IN GENERAL torsionally soft enough for this decoupling function?
There is no general. But that doesn't mean that they can't be used. You'd need to calculate the required compliance and see if there is a suitable one.
 

wsimpso1

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I realize it's difficult, dangerous even, to make general statements in this area, but are "Giubo" AKA "Rotaflex" couplings IN GENERAL torsionally soft enough for this decoupling function?
You can not just say "any will do". Let's remember that they are not designed to do what you are planning on. We are taking advantage of some lucky coincidences. Several characteristics all have to be acceptable:
  • Torque Capacity - The ones that are OEM drive shaft and half shaft joints will usually have way more strength than needed, but there are other rubber couplers that may be too small. Then there is the issue of at what torque will the guibo go to very high spring rate - bottoming out? This has to happen at least above 1-1/4 of max engine torque;
  • Benign Failure Mode - In the unusual circumstance of one of these failing, we still want the system to transmit flying power. If it just goes "twing" and you are now flying a glider, that is bad. This is both a function of guibo shape and design of the rest of the coupling;
  • Inspectable - Can you check it during preflight for issues?
  • Spring Rate - It needs to have a low enough spring rate (combined with engine side inertia and prop side inertia) to put resonance at least 1-1/2 octaves below your engine's min firing frequency. If it is a rising rate spring, the resonance at all engine torques still has to stay 1-1/2 octaves below min firing frequency as you go up the torque curve. This is not trivial. Examples:
  • One builder of turn key powerplants has told us that flywheel inertia and ground idle speed in his engines are set to avoid resonance, and that they deliberately used a heavier flywheel to achieve this;
  • Another builder's coupling used cut up guibos to make his coupling - info obtained during inspection at a fly-in;
  • And our own RV6EJguy modified his EJ22 flywheel to get more vibe margin at low engine speeds, with the benefit of smoother idle.
If you intend to use OEM guibos, good practice is to build a test rig to get spring rates and demonstrate lots of excess torque capacity, then do the math to establish resonance frequencies and safe margins. You may have to buy and test a few of these guys or even start modifying them to get acceptable behavior.

Billski
 
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Billrsv4

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Billski, Karma,
I have used "giubo" couplings on race cars and some old road cars. Some Rotoflex couplings as well. These usually begin well but I have seen way to many failures with extended use. I should mention that I did not design these parts into these vehicles, they were applied by others. Something to note is that even the manufacturers of the cars using these couplings will often add a "safety" by making a loose ball and socket joint that fits within the Giubo. these were frequently used in place of a CV joint on half shafts on some small formula cars. While these devices can be well designed, the Rotoflex coupling with two direction bolting being more to my liking. I would never use one on my airplane. Their failure can be unexpected and catistrophic. The older ones with many heat cycles most of all. Not saying they can't work, just that in my experience I've seen too many failures
Bill Jepson
 

mullacharjak

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Rotoflex or flexible couplings are used on the Rotax C gearbox.I havent seen any common information that they fail routinely. I have flown behind a C gearbox for 250 hours (Rotax 582) without any problem or maybe our engine was the one lucky exception.
That airplane is still being flown but not by me with the original rubber.
Rotax gearbox coupling has no safety backup.
 

pictsidhe

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Rotoflex or flexible couplings are used on the Rotax C gearbox.I havent seen any common information that they fail routinely. I have flown behind a C gearbox for 250 hours (Rotax 582) without any problem or maybe our engine was the one lucky exception.
That airplane is still being flown but not by me with the original rubber.
Rotax gearbox coupling has no safety backup.
Rotax does the kind of engineering for their products that no homebuilder has the resources for.
 

Billrsv4

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I want everyone to know that when the proper coupling is sellected you will likely get many trouble free hours. The rotax 582 is not a high horsepower (relatively) engine which makes selection much easier. The engines and vehicles I have been dealing with are generally 200 HP and up. One other big factor on these parts is how long they have been sitting on a shelf somewhere. I installed one on a drive shaft that failed within 20 hours. The part in question was in a Fiat, obviously had some shelf time and was manufatured in Turkey. The Rotoflex used on your Rotax was likely fresh and new, and sized large enough to allow you some margin for use with that engine. I would also be willing to bet that Rotax took the specs from the manufacturer and never did a multi-year test on it. What I am saying is don't try to use tha lightest possible version of the coupling for your horsepower. Any engineer will allow for SOME margin depending on their risk acessment. Also many of these parts are on ultralights. Service life will depend a lot on where you keep the plane. Do you take it down, (folding wing version?), and store it in your garage? In a hangar? Or outdoors in a tiedown? exposure to the elements can effect flex couplings a lot. And even if you have had good service I'd suggest making it a regular matenance item. Replace them every time you get a BFR just to be safe.
T.O. Bill
 

