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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
>
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