Is there a certain position the prop should mounted to the engine in reference to degrees? Seems the engine would be smoother and possibly easier on bearings if there was a correct relationship between the crank counter weight and the piston and the prop.

Any thoughts?

The prop acts like a flywheel so it does not matter what position it is. The prop is usually placed in a certain position for hand proping reasons.

Pathogen202,

A balanced prop is not and must not be a balancer.
It must have totally neutral balance.

For easier hand starting and priming, it is best to mount the prop so as the exhaust port is just closed, the prop is horizontal.

Compression begins as you start flipping the right hand blade over the top, to the left (with you facing the front of the engine).
The engine fires as you follow through, to your left.

An unbalanced prop can can smooth out the vibration of an unbalanced crank if it is put on right.

Spicoli,

Please look back in RCU... Never mind, I copied it.

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http://www.rcuniverse.com/showthread...procating+mass

Balancing... (post # 13)

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....
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So, no it will not help....

I agree that the prop balance should be neutral. Although it seems to me that the prop would impose different forces when parallel to the ground than it would when perpendicular to the groung. I guess those forces could be nullified by the spinning of the prop, especially at several thousand RPM.

If it were true, we would have to change the blades on our pushmowers also, same principal.

Pathogen...I spent about three hrs. one day puting the prop at different locations and found out that I could not feel any difference in vibration. This was a very easy thing to do and it was not very scientific but I could not tell it made a change at all. Now if we were turning much higher RPM's then everything would need to be ballenced so much better which is only going to get so good with a single cyl. engine. Even ducted fan engines are prone to some fibration and they are spinning a different rotating mass then we are. about the only way to get a smooth engine is to buy a twin cyl because they both cancel each other out for the most part.

Everything is running rather smooth now. Just wanting that extra little assurance. Ill end up pyle driving the engine into the ground learning aerobatics before i wear it out anyway

Proteus, Nice username. Good old ship.

Yes...I know what you mean...I don't have anything that I had to stop using because it was worn out...I usually end up tearing it up before I use it up.

Hobbsy,Darzeelon,

I am no expert on engines but I would tend to believe Dave Gierke over you 2.
Read page 82 in MAN JULY 2002 under the heading "Smooth A Vibrating Engine With An Unbalanced Prop"

So do you guys figure you no more than Dave Gierke?

I am pretty sure I read the same thing from Clarence Lee somewhere but I haven't found it yet to show you?

Hobbsy,
I don't mean to offend you but you are wrong.Dave Gierke can explain it better than me but I will try.
Okay,assume you have an engine that vibrates more than it should because the counterbalance on the crankshaft is not heavy enough.[common problem on older engines]
This is what you do,
1 make sure piston is at top dead center
2 put on a unbalanced prop, heavy blade down

That is it ,pretty simple.

A single cylinder engine is inherently unbalanced but some of the better engines (such as Rossi) use tungsten carbide inserts for the best compromise. There's a detailed thread on it at http://www.rcuniverse.com/showthread...and+crankshaft

For lesser engines that are way off it's sometimes suggested to fit the heavy blade opposite to piston travel (or if you like, to add to the counterweight) but personally I don't like the idea because then that out of balance acts directly through the front bearing or the front of a bushed bearing. Plus there's an out of balance coupling to the engine mounts which could add vibration to the entire model (if possibly slightly less for the engine itself)..

The manual that come with my new Thunder Tiger PRO .46 says to bring the piston to near TDC, as in just as the engine starts to meet resistance due to compression, then to attach the prop at 2 o'clock and 8 o'clock positions. I would assume that this is for hand starting purposes, although it doesnt put the prop in the best position for starting. Im using an electric starter anyway. So i dont know. I do know that im not experimenting with it until the warranty period is up on this engine, if i ever do.

That makes sense also. There are a lot of opinions on this subject. I guess you can also read into something too much also. So the clock position is mainly to protect the prop rather than the engine?

Did you guys read what Dave Gierke wrote?
Do you think he doesn't know what he is talking about?

Hobbsy,
I never said he wrote the laws of physics or controls them.
I asked if you read what he wrote about the subject?
and if you believe him?

I haven't read the article either; but hobbsy's example of a helicopter's main rotor says it all.

If a rotating mass isn't balanced, it WILL produce vibration that WILL be transmitted to the airframe back through the main shaft, crank shaft, whatever.

This is critical on a heli; especially the larger ones. That's why we spend whatever time it takes to get a pair of main blades perfectly matched. I just haven't seen it as a big issue on planks, in 16+ years of flying them; from .049s up to 1.50 four bangers.

Mount your prop where it is convenient; and know that, if you don't balance it, you will be adding to the vibration produced by the engine.

Steve

This argument is almost funny. Almost.

It seems as though we have some people that are somewhat educated in physics, insisting that their theories are absolutely and most totally correct, without exception.

On the other side are individuals with hands on experience that know better.

Who is right?

Both and neither.

The problem with theories is that they do not take into consideration human perceptions, or lack of same.

If a model airplane is shaking a bit more than I like and I put on a prop with one heavy blade in a position that counters and reduces the shake, am I hallucinating?

If we accept that the above situation has indeed occurred, does that make the theoreticians wrong? Of course not. The heli boys know what they are doing as is proven by their success.

On the other hand, Dave Gierke has been proven correct in at least an occasional instance by the success of the model with the properly positioned heavy blade cancelling out some of the vibration. Neither position cancels the other. In fact, they help prove each other if enough examination and analysis is performed.

Clarence answered a question about this very subject in the RMC I got today, his totally ambivalent answer was "sometimes it works and sometimes it doesn't" so even experts who defy logic only get it to work when they are lucky.

Artisan:Hobbsy not talking theories, he flat out says it is impossible because it is against the laws of physics.

If Clarence Lee and Dave Gierke can make it work sometimes,I guess they can bend the laws of physics.

Well I have had enough of this one.I think I have proved my point with the help of Clarence and Dave

Actually this is incorrect. An unbalanced centripetal force from each blade will cause vibrations. Centripetal force is calculated by F=m*v^2/r but because each blade is (hopefully ) turning at the same speed then we can ignore the v^2 term. This leaves us with m/r. Therefore a blade that's heavier inboard ( reduced radius) will completely balance the centripetal force of the other blade. The laws of physics dictate these things

An extreme example is a single bladed CL speed prop. These have a counterweight that's enclosed inside the spinner yet turn over 40K revs very smoothly.

I just don't see how this is possible, if one balde has an imbalance of .25 of an ounce on the blade tip and another has an imbalance of .25 of an ounce at the root but the two weigh the same, the extra .25 ounce at the tip is going to be influenced by far more force than that at the root.

hobbsy....I see what you're getting at. If two blades weigh exactly the same then to be in balance they must have the CG for both of them located at exactly the same distance out from the root. With the single blade example the CG of the counterweight is probably about 1/2" out from the crank centre line while the much lighter prop blade is maybe 3 or 4 times further out.

Your example would change the CG of the prop. Any object always rotates exactly aroung its CG. This is true at ANY RPM. An apple floating in the space station, when spun slow, has NO DIFFERENT vibrational node than if spun fast. It will spin around the same CG, slow or fast. So does a propeller. It doesn't matter HOW you set that CG. A small mass far out, or a big mass close in. One side can have a big mass close in, and the other a small mass far out and the CG will be right in the middle. The prop will still want to spin at the CG, period. Once you are mounted off the CG, the prop will still want to spin around the CG. The difference between mounting position and CG and the forces trying to spin the prop on the CG we know as vibration.