I'm trying to eliminate as much vibration as possibe in a 1.20 FS. Will positioning the prop a certain way in relation to the piston at TDC reduce vibration? Will the same technique work on 2 strokes? Thanks for your help.

This topic has been much debated.....
One side says to leave your prop unbalanced and put the heavy end of the prop opposite to the piston( piston on TDC)
The other way is to balance your prop and put it on any which way...I like to mount it so that when the engine dies- either by me or by fate- the prop stops horizontal. Takes some of the fear out of having a prop strike.
I've read posts here of some guys that disassemble the engine and start shaving cranks, rods, etc. I haven't done anything like that yet. Guys that did it, swore by it.......

Prop position, whether balanced or not, makes no difference to any of the forces going on in the engine itself.

Balancing, as such, does make a difference, and you should do it.

There are no out-of-balance forces in an engine that can be counteracted by the prop, wherever it's positioned.

-David C.

The counter weight on the crank balances most of the weight of the rod and piston but because the rod is continuously changing angles only a countershaft balancer can come close to balancing it all. Leaving your prop unbalanced three inches away will only strain your bearings and beat up your airframe.

I feel that hobbsy is right on........ but....... I can "feel" the keyboards clicking out there. There are many guys, (as discussed in other threads), that say balancing is a waste of time.
A lot have said... "a waste of time on smaller props".. like under 16". I just can't understand the reasoning in that statement. I am not an engineer, but, I just feel better when that prop is balanced, (any size)!! I mount mine horizontal too. .......lownslo...

Bob

Anyone who says they don't balance props, has not run into a bad one, and had the plane jumping up and down. Or didn't know what the problem was, when it happened. .... And even a little out of bal. is hurting your engine, and taking years of it's life.O.K. maybe I baby my engines but treat me pretty good.....



bobbdd

AMEN bobbdd !!!

While I check the balance of my new props before I use them I don't spend a whole lot of time balancing them. I just line up the heavy blade opposite the piston. But then I don't really use anything bigger than 12 inches. On occasion I have had to throw props out. They were just too far out for my standards.

The worst vibration I have ever experienced, other than a 4-stroke, was with a Supertigre S-75. These engines are notorious vibrators. (Wonder if the G-75 is just as bad?) I took it apart and checked the weight of the piston, rod, wristpin, and counter balance using a digital scale. The counter balance was way too small to correctly balance out the weight of the piston, rod, and wristpin. I believe they used the same crank as the S-61 and simply relocated the crank pin for a longer stroke. The S-75's piston and wristpin are significantly heavier than the S-61's. I did some grinding on the crank and generously chamfered the wristpin to lighten it up and got the engine to run noticeably smoother. It wasn't a great improvement but it was better. I think the only way to get the engine any smoother would have been to add heavy metal to the counterweight or make a counterweighted prop drive plate. I'm sure a properly indexed out of balance prop would have helped a bit, too. Placing a counterweight outside of an engine is not a big deal. Many car makers do this with counterweighted harmonic balancers and flywheels. The Chevy 400 small block and 454 big block are two good examples of this practice. Also, when an automotive machine shop balances a crankshaft they only add or subtract weight from the front and rear counterweights. Never from the inner ones.

You should have heard the decrease in engine-airframe noise after I balanced the APC 8x8 and 8x10 props that I'm running on my West eurotec .50 engine on razor blades instead of on my balancing tool I use for the big props...
This thing spins an 8x10 at close to 20000 rpm on the ground... and it just runs much more quiet now the props are balanced to the milligram... so something must have been wrong there before...

The external counterweights on an auto engine are mounted on a 50 lb. crankshaft in a 600 lb engine that is isolated from the chassis by soft mounts. The out of balance prop is mounted on a one lb. or less engine three or four inches from where the engine is attached to the plane and turns at twice or more rpm as the auto engine. The automotive analogy does not work here. I balance props until they will randomly stop in any position on the balancer then check them on a second balancer. APCs are the worst culprits and the hardest to balance.

From my earlier post....
There are just as many that feel the opposite, and there is not much middle ground. This also happens when fuel/oil needed is discussed. It is interesting to read the different theories and analogies.
I used to use the analogy of .... have you ever been beside a car on the highway with it's tire bouncing up and down? I don't put tires on my truck without having them balanced. Of course.. there was someone out there with a whole lot more of technical training that shot that analogy down. ... and that's OK. I will continue to balance. (I have more time than money)
....lownslo......Bob

I have done it both ways, but if for some reason, you change props, or forget which end is the heavy end, you could have a vibrating mother. I think its safer to balance the prop, and let it line up any which way. I like the horizontal method myself, but thats just me.

