LED and photo transistor in same housing. I put it inside  a 1/2" CPVC tube so I can't read the part number.

In an ideal world, a mass rotating around a center point should produce a sine curve. But what if you have more than one mass? You should be able to use vector math to find the resulting single equivalent mass.

However, looks like a combustion engine is not that simple. I'm now fairly confident that the near-sine curve I see on the scope is mainly the combustion event. Someone need to refresh my memory: how many times does combustion occur per flywheel rotation? Looks to me like one. Rotor 1, then next rotation rotor 2, then next rotation the next surface of rotor 1, etc. Is that right?

To make matters worse, you also have other sources of vibration: alternator, water pump, etc. Their vibrations (different frequencies) are added to the basic one period per flywheel sine curve. If I'm right, the combustion event won't be a simple sine curve either: a pulse resolves into multiple frequencies (spectrum).

When you look at a scope the signal is not stable: any little misfire will jar it up or down. Makes is really hard to read amplitude on scope.

In theory, a real effective low pass filter should remove all other frequencies than the one you're interested in but doesn't seem to work all that well in practice. So, back to FFT on my Arduino Nano. Basically using FFT to produce a frequency spectrum.

My Arduino program takes 128 samples with a sampling frequency of 300 Hz. Sampling starts at the trigger pulse (may or may not be a good idea). This is repeated 8 times and averages calculated. Then the FFT transforms the 128 sample values into 64 frequency "bins", the magnitude of each  are then plotted.

Using Ernest's method, I took 5 readings (no nut, then a 1/4" lock nut zip-tied to each bolt nut in turn), running engine at or just below 3,000 RPM (50Hz). (Varied RPM slightly using mixture knob to ensure got result in the 50 Hz bin only) (Basically using the FFT as a band-pass filter.)
Result was that the 4.4g weight worsened the amplitude at bolt 1, 2, 3 and 4.
No nut: 5-10 (varying between 5 and 10 on successive plots)
Bolt 1: 20-22
Bolt 2: 18-20
Bolt 3: 30
Bolt 4: 20-22

So if adding a 4.4g weight increases vibration at least by a factor 2 no matter where it's added, I decided that's good enough, removed it and called it quits. Onto prop balancing. (Except, should probably remove the PSRU and replace the oil seal now that the prop is off ...)

Now, my circuitry doesn't match theory. The LP filter op-amp should have had a 3x amplification and 3db frequency at 120 Hz. A following op-amp should have 100x amplification. Hooked up a generator to the input instead of the ADXL335 and measured input and output. Filter amp has no amplification and the filter seems to cut in way lower than 120Hz. The following op-amp only seemed to have 80 to 90x amplification. I should probably use low-noise op-amps as Charlie suggested instead of the LM358. The LM358 runs on single-supply voltage (5V) but that requires caps between filter output and amplifier input. I guess I should make a schematic and submit for critique. Got the theory from my electronic tech apprenticeship papers back in 70's.

As for measuring phase or angle on the scope, sort of can be done. But again, this is not an electric motor!

Also the FFT gives both amplitude and phase info for each frequency "bin", but I have not been able to relate that to the trigger pulse. Hmmm, now I think I get it. Supposed to have two channels being sampled simultaneously: one the trigger pulse, two the accelerometer. Not sure I want to spend more time on it at the moment. The "nut" methods appear a lot simpler (although require more runs).

Finn

On 3/17/2021 10:30 PM, Stephen Izett stephen.izett@gmail.com wrote:

Hi Steve B and Finn

Appreciate learning from your minds.
I’m trying to understand your process.
I have a new prop and plan to balance engine at ring gear prior to balancing prop as you are doing.
I’m fortunate to have access to a prop balancer but would like to understand your process.

I have some questions:
What device are you using to get your trigger sig from the aluminium tape reference point?
Should the cro image look sinusoidal, period being equal to rpm and amplitude proportional to imbalance at the particular angle of rotation relative to the trigger position?
Should the X and Y signals from the ADLX335 (breakout board mounted on its edge) be the same but 90deg out of phase?
I'm trying to remember my filter theory! Won’t a 100hz filter attenuate your signal at 6000rpm by some 3db, so your higher rpm readings may look better/more balanced than lower rpm readings?

Steve B. I’m trying to follow your logic.
Having taken 4 readings with a nut moved around the 4 cardinal positions, if the nut is not way to small or large, and the system was not initially at best balance, then 2 of the readings (adjacent positions) will show less amplitude than initial reading, and the other 2 higher amplitude.

So after the 4 test readings, if the 4 signals are equal and higher amplitude, wouldn’t this point to the initial balance being better, and so suggesting a smaller nut be used and retested? Is this what you are saying?
Sorry if I’m confused.

Could you measure the required angular position of a correction weight directly from the cro image? That being the angular distance between the trigger position and the maximum amplitude of the signal minus 180deg?

Appreciate you guys and the mental stimulation.

Steve Izett

On 18 Mar 2021, at 1:23 am, 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:


◆ This message was sent from a non-UWYO address. Please exercise caution when clicking links or opening attachments from external sources.


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