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Engine Balancing

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Old 10-30-2002, 12:13 AM
  #1
PTERODACTYL
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Default Engine Balancing

Hello
I'm looking for a article on engine balancing, can't remember where I read it but its been about a year ago or so. It talked about balancing a engine for more power and smoother runing
Please let me know if someone else read this articale it was in one of the leading magazines, model airplane news , RCM , Jet International,ect If you know about it please E-Mail me and let me know where to find it.
Thank you
Thane
tmckinse@wyoming.com
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Old 10-30-2002, 01:19 AM
  #2
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Default Engine Balancing

Why not do a search on rcuniverse? It's talked about often on here.

Enjoy,

Jim
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Old 10-30-2002, 01:50 AM
  #3
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Default Engine balancing article..

Is in the March 2002 RCM in the "Hover" column. I'd post it here, but I don't know my membership code or whatever and can't get to the article online..I guess I'll have to wait til RCM comes to get the number LOL
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Old 10-30-2002, 03:04 AM
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Default Actually..

Actually, John Benario in RCM's Hover column was describing how he read about rebalancing cranks from an article in Model Aviation (the AMA magazine) in one of the control line speed columns. I read them all religiously! DOnt know the exact issue but it would have been in 2001 or early 2002. Definitely in the CL speed column.

For all those rc guys that think there isnt anything to gain by reading the control line and free flight articles, those guys do more engine rework than anyone else! Read and learn... its all good stuff!

Hope that helps...

Andrew Coholic
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Old 10-30-2002, 03:39 AM
  #5
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Default Yes, I read that article, too..

Good heads up!

I seem to recall Benario going thru all the steps of weighing, balancing, grinding..the whole nine yards...it was just the article that stuck in my mind.
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Old 10-30-2002, 05:17 PM
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Default Engine Balancing

Chuckauger,
When I saw the picture of his finished crank, I said to myself that pin is going to break within 10 minutes of run time, he ground a lot off of it!

Andrew
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Old 10-30-2002, 05:24 PM
  #7
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Default I heard that!

I thought the same thing..but then in the August issue he went on to say how much he liked the mod..so I guess it worked OK...of course it was in a heli, so that might make it a little easier on it. Sure as heck if I did it it'd break! LOL

And AJ..what is the plane in your avatar??
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Old 10-30-2002, 05:36 PM
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Default plane pic

Chuckauger,
That plane is my turbine powered Super Reaper. Its my first jet, and is a Mick Reeves (from England) kit and I built the 12 pound thrust MW54 turbine (also from England) from a screw together kit.

Its about the cheapest way to get into turbines, and is a nice large (72" wingspan) all wood (foam core wings) jet. Has fixed gear for simplicity/durability and will do 100mph tops. Lands very slowly too, and takes off on rough grass in <200 feet.

well time to go back to work, lunch is over!

Andrew
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Old 10-30-2002, 09:51 PM
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Default Engine Balancing

Thanks to all for the info
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Old 11-15-2002, 11:11 PM
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Default engine balance articles..?

Hi guys,
cannot easily get a back issue of RCM with Mr Benarios article on engine balancing. Someone out there who can help please..??

Attach a photo of one of my "swiss cheese" Saitos for you to comment upon. Wonder how far you can bore out the con rod before....SKRUNKK..? This particular Saito 65 sure purrs like a cat and has now a few hours running time without mishap.
Anyone out there who would like to compare notes on engine balancing - please...??..Cheers/Harald
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Old 11-16-2002, 12:10 AM
  #11
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Default I can try...

Engine Balancing

One thing that everybody with a 60 size helicopter notices is how the engine vibrates and shakes at idle. The shake does not go away at high rpms, it is simply masked. Remember with vibrations the amplitude of response (how much it shakes) decreases with increasing frequency (rpm). The reason smaller engines don't have the buzz is there is less mass moving in the engine.

My readers who have followed me over the years know that I have had some success with modifying engines, but one thing that I have never had good information on is balancing. When one removes weight from the piston or crankshaft what changes with the balance? I could never say one way or the other.

That changed a month ago. Model Aviation has a new U-Control Speed columnist who is very smart, Scott Newkirk. See, you really should read everything out there, not just helicopter stuff. In the September 2001 issue Scott discusses crankshaft balancing and gives an analytical formula. I talked to Scott to get more details. As with any formula dealing with single cylinder engines it would be difficult to prove, but Scott says it has worked very well for him over the years. Oh, by the way Scott, what rpm do you guys run your engines at? The answer: 42-44,000 on the ground (no that is not a misprint).

At last a crankshaft formula that can be measured. The Thunder Tiger 70 engine in the Raptor was a great engine to try out Scott's formula on. While the 70 runs well, it does have a vibration that is very noticeable at idle. This is not a slur on the Thunder Tiger engine, the YS and most 60s have a bad vibration at idle too.

The formula is the crankshaft counterbalance should balance out the weight of the piston, wristpin, piston ring if used, circlips if used, the small (piston) end of the conrod, and half the weight of the large (crankshaft) end of the conrod.