On a slight off topic note, Nelson engines now come with counterbalanced thrust washers. You set the engine at TDC, and line a notch on the thrust washer to 11 O'clock. I was skeptical of these at first, and resisted the switch, fearing I would forget to line it up some day at a race, and it would cost me a heat. I've finally switched all my racers over to these, and lo and behold, have gained 100-200 extra rpms.

I'm suprised a manfucture hasn't started producing something similar for sport engines....

Could be a new cottage industry.

The first thread I entered talked about prop balancing.

A single cylinder engine is NEVER balanced. It has a revolving crankshaft counterweight to balance both a rotating mass and a reciprocating mass.

The rotating mass is the connecting-rod's lower end mass.

The reciprocating mass is the connecting-rod's upper end mass, the piston, the gudgeon (wrist) pin and its retainer(s) and the piston ring (if the engine has one).

The rotating mass is naturally, fully balanced.
As for the reciprocating mass; if not balanced at all, it will induce strong, up-and-down vibrations. If fully balanced, the crankshaft counterweight will induce strong side-to-side vibrations.

This, because it must counteract the reciprocating mass acceleration and deceleration, which is close to zero when that mass is around midway up and down. At these times the counterweight is just off-balance slag, either to the right, or to the left of the engine centerline, hence the side-to-side vibrations.

So the counterweight only partially balances the reciprocating mass. As a result, the engine vibrates both up-and-down AND side-to-side, but to a lesser degree.

A non-balanced prop can induce its own vibrations, aiding the counterweight, or interfering with it. A prop must be balanced, but you do not have to invest in super-duper balancing equipment.

Using the common fingertip balancer, or even a knife edge, should do it sufficiently well, as any very small imbalance left, will be masked and overwhelmed by the much stronger, internal engine vibrations.

In theory, a single cylinder engine can be balanced, using two concentric, counter-rotating shafts, with counterbalance weights, in front and behind the crankshaft. Very complex, heavy, hard to put into practice and will make your $60 .40 engine cost $1,000, or more. Just a thought....

Sincerely,

Take a look at this pic... it is one from a new MB profi F3D engine... ok, it is not fully balanced, that can't be achieved, but they do go as far as they can to get imbalances out of that engine.
Less stress on bearings equals more power and longer bearing life.

By the way, what you see in the picture is a spinner backplate... lightened at one side in the form of holes, and made heavier at the opposite side bith brass inserts. Like it is shown in the picture, is the position with the engine in BDC. The meaning of this is to balance for the induction port in the crankshaft. The effect of putting on out of balance prop on a front intake engine can be the same... but IMO it is a little more complicated than just not balancing your props and putting them in a certain position...
The out of balance weight of a heavy prop blade is much further away from the shaft centerline, so if you want to balance for the induction port, very little weight is needed.

I start my engines by hand mostly, so this means my props are positioned horizontally mostly too... so I have to balance them....

Rudeboy,

The photo you sent illustrates Mallory metal (I believe it is either a Tungsten, Osmium, Iridium, or Uranium alloy) slugs, embedded into a balancer plate.

They may better balance the engine than solely the crankshaft counterweight, but they have little effect, if any, on what I illustrated in my message.

To come to a real balance, you would need two such balancer plates; each with one quarter of the needed, full counterweight; one on the front of the crankshaft (around the propeller end) and one on its rear end, both geared to rotate at crankshaft speed and both in opposite rotation to the crankshaft, which has half the needed, full counterweight.

The gearing must put all three counterweights down at the same time and up at the same time, but fully counteracting to the left and to the right. When the crankshaft counterweight is on the left, the balancer plates' counterweights are both on the right and vice-versa.

This way, the reciprocating mass is fully balanced up-and-down. Only much lighter second order vibrations, remain unbalanced. And at the same time, no left-to-right vibrations result, because the crankshaft and the two balancer plates counteract each other laterally.

Sincerely,

... the people who built this engine say they are brass inserts... or at least, it says so on their website...

Next time I see the man, I'll ask him...

I wasn't reacting on what you said, I know a single cilinder engine can't be balanced completely without auxiliary balancing shafts.
I was just pointing out that positioning your (out of balance) prop in a certain way can have a good effect in some very rare case, but that it is very unlikely to get it right by just putting an unbalanced prop on the engine.

Rudeboy,

If this is an aluminium balance plate, it could very well be brass.
Mallory metal is used in iron, or steel crankshafts.

What you say about the prop positioning is true.

I just balance it, so it is a neutral counterweight and tighten it at a horizontal position, when the compression starts, so it could be easily hand started.

Sincerely,

The MB FAI engines balanced thrust washer, is similar to what is provided with the Nelsons (same concept anyways). Its definately not going to counter react all the forces, but I do believe it helps somewhat.

Going by the colour of the weights in Rudeboy's photo they're brass. Mallory metal (tungsten carbide) is almost black and they're used in the crankweb of things like the Rossi 45.