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Variable Oil Pressure Reading

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JohnF

Joined: 13 May 2010
Posts: 124

Posted: Mon Jun 19, 2017 5:35 pm Post subject: Variable Oil Pressure Reading

I just changed the oil lines on the Rotax in my RV-12 and now see oil pressure varying from 33 psi to 60 psi. I moves around like that in flight.



I wonder if air is in the oil line to the oil pressure sensor...if so how to check and bleed it out since it goes to a 'dead-end' at the sensor.



Best I can think of is to open that line and let oil run out and quickly replace the oil line to the sensor in