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11-02-2014, 06:19 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by PCHunt
And just to add fuel to the fire, the "counterweights" aren't really counterweights at all.
They are torsional vibration dampers.
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Actually pendulum absorbers, not dampers, but bless you Pete. Labels are important if readers are expected to really understand.
Folks, study the location of the pendulum absorbers on a Lyc crank and you'll see they are 90 degrees to the crank throws, not opposing them. They "counterweight" nothing but themselves. However, other engines can and do use the pendulum mass as a counterweight for piston and rod mass. In all cases the effective order is determined by pin and bushing dimensions.
The Chilton patent:
http://pdfpiw.uspto.gov/.piw?PageNum...3DPN%2F2112984
Quote:
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I am going to assume (foolish I know) that if you call an engine shop and say I want X engine and it will swing Y prop that they might be able to advise which kind of crank is better suited for you chosen combination?
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I think you'll find a lot of shops know about the parts, and not much about how they actually work, or how effective they are. The engineers who measure propeller stress have the answers to your question.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 11-02-2014 at 06:23 AM.
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11-02-2014, 07:39 AM
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Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 5,686
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And some more info From Lycoming SIL 1012G:
Crankshaft counterweights are calibrated to absorb torsional (twisting) vibrations on the crankshaft at critical frequencies between power strokes during engine operation. Each crankshaft has its torsional frequency which is a function of crankshaft length, crankshaft stiffness, stroke, mass, and moments of inertia driven by the engine. A crescent-shaped counterweight is attached to top and bottom lugs on the crankshaft by a pair of rollers. The diameter of the holes on each counterweight is a specified size that corresponds to the pair of rollers as a matched set. The rollers rock back and forth inside the holes of the counterweight to absorb torsional vibrations as the crankshaft rotates. This dynamic causes dampening of the resonant frequency of the engine/propeller combination. The counterweights turn opposite the crankshaft’s vibratory torque energy to decrease torsional vibration.
There are also many notes from Lycoming about abrupt throttle movements possibly causing damage to or detuning counterweights, something to consider if you operations call for this (aerobatics, formation come to mind or if you're just a ham fisted operator).
__________________
Walt Aronow, DFW, TX (52F)
EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 1700+ hrs, New Titan IO-370, Bendix Mags
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
Last edited by Walt : 11-02-2014 at 07:44 AM.
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11-02-2014, 09:28 AM
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Join Date: Jan 2005
Location: KPYM
Posts: 2,686
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Quote:
Originally Posted by rzbill
Thank you for that. I know the "counterweighted" -6 crank will have less vibration issues but I had not internalized the practical aspects of it.(meaning getting rid of the prop restrictions). I remember those restrictions on the IO-360-A1A in my dads Mooney. Opening a can of worms here but I am going to say that being able to run in the 2000 to 2350 RPM range is important to me because I have signed on to running oversquare LOP at low altitude as is the current rage  . I was lucky to learn from Les Dowd that my non-counterweighted engine and Hartzell prop don't have any restrictions.
If it were not for that particular combination, I think I would think very hard about doing what is needed to clear the restrictions, even buying the -6 crank. 
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Here is a copy of the aforementioned restriction/recommendation from Hartzell.
It seems that if your engine has modifications like electronic ignition or FADEC, the restriction is still in place.
So for someone like me with an ECi engine and electronic ignition without a counterweighted crank... How would this all apply?
I dare say that it is just like it but different! That is the experiment in experimental category!
 CJ
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RV-7 Flying - 1,200 Hours in 5 Years!
The experiment works!
TMX-IO-360, G3i ignition & G3X with VP-X
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11-02-2014, 09:34 AM
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Join Date: Jan 2005
Location: KPYM
Posts: 2,686
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Quote:
Originally Posted by DanH
Actually pendulum absorbers, not dampers, but bless you Pete. Labels are important if readers are expected to really understand.
Folks, study the location of the pendulum absorbers on a Lyc crank and you'll see they are 90 degrees to the crank throws, not opposing them. They "counterweight" nothing but themselves. However, other engines can and do use the pendulum mass as a counterweight for piston and rod mass. In all cases the effective order is determined by pin and bushing dimensions.
The Chilton patent:
http://pdfpiw.uspto.gov/.piw?PageNum...3DPN%2F2112984
I think you'll find a lot of shops know about the parts, and not much about how they actually work, or how effective they are. The engineers who measure propeller stress have the answers to your question.
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Dan,
In the Hartzell document they refer to the design as an "undamped engine".
What is in a name? I am seriously inquiring what the difference is between a damper versus an absorber.
...just curious.
