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07-22-2007, 08:33 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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State-of-the-art torsional modeling:
http://www.torsional.com/images/pdfs/marine_app.pdf
Diesel powered branched system with multiple gear ratios. Ack!
Note that current software of this kind makes allowance for bearing friction at every shaft, the small damping value found in the torsional soft couplings, etc. In the absence of a dedicated damping device, this is the real truth of damping. It is the sum of the small frictional values that keeps a resonant system amplitude from reaching infinity.
Compared to modeling this marine drive system, ours are very simple....and we can get better results too.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 07-23-2007 at 01:41 PM.
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07-23-2007, 11:18 AM
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Join Date: Feb 2006
Location: Sherwood, Oregon
Posts: 236
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<<It is the sum of the small values that keeps a resonant system amplitude from reaching infinity.>>
This is what I was getting at in my earlier posts. I know that steel is really good at handling flex, as long as max strain is below the elastic limit. So, it might not take much much elasticity in a connecting element to limit the max strain to a safe value. The area under the strain curve wouldn't change, but the peak value would.
What I'm really curious about though, is how strain in a belt might be measured. With one sprocket on the crankshaft, and a typically short and beefy prop shaft, it would seem that if distructive resonance occurs, itwould occur in the belt. How to strain guage a belt? Looking at witness marks on the belt sprockets, stroboscopic analysis could show resonance occuring, but it might be very difficult to gauge amplitude. Is there an industry standard way this is done?
Ted Johns
RV7 plans preview
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07-23-2007, 02:00 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Ted,
It is possible for a node to be located at the belt. There is no requirement to place a stain gauge at a node.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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09-22-2007, 12:24 AM
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Join Date: Sep 2007
Location: BC
Posts: 1,674
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Thanks to Dan for this wonderful thread. I'm sure I speak for many reading but not posting simply because we're on the steepest part of the learning curve and have nothing to add. I am learning a lot and always knew there must be something to understand about this topic. Now I can just barely keep up with the topic but really need to ask a question. I am close to making the final engine decision for my RV7A. I like the idea of the Egg H6 for its smoothness, dual-mass flywheel, heavy gear box etc etc. But have always been concerned with the lack of real math, testing, lack of history ,measured and posted results. Maybe they don't think we would understand it anyway. Maybe it hasn't been done and maybe they don't even understand it. Question: Dan, in your personal opinion, would a Lyc clone with a fixed Sensenich prop (tested on Lycs) with electronic ignition be a riskier or safer approach in terms of harmful vibrations of all kinds than the Egg Subaru with 2.02 drive and three blade carbon fiber prop, or would they be equally unknown? Thanks for your insight.
Bevan
RV7A wiring, engine decision time and scrambling up the steep part of the learning curve.
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09-22-2007, 12:34 AM
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Join Date: Sep 2007
Location: BC
Posts: 1,674
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Dan,
Further, would you say that a lyc clone will have (for all practical purposes) the same torsional profiles (modes, nodes, amplitudes, natural frequencies, restricted RPM ranges etc) as the eqivalent certified Lyc? If so, would adding electronic ignition with variable timing (Lightspeed) significant alter same?
Bevan
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09-22-2007, 09:18 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Bevan,
In the strict context of torsional vibration, there shouldn't be any difference between a measured Lycoming/propeller-X combination and the same prop on a Ly-clone, assuming it is really a clone.
The addition of an electronic ignition has been shown to have an effect on metal propeller vibration. Search around and you'll find bulletins that address the matter. The bulletins more or less say "don't do it", but fail to supply hard data so we're all in the dark. I suspect (caution; unproven assumption!) the observed stress increase indicated a reduction in expected (long) service life rather than likely short-life catastrophic failure. Obviously there are lots of EI units flying and they are not flinging blades....yet.
Ly-clone safer than an Egg? I'll probably make somebody mad here, so please try to remember that I like and support alternative engines, have flown my own experiments to OSH and S&F, and have more of the same planned for the future.
Currently I think the Lycoming and the Ly-clones far less experimental than any alternative. I also think the average homebuilder is more likely to achieve a safe installation with a conventional engine; the auxiliary systems are less complex and the support network (# of qualified eyeballs)more extensive.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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09-22-2007, 10:24 AM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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I'd agree with Dan on this point and like to add that much of the don't do Lyco/ Clone/ prop combos revolve around long term fatigue issues with metal props. I'd concur that you are probably in safer territory with a wood or composite prop on a Lyco/ Clone from a prop and TV standpoint than most relatively untested auto conversions.
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09-24-2007, 11:29 PM
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Join Date: Sep 2007
Location: BC
Posts: 1,674
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Thanks!
In respect to your comment "If it really is a clone", My clone would be an Aerosport power built IO-360 made from a combo of ECI and Superior parts. These parts (as I understand it) are "equivalent" dimensionally to a Lyc as they have to fit same. True? and is this a true clone in your opinion?
Thanks
Bevan
Abbotsford BC
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09-25-2007, 07:21 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Bevan,
Ask Aerosport Power if your engine would in fact be identical to a particular certified Lyc. Then ask Sensenich how they view such a clone. Then report back here <g>
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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09-25-2007, 08:05 PM
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Join Date: Jun 2006
Location: Colorado Springs, CO
Posts: 50
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i was wondering if we might be able to get back on subject with this post...
i thought it was about Belted Air Power...
please dont flame me, i just am interested in BAP and would like to get more info.....
thanks!
jeff
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