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using and unbalanced prop on purpose

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using and unbalanced prop on purpose

Old 04-15-2004, 04:17 PM
  #26  
Stick Jammer
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Default RE: using and unbalanced prop on purpose

Just to through out some more Ideas. I build alot of race engines for cars and there are several ( chevy 400 chrysler 360 and cast crank 400s and 440s to name a few) that are externally balanced. there is not enough weight on the crankshaft counterweights, because of size limitations inside the crankcase , to properly balance the rotating assembly. So weights are added to the flywheel and harmonic balancer to help to balance out the problem,
Not to get off the main topic, but any weights that are added to the flywheel or harmonic balancer of an eight cylinder engine are usually there to simply balance themselves. If the crank needs external weight to balance, it was a poor job of machining as material can be removed if there is no room to add. To my knowledge, lack of room is not a problem in a small block Chevy.
Old 04-15-2004, 06:26 PM
  #27  
Duane-RCU
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Default RE: using and unbalanced prop on purpose

Here is what I know about mine. I had two different engines, same plane and prop. At wide open, you could barely touch the fuel line, and the engine would slow down. It took me along time to figure it out, but my prop was not balanced, and at high RPM, the fuel was aerating from the vibration, causing a lean condition, and when I touched the fuel line, not pinching, just laying my finger on it, I would absorb the vibration, and it would richen up. I don't know how un-balanced it was, but all I did was change props, and balance the new one, and it was fixed. Plus, the plane did not rattle, and better RPM's. (I hope I explained this so you could understand)
Old 04-15-2004, 06:26 PM
  #28  
Azcat59
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Default RE: using and unbalanced prop on purpose

Jim Messer has summarized this well. We are not searching for balancing perfection in the engine. My approach is: I will make a rough attempt to balance if it is a wood prop. If too much needs to be taken off (usually due to a variation in wood grain) I mark and put the heavy blade opposite the piston. I also use a lot of APC props, and I think they are generally pretty well balanced, but if not, I'd put the heavy side "down" as well. Works for me.

As for getting a four-stroke to quit vibrating, it will probably take more than the heavy blade down to smooth out a single-cylinder four stroke!

Clair
Old 04-17-2004, 02:47 PM
  #29  
Jim Messer
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Default RE: using and unbalanced prop on purpose

To moparcolt and stickjammer:

This is off the subject of model plane engines, but related to your responses. I spent most of my life designing two-cycle natural gas engines up to 20,000 horsepower (100 HP/cyl), and one thing I do know is that 4, 8, 12, 16, and 20 cylinder engines of the V-configuration, could be perfectly balanced without the need for external balancers. All others, 6, 10, 14, and 18 cylinders required a harmonic balancer to keep the engine on the foundation. Also - the 10 cylinder engine was the most difficult to balance.

Inline engines were similar; the 4, 8, and 12's had perfect balance. All of the others, 5,6, and 10 cylinder engines required external balancing. We did not build a 7, 9, or 11 cylinder engine.
Old 04-22-2004, 02:57 PM
  #30  
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Default RE: using and unbalanced prop on purpose

ORIGINAL: Duane-RCU

Here is what I know about mine. I had two different engines, same plane and prop. At wide open, you could barely touch the fuel line, and the engine would slow down. It took me along time to figure it out, but my prop was not balanced, and at high RPM, the fuel was aerating from the vibration, causing a lean condition, and when I touched the fuel line, not pinching, just laying my finger on it, I would absorb the vibration, and it would richen up. I don't know how un-balanced it was, but all I did was change props, and balance the new one, and it was fixed. Plus, the plane did not rattle, and better RPM's. (I hope I explained this so you could understand)
Would you not think that someone, somewhere and at sometime would have taken a single cylinder model engine and put it on a test stand and measured 3 or 6 axis dimensional vibrational levels v's various prop positions, unbalanced, "balanced", two blade, three blade, etc., etc., etc. Then we could really see if modeler's are wasting their time and money balancing their props to unmeasurable precision or if it is really necessary and for what reason? Meanwhile, I will go about my flying with both electric, gas and also glo using the plastic and wood props as purchased and install them "unbalanced" in any position and know that my experience has yet to demonstrate a significant reason not to do just that! This would be a great challenge for one of our various engine gurus who publish every month in 4 or 5 different magazines and seem to have too much repetitive material and maybe because I have read too many of their articles over too many years!