 CJ
__________________
RV-7 Flying - 1,200 Hours in 5 Years!
The experiment works!
TMX-IO-360, G3i ignition & G3X with VP-X
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11-02-2014, 05:50 PM
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Join Date: Feb 2006
Location: Sydney, Australia
Posts: 251
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Looking at the original poster's signature line I think he may be wanting some practical help to decide which engine to buy. I've noticed there are a number of counterweighted and non counterweighted engines appearing over at Aerosport Power and Titan Engines.
Counterweighted engines are likely to run more smoothly and reduce torsional vibrations on the engine and prop. This might be a good thing if you intend on departing from tested and approved engine/prop combinations.
On the other hand, the heavier crank and counterweights themselves add additional weight to the engine (I would guess 7 lbs total). As others have said they can be detuned by rapid throttle changes and generally one more thing to go wrong and cost money at overhaul.
The only sure way to know if a given propeller is compatible with a given prop is to test it and analyse the vibrations. Although I have never heard of a prop departing an RV due to electronic ignition or high compression, that doesn't mean it is impossible.
In the past Experimental engines were built from PMA'd parts to established (certified) design. In the last few years the experimental engine market has expanded and there is a lot more choice. It seems like it is going to be impossible to test all of them with every possible propeller. Is that an issue? I guess we will find out in time.
__________________
Richard Talbot
RV-7A
Sydney, Australia
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11-02-2014, 06:09 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by Captain_John
Dan,
In the Hartzell document they refer to the design as an "undamped engine".
What is in a name? I am seriously inquiring what the difference is between a damper versus an absorber.
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Bad nomenclature is common, even with factory folk. Consider "shock absorber", which is really a damper, or "radiator", which is really a heat exchanger.
To your question; a damper is usually considered to be a device that removes energy from the system, most often as heat. A pendulum absorber removes no energy. Instead, it changes the length of the pendulum when the mass rocks on the pin/bushing.
Quote:
Originally Posted by rwtalbot
The only sure way to know if a given propeller is compatible with a given prop is to test it and analyse the vibrations......It seems like it is going to be impossible to test all of them with every possible propeller. Is that an issue? I guess we will find out in time.
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Well said Richard.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 11-02-2014 at 06:15 PM.
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11-02-2014, 06:31 PM
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A standard eci crank costs a little less than 4K, 6K for Lycoming. A counterweight crank is about triple that new.
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11-02-2014, 07:03 PM
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Join Date: Nov 2005
Location: Asheville, NC
Posts: 2,692
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Sigh....Semantics.
Engineering terms getting in the way of teaching.
I can make an equal case that the bifilar mounted masses on the -6 crank are counterweights in that they are weights and their action is counter to the loads they are intended for.
__________________
Bill Pendergrass
ME/AE '82
RV-7A: Flying since April 15, 2012. 850 hrs
YIO-360-M1B, mags, CS, GRT EX and WS H1s & A/P, Navworx
Unpainted, polished....kinda'... Eyeballin' vinyl really hard.
Yeah. The boss got a Silhouette Cameo 4 Xmas 2019.
Last edited by rzbill : 11-02-2014 at 07:23 PM.
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11-03-2014, 02:24 AM
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Join Date: Mar 2010
Location: Netherlands
Posts: 18
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Counterweighted crankshaft
Quote:
Originally Posted by ccsmith51
The O-360-A1F6D on my RV-4 has a counter-weighted crank.
I was concerned about any issues with the counter-weighed crank and using a prop that was different than what the engine and crank were designed for, so I called and talked to a tech at Lycoming. I told him that I was using a wood prop and he said I should have no problems.
I don't know what his answer would have been if I had been using a metal prop.
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I have put a wood/composite prop on a IO-360C1D6 and have often wondered about the wisdom of this. My concern relates to propellor Moment of Inertia which for a light wood/composite prop is much lower than a metal prop, and the potential to damage the hangers/rollers of the counterweights at low rpm (starting) & by changes in rpm in higher regions.
Never had anybody provide a technical/scientific answer to this but any empirical conclusions could be equally valuable.
__________________
Jan Rijkers
6106 NW 41st Drive
Coral springs
mob 9547563551
RV6A
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11-03-2014, 08:29 AM
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Join Date: Jan 2005
Location: KPYM
Posts: 2,686
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Dan, thank you for the description.
Bill, I hear you! I teach electricity and you must choose words wisely!
All of the guys on this forum compose an incredible resource!
How did we ever get by without the internet!?!
 CJ
__________________
RV-7 Flying - 1,200 Hours in 5 Years!
The experiment works!
TMX-IO-360, G3i ignition & G3X with VP-X
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