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Fluid crankshaft damper?

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Aviacs

Well-Known Member
Joined
Mar 16, 2019
Messages
912
Has anyone explored or installed a fluidic crankshaft damper on a flat 4 VW for aviation?

For the casual naysayers - I understand that in theory short flat engines "don't need" crank dampers.
However, aviation use has proven that they are much more susceptible to crank failure from torsional resonance, even forged cranks, than as stock units installed in cars.

I'm strongly considering adding an electric starter back onto mine. It currently has a Diehl case; I also have a removed complete Aero Vee set up/mount/alternator etc with some failed components. So the thought is that while in there and possibly changing the motor mount extension anyway, why not incorporate an aftermarket fluid damper. Machining such as mix 'n match old pieces, or complete new parts is possible here. I am aware it could entail some "interesting" accommodation or adaptation of the magneto drive, considering this is a dual ignition engine.

smt
 
To answer your question, I'm not aware of any installed dampers. But neither am I aware of any failed cranks in, oh, I'll say the ten years since mine's been flying, quite possibly much longer than that. Stick with a wooden prop and the damper isn't necessary in my opinion. I'm not an expert in these matters, just one data point in the universe.

Ed
S2 Stretch with GP 2276
 
Molt Taylor used some type of vibration damper on his Aerocar, but it used a drive shaft to a pusher prop. I don't know of any application for a VW based motor.
 
Molt's used shot as the "fluid". It was also the direct soft coupling to the driveshaft.
I was very interested in the IMP & belonged to the Yahoo group; but recognized that such a project would never see completion here. :(

smt
 
I'm not aware of any installed dampers. But neither am I aware of any failed cranks in, oh, I'll say the ten years since mine's been flying, quite possibly much longer than that.

http://www.greatplainsas.com/phubhistory.html

Not mentioned - cranks in applications with known torsional resonance issues tend to be hard on the bearings, and cause things like accessory drives, keys, and such to have short lives. Fairly familiar VW issues: consider current discussion of magneto keys, puck.

Stick with a wooden prop and the damper isn't necessary in my opinion.

It seems a nuisance that one cannot experiment with simple, light, electric controllable props as well. Forget gear-reduction to keep the prop in the same center or near, as the current crank location in the AC. Sadly, my silly pre-occupation with the subject was induced at birth (of pilot-hood) by 6 years behind a wonderful GO-300 in my C175. Might not be curable but assistance is not un-welcome. :-\

smt
 
GreatPlains0702d.jpg



Over the years, Steve has accommodated nearly every whim of the VW-engine market. For instance, Great Plains now offers accessory cases with the starter mounted at 12 o'clock, 6 o'clock, 3, and 9 o'clock. There simply aren't any more choices -- unless you go to running the prop from the other end of the engine (and Steve has a system to do that, too).

Not often seen as a fan of reduction drives, a lightweight, belt-drive one's available for certain applications; and there's also a gear-drive version, that gives those front exhaust pipes enough room to make the turn rearward. That's something that has always been a point of trouble, especially when a short prop hub has been employed. In addition to the clearance, of course, is the increased available horsepower, due to the gear reduction.
 
Torsional vibration problems require actual system level tuning. Aircraft piston engines that used reduction system needed significant testing and tuning in the drive train to achieve proper results. Its not a simple exercise.
 
Aircraft piston engines that used reduction system needed significant testing and tuning in the drive train to achieve proper results.

e.g. "Factory" engines such as GO-300 & others used dynamic counterweights. People who flew them like direct drive (low rpm) had premature parts life issues because they were not flown in the design range (2800 - 3200 on the tach)

"Proper" results might also informatively be considered from the perspective of efficient results.

Its not a simple exercise.

Happy to hear what you know on the subject.

Most VW research for the past 40 years seems to have been essentially "try it, sell it, and see if it breaks". Vibration issues that would not be tolerated elsewhere are still a regularly seen and reported factor in the best aviation VW's. Parts installed and then removed from mine by the PO show the effects. Notes in the logs are informative about the life. Most of us can never explore such things due to the old bugaboos of time and cost. Steve Bennett seemed to constantly bootstrap himself and GP to the next level. I'd have liked to have been aware of him & his work when he was alive.

I do take the point that in reference to my original Q, apparently no one known to the group has experimented with a fluidic damper. Or possibly the results were so bad they were embarassed to report. ;)

Vibratech has included some supportive emails. Of course it could be observed they have product to sell whether it works or not. :) I'll be happy to hear more input. Now it's back down to time and money as to whether to take the next step. From a practical standpoint (the airplane is more or less operational) i need to pursue a canopy solution first anyway.

smt
 
Aviacs said:
Happy to hear what you know on the subject.

You can read the presentation I wrote on the topic, a long time ago. It was a short presentation on the topic, but a good overview for an engineering audience.

