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  #1  
Old 11-01-2014, 01:32 AM
xblueh2o xblueh2o is offline
 
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Location: SF East Bay
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Default Counter/non-counter weighted engines?

OK,
I am going to admit my own lack of knowledge here.
Aside from the obvious, just what exactly is the difference between a counterweighted and non-counterweighted engine? Under what conditions would you want one or the other? What are the relative advantages and disadvantages to either and any other info you think is relevant.
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  #2  
Old 11-01-2014, 09:12 AM
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rzbill rzbill is offline
 
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This is somewhat of a semantics issue so I will try to give more info for your understanding.

Engines have multiple vibration orders (meaning combined sets of frequency and amplitude) that are not all simply the same frequency as the crank RPM. For instance, since a 4 stroke fires every other rotation, there is a 1/2 order vibration (1/2 RPM) and it is followed by basic balance of the crank or prop that would be 1st order (same as RPM).
The whole vibration formula has multiple order terms starting with the primary order vibrations (above) and then the secondary order etc etc and the influence gets smaller and smaller as the terms go off to the right side of the equation as they describe smaller and smaller physical influences.

All aircraft engines are counterweighted for the first few modes (sorry I don't remember how many). The term "counterweighted" as generally applied to Lycomings is in reference to a specific crankshaft design that includes extra weights that actually move in relation to the crank. They are in a kind of pendulum arrangement on the crank webs that allows slight movement of the weight. The movement is needed to address higher order (further to the right in the equation) and more complex vibration modes. A quick google search says 6th and 8th order.

My understanding of history is that the "counterweighted" crank was introduced with the angle valve Lycomings and it was signified by a "6" in the engine suffix. With the advent of so many EXP Lycoming clones, both angle and parallel valve, I don't think one can tell what is counterweighted and what is not without knowing the engine details from the builder.

Since I am shooting from memory, I welcome any corrections or clarifications to the above from other members here.
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RV-7A: Flying since April 15, 2012. 850 hrs
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Last edited by rzbill : 11-01-2014 at 09:55 AM.
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  #3  
Old 11-01-2014, 12:36 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Default

To add to Bill's (excellent) description, a possibly more direct / simple explanation.
An engine is not built with a counter weighted crankshaft to make it better, it is generally done to make it work.
The vibration modes induced into a crankshaft are a function of the crankshafts size, stiffness, the compression ratio of the engine, stroke of the pistons, ignition timing, what propeller is installed.... and on and on....

Most of these influences are fixed by design. But some of them we as builders can influence, such as installing a previously untested propeller, or an ignition system that is able to advance the ignition timing beyond what is typical with magnetos. When that is done, it is possible to take any engine (counter weighted or not) outside of the safe zone of propeller/crankshaft vibration interaction.
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  #4  
Old 11-01-2014, 03:18 PM
Bevan Bevan is offline
 
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My parallel valve 180HP 0-360 (0-360-A1F6) has a counterweighted crank as built by Lycoming. So this is not just for angle valve engines.

Bevan
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Last edited by Bevan : 11-01-2014 at 03:19 PM. Reason: added engine model number
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  #5  
Old 11-01-2014, 03:27 PM
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ccsmith51 ccsmith51 is offline
 
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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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  #6  
Old 11-01-2014, 03:59 PM
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RV8Squaz RV8Squaz is offline
 
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Quote:
Originally Posted by rzbill View Post
This is somewhat of a semantics issue so I will try to give more info for your understanding.

Engines have multiple vibration orders (meaning combined sets of frequency and amplitude).
The whole vibration formula has multiple order terms starting with the primary order vibration and then the secondary order etc etc and the influence gets smaller and smaller as the terms go off to the right side of the equation as they describe smaller and smaller physical influences.

All aircraft engines are counterweighted for the first few modes (sorry I don't remember how many). The term "counterweighted" as generally applied to Lycomings is in reference to a specific crankshaft design that includes extra weights that actually move in relation to the crank. They are in a kind of pendulum arrangement on the crank webs that allows slight movement of the weight. The movement is needed to address a higher order (further to the right in the equation) and more complex vibration mode. Memory says 4th order but I'm not sure.