Weighing the large end of a conrod. The wristpin is used to support the small end on the balsa support. The balsa is sized to hold the conrod level when the balance pan is in the balanced position. With the weight of the large end known, the weight of the small end is then the total weight minus the weight of the large end.


This is an easy formula for helicopter fliers to use. We already have High Point balancers and triple beam balances which are the tools used. Photo #2 shows how I weighed the large end of the conrod. The weight of the small end of the conrod is then the total weight minus the weight of the large end. On the 70 the small end weighs more than the large end because it is wider. A bob is then made that equals the required weight and is hung from the crankpin with the crankshaft on the High Point. Photo #3 shows a crankshaft on the High Point.




A crankshaft on the High Point. This is an O.S. .46 crank and conrod. Note the crank balances the weight of the conrod and wristpin but no piston. The counterbalance will not balance the weight of the components when the piston is added, similar to the Thunder Tiger 70. Again the counterbalance is as big as it can be which means that weight must be removed from the crankpin side to attempt to make the counterbalance meet Scott Newkirk's formula.


How did the stock Thunder Tiger components measure up to this formula? The crankshaft counterbalance did not come close to balancing out the components listed. The design of the counterbalance is such that it is already as big as it can be so it seems that it is a manufacturing decision to not balance the crank better. Since the counterbalance cannot be made bigger the alternative is to remove material from the crankpin side, which may be difficult to do in a production process and still keep costs down.





My Thunder Tiger 70 crankshaft after balancing. The outlines show where the metal was when I started. It required a huge amount of grinding to meet Scott's formula. The engine runs much better with the balanced crank than with the stock crank.

Photos #4, #5, #6 show my crankshaft after I removed enough material to satisfy the formula. I do not have a before picture but the drawn lines on the picture show where the original outline was.

With this crankshaft the engine runs great and has very little vibration at idle, lending credence to the accuracy of Scott's formula. Scott tells me he got this formula from a Russian technical publication on U-control speed engines.




My Thunder Tiger 70 crankshaft after balancing. The outlines show where the metal was when I started. It required a huge amount of grinding to meet Scott's formula. The engine runs much better with the balanced crank than with the stock crank



.
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Old 11-16-2002, 02:20 PM
  #12
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Default Engine Balancing

Hey ChuckAuger(engine guru), I haven't written to you in a while. Excellent photo essay! I have a MAG .91 that vibrates like crazy at idle. It shakes the whole plane.

In photo #2, with the crank on the High Point Balancer, it looks like you have only the rod and wrist pin?

If properly balanced, the crank with the attached rod and wrist pin, should rotate to any position w/o rolling back?

Why don't you use the whole assembly(piston to rod), and balance this against the crank counter balance?

I thought the shape of the counter balance, helped the flow of fuel, by throwing it up the ports, with those flat parts? Is it just a counterbalance?
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Old 11-16-2002, 02:46 PM
  #13
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Default I take no credit..

The text and photos are from RCM. Yes, the 2nd photo shows the crank balancing only the rod and wrist pin. That is why he ground all that material off the crank around the crank pin, to lighten the side of the crank opposite the counterbalance. By making the crank pin side lighter, you enable the counterbalance to balance out more of the piston weight.

Maybe he should have taken a shot after with the piston, rod, and pin to see how much he got it to balance.

And as far as the shape of the crank throwing mixture up into the ports..some RI engines have totally round disks for counter balances and they run fine.
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Old 11-17-2002, 01:29 AM
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Crankshaft shape plays no part in throwing the mixture up the transfer ports. The flats on the counterweight are just a cheap way to machine a crank for reasonable balance. If you look at a Rossi 45 crank though it appears to be a solid disc but it's machined internally for balance and then to make it even better they insert tungsten (from memory) on the side opposite the crank pin. That's the sort of detail you get in a high quality engine
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Old 11-17-2002, 04:48 AM
  #15
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Default Tungsten

tungsten you say that sounds alot easier then what I did, I welded more metal on the counter balance and then machined it, also ground down the rod to a airfoil shape and ground out some of the piston. I was able to remove about 1 gram, still barely got it to balance [OS .32 FH] did some work on the ports too, chamfered the outside bottom of the ports to get better flow. Haven't had a chance to run it yet. On automotive racing engines the counter balances are airfoil shaped to reduce drag.
Thane
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Old 11-17-2002, 05:45 PM
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Default Engine Balancing

a caption in a UK mag here had a MDS mercury balanced, it ran at 21,000. Surely a engine would sink if you dropped it in mercury
So what can us everyday modelers do with our dremmels?
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Old 11-17-2002, 08:23 PM
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Default engine balance and 4-strokes..?

