Hey, it works!Now if we could just convince you to (1) machine a new flywheel with a nice high-inertia ring at its perimeter, and (2) install a real honest-to-gosh engineered soft coupler to replace those horrible urethane bushings...
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Sort of. Ever work with urethane bushings and drive pins? The urethane has so much hysteresis that it compresses at the first power stroke and stays compressed in the hertz range, result being lots of system lash (free play at the drive pins), in particular when in resonance.
Big difference with an engineered coupler like a Centaflex. Rubber has far less hysteresis, it's preloaded in compression, cannot develop lash of any kind, and the stiffness is a known spec sheet value.
I'd have liked Ross incorporate one, at a softer spring rate. He will cheerfully tell you it still resonates well above idle. Adding the perimeter slugs (adding flywheel inertia) lowered the amplitude some, but doesn't move frequency very much.
How much? If I drag out real data for an early iteration of my Suzuki drive and plug it into the Holzer code, a 50% increase in element 1 inertia (crank and flywheel) results in a 3.5 hz frequency reduction. That's only 131 RPM for the 3 cyl, or 105 RPM for a 4 cylinder. Not much.
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Heliano
Well-Known Member
Maybe I did not properly understand the intention. Here´s my question anyway (sorry if it is stupid): wouldn't it be better to make a solid model, and use the inertia/material data with finite elements to do a modal and frequency response analysis? The solid model could be built for example with Catia, Solidworks, Solid Edge; the modal/frequency response analysis could be run for example with Femap, ANSYS or Comsol, just to name a few.
In my case I don't know how to use any of the programs and can't afford to learn or purchase them. Also I would hate to have a model that made an assumption that was not clear until something broke. When going through the older literature that has been mentioned it becomes a matter of interpreting the numbers the formulas provide and using that data. Some of these guys here have used these programs. As they have said they chose not to go the development route.
The couplers are rubber, the engine mounts are urethane which were probably not the best idea however it's run almost 450 hours now with minimal issues in these areas. No space to install an engineered coupling without machining a new drive plate with splines. I don't have that machining capability in house so that will probably never happen at this stage. The thing is reliable (so far) and I'll leave it at that.Now if we could just convince you to (1) machine a new flywheel with a nice high-inertia ring at its perimeter, and (2) install a real honest-to-gosh engineered soft coupler to replace those horrible urethane bushings...
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As I said, these are not urethane. I can't go any softer here as the present setup has a deflection of 1 deg per 25 lb./ft. at 15C. The only reasonable course without massive time and expense was to add flywheel mass. Russell Sherwood did the same thing on his 6 cylinder Sube a few years back and we've both been happy with the results. He has about twice the time on his setup as I do. In both cases, the hammering was much reduced down around idle. The amplitude is what will break things and that's much lower now for both of us.
Much as I hate to add weight to an airplane, this was the best solution for me. I did a big weight reduction program at the same time I did this mod so came out lighter than before.
Hmm. My memory of Ross' details is fuzzy, so I hit the advanced search tool. Look at Ross' input: More Thoughts on PSRUs post 166.
Rubber bushings, not polyurethane, were specifically mentioned in the post, as was doubling flywheel inertia. Original mode was at 850-950 rpm and another mode was present at 1100-1400 rpm, smooth at 2000 rpm. Modified flywheel gave him smooth at 1300 rpm. If he doubled flywheel inertia and changed nothing else, we would expect everything to move downscale somewhere around 65-70% of original (flywheel inertia dominates the engine side, f = sqrt(k/m), m is doubled, so sqrt(1/2) is 0.707). Sounds like it is in the ballpark.
If engine idle speed is set somewhat above resonance, we are are already on the isolating side of the curve. Adding inertia to the engine side does two things: It reduces the angular amplitude of the firing pulse, and; it reduces the resonance frequency. Both are good things if you are keeping the idle speed the same. Let's get scale. With resonance at 900 rpm, idle at say 1100, f/fn is 1.22, and amplification ratio on the standard curve is 2.5. move resonance down to 636, f/fn = 1.73, amplification goes to around unity. Amplification is reduced to 40% of what it was before. That is a bunch of improvement for a system that did work and live before modification.
