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Periodic Vibration RV4 O-320 wood prop

JBurf

Member
Does anyone have some insight as to what the root cause of these symptoms might be....? (I'll do my best to describe without going on infinitum...)
It all starts with trying to get to lean of peak.
At cruise power the mixture can be leaned back to about 7.1 - 7.2 GPH.
As it is leaned further, a periodic vibration runs through the airframe... can fell it particularly in my feet. The period is approx 2-3 sec.
The more lean, the more severe the vibrations.
Typically one would lean until the engine runs rough, then add a bit of mix to yield a LOP condition... but this engine seems to start running rough on the rich side of the max EGT.
I have captured EGT for each cylinder and the fuel flow as shown in the attached plot (manually captured on video then entered into Excel) which shows a 175°F difference between cylinders 1/2 and 3/4... not sure this is significant but the peaks and shape of curves are.
The vibrations start when leaning back to 7 GPH and worsens thereafter.
My thoughts are; one (or more) cylinders are running leaner than the others (intake leak(s), and or propeller balance are the contributors to this issue.
Thoughts?
 

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  • 7Jun EGT vs FF.png
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Lean of peak with a carb usually isn’t possible due to uneven mixture distribution. I have better luck getting them more even by throttling back a little from WOT. What ignition do you have? I’m assuming conventional mags.. I believe Savy has a “intake leak” test, have you tried that?
 
Conventional mags (Slick).
Next test is to throttle back to idle in base leg and watch to see if any EGT numbers climb. This is reported to indicate a leak in the corresponding intake passage.
 
Air distribution is very poor on these engines. THerefore, with a carb, it is very common for 1/2 & 3/4 to get different amounts of air fuel mix and therefore create struggles when going leaner than peak EGT. Various trick to levearage turbulence to improve things like throttle off of WOT or carb heat. Most just accept that a carb'ed lyc is not suited for LOP operations. So, you have 2 pairs of cylinders producing different levels of power due to imbalanced AFR's. Not surprising that this might create vibration with the prop. We know that small difference in power pulses can create issues for the prop, as various prop makers have seen issues with Elec Ign and warn against using them. Look at an HP vs Leaning chart and you will see that on the rich side of peak, the line is quite flat and curves agressively on the lean side. When dealing with imbalance issues, you want to be in a flat area of the curve so that different AFRs have only a small impact on power balance.

Larry
 
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Thanks for the thorough explanation lr172, helps to put things in perspective.
If the power imbalance is unavoidable, then the only solution is to run ROP then. I suppose the effect on the prop may be changed by installing an aluminum prop with different harmonic vibration characteristics than the current wood, which might allow a bit more leaning than can be achieved currently, but for the expense, is much gained? ...probably not, but theory is sound yes?
 
Thanks for the thorough explanation lr172, helps to put things in perspective.
If the power imbalance is unavoidable, then the only solution is to run ROP then. I suppose the effect on the prop may be changed by installing an aluminum prop with different harmonic vibration characteristics than the current wood, which might allow a bit more leaning than can be achieved currently, but for the expense, is much gained? ...probably not, but theory is sound yes?

Most carb'ed users don't talk about prop vibration when trying to lean, just roughness, so suspect there is something a bit unique with your prop that makes this noticeable. However you may want to investigate a bit further. You are showing a 200* EGT delta between the cyl pairs and I thought that most have a smaller spread, It has been a long time since I had a carb on my engine, so probably not the best to help in this area. It would be best if you gave us the full chart showing the richer areas. Also, the 320 on my 6, peaks well above 8 GPH in cruise (2700 RPM), so wondering a bit. At 8K, I am turning 2730 and burning 8.1, which is around 40* LOP. It is a good bit higher down low.
 
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Air distribution is very poor on these engines. THerefore, with a carb, it is very common for 1/2 & 3/4 to get different amounts of air fuel mix and therefore create struggles when going leaner than peak EGT. Various trick to levearage turbulence to improve things like throttle off of WOT or carb heat. Most just accept that a carb'ed lyc is not suited for LOP operations. So, you have 2 pairs of cylinders producing different levels of power due to imbalanced AFR's. Not surprising that this might create vibration with the prop. We know that small difference in power pulses can create issues for the prop, as various prop makers have seen issues with Elec Ign and warn against using them. Look at an HP vs Leaning chart and you will see that on the rich side of peak, the line is quite flat and curves agressively on the lean side. When dealing with imbalance issues, you want to be in a flat area of the curve so that different AFRs have only a small impact on power balance.

Larry

What are AFRs?
 
What are AFRs?

Yes, air fuel ratios. It is how most of the world measures mixture. We use EGT to approximate, as the lead in our fuel prevents the use of O2 sensors that indirectly measure AFR (they measure the qty of oxygen in the exhaust and from this can deduce the AFR pretty accurately - the wide band ones anyways) . EGTs are tough to use as an AFR substitute, as several things other than mixture can influence them.
 
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