I'm pickin' up good vibrations

OkieFlyer

OkieFlyer

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Andrew L.
Actually more like pulsations, and definitely not good.

A while back, I mentioned I had a bit of a wiggle in my oil pressure gauge. Nothing terribly concerning, but when the engine got good an warm, say 15 minutes in to the flight, the gauge would start swinging rapidly maybe 5 psi. Haven't worried much about it since it always stayed on the high end of the green and I couldn't detect any other issues.

Since then, I've started to detect a small swing in the vacuum gauge, again very small, but rapid. On on the way back from Oshkosh, I noticed the attitude indicator also bouncing just a little bit with the same rhythm.

Honestly, I think I'm starting to feel the vibration a little too. Could be in my head, but it's fair to say that whatever is at the root of it is getting worse.

What to you think? Vacuum pump going out? Blow-by? Something I don't want to think about?


Engine is making lots of power and running well. Had the prop dynamically balanced back in April. Oil consumption is up to maybe a quart in 5-6 hours, so that's not real good. Not blowing any smoke out the exhaust as far as I can tell. My annual is coming up next month and I need to look into this, but I'd like to try to get some ideas before I turn the mechanic loose on it.
 
OkieFlyer said:
Actually more like pulsations, and definitely not good.

A while back, I mentioned I had a bit of a wiggle in my oil pressure gauge. Nothing terribly concerning, but when the engine got good an warm, say 15 minutes in to the flight, the gauge would start swinging rapidly maybe 5 psi. Haven't worried much about it since it always stayed on the high end of the green and I couldn't detect any other issues.

Since then, I've started to detect a small swing in the vacuum gauge, again very small, but rapid. On on the way back from Oshkosh, I noticed the attitude indicator also bouncing just a little bit with the same rhythm.

Honestly, I think I'm starting to feel the vibration a little too. Could be in my head, but it's fair to say that whatever is at the root of it is getting worse.

What to you think? Vacuum pump going out? Blow-by? Something I don't want to think about?


Engine is making lots of power and running well. Had the prop dynamically balanced back in April. Oil consumption is up to maybe a quart in 5-6 hours, so that's not real good. Not blowing any smoke out the exhaust as far as I can tell. My annual is coming up next month and I need to look into this, but I'd like to try to get some ideas before I turn the mechanic loose on it.
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Worn out engine and bad gauges.
 
OkieFlyer said:
when the engine got good an warm, say 15 minutes in to the flight, the gauge would start swinging rapidly maybe 5 psi. Haven't worried much about it since it always stayed on the high end of the green and I couldn't detect any other issues.
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As the oil warms the indicated oil pressure will drop. Since you mentioned this was within the first 15 mins... Their maybe pockets of oil that have not yet fully circulated that are cooler and heavier.

Check your oil screen. Surprising how many planes have not opened and cleaned the screen in a while. A quart every 5-6 hours is fantastic and way OK.
 
At least on my plane... the pressure regulator keeps the pressure in a operating range as opposed to a fixed value. I see the pressure slightly higher when the engine is cold and lower when it's hot.
 
NordicDave said:
At least on my plane... the pressure regulator keeps the pressure in a operating range as opposed to a fixed value. I see the pressure slightly higher when the engine is cold and lower when it's hot.
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Either way, the oil pressure stays in the right range, plenty of pressure, it's just the flutter or pulsation I'm worried about. What began as just a little oil pressure flutter has spread to suction pressure flutter, and now attitude indicator flutter. It's something other than just the instruments themselves, and they all flutter with the same rhythm.
 
NordicDave said:
Does the Prop RPM or manifold pressure change as well?
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Nope.


Does anyone think an issue with a wet vacuum pump could cause this? With the absence of the flutter in any of the other engine instruments, seems plausible to me.
 
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Could be some of the gauges are gong wonky. Could be a combination of vacuum pump and wonky oil gauge.
 
denverpilot said:
Did it start right after the prop work? Just asking the obvious just in case...
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Yep. And there is the possibility of a resonant frequency; fly with the RPM on the engine and/or prop at slightly different speeds and see if it changes. Also, get another oil pressure gauge on that thing, and see if it does the bouncy-bounce.
 
Is there any way for an engine driven vac pump to have a leak to the crank case? If so, there is your relationship.
 