Now, would I like to have seen a new high inertia flywheel machined from one piece of 4130 instead of the bolted on weights? Sure I would, and for several reasons. First and foremost is Ross could have doubled his flywheel inertia at less added weight, or gotten even more inertia at same weight. Since I am the guy preaching "weight is the enemy", I gotta say that. Having the engine continue to swing the prop is important too. The other reason I would prefer a one piece flywheel is that the bolted on weights give so many places where a nasty failure can be initiated. Yeah, properly designed and assembled, they will stay put until Ross wants them to come off, but I believe in human errors, so like to see less opportunities as good design. I am also used to building millions of copies of my stuff where even rare human errors is a lot of warranty.
Ross ran 400 hours in the original form without breaking anything, so it was, at minimum, somewhere around acceptable. The new form is smoother everywhere it is operated and has to be easier on stuff like bearings, gear sets, and props, not to mention more pleasant to operate. The only down sides are maybe a little excess weight and some risk of one of those many joints coming undone. And that "works" for most of us. Most of all, it works for Ross, and he is the guy flying it...
Billski
The flywheel weights were nested in tight fitting recesses so the bolts are not under shear. Russell Sherwood had Guy Marcotte machine a custom steel flywheel the 2nd time around on his EG33. I think it was 12-14 pounds. Mine is 15 pounds now, up from 7.
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Why not a solid ring bolted on?So many less
parts and it could be balanced after install.
Cost of a slice of 12" shafting and machining time to take the center out of it. The CNC lathe with bar puller can spit out 16 of these weights in about 45 minutes from cheap material. The mill can cut the pockets and drill the holes in less than 20 minutes. Plus this looks cooler... Balance wasn't affected. Also if this didn't work, I could remove all the weights without removing the gearbox.
Ahh, please excuse me, faulty memory. Found the photo. Is that a Malcorp suspension bushing? Got an application, part number or catalog page?
That's soft....but it maintains that rate for how many degrees of angular displacement before rising very rapidly?
And there we arrive at the key. Folks here are interested in designing drive systems, and you're a respected man. So, some will look at the flywheel photo and think "Oh yeah, pins and rubber bushings must work fine", not realizing it was a design feature retained only because improvement would have required modifying the Marcotte. Yes, it works, sort of. An engineered coupler would perform better, torsionally speaking, and on the practical side, it would last for more operating hours.
Now, are you finally coming to OSH this year to have a beer with me?
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Was one of your considerations how close to the edge you put the weights,to prevent cracking?
And if so ,is there a suggested distance to keep under?
The rate was pretty linear to about 100 lb./ft. then started ramping up. I only went to 150 as that was the limit of the torque wrench. That's pretty soft and doesn't do much at resonance where I'm guessing torque may exceed several hundred lb./ft.Ahh, please excuse me, faulty memory. Found the photo. Is that a Malcorp suspension bushing? Got an application, part number or catalog page?
Rubber Automotive Parts Manufacturer & Exporter - Malcorp
We manufacture & export various type of rubber automotive parts including engine mounting, rubber bushes, center bearings & more. Contact us for more info.malcorp.net
View attachment 113229
That's soft....but it maintains that rate for how many degrees of angular displacement before rising very rapidly?
And there we arrive at the key. Folks here are interested in designing drive systems, and you're a respected man. So, some will look at the flywheel photo and think "Oh yeah, pins and rubber bushings must work fine", not realizing it was a design feature retained only because improvement would have required modifying the Marcotte. Yes, it works, sort of. An engineered coupler would perform better, torsionally speaking, and on the practical side, it would last for more operating hours.
Now, are you finally coming to OSH this year to have a beer with me?
The bushings are Moog K6176.
I agree, the pins and bushings are not great for sure but offer a fail safe design which fortunately I haven't had to test yet... I suspect Guy Marcotte didn't have a good understanding of TV and that applies to most of us in our younger years as well without the formal education and vast real-world experience that Bill has. TLAR was almost certainly used in this part of the PSRU design.
That being said, Guy was making drives to fit several different engines with many different props where detailed vibe analysis would be impossible given the one man operation. He has produced one of the most reliable drives on the market which is quite an accomplishment for a non-engineer IMO. His background was as a machinist, working on and repairing industrial gearboxes if I recall correctly.
I'd compare the use of my setup with the Rotax 9 series setups which also have a severe TV period at low rpm (and much wider rpm range than my Sube). Despite the money and techical resources they have, they didn't solve it, they just told folks not to run below 1400 rpm. The Rotax has no effective flywheel mass at the prop drive end.
Lycoming has several models with restricted rpm zones using certain props- also not solved despite their resources. If I have to change my bushings out every 350 hours or so, I'm ok with that. At the rate I'm flying now, that will see me out.
