Okay, here's one for you...

I mounted a new OS Max .60 FP to a stand which I made for break-in purposes. The manufacturer suggested a 12-6 prop for break-in and I made the stand from 1/8" aluminium. It stands high enough to allow the prop to spin and is backed up by two 1/2" gussets fro strength. It is solid enough that the glass/nylon prop I used first warped forward at full throttle when the engine was leaned out to give full performance.

At work, we have alot of rotating equipment so I asked a machinist to do a vibration scan on the engine with the probe they use to check for vibration in pump motors. Attached should be the print out, if I did it right... The prop used for the readings is a 12-6 wood Top Flite.

Anyway, as you can see, there are several small increases of vibration, but I am concerned about the large spike at approx. 8000 rpm on the graph or 4000 rpm of prop rotation. The machinist explained that the small spikes appear normal as they are harmonics of each other and well within acceptable parameters for the pumps he normally checks. The checks are for worn bearings to prevent caststrophic failure.

To read the graph, the vertical axis represents the linear movement in inches per second, which if I understood him correctly, means how fast the vibration would make the item travel if it were not secured. the horizontal axis represents the rpms of the prop when divided by 2 because the machine assumes one blade and not two. This means that the small spike at 33.94k is actually at 16.97 rpm.

The actual question after all this set-up is: Is it normal to have a spike of vibrations such as shown on the graph? It is noticable by ear and eye, but I am unsure how bad it is because the engine was securely mounted and could not transmit any vibration to a plane body which might dampen it, yet was not clamped to a solid table, like you would see it a commercially purchased stand that sits much lower on the table. Obviously, the vibrations smooth out with an increase or decrease in throttle and only occurs at the 4k area.

The prop was balanced with a Top Flite magnetic balancer and it stays where I put it in any position with very minmal movement.

Did it happen to be fourstroking at the rpm of the high spike.

Dave,

I don't know what you mean. The engine is a two-stroke, but I guess you knew that by the make and model. Please explain so I can supply that information.

I ran the engine on the plane today. It seemed to greatly dampen the vibration, but the plane did jump a bit at the 4000 rpm level, but no where near as bad as it did on the stand. It may be a poor stand design set up a bad resonance. Unfortunately, the trim needs a bit of tweaking...

Ground-2 Props-0...

The guy at the LHS is beginning to love me...

Most all single cylinder engines (I'm speaking broadly here about model engines without counter rotating counter balances!) will have a range where the vibration is higher than the norm. This is just a compromise by the manufacturers to make a light engine..they don't have enough crankshaft counter-balance to balance the entire weight of the piston/wrist pin/rod. The trick is to not prop them where they run in that range all the time. But I doubt you will be propping your 60FP to run at 4K, so just put on another prop and go fly !

Four stroking is when the engine misses and fires about every other stroke from being a little rich. All two strokes do it at some rpm, even Lawnboys. Also Chuck is spot on.

Not being a balancing expert I'd guess it was a combination of things such as an insufficiently rigid test stand and maybe the prop being used. The spikes are obviously harmonics and possibly the large spike might have been from the natural harmonic of the wood prop interacting with the natural harmonic of the test stand. But like I said, this is purely a guess.
One thing that intrigues me though is how you managed to get rpm figures that are so high. You've mentioned the small spike at 17K actual revs and no 61 will spin a 12x5 prop that fast. The graph goes to 20k.......
A 2 stroke engine will go into what's called 4 stroking mode if the mixture is so rich it can't fire every cycle but skips a cycle.
BTW, another area that causes vibration is that the prop doesn't turn at a fixed speed. It varies in revs through each cycle to the extent of a couple of hundred rpms's so the engine tends to rock sideways in both directions each cycle.

Wouldn't that make a linear graph since it would be more pronounced at lower RPM than high?

Downunder,

The manufacturer's data sheet states the practical RPMs for this engine as 2000 to 16000 RPM. That is practical, so I assume it can go higher. I had the engine leaned out alittle too much by ear, and I attained those RPMs verified by a cheap hobby shop tach my brother loaned me and by a more sophisticated tach used by the machinists that is used to double check the shaft RPMs of the pump they are working on. The 20k (or 40/2) figure was entered into the meter as a high point because the machine needs to know what parameters it is working with. To be honest, the engine was absolutley screaming on that run and this was the only time I had it rev'ed that high. It was a bit scary... It was running very lean at the time, so I don't think it was four-stroking.

I am also starting to believe that is was a poor test-stand design, because as I said earlier, the vibration seems much more normal when the engine runs on the plane and my experiance with these things is limited to 25 years ago, as I am just starting this obsession again adn wanted the benefit of you guys who know alot more about it than I do.

I think the stand did not allow the engine any dampening and niether did it hold the engine solidly enough. The only reason I posted the graph was because the large spike seemed so out of proportion to the rest and the machinist that ran the tests seemed very apprehensive about the spiking. He indicated that if a pump showed that much linear movement, it would be brought off-line immediately and overhauled. That sort of worried me a bit...

I greatly appreciate everyone who posted to this thread. You've given me some extra knowledge and made me feel better about the thing. Now... To crash, er I mean fly it!

It would be interesting to do your test with a few different props. See if your spike hits at the same time. As for the practical RPM range, the only way this engine is going to see anywhere near 16,000 is with a much smaller prop, much much smaller.
It doesn't sound like your stand has any dampening at all. Most any engine that is mounted to metal has some sort of isolation, so your test might be better served if you at least went to a composite mount on an aluminum stand/frame..
If you think about, even if you take an aluminum mount and mount your engine. What are you mounting it on? Wood, that's right. Unless your using an aluminum airframe(kidding, I know your not) , full scale planes that are of a metallic airframe definately have dampeners, whether it be rubber, or something that has dampening characteristics somewhere.
The OS 60FP is/was one of the smoothest bushing motors on the market. Another interesting test would be to test a .61FX with the same prop you tested the FP.
Sounds like youv'e got some cool things going, and a good job that allows and supports a good hobby. Thanks for sharing your finding, looking forward to more when you find something out.

You tell em Chuck! It's wonderful to have an inquiring mind, but don't let it ruin your flying

This is a bit of a misnomer really because it comes from the fact that the manufacturers tend to port their engines to get as much power as possible to sway modellers into thinking they're buying a really powerful engine. The max practical revs they give is usually where it gets peak HP...they'll certainly rev faster than that but with less HP. But the prop you need to use to get those revs makes it useless in just about any model.