Meanwhile, those who can precisely "static " weight balance their props should never have to use foam around their fuel tanks to prevent fuel foaming as now vibration would not be a problem? Also, remove the foam from their on-board radio components and generally eliminate vibration from their thoughts! Perhaps the issue as above is the lack of significant recorded data to make any scientific judgment pro or con. Perhaps what we would find out is that static balancing a severely unbalanced wooden prop would reduce or maybe increase vibrational amplitudes by 5 or 10% on some axis of measurement depending on prop installation position and the net effect on reducing total vibration would have little impact on the real world of single cylinder reciprocating engine vibration. Foam protection for this and that would probably still be a smart move for on-board components. [[:
Old 04-22-2004, 06:26 PM
  #31  
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Default RE: using and unbalanced prop on purpose

i never balance my props,i was a motorcycle mechanic for 17 years and i never came across a single cylinder engine that didn't vibrate like hell!old panther 650's almost jump off the ground when piston hits t.d.c.,i know model engines are smaller but what difference is a prop going to make on something that is inherently unbalanced?
Old 04-22-2004, 06:30 PM
  #32  
CafeenMan
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Default RE: using and unbalanced prop on purpose

ORIGINAL: bigchap

i never balance my props,i was a motorcycle mechanic for 17 years and i never came across a single cylinder engine that didn't vibrate like hell!old panther 650's almost jump off the ground when piston hits t.d.c.,i know model engines are smaller but what difference is a prop going to make on something that is inherently unbalanced?
umm.... well, there's things like broken firewalls, shorted R/C equipment, short-lived airframes, fuel foaming, etc. Just about anything that can be caused by vibration can happen. The more the prop is out of balance the worse it is.
Old 04-22-2004, 06:35 PM
  #33  
bigchap
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Default RE: using and unbalanced prop on purpose

start an engine without a prop and see how much it vibrates,then put an unbalanced prop on and do the same,there is hardly any difference,besides,if you use good quality props you shouldn't need to balance anyway.
Old 04-22-2004, 06:38 PM
  #34  
CafeenMan
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Default RE: using and unbalanced prop on purpose

ORIGINAL: bigchap

start an engine without a prop and see how much it vibrates,then put an unbalanced prop on and do the same,there is hardly any difference,besides,if you use good quality props you shouldn't need to balance anyway.
I've run engines with a spinner only so I could polish the spinner. It didn't vibrate anything like what I've seen with an unbalanced prop on it.

I agree that most props are pretty well balanced as they come from the manufacturer, but I balance mine anyway. Doesn't take long and it's worth it in my opinion - especially wood props.

- Paul
Old 04-22-2004, 06:40 PM
  #35  
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Default RE: using and unbalanced prop on purpose

fair comment.
Old 04-28-2004, 06:39 AM
  #36  
flicka5
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Default RE: using and unbalanced prop on purpose

ORIGINAL: CafeenMan

ORIGINAL: bigchap

i never balance my props,i was a motorcycle mechanic for 17 years and i never came across a single cylinder engine that didn't vibrate like hell!old panther 650's almost jump off the ground when piston hits t.d.c.,i know model engines are smaller but what difference is a prop going to make on something that is inherently unbalanced?
umm.... well, there's things like broken firewalls, shorted R/C equipment, short-lived airframes, fuel foaming, etc. Just about anything that can be caused by vibration can happen. The more the prop is out of balance the worse it is.
Other than fuel foaming, have never seen the the additional problems. I suppose a three blade prop with a missing blade could break a firewall? It is a marvelous feature of a nice wooden fuse to dampen those uncontrolled engine vibrations which makes one wonder why people use soft mounts. Of course, soft mounts are another bag of worms to argue about with no conclusive proof one way or another. Fuel foaming is easily controlled by proper tank insulating and it also helps immensely to put a few drops of Armorall in you gallon jugs of glo fuel.
Old 04-29-2004, 01:07 AM
  #37  
Bob101
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Default RE: using and unbalanced prop on purpose

I'm waiting for someone to bring up V-6's. How those with an offset ground crank balance compared to those that aren't offset ground (2 rods on different centers sharing the same connection between 2 main journals).

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