My understanding of history is that the "counterweighted" crank was introduced with the angle valve Lycomings and it was signified by a "6" in the engine suffix. With the advent of so many EXP Lycoming clones, both angle and parallel valve, I don't think one can tell what is counterweighted and what is not without knowing the engine details from the builder.

Since I am shooting from memory, I welcome any corrections or clarifications to the above from other members here.
As Bill said, the engine model number will have a number, typically a "6" in the suffix, such as IO-360A1B6. However, it is much more than semantics. A counterweighted crank has moveable weights that do as Bill said, but the crankshaft is about 6 pounds heavier and is more costly. The advantage is that the engine will have none or lesser RPM restrictions depending on the prop.

I have an IO-360A1A that does not have a counterweighted crank with a Hartzell HC C2YK prop. This engine/prop combo has 2000-2350 RPM restriction, but it's an rpm range that is rarely used so it's of no consequence. To be sure about any rpm restrictions for your proposed engine/prop combo you must refer to the prop type certificate data sheet. The 200hp angle valve engines currently sold by Vans have the counter weighted crank.


I hope this helps.

Jerry Esquenazi
RV-8 N84JE
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  #7  
Old 11-01-2014, 04:05 PM
Bevan Bevan is offline
 
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I would think, if you had a metal prop, you would contact the prop manufacturer to ask if they had done a vibration survey with that particular prop and engine combination. If they did, they would give you the resulting recommendation (restrictions). If they did not test that combination, they may say "don't know".

I might surmise that a counterweight crank would be an effort to make a minor change in the vibration survey result so as to make a particular engine/prop combo more practical. ie move the RPM restricted range away from where one wants to cruise at.

Bevan
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Last edited by Bevan : 11-01-2014 at 04:09 PM.
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  #8  
Old 11-01-2014, 08:45 PM
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rzbill rzbill is offline
 
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Quote:
Originally Posted by RV8Squaz View Post
As Bill said, the engine model number will have a number, typically a "6" in the suffix, such as IO-360A1B6. However, it is much more than semantics. A counterweighted crank has moveable weights that do as Bill said, but the crankshaft is about 6 pounds heavier and is more costly. The advantage is that the engine will have none or lesser RPM restrictions depending on the prop.

I have an IO-360A1A that does not have a counterweighted crank with a Hartzell HC C2YK prop. This engine/prop combo has 2000-2350 RPM restriction, but it's an rpm range that is rarely used so it's of no consequence. To be sure about any rpm restrictions for your proposed engine/prop combo you must refer to the prop type certificate data sheet. The 200hp angle valve engines currently sold by Vans have the counter weighted crank.


I hope this helps.

Jerry Esquenazi
RV-8 N84JE
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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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 : 11-01-2014 at 09:14 PM.
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  #9  
Old 11-01-2014, 10:33 PM
xblueh2o xblueh2o is offline
 
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All,
great stuff.
Thanks to all for the education.
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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RV-8 with the Showplanes Fastback conversion
Emp completed except for glass work
Wings completed except for bottom skin and glass work
Fuselage underway
N18451 reserved

Last edited by xblueh2o : 11-01-2014 at 10:35 PM. Reason: typo
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  #10  
Old 11-01-2014, 11:50 PM
PCHunt PCHunt is offline
 
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Default Torsional Dampers

And just to add fuel to the fire, the "counterweights" aren't really counterweights at all.

They are torsional vibration dampers.

A true counterweight is firmly bolted to the crankshaft, and is there to balance the spinning mass.

A torsional damper is added to absorb the twisting motions in the crank due to firing pulses. See: http://en.wikipedia.org/wiki/Torsional_vibration

A great read is how the engineers solved the torsional vibration issues on the Pratt & Whitney R-2800 during WWII. See: http://www.enginehistory.org/NoShort...Crankshaft.pdf

More than you wanted to know!
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