Due to the return restrictions back into Canada now, I can't travel to the US at all. Looks like I'll miss Reno too also unless something changes soon.
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Thank you, that helps. The report was:
My setup with the original light flywheel was bad around 850-950, smooth at 1000 (good luck there) but went through another worse period between 1100 and about 1400. Above that, smooth. When I doubled the flywheel MOI, the amplitude was massively reduced at both points and the smooth zone now starts at about 1300, good above that. I can still feel and hear the "buzz" as I transition from idle through that 1100-1200 rpm range to taxi but it's only about 30% of what it was with the light flywheel.
1400 vs 1300 if I understand correctly, a ballpark 100 RPM reduction in frequency.
But here idle speed is not above resonance. Note "... as I transition from idle through that 1100-1200 rpm range to taxi.."
Absolutely. My hope for his increased flywheel inertia is reduction of crank oscillation in the resonant period, ending the apparent cavitation damage to the rod bearings. The original wasn't going to go much further.
Thanks. I'm sure we talked about all this back when, but I've forgotten.
100 ft-lbs is well below nominal torque for the turbo Soob. I suspect your rate is far from linear, quickly going very high, just because the wall thickness of the bushing doesn't allow significant angular displacement. Got the angle vs torque data handy? Here's one for an NA Legacy 2wd clutch center; note 6 degrees before it hits the stops. The Centaflex elements are 7.5 to 14.
Count me among the "most of us". Still a student.
Oh wow, I didn't realize Canada was still so locked down. Bummer. How many times have we said "next year"?
Stock torque on my engine is 181 lb./ft. I run a bit less MAP than factory but have a far more efficient turbo system plus higher CR pistons so I'd call that figure still close but likley at a higher rpm than the stock engine due to the looser turbine AR and larger wheel.
So as I operate now, the idle is reasonably smooth at 1000 rpm. I still try to avoid the 1100-1200 rpm range as I transition to taxi power (2.2 to 1 reduction ratio). It's very smooth at 1300 (591 prop rpm) and above so nothing is impacted much now. It's so much better than before where the wingtips would oscillate several inches up and down and the panel was a blur while passing through the resonant zone.
Canada hasn't followed any science on this crap and has trampled on all our "guaranteed" Charter rights. Lawsuits are before the courts but it could take months for those to process with unknown outcomes.
So as I operate now, the idle is reasonably smooth at 1000 rpm. I still try to avoid the 1100-1200 rpm range as I transition to taxi power (2.2 to 1 reduction ratio). It's very smooth at 1300 (591 prop rpm) and above so nothing is impacted much now. It's so much better than before where the wingtips would oscillate several inches up and down and the panel was a blur while passing through the resonant zone.
Canada hasn't followed any science on this crap and has trampled on all our "guaranteed" Charter rights. Lawsuits are before the courts but it could take months for those to process with unknown outcomes.
Looked them up. Technical point...to operate as a suspension bushing, the rubber is compressed into a steel sleeve. That compression is an important design feature. Looks like the sleeve is removed for the Marcotte.
Next time the Marcotte comes off, consider a Centaloc splined hub in a Type 1S Centaflex A. Offhand, it looks like the Marcotte input shaft is a SAE CC 17 spline, and if so, a Centaloc hub is a direct replacement for the stock input disk. "S" means the Centaflex slides over flywheel pins to mount the Marcotte on the engine.
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Later versions of the Marcotte drive leave the outer and inner steel bushings in place and dispense with the aluminum drive pins in the earlier ones. The one in my -6A doesn't use the bushings.
I'm sure Moog never intended them for this application. They were merely convenient for Guy to use. Yes, the extra pre-compression and bearing area of the sleeves would make them effectively stiffer- if that's what you wanted.
I'm sure Moog never intended them for this application. They were merely convenient for Guy to use. Yes, the extra pre-compression and bearing area of the sleeves would make them effectively stiffer- if that's what you wanted.
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Anyone thought of a Simple Tunable Pendulum Damper along the lines of 6ejguy flywheel weights, but more mobile, like ball bearing within a contained void.
It would have to be tuned to the host engine. Powersport developed their own Pendulum Damper but it was expensive to manufacture, maybe we could take this idea and make it much more simple, and adaptable.
I have some ideas if people think it worthy of discussion.
George
It would have to be tuned to the host engine. Powersport developed their own Pendulum Damper but it was expensive to manufacture, maybe we could take this idea and make it much more simple, and adaptable.
I have some ideas if people think it worthy of discussion.
George