Many thanks Chuckauger for you informative overview.
1) Nice job and I´m happy to see the "balance factor" now being defined as the whole assembly but half the weight of the big end of the rod. I will certainly dismantle some of my modified 2-strokes to see where I ended up in my effort to take away as much as I dared on the crankpin end.
2) rsieminski commented on your nice "scoop" and it sure looks good with some further grinding just below the crankpin to facilitate breathing. Nice job which will certainly inspire me to go further in my own modifications.
Attach a photo I saw in a german magazine showing the grinding of a Rossi crankshaft. Comments..??
3) Chuckauger(or anyone else) do you know if the "balance factor" as described by you and Scott Newkirk above also does apply to a 4-stroke engine..??
4) w8ye suggested Pteracodyl do a rcuniverse search on engine balancing. Well, I tried that but did not find much beyond this thread.....my fault maybe...??
hope to hear more from you all...........Cheers/Harald
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Old 11-18-2002, 03:55 AM
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Default Engine Balancing

Interesting photo and even though it's a Novarossi you can still see where they've inserted one of the tungsten balance weights. Actually I get the impression there may have been another where the web has been scooped out.
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Old 11-18-2002, 09:48 PM
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Default tungsten insert of course...!

Many thanks downunder for pointing out the tungsten insert in the photo I attached. Did not see that myself until you made me aware of it.......Cheers/Harald

PS...here is another one from a Jett.65 engine....DS
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Old 12-15-2002, 06:25 AM
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Default Balance talk....

...I've got a S.T. 3000 that is literally shaking my
plane apart. The prop came in perfect balance....
an 18-8 Zinger. The spinner wasn't bad, but the
back plate was way off. Regardless....the plane
shakes beyond belief. I know rubber mounting
might help....but I'm not looking for a Band-aid.

The problem is, the motor is very much out of balance.

My question is....why can't the crank be balanced
100% with the rod, piston, pin and related parts all
there.? What's with this 1/2 of the weight of the big
end business ?
Why can't the 2-stroke crank/piston assy. simply be
BALANCED ???

Am I missing something ? What are the counter weights
for on the crank ? Is there an imbalance/firing pulse
thing ....that I'm not awhere of ?

Take a V-8 auto engine for example....the crank is
balanced so that it will not rotate in any position on
centers....it's balanced. The pistons, rods, pins, ect.
are all weighed and made the same. The flywheel and
front pulley are balanced. The end result is that the
engine is BALANCED.

Not rocket science. So...what is the difference between
any other engine on the Planet....and a single cylinder
2-stroke engine ???

Wouldn't one assume that a single 2-stroke would be
"simply in balance" also ???

I'm not trying to make conversation about this. I have
two other planes set for the S.T. 3000's, including a 1/3
Pitts, and a Bridi Killer Bee. If this engine is simply a
shaker....I'm gonna hafta' be spending some coin for
some Moki's, I guess. (something in the 1.80 size).

Any, and all serious experience with balance issues
are welcome. Please....no theories about the rotation
of the Earth, or the position of the stars, ect.
I'm gonna start grinding on something....

Thanks....Dave.
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Old 12-15-2002, 11:20 AM
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Default Engine Balancing

Hey Dave,

There is an article in the Jan. 2003 issue of M.A.N. by Dave Gierke relating to engine vibration I think you will find of interest. The Machinery's Hand Book has the basics of balancing if your into the mathematical end of it. This book may be available at the local library along with other mechanical references.

Happy Fly'n....
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Old 12-15-2002, 02:30 PM
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Default Engine Balancing

because the piston,pin rod, are oscilating? up and down in the cylinder they are only considered 'part' of the crank weight. A single cyl engine can never be perfectly balanced. All compromises.

Tried to get my kamikazi pilots licence but I flunked the test.
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Old 12-15-2002, 02:36 PM
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Default Engine Balancing

No single cylinder engine (2 stroke or 4 stroke) can be perfectly balanced because the requirements for perfect balance keep changing as the crank rotates. The purely rotating bits are easy but the reciprocating parts make life difficult. You could weight the crank web for perfect balance of the piston at top and bottom of the stroke but at half stroke the counterweight will be at 90 degrees to the axis of the cylinder. The compromise for minimum vibration is 50% of the weight of the reciprocating bits. The conrod however is peculiar in that basically the big end is purely rotating while the small end is reciprocating. All other points of the rod are various mixtures of the two.

Multi cylinder engines don't have this problem because as one piston is coming down there's another on the way up so their individual reciprocating forces balance each other out.
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Old 12-15-2002, 10:05 PM
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Default Fox engine crank

Howdy, yes indeed balancing a single cylinder engine is not trivial, maybe that´s why people approach it so differently. Attach a photo from an RCM article by Clarence Lee describing how a Fox .40 crank has been milled. Must admit that I do NOT understand the reasoning beind that particular scheme. Anyone who can explain..??.....Cheers/Harald
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Old 12-15-2002, 10:20 PM
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Default Good Vibrations!

I agree with you Downunder, except in the case of my V-twin Harley-Davidson. lol.
Actually, I have the new Twin Cam 88B engine, with the two counter balance shafts and its a very smooth running engine. It's a very ingenious setup. You maybe able to come across it at their web site if your interested?
For those who are not familiar with the Harley V-Twin. The connecting rods share the same crankpin and vibration is what they're all about, thats if your into that kinda thing? The Ladies sure seem to like it!

Happy Fly'n....
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