Lendo

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Billrsv4,
There is a planetary available here in Australia but looks to be designed and parts manufactured in the States, and looks to be well made. Their in the process of changing their Damper (sounds ominous) and I asked for a photo of it as they claim the drive stays engaged, even if the damper fails. I guess it is allowed to bottom-out in the event of failure.
BTW I haven't had a response on that request - sadly!
George
 

Billrsv4

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George,
I looked over that planetary. The manufacture of that planetary looks excellent. They don't show the asorber/dampener in their video. I hope they find an adaquate solution. I wish them well.
I must mention though my friend that was one of the partners of the original Powersport has told me REPEATEDLY that every planetary they ran had problems. Even the carefully constructed one off straight cut tight tolerance units. They eliminated the problem with a very complicated pendurolus dampener. This is just like the dampener used in large radials. It worked but cost a ton of money. The problems they had, caused their complete reversal to an internal ring gear stiff system. I have the utmost respect for Steve and Everett because very few people would have pulled back and committed to a redesign. Their problems were only showing up after around 100-200 hours. They had more time behind a wankel rotary in an aircraft than anyone. They are and were true professionals that would only commit to selling their product when it was trouble-free. I maintain that if Everett had not been involved in a fatal accident, (not related to the engine package). They would be the most common alternate engine on the market today. Lots of folks on HBA and elsewhere discount the problems of torsional vibration citing "a friend" or "someone they know" who is flying successfully on a shoestring. When you investigate you find they haven't flown much. Or at high power levels. You must be able to fly at 100% for hours or you must accept the risk of failure. I am not timid and really know my powerplants. I do not discount the need to have the package checked for resonances in the operating range.
That buzzkill stated I must say that I like the CAD design renderings of the Kiwi/American gearbox I've seen. I hope they overcome any teething problems and are very successful. I know it isn't easy as I've been trying to just bring back a successful design and so far haven't been able to afford to.
T.O. Bill
 

Lendo

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Thanks Bill,
Good information for the discussion. Financial limitations are always the issue, I haven't been able to win Lotto yet - I'm sure it's my turn :).
If I win your the first man I'm talking to. Have you tried 'Go Fund Me'- although the target customer base is limited.
George
 

Billrsv4

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George and Group,
As stated before I'm working on a New/Old PSRU myself to the point of doing blueprints and getting quotes. Steve who owns the intelectual property requested that the gearbox be self financed and tested before offering it to the public. NO VAPORWARE. I am honoring his wishes and avoiding a "go fund me" or other online fund raising effort.
Bill
 

GESchwarz

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Wow! So much to read and absorb! I have been building my airframe for 10.5 years and it is getting to be time to work fwd of the firewall. I have lots of questions. I would like to speak to someone who has got their system up and running nicely and can tell me how to do the same.
 

rhbelter

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Ahoy, GE,

I suggest that you go to: http://www.epi-eng.com/ The web site of Jack Kane, who is THE foremost PSRU designer/builder existent. Digest his 'advanced-degree-material' and then continue.
Enjoy /s/ Bob Belter EAA 8444
 

Flow

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Another anecdote from the automotive world I remember talking to Bill about is in the Mazda B power train racing where I have seen multiple examples of race track reliability for years with gearboxes being run at double the factory torque specs only to destroy two gearboxes in one weekend after switching to lighter twin plate solid center clutch / flywheel systems. Stepping back to single plate, sprung center systems, still with lightened flywheels I might add and straight back to reliability. Pulsed torque deliveries are such a great way to shatter hardened metal aren't they.
 

Geraldc

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What is the difference in loading between a hydraulic pump and a propellor?
25 years ago I built a logsplitter with a 4 cylinder 60hp diesel coupled to a hydraulic pump with a centafex coupling.
The machine has been in commercial use in all that time and has had the motor replace but is still on the original coupling.
It has always run at a constant speed .
 

wsimpso1

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The prop has orders of magnitude more inertia. The pump has pulses per rev based on the number of chambers it has, while the prop can have a variety with the biggies being things like p-factor at the blade count per rev. I bet the engine and the pump have a different number of pulses per turn of the engine. The coupling's main job is probably to take up the inevitable alignment errors between engine and pump axes.
 
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