Is it possible to offset a less balanced engine with a less balanced prop?

[Target]

I know that the subject question is somewhat like fixing a dent with a hammer, but some dents get all but fixed with a hammer....

So, I am wondering if an engine is known for shaking, would having a prop with one heavier blade placed in the "right orientation" make it possible to get less vibration from the engine with the mounted propeller?
I know that some high end tire shops offset the heavy side of a tire from the heavy side of a wheel to try to minimize the balance weight needed. Seems kind of similar.

Go!

[peabody]

I would imagine that static balance could be achieved, by dynamic would worse. Something about "rocking couple" or some such. Brett will explain....not rolling couple...hence the edit

[Target]

I'm definitely interested in the answer!!

[Bootlegger]

 Seems that several years ago Fast Richard Mathis had some ideas along these lines when he suggested to put the light blade on the right and it wouldn't shake as much.  If memory serves this was in Flying Models in the  late Sixties or early Seventies..

[Dave_Trible]

In the ‘classic’ era it was sometimes a practice to loosen and rotate the prop 180 degrees when an airplane seemed to vibrate more than usual.  Quite often it worked to smooth out the run.  Nobody spent much effort to balance props and the engines were anything BUT balanced.  So to answer the question,  yes it may help but just the suggestion might drive some crazy.  You can just try it and see- 50/50 odds.  You can balance props but the average engine?

Dave

[BOB ALLAN]

Must work on cars !

[Target]

I guess it could be feasible on some level.
I'll hold out for someone that thinks they have a more definitive take on my question.

[Dave Hull]

Maybe.....

Again, you could static balance about the rotating axis down to your ability to measure it. That might not make it better, it might make it worse.

Think in terms of symmetry and work your way aft. If the prop nut is perfect, the prop is perfect, the crank is perfect back to the intake window, and then perfect back to the counterbalance. Now the problems start. You could static balance the crank, but then the weight of the rod which is partially constrained to the crank by the crank pin would be !00% off. So instead, counterweight the crank for some fraction of the rod weight--say 40%. Other than the intake window, all of the unbalance is near the back. So putting an additional, compensating unbalance at the front (ie. the prop) means that you are increasing the "out of plane" shaking forces.

Not sure if I can easily describe this without a force diagram (engineering again....) but draw a stick figure of the elements. Then concentrate the mass of the crank disk, and big end of the rod a bit off the axis in the plane of the cylinder. Then do the same with an intentionally unbalanced prop, but 180 degrees of shaft rotation in the plane of the prop. Now draw a line between these two. If you could rotate the axis thru that line--instead of along the cranks centerline, you would reduce the shaking forces.

Balancing a single cylinder engine is at best a compromise. There is no theoretically perfect solution. But making the rod lighter, the piston lighter, the wrist pin lighter all help a lot. These have the effect of moving the center of mass in that shaking couple closer to the rotating axis.

I have a profile plane that absolutely did not like the LA46. The nose was stiffer than normal and it just buzzed. When I put an LA40 in it, it was more manageable. Not great, but no longer horrible. I think part of the reason is that the LA40 piston is a bit lighter.

I had no luck using unbalanced prop and rotating it around looking for a sweet spot on a Fox 35. Spent an afternoon trying. One of the more experienced flyers came over somewhere during this episode, and was outraged I was using an unbalanced prop....

Dave

PS--I'm not sure what all is done to high performance car wheels, having never owned one (yet), but believe I heard that in the past, Porsche technicians would measure the runout of a mounted tire, and then rotate it on the rim to compensate for eccentricity. The only other option for a given wheel/tire combination would be to shave the tire. Once true, then the balancing can start. A bubble balance or static balance is not good for high performance, so you spin balance (dynamic balance). But if you can't or don't want to put balance weights on the outside of the rim when that is where they really are needed, then you are not taking full advantage of dynamic balancing. O f course, if you are lucky, and your wheel assembly is heavy to the inside, then putting the balance weights on the inside where no one can see them is also the right answer for best balance....

[Target]

I haven't run the particular engine yet myself, but listening to some of my OFB's, they say it's a shaker- the Cox .09.

At 2.70 oz, it's pretty light so, I'm wondering just how much it would shake.

As for shaking, am I likely to notice it more with a smaller or lighter (balanced) prop turning more RPM's or a larger and heavier (balanced) prop at lesser RPMS?

Yet another debatable question.

And, GO!

[Lauri Malila]

 It's been done in some engines, but usually just in higher revving racing ones.
Necessity to do it also partially depends on the decisions if you focus on balancing rotating or reciprocating masses.
I wouldn't do it with prop, a smaller radius is petter and more repeatable. Here is an example of MB pylon engine:

L

[Peter Grabenstein]

I haven't run the particular engine yet myself, but listening to some of my OFB's, they say it's a shaker- the Cox .09.

At 2.70 oz, it's pretty light so, I'm wondering just how much it would shake.

As for shaking, am I likely to notice it more with a smaller or lighter (balanced) prop turning more RPM's or a larger and heavier (balanced) prop at lesser RPMS?

Yet another debatable question.

And, GO!

Cox .09 a shaker ? , my doubts,  did you check the crankshaft ? could be slightly bended .