glafond

I was researching prop balancing techniques today and I came across an interesting article by Joe Wagner. His article cited that on a single cylinder engine, prop balancing is largely a wasted effort. This got my attention in that logically I can follow where this theory could be correct. He talked about a friend who would set the heavy side of a prop down, while the piston was at Top Dead Center. This seemingly counteracted some of the vibration of the engine and reduced vibration even more than having a perfectly balanced prop. Has anyone tried this out with any success? As a novice I wonder if this would be something I could do, or should do, in order to save time and effort and perhaps even enhance my planes performance. Comments, stories, ideas, all welcome. Thanks!

glafond

Just a correction, the prop was set heavy side down in opposition of the crankpin at TDC. I'm not sure if thats different then what I said but just thought I would clarify.

Stick Jammer

That theory is bunk. The crankshaft is balanced with a counter weight. Anything you bolt to it should be balanced too. If an off balance crankshaft can make the engine run smoother I think they would build them that way.

ifixairplanes

My Feedback: (9)

i heard that theory too. i think it is mostly to help off set an unbalanced crankshaft. always balance props.

sean

Strat2003

My Feedback: (1)

I've heard that story about old McCoy C/L speed engines. It seems some of them didn't have enough counter balance on the crankshaft, so some guys had some luck setting the heavy blade that way. I haven't heard of it being useful on anything modern, though.

DBCherry

My Feedback: (4)

NO engine's piston and con rod can be balanced with a weight on the crankshaft, there is just too little room and the "moment arm" of the piston and connecting rod is so long in comparison to the counter balance that it's impossible to do.

If it were possible to offset all the imbalance, then our engines would be vibration free, like an electric motor.

There is good argument to be made for off-setting some of the piston's imbalance with the heavy blade of the prop.
Dennis-

Jim Messer

My Feedback: (2)

I introduced the Quadra 35 engine to the USA marketplace back in 1976. It was designed with a single counterweight on the inside side of the connecting rod. The engine vibrated like crazy, especially at low RPM's. Dario Brisighella got into the business of making special flywheels for this engine to reduce vibration. Actually what he did was to dynamically unbalance the flywheel so that it acted more like a second counterweight on the side of the crankshaft that had no counterweight. I purchased hundreds of these flywheels for resale, and no two ever came back alike. But Dario's wheels did cut down the vibration by a huge amount. At the same time, we experimented with prop balancing, and on an engine like this, a balanced prop didn't improve the vibration level at all. What we did find however, was that an unbalanced prop, put in the position of the heavy blade down when the piston was at top dead center - reduced vibration considerably. You can call it "bunk" if you want to - but I have been flying nothing but gasoline engines since 1975, and I put all my props on this way. I don't balance - I just find the unbalance, and put the heavy side down. It works for me. Should work for others as well. At the present time, I mostly use Zinger and Mejlyck props, and it is safe to say that they really aren't far out of balance at all - so we aren't talking "big gobs of unbalance" here. If I had a prop that was "really heavy" on one side, I would throw it away. I remember Joe Wagner's article, and I'm guessing that it was probably dated about 25 years ago - same time as we were doing our work. As I remember, Joe pretty much confirmed what we found out during our tests. Don't sell the article short. It has merit.

glafond

Wow, there is a wealth of information here to go through. I guess the bottom line is I have to find out what works for my engine. I'm using a Super Tigre G-61 that I just purchased and have not even broken in yet. I was told by my future instructor that we will do that at the field. I think I may try both options out and see if my engine performs differently either way. I'm running on the assumption that there will be a visible difference in vibration. I'm not sure if that is a correct assumption or not but I guess I will find out. It would be interesting to hear if anyone has tried this on glow engines. I do see points on either side though, if you have a prop close to balance the vibration change will be very minimal, if you have an out of balance prop its going to either help, or possibly hurt the engine leaving no middle ground. At the moment, balancing seems like the safer option. I love good discussion like this though. It's what these forums are all about!

