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  #101  
Old 12-06-2022, 11:22 AM
Tomcat RV4 Tomcat RV4 is offline
 
Join Date: Nov 2007
Location: Jacksonville,Fl. 32246
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Ross , you are so CORRECT ,my subaru 2.5 idled at 750 rpm, with NO SHAKING,
and my 1.8 Honda civic in my Zenith 701 does likewise. A friend with rotex is
constantly fussing with carbs even with 1400 rpm idle. Eventually someone will address this with not only gearbox, but good flywheel/coupling system. I donít want to get in trouble listing business that unfortunately doesnít advertise here,
but they supply 3 different gearboxes, depending on engine size and HP, as a
result have been very successful. Tomcatrv4
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  #102  
Old 12-06-2022, 11:31 AM
KeithO KeithO is offline
 
Join Date: Jul 2010
Location: Jackson,MI
Posts: 149
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Dan, potentially they are designed to a price point and reliable "enough". How often to they require the gearbox to be overhauled ? Things like that can be quite profitable and keep A&Ps in work too... Rotax doing spur gears vs helical. Cheaper to make, stronger but noisier. Potentially allowing for bushings vs roller bearings.

Quote:
Originally Posted by DanH View Post
I dunno gents...does anyone in the alt engine world really have standing to knock a 912? It has been a rather successful design.

Propeller inertia is the relatively immovable object against which the rest of the system oscillates. Given the same shaft load, low engine mass means the block oscillates more than a Subaru with a 50 lb gearbox.

The metal-to-metal rattle in the resonant range is the ramp-type dog clutch doing exactly what it was designed to do. There are several styles, and there is also an available friction clutch to further limit maximum shaft torque in some versions.

Photo here, two different dog clutch hubs. Driven side stacked on driving side, which is integral with the spur gear. Left is no-clutch, right is driven hub with external spline for overload clutch plates:

http://contrails.free.fr/images/rota...5156.sized.jpg

Last edited by KeithO : 12-06-2022 at 11:38 AM.
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  #103  
Old 12-06-2022, 12:26 PM
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DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
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Quote:
Originally Posted by rv6ejguy View Post
I bring it up only to illustrate that even with all of their engineering and funds available, Rotax didn't solve the TV issue down low, they do what all of us are doing- avoid idling in the resonant range with the recommendation to be over 1400 rpm.
Remember, design is the art of considered compromise. Rotax incorporated soft elements on the C and E boxes, but ultimately elected to go with the ramped dog/overload clutch system for the 912. It is, for practical purposes, a torsionally stiff system until excited at its fundamental natural frequency, when it loads the dog springs and slips the clutch. The result is noisy in resonance (the dogs) and results in a higher frequency. On the flip side, it probably pushes the F2 above the operating range. I only say "probably" because I have not instrumented one, but it is what the theory suggests. And doesn't require a flywheel, which brings us to...

Quote:
The 912 I ran on my test stand was much worse than my Subaru ever was below this rpm.
I am reminded of the old joke which goes "Doctor, it hurts when I do this", to which the doc says "Well, don't do that".

Rotax clearly asks operators to stay above the resonant RPM. With 2.43 or 2.27 ratios, 1400 at the crank is 600 at the prop.

Hey, I'm not here to play fanboy for Rotax. I'm just sayin' the design has reasons, and they may not be real obvious.
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Last edited by DanH : 12-06-2022 at 12:34 PM.
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  #104  
Old 12-06-2022, 01:11 PM
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rv6ejguy rv6ejguy is offline
 
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I suspect the Rotax case was pretty challenging without flywheel inertia. The gearing helps to keep residual thrust low but some folks still wish it could be less. I have some time in a CT and it was a real floater in ground effect due to the high residual thrust.

Yes, a compromise and not ideal. I just expect something better from a mass produced aero engine.
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  #105  
Old 12-06-2022, 02:19 PM
KeithO KeithO is offline
 
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Location: Jackson,MI
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I downloaded Freecad last night and was able to get a few FEA iterations run in 3-4 hours, learning how to make a model and then go through the steps going from model to mesh, applying loads etc. Im still learning how to drive it, but it is a heluva tool for the price ($0)...

