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.