Any fluid damping devices has the distinct down side of generating heat and decreased efficiency compared to other solutions. The amount damping in a system is a critical value to select.

Vibratech is not selling anything new, this is very old tech.
 

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As you note, decent basic intro to the subject for people who have never considered or been exposed to it.
Based on some of the graphics i have to ask, were you associated with Orenda, EPI, or the Bugatti P100 project?
Who exactly did you make the first presentation to?

There are some points that could use a bit more elucidation: e.g. hard drive vs soft drive. For instance, your examples of hard drive actually utilize (more or less) tuned quill shafts between the crank and the pinion to "soften" the spring rate. Though i have been "all over" EAA's P100, i am not familiar with the original drive train. I would be interested in more info.

The GTSIO also uses 2 sets of dynamic counterweights. I am not familiar with the power section of the RR's.

Well designed fluid dampers are sort of a "bandaid" approach that works fairly well when there is no means or money to develop other approaches to target specific RPM ranges. (Such as dynamic/pendulum counterweights or other absorber system). If a given fluidic solution works at all, it tends to work to improve things over the entire rpm range. IIUC they had a bad rep in Nascar for a while due to poor "fluid" choices in which they functioned quite well first serious run, but the heat issues partially polymerized or otherwise degraded the medium & it would settle and fix off-center when the cars were parked. Next run the off center mass would explode the engine. This is poor design and materials choice, not a direct refutation of the system.

Absorbers, such as the typical aircraft dymnamic counterweights are ideal for that use - narrow constant rpm range with specified attachments (propellers and accessories). Elastomeric absorbers which are (loosely) another sort of spring pendulum work well on almost all automobiles when tuned for the worst resonant band. They also dissipate as heat some of the energy, similar to fluidic dampers.

Vibratech is not selling anything new, this is very old tech.

That is what Vibratech claims as well - that through the Houdaille connection in their early days, they were in at the dawn of fluidic damping history in the 30's/40's.

As a historical side note to your inclusion of the Bugatti airplane, Bugatti himself must have had torsional vibration issues deeply ingrained in his consciousness during the late 19-teens. Fortunately for him, all funded by the US government for what would be $millions in todays money.
 

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Aviacs said:
There are some points that could use a bit more elucidation: e.g. hard drive vs soft drive.

A Hard drive solution is one that has a high torsional spring constant at the output of the engine to boost the resonant freq of the drive train as a whole above that of the engine excitation frequencies. Its the brute force method and most common with the least amount of damping.


The basic thing to get out this is there is no magic bullet solution that lets you close your eyes and bolt parts together. Every component from piston to propeller has a transfer function with feedback that needs to be evaluated as whole. You wont get rid of all the vibration, the goal is to get the amplitude under the fatigue life of the material for each comp. In vw or any auto conversion you can't tune the engine as part of the system so you need to look at building the drive train as an effective transfer function. Also "constant" are not really constant in the real world.

This is to long of a topic for me to write about in a forum. I would recommend looking at matlab and simulink as means of developing a working computer model and data capture from the real world. Much cheaper way to start then experiments on real hardware.
 
I don't think anyone who is aware of the subject imagines a magic bullet.

Thank you for the summary of "hard" drive - It is interesting that the examples of hard drive in your presentation, and the successful more or less modern (WW2 & post-) have quill shafts to lower the spring rate of the total system. I'd still like to know more about the soft drive of the 100P. Understood, that is probably searchable online.

I need to let it go for now & get back to resolving my canopy issue.
The current torsional resonance stuff was inspired by vacation last week. We spend an annual week with friends at their lake house. Being a physicist, my friend often has interesting projects to discuss. This year it included a contract white paper for the solar industry exploring best practice and optimization of solar tracking systems for solar farms. To the aggravation of our wives, we spent several days discussing construction and mechanical considerations, and, - wait for it - torsional resonance in tracking systems! It can be a real problem if not considered at the design and engineering stage. I had suggested that flutter could be a problem based on the way some arrays are balanced. But apparently, torsional resonance (induced by wind/vortices) is the larger issue. I'm not in the least familiar with the industry, but pictures and hardware specs indicate that some (many?) arrays have fluidic dampers. (Albeit the "shock absorber" style)

Torsional resonance is a potential factor is any largish mechanical system or "excitable" structure. Here's a link, of links, to the problem in diverse areas.
https://www.sciencedirect.com/topics/engineering/torsional-vibration

Getting back to IC engines, this is all rather a nostalgic exercise anyway.
Within 20 years, and quite likely sooner, we at the lighter end of GA will all be flying behind electric motors.
Electric motors, especially driving propellers, certainly have torsional vibration to consider. Generally it is simpler to resolve, though.

smt
 
Aviacs said:
Thank you for the summary of "hard" drive - It is interesting that the examples of hard drive in your presentation, and the successful more or less modern (WW2 & post-) have quill shafts to lower the spring rate of the total system.