I dont have the tangential nor radial loads applied at exactly the right angle yet, I probably have to add some wireframe elements to align the loads to, but the location of the stress concentrations and the magnitude is starting to look very close to the failed parts...

Peak Von Mises stress is at 417MPa or 60.5ksi Certainly high enough to cause fatigue without the benefit of TV problems, misalignment etc yet Im sure the Viking 110 has some of each of those, just like any other engine. I also think that at these levels one needs really good surface finishes and some of the flanges are clearly horrible in this regard.

I need to finesse the design some more, figure out the wireframe elements and correct the load angles and then look at some larger blend radii, thicker web and the original design for bolts to see how they compare.
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  #106  
Old 12-06-2022, 04:04 PM
KeithO KeithO is offline
 
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The quote finally came in for the Centaflex coupling. Will order it tomorrow.
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  #107  
Old 12-06-2022, 04:09 PM
Cumulo Cumulo is offline
 
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Location: KHMT
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Great start, Keith on the FEA. Would take me days to get that far from a cold start.
I wish there was a free engine dynamics program as well. I have used an electrical circuit simulator LTspice to do it, but the results are hard to interpret. It involves converting mass, elasticity, friction into capacitance, inductance and resistance. The resulting circuitry is fairly simple, but as I say, hard to interpret and no real visual output.

As an aside, I flew behind a geared engine decades ago for many hundreds of hours, a Lycoming GO-145, as smooth as an electric motor. It had problems, but nothing to do with the gearing. Loved it. For the TV solution, it had four enormous pendulous dampers on the crank. The crank gear ran inside the prop gear at a ratio of 17 to 27. No shutdown shudder, no soft components, but a lot of clunk sounds starting and stopping.

For an auto conversion using tuned dampers on a four cylinder, the 2nd order frequency is the big one, and therefor heavy damper weights. And since they would be busy all the time at all RPM's, would need to be well lubed. So, maybe they could be inside a hollow, lube containing flywheel. All 2nd order vibration to the gearbox could be cancelled. The idea could be carried to the 4th order at which point other frequencies would require very little mechanical consideration. Basically just DC twist going to the prop.

As I typed this out, I have begun to think this might actually be very feasible. hmm

If I where younger and had the skills and hands-on ability of Dan, I'd be on my way to the junk yard to get a good motor.

A bit OT, I know, but maybe a worthy idea. It seems Keith now has the bull by the horns solving the serious problem addressed in this thread..

Ron

Last edited by Cumulo : 12-06-2022 at 04:31 PM.
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  #108  
Old 12-07-2022, 04:18 AM
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Lufthans Lufthans is offline
 
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Quote:
Originally Posted by DanH View Post
I dunno gents...does anyone in the alt engine world really have standing to knock a 912? It has been a rather successful design.
No knocking on the 912 intended from my side either Dan. I like that engine. I learned to fly behind one. They are good, solid engines and have done the LSA/ VLA/ EU Microlight aviation a tremendous service, basically single-handedly enabling these categories. If they'd do a 200 hp version (6-cylinder 915i?), then Lycoming might be in for some hard times.

Like Ross, my remark was only to confirm that they too have to content with a significant rattle at these lower RPMs.
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  #109  
Old 12-07-2022, 05:53 PM
KeithO KeithO is offline
 
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I did an analysis of the original bolted design too. Here is how that came out.

Von Mises stress at 454Mpa, not surprising given the 1/3 reduction in cross section caused by the counterbore. Highest stress concentration at the OD of the giubo bushing. Unlike the welded pin design, a much lower stress concentration tangent to the central boss. I think the reason why the sample lasted as long as it did was because of a very good surface finish, the best of all the samples in my possession.
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  #110  
Old 12-07-2022, 05:57 PM
KeithO KeithO is offline
 
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What the actual part looked like
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