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Seems like you're over thinking it.

Take a reading with nothing added.

Add a nut and take a reading at each of the four points.

Two of the points should show improvement (less vibration), and two of them should show degradation. Each set should be adjacent. If this is not the case, you're done, because your motor is balanced better than your hardware can detect. The one exception is if all readings got worse. In that case, repeat the operation with a half sized nut, like maybe jam nut.

If you did get improvement in two adjacent points, you've identified the point of imbalance. Now you just have to determine how much weight to remove it. I'd try another weight scaled to the nut you used and the amount of improvement it produced. For instance, if position A cut the ips in half, and an adjacent position B by 75%, the imbalance is going to be between those two. I'd try the nut and a washer on A, and a washer on B. Keep doing that until you find it to tedious to contine.

This process also works to balance a treadmill motor that you add to a bench top mill, btw.

On Wednesday, March 17, 2021, 01:23:56 PM EDT, Finn Lassen finn.lassen@verizon.net <flyrotary@lancaironline.net> wrote:

I'm now on day 5 (newly) trying to balance the engine with prop removed.
After a trip chasing a wild goose into the weeds with FFT, I'm now trying the spreadsheet method: measure amplitude at these four points:
Unbalanced
Adding a 7.9g weight (nut) at one of the four bolts (reference)
Move the nut +90 deg from the reference bolt
Move the nut -90 deg from the reference bolt

Tried it at 3000 RPM.
Plugged the numbers into the spreadsheet, it told me to add 28g at 4.5 deg. Seemed like a lot. The result was a lot worse vibration that I could actually feel.

Tried again at 4,500 RPM, this time using P-P voltages.
Now wants me to add 183g at -143 deg.

Very hard to get consistent readings.

Really doesn't make any sense that the addition of a 8g weight influences the amplitude of the scope curve so little.
The big washers on each side of the rubber donuts are 30g each.

I'm getting a good trigger signal from a piece of alum tape near rim of flywheel.
Using a ADLX335 accelerometer followed by a 100Hz low pass filter and 300x amp. ADXL335 has sensitivity of 300mV/G. With 300x amp, that's 90V/G.
0.7Vp is about 0.008 Gs or 0.005 ips at 6000 RPM if my math and circuit is correct.
V (ips) = A (Gs) * 3690 / RPM

Is it possible that the near-sine curve I see on the scope is actually the reaction (acceleration/de-acceleration of the rotor) to the combustion event?

Steve, how did you balance your engine?

Finn

On 1/28/2021 12:39 AM, Sboese sboese@uwyo.edu wrote:

Matt,

One cannot balance the prop by placing the balance weights on the flywheel due to the gear ratio of the psru. I chose to dynamically balance at the flywheel with the prop removed, then install the prop, and finally dynamically balance at the prop. That appears to have given good results for me.

Steve Boese

On Jan 27, 2021, at 6:09 PM, Matt Boiteau mattboiteau@gmail.com <flyrotary@lancaironline.net> wrote:

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How does everyone balance their prop, do they gain access to the flywheel and add bolts/washers there like a standard aircraft engine? I'm looking to balance the prop around X60 (Williston, FL), and I forget who, but someone said we could rent one nearby?

Had a 301 ss slip joint bellow for the exhaust and it cracked, which set me back a few months. The end of my exhaust must have too much vibration, I will figure out more rubber mounts with the welder to stop the end vibrating. Bought a 2.75" inconel split joint bellow from SPD exhaust that should handle the heat.
http://www.spdexhaust.com/pdfs/AccPDFs/Bellows.pdf

- Matt Boiteau

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