From: "Stephen Izett stephen.izett@gmail.com" <flyrotary@lancaironline.net>
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Subject: Re: [FlyRotary] Balance prop
Date: Sat, 20 Mar 2021 05:09:52 +0800
References: <list-1325548@logan.com>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
In-Reply-To: <list-1325548@logan.com>
Message-Id: <5C9B19D8-9B6D-4D33-9CDA-4D1E4F92A632@gmail.com>

Ha. Sounds like what I would do Finn.
Where is wisdom when you need her!=20
But I find the journey stimulates the grey cells.
All the very best as you tick off the todo items.

Steve I


> On 19 Mar 2021, at 11:03 pm, Finn Lassen finn.lassen@verizon.net =
<flyrotary@lancaironline.net> wrote:
>=20
> Found a soldering error on my circuit. So only 100x amplification and =
a Q of 0.5 on the RC filter (not sure what that translates to in =
dB/octave).
> (LM358 datasheet "DC-coupled low-pass RC active filter). R4 not =
connected to ground. A =3D 1 + R3/R4. Q =3D 1/(3 - A). What does a =
negative Q mean? Unfortunately with A =3D 3 filter starts oscillating.  =
So need to lower or increase filter amplification.
>=20
> Well, if there are two combustion events per rotation the near-sine =
curve doesn't make any sense. Then there should be two cycles between =
trigger pulses. If not combustion-related then a weight on one of the =
four bolts should have reduced amplitude somewhat, but on each bolt it =
got worse but didn't influence the curve much. Now I have no idea what =
the curve comes from.
>=20
> I like the old school idea.
>=20
> As for flying off the hours and then flying to your place, the reason =
I'm revisiting balancing now is that on my last flight I noticed a =
vibration at 144 mph and 5,600 RPM. Took the prop off to recheck static =
balance (didn't note where to put washers when I last did it months =
ago). With the prop off, I thought I might try to check engine balance =
(You mentioned reports of problems balancing geared Rotax and Steve had =
mentioned that he first balanced the engine, then the prop.) A week =
later I'm into filter theory, FFT (spectrum analysis) and so on, trying =
to make my own instrumentation and balancer. Typical for me :(
> Should probably just have remounted the prop with the washers needed =
from static balancing and gone flying. Oh, well...
>=20
> Then there's cooling. Tracy thinks my rad coolant deltas are still to =
big. Need to get rid of 90 deg elbow in return line from one rad and =
another in the supply to the other rad. Then there's oil and fuel leaks =
...  What to do first?
>=20
> Finn
>=20
> On 3/18/2021 7:38 PM, Charlie England ceengland7@gmail.com wrote:
>> Each rotor fires once per rev, so the engine has one torque pulse =
every 180 degrees of E-shaft rotation.
>>=20
>> If the sine wave might be combustion related, can you move the sensor =
array (both trigger and vibe sensor) 90 degrees & compare timing of the =
peaks to the original configuration? If it's combustion events, then =
sine wave will 'move' 90* earlier or later related to the trigger point. =
If it's actual imbalance, the peak should follow the sensor array.
>>=20
>> Filter frequencies are typically spec'd at the -3dB 'knee' point. So =
if the filter was constructed based on frequency 'x', then rolloff will =
start significantly sooner than the spec freq. How much sooner depends =
on what 'order' the filter is. 1st order (6 dB/octave) will begin to =
roll off *much* earlier, removing more in-band and *not* filter out as =
much out-of-band as a 3rd order filter (18 dB/octave), which has a much =
steeper slope. If you want to avoid affecting in-band levels, the cutoff =
must be significantly out of band. See the 'Butterworth' filter graph on =
Wikipedia. https://en.wikipedia.org/wiki/Low-pass_filter
>>=20
>> Have you thought about going 'old school' with balancing? Rig one of =
your LED landing lights to trigger off the peak pulse of the ADXL335, =
put a 'witness mark' on the face of the flywheel, and the strobe will =
show you the rotational position of the flywheel when the ADXL335 feels =
the peak out-of-balance.
>>=20
>> Or, you could let an A&P do it. Or get on with flying your time off, =
and then fly it up here; I've got our EAA chapter's Dynavibe in my =
hangar...
>>=20
>> Charlie
>>=20
>> On 3/18/2021 3:38 PM, Finn Lassen finn.lassen@verizon.net wrote:
>>> LED and photo transistor in same housing. I put it inside  a 1/2" =
CPVC tube so I can't read the part number.
>>>=20
>>> 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.
>>>=20
>>> 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?
>>>=20
>>> 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).
>>>=20
>>> 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.
>>>=20
>>> 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.
>>>=20
>>> 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.
>>>=20
>>> 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
>>>=20
>>> 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 ...)
>>>=20
>>> 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.
>>>=20
>>> As for measuring phase or angle on the scope, sort of can be done. =
But again, this is not an electric motor!
>>>=20
>>> 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).
>>>=20
>>> Finn
>=20
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>=20
>=20
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