I know this is hard to estimate, but about what is the frequency of vacuum oil pressure gage/indicator vibration? 1/2 minute? Two Hz? 20 Hz blur? There's a lot of range of frequencies in this world. I realize the amplitude(s) are different, but are the frequencies different too? Does it change if you adjust rpm? Do you think it might be tied directly to rpm? Do the oscillation indications move together/synchronize?

My guess is this is something subtle but important.
 
nrpetersen said:
I know this is hard to estimate, but about what is the frequency of vacuum oil pressure gage/indicator vibration? 1/2 minute? Two Hz? 20 Hz blur? There's a lot of range of frequencies in this world. I realize the amplitude(s) are different, but are the frequencies different too? Does it change if you adjust rpm? Do you think it might be tied directly to rpm? Do the oscillation indications move together/synchronize?

My guess is this is something subtle but important.
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It does change slightly with rpm. Don't know what frequency, but the needles are a blur.

455 Bravo Uniform said:
Is there any way for an engine driven vac pump to have a leak to the crank case? If so, there is your relationship.
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I think that is a possibility, which why I asked about the vacuum pump. Didn't get much support on that theory though.
 
denverpilot said:
Did it start right after the prop work? Just asking the obvious just in case...
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Kenny Phillips said:
Yep. And there is the possibility of a resonant frequency; fly with the RPM on the engine and/or prop at slightly different speeds and see if it changes. Also, get another oil pressure gauge on that thing, and see if it does the bouncy-bounce.
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Really?

How can adding a couple of washers to a spinner make one iota of difference to anything else?

...negatively, anyway.

Please respond in engineering terms. I don't understand much else.

It's my understanding that "resonance" is a result of an unbalanced engine (or in the case of a twin, two engines that are not sync'ed) and not a result of a "more balanced" engine.

But aviation isn't my discipline of engineering so...I could be wrong...again...
 
timwinters said:
Really?

How can adding a couple of washers to a spinner make one iota of difference to anything else?

...negatively, anyway.

Please respond in engineering terms. I don't understand much else.

It's my understanding that "resonance" is a result of an unbalanced engine (or in the case of a twin, two engines that are not sync'ed) and not a result of a "more balanced" engine.

But aviation isn't my discipline of engineering so...I could be wrong...again...
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I believe there was more work done to that prop than just putting balance weights on. Isn’t this the prop that was refinished completely with a long thread about it?
 
denverpilot said:
I believe there was more work done to that prop than just putting balance weights on. Isn’t this the prop that was refinished completely with a long thread about it?
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Yes, that is correct. A prop balance was done immediately following the prop dressing. Balanced to .10 IPS @ 2300 RPM.


Just had the plane in for annual, and I asked the guys to check the prop balance again (a different guy did the last balance). They measured .38 IPS at cruise power, and were able to get it down to .01 from idle to 2000 RPM, and .14 at balls to the wall. I flew it home today and the felt vibration is much, much less. I am pleased with that result, but I must say I'm a little perplexed as to why it went from .10 IPS to .38 IPS since it was balanced in April. Perhaps the last guy didn't do something right. I dunno.

That said, the bobble in the oil pressure was still there when I flew it home from annual today, so I'll proceed with getting a new gauge and/or sender.
 
Wouldn't it Be Nice if fixes were always that simple?
God Only Knows you'd rather be Surfin' USA in Kokomo



Now I have ****ing Beach Boys songs stuck in my head
 
OkieFlyer said:
I'm a little perplexed as to why it went from .10 IPS to .38 IPS
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If the original balance was in fact .1 then several things could cause a shift. For example, simply removing the spinner and reinstalling it 180 or using new hdw could do it. Or tightening a loose alternator can tweak things. Was the balance checked after the annual or before? Regardless, .14 is good. Maybe get it checked in 6 mo an see if it shifts again.
 
Bell206 said:
If the original balance was in fact .1 then several things could cause a shift. For example, simply removing the spinner and reinstalling it 180 or using new hdw could do it. Or tightening a loose alternator can tweak things. Was the balance checked after the annual or before? Regardless, .14 is good. Maybe get it checked in 6 mo an see if it shifts again.
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The first balance was done in April of this year. Nothing had been touched until annual this past week. I believe the mechanics did the balance at the end of the inspection, so obviously, the annual itself could have caused a shift. However, the vibration started showing up during the period between balances, when nothing in regards to the rotating equipment had been messed with. It's interesting, and I will be paying close attention. To be honest, after feeling what .14 is like, I have a hard time believing that the last balance was actually .10.
 