Lower relative to what?

You can achieve the same spring constant with different geometry and material selection. Just because its a quill doesn't mean its low. Also your neglecting the components mass moment of inertia and its own resonant freq.

Its an interplay of different things and why you cant focus on just one thing.
 
The equation for a shafts spring constant is as follows

Kt = (G*J) / L

G = Modulus of the material

J = Mass moment of inertia

L - length of shaft

All of the hard drive examples have very short shaft lengths, smaller than the crankshaft, So the Kt the would be higher then the crankshaft.

Your incorrect in saying a quill shaft has lower spring rate, because it doesn't account for length, material property, or mass distribution. Its a description of particular geometry but that does not mean its high or low relative to something.
 
Lower relative to what?

You can achieve the same spring constant with different geometry and material selection. Just because its a quill doesn't mean its low. Also your neglecting the components mass moment of inertia and its own resonant freq.

My actual statement:
.... to lower the spring rate of the total system.

The system has been lengthened (with the quill shaft) the diameter of which is smaller (less moment of inertia), they are all steel in examples (so the modulus is the same)

Per your equation: G is the same, J average has declined, L has increased "significantly".
Or another way to look at it is Hooke's law (equation referenced in your presentation) : total spring rate of springs in series is less than lowest of the component springs. Practically, this means things can twist further before exceeding the elastic limit (going kablooie). How much extra, I don't have a clue how that is decided to be efficient (enough to protect the crank but limited so as to minimize or exclude other problems.)

smt
 
Aviacs said:
total spring rate of springs in series is less than lowest of the component springs.
smt

Also incorrect.

K total = SUM(1/K individual)

They add like parallel resistors when part of the same body.

The system has been lengthened (with the quill shaft) the diameter of which is smaller (less moment of inertia), they are all steel in examples (so the modulus is the same)

The mod E may or may not be the same but the geometry is not the same, so individually it has a new kt. Also do not forget that gears, like those in the redrive of the P51 or other psru, have high kt.

You do not have enough information to know what the kt of the quill shaft is, it could be harden which would boost Kt significantly.
 
Also incorrect.

K total = SUM(1/K individual)

mathematically that implies the system total decreases compared to any individual unit.

They add like parallel resistors when part of the same body.

You've discovered a new physical principle.

You do not have enough information to know what the kt of the quill shaft is,

There may be minor differences between alloys, but youngs modulus is practically the same for all carbon steel & will be even closer for the alloys used for forged cranks and quill shafts.

it could be harden which would boost Kt significantly

Another new principle, apparently.

"Hardness" is a proxy for tensile strength and does not change elasticity
of a given steel meaningfully up to the yield point.

Look - I am going to stop here.

I posted a very simple question about whether anyone had experience or technical acumen with fluidic damping of flat 4 aircooled VW engines. None of your posts have addressed that usefully. As far as i can see, most of your posts are an attempt to show that i asked a dumb question or am too dumb to understand the implications, that you have superior knowlege but no time to derive or share it at a practical level. You use formulas to bolster your position, but don't seem to understand what they imply. You further suggest that if i spend a few thousand$ on higher end engineering programs, all will be clear. I'm aware of that possibility, but it is not an insightful or useful answer on your part.

smt
 
Aviacs said:
They add like parallel resistors when part of the same body.

You've discovered a new physical principle.

The equation for resistor in parallel is of the same form as the equation total Kt.

1/Total Resistance = sum(1/individual resistance)

Its called an analogy not a physical principle.


Aviacs said:
Also incorrect.

K total = SUM(1/K individual)

mathematically that implies the system total decreases compared to any individual unit.

Correction I made a typo, it should be as follows, sorry for the confusion.

1/ Ktotal = SUM(1/K individual)

Yes you are correct about the math. I had to read you first post on "series of springs" again and I really misread it. I was trying to bang out a response before going to a meeting at work. I must have seen the word series and my mind was like the math is like parallel resistors, which it is. Sorry about that I was not trying to make you look dumb.

merlingearsmodded.jpg


How ever looking at this photo of Merlin Engine the output shaft is much larger in diameter and shorter than the Quil/crankshaft, in other words it has a higher spring constant then the engine and is not in risk of entering resonance compared to the engine. A p51 as a whole is still a much stiffer drive sytem, quill included, then Bugatti P100 or BD-5. A BD-5 according to the people working on it entered resonance around 600 rpm on the engine.

The term hard drive and soft drive system are defined on pg 173 of Alternative Engines by Mick Myal published by EAA.


You further suggest that if i spend a few thousand$ on higher end engineering programs, all will be clear. I'm aware of that possibility, but it is not an insightful or useful answer on your part.

Matlab home edition is $150, not "thousand$", its why I recommended it. Much easier/cheaper to engineer the system in a computer and play around with values. There is also vibration analysis code for matlab on the internet for free.
 
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