Stick Jammer

I don't believe I stated that the crankshaft counter weight will balance the added weight of the piston and conrod. The counter weight is there for rotational balance of the crankshaft as it is in any piston engine. This means that the crankshaft will remain in any position without wanting to rotate one way or the other when at rest with no friction to restrain it. The piston and most of the conrod are not part of the rotational mass, they are linear. That linear mass has no effect on the crankshaft balance. It is the linear mass of the pistons that produces the unwanted vibration due to the forces imposed by combustion, and is also the reason that an electric motor doesn't experience this problem. Precisely the reason why the compression strokes of a multi- cylinder engine are offset from one another, to dampen the linear forces. A balanced flywheel of one sort or another will also dampen the vibrations caused by these linear forces imparted on the crankshaft. Try running a race built small block Chevy at 10,000 rpm,s with an unbalanced crank or flywheel. It won't stay together for long. A balanced prop acts as a flywheel. The problem with an unbalanced flywheel is the inconsistency of the centrifugal force it creates. The faster it spins, the greater the out of balance force it transfers to the crankshaft bearings even when the piston is on it's way back up and the power stroke is over. Now granted, some may have observed what appears to be a smoother running engine when an out of balance prop was installed in the correct position due to the flywheel effect or possibly the crankshaft wasn't balanced to begin with, but I can't see where you are doing the engine any favors by deliberately installing anything to the crankshaft that isn't balanced. I'll stick with balancing my props.

Live Wire

Model aircraft engines can be balanced but it is cost prohibitive in production. There are articals in mag's that explain how it is done and the advantages. It does weaken the crank shaft because you have to remove the weight from the rod side of the crank. The rod piston and pin make the crank heaver on the rod side and they can not make the counter balance heavy anough to balance.

Azcat59

I agree with DB Cherry above. I note that Clarence Lee usually compares the counter-balance weight to the weight of the piston, wrist pin and rod, and of course the counter balance is never quite enough to balance out the other parts.

(Clarence Lee is considered the "Dean" of model engines, and has written for RCM for the last 30 or 40 years).

Clair

aimmaintenance

As for the smaller glow engines (up to 60 size or so) balancing is a monumental waste of time in my opinion.
I've seen a thousand unbalanced props go on with no ill effects.

MinnFlyer

My Feedback: (4)

Double ditto.

FLYBOY

My Feedback: (11)

I never ballence props for the smaller engines. I do on the larger ones because I have noticed a huge diff on them, but never saw a problem on the little ones.

Rodney

DB Cherry is correct, none of the model engines have a perfect balance and putting the heavy side of a prop down most often helps.

Stick Jammer

For anyone that agrees with this statement, I'm sorry but this is apples to oranges. It is not an internal "imbalance" that causes vibrations in an internal combustion piston engine. And to reiterate, the counter weights on the crankshaft don't have to and are not designed to compensate for the weight of the piston. The crankshaft is or should be machined to balance it's own rotational mass. The piston going up and down is not part of the rotational mass.

BlastOHotAir

Even if the engine did not have the correct counterbalance weight on the crankshaft, placing an unbalance propeller at the front of the engine would do no good because it is in a different plane than the piston/crankshaft. Think about it. When you take your tires to be balanced, weights are installed both on the inside and outside of the tire, and they are usually never at the same point of rotation. Many years ago, some gas stations would use a machine called a "bubble balancer" and place weights only on the outside of the tire rim. The tires would still vibrate, and now the vibration isn't just up and down, but now also in and out (top of the tire vibrates outward, while the bottom of the tire moves inward). Same thing with dynamic balancing of your model engine.

Never knowingly run a model engine with an unbalanced propeller.

Spicoli

I have never done it myself but I have read articles from Clarence Lee,Gerry Yarrish and Dave Gierke on how to do it.It can be done.I agree with DB Cherry.So do the above experts.