OkieFlyer said:
To be honest, after feeling what .14 is like, I have a hard time believing that the last balance was actually .10.
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That's possible. Could also have been an error during the final weight installation. Sometimes weights are temporarily put in locations during balance then the final weight amount is recalculated for installation at a permanent location. I've screwed up the math too. And just as an FYI, with a little extra work you might be able to get that .14 down to less than .07. The best I ever got (on a 182) was .04, but that was more a challenge by the owner than a necessity--but it was smooth.
 
What airframe and engine is this?

Verify the gauge first. Clean the oil pressure regulator and blow out the line. Using a known good gauge, what is the oil pressure doing? Trying to isolate airframe from engine possibilities.

What condition are the mounts in? What condition are the baffles and cowling in? Spectrum analysis is available if the vibration comes back.

Was the prop removed at some point? Is it indexed properly?

Just off the top of my head!
 
Bell206 said:
That's possible. Could also have been an error during the final weight installation. Sometimes weights are temporarily put in locations during balance then the final weight amount is recalculated for installation at a permanent location. I've screwed up the math too. And just as an FYI, with a little extra work you might be able to get that .14 down to less than .07. The best I ever got (on a 182) was .04, but that was more a challenge by the owner than a necessity--but it was smooth.
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I once got down to .01. More of a fluke than anything I think. More typical is .05-.07.
 
Bell206 said:
That's possible. Could also have been an error during the final weight installation. Sometimes weights are temporarily put in locations during balance then the final weight amount is recalculated for installation at a permanent location. I've screwed up the math too. And just as an FYI, with a little extra work you might be able to get that .14 down to less than .07. The best I ever got (on a 182) was .04, but that was more a challenge by the owner than a necessity--but it was smooth.
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Dang. Was that at WOT or cruise RPM?

Just going by what I feel in my feet, there is a noticeable difference in the most recent balance compared to the last. I'd be willing to bet that the last guy just didn't put everything back like he should've. Admittedly, I wasn't able to let him spend a lot of time on it the first time, as I stopped in on the way to somewhere else.
 
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JAWS said:
What airframe and engine is this?
C-182E Cont. O-470-R

Verify the gauge first. Clean the oil pressure regulator and blow out the line. Using a known good gauge, what is the oil pressure doing? Trying to isolate airframe from engine possibilities.
Will do.

What condition are the mounts in? What condition are the baffles and cowling in? Spectrum analysis is available if the vibration comes back.
Mounts are good, thus sayeth the mechanics. Rubbers look like they are in good shape.

Was the prop removed at some point? Is it indexed properly?
Prop hasn't been off in many moons.

Just off the top of my head!
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OkieFlyer said:
Was that at WOT or cruise RPM?
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Cruise and more on the low side of cruise rpm.

OkieFlyer said:
Just going by what I feel in my feet,
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Unfortunately you can't determine the "mechanical" condition of engine mounts by only a visual check. The rubbers are technically vibration isolaters or dampers and are usually "tuned" to the 1/2 per to 3 per vibration frequencies to dampen the inherient vibrations produced by the engine and prop assemblies.

So even if you balance the prop to .1 but still feel something in the airframe it could be the rubbers have lost their vibration qualities or you have a vibration outside the frequency range of the rubbers. However, the only way to check is through a spectrum vib check from engine side to airframe side of the mount. Though most people don't go that far.

However, if your vibration continues to occur especially in the pedals, you can fly with the balance gear with a spectrum set up and troubleshoot different vib freqs. I've done it several times to isolate powertrain from airframe issues.
 
Bell206 said:
Cruise and more on the low side of cruise rpm.


Unfortunately you can't determine the "mechanical" condition of engine mounts by only a visual check. The rubbers are technically vibration isolaters or dampers and are usually "tuned" to the 1/2 per to 3 per vibration frequencies to dampen the inherient vibrations produced by the engine and prop assemblies.

So even if you balance the prop to .1 but still feel something in the airframe it could be the rubbers have lost their vibration qualities or you have a vibration outside the frequency range of the rubbers. However, the only way to check is through a spectrum vib check from engine side to airframe side of the mount. Though most people don't go that far.

However, if your vibration continues to occur especially in the pedals, you can fly with the balance gear with a spectrum set up and troubleshoot different vib freqs. I've done it several times to isolate powertrain from airframe issues.
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Thanks. If the vibration comes back, we'll do some more checking. She's back to smooth for now though.
 
Andrew,

I'd suggest letting Kent know what happened if you haven't. He may have some good insights.
 
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