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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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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). |
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![]() 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! :p CJ |
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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. :confused: CJ |
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. |
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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:
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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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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. |
Counterweighted crankshaft
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Never had anybody provide a technical/scientific answer to this but any empirical conclusions could be equally valuable. |
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!?! :p CJ |
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