BlastOHotAir

I find the use of the term "experts" in relation to the responses to this thread to be very interesting. Quite honestly, I have been extremely amused that no one in this thread seems to realize that balancing an engine is not an art, and it is not done by trial and error. It is part of machine design. It is simple mechanical engineering. The counterbalance of a crankshaft DOES compensate for the linear motion of the piston. The problem is that it moves in both the x and y direction, so although it does compensate for the piston moving up and down (x direction) it also moves its weight side to side(y direction). Ever wonder why there are very few car engines with an odd number of cylinders? Harder to balance dynamically. Manufacturing tolerances determine the perfection of the engine balance. The slight imperfections in engine balance have a huge impact on how fast an engine will run. One reason why a Jett 50 makes so much more power than an OS50 is better balancing from tighter manufacturing specifications. It isn't all about compression and ignition timing. Model engines also vibrate because of rotational acceleration and deceleration due to the fact that they only fire once per revolution, whereas a V8 engine does it 4 times per rotation, and makes much less power per pound of drivetrain weight than a model aircraft engine. Automobile engines have wonderful dempening motor mounts to help reduce the transmission of vibration, and they have a great big flywheel and harmonic balancer that practically eliminate rotational acceleration and deceleration due to the firing sequence. Our little model engines have none of that.

Anyone ever driven a 4-cylinder Ford Pinto, and then noticed the difference when they climbed into a Cadillac?

So stop thinking that the balance of a propeller has any impact on the engine balance. Your one-cylinder aircraft engine is balanced as good as it is going to get by its manufacturer. If a person writes a magazine article saying that they can balance the engine with the propeller, that person has likely never taken a college level mechanical engineering class in machine design. Is it possible? Yes. But it also requires that the person doing the balancing be able to modify the center of gravity of the propeller accurately. Is it possible without a significant amount of math and exact propeller placement? No.

Small propellers have very little inertia (they don't weigh very much), so the impact of one being out of balance is small. But anyone who wants to win in SSC combat knows that balancing the small propeller on a little .15 counts for 150-200 additional rpm, and significantly reduces the potential for fuel foaming. Balancing your propellers significantly improves the reliability of your aircraft.

If you don't want to mess with balancing your propellers, you don't need to, because the airplane will still fly, and you will probably crash it before you see the damage from engine vibrations. As for me? I like my planes to fly troublefree. There is nothing I hate more than spending my precious free time trying to figure out why my expensive toys don't work when I could be flying.

CafeenMan

I've heard the theory too, but there are two reasons why I don't do it. The main reason is that I like the prop to be horizontal when the engine begins to enter compression. That's the place where I'm most comfortable hand starting the engine and it also gets the prop away from the ground if I come in dead stick.

The other reason is that how in the heck would you know how much it should be out of balance and who wants to spend time getting it right? I seriously doubt an out-of-balance prop is going to be automatically out-of-balance to the right degree.

Too much trouble for me to bother with it. I'd rather just balance it and bolt it on.

- Paul

Azcat59

Cafeen Man: I too like the prop to be at 3 and 9 o'clock when starting. If you do this, and put the heavy blade on at 9 o'clock, it will be straight down when the piston is at top dead center. And lined up with the crankshaft counter-balance.

I don't think using a heavy blade is a matter of getting the engine into balance. I think it is a matter of getting it CLOSER to being in balance.

Clair

moparcolt

My Feedback: (2)

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, very similar to a prop that has more weight on one side. I will have to admit this is not exact science as you cannot check the balance on a small engine like you can, on special equipment,[8D] for a car engine but it could definatly have an effect that could have positive effects if the weight is not so much that it throughs the engine out of balance the other way.

CafeenMan

Yep. I didn't think about that. But that still leaves the problem of knowing how much out of balance the prop should be and then after you know, actually achieving it.

Jim Messer

My Feedback: (2)

To Cafeen Man: Like I said before, I do not balance any of my props. I simply find the heavy blade, if there is one, and install it at the 9:00 position. I don't think anybody is smart enough to calculate how much out of balance a prop should be to make an engine run smoother, and I know of nobody that intentionally unbalances a perfectly balanced prop. That would be ridiculous to say the least. So make it easy on yourself and just do what I do, and just for fun - give it a try sometime. If you don't like the results, then go ahead and balance the prop to your heart's content. When you do that, you can also install it any way you want. That's one advantage of balancing, but is it worth it? On the single cylinder gasoline engines that I run, I think not.

CafeenMan

Jim - I'll give it a try on one of my 4-strokes that seems to have a lot of vibration. I need a Vibra-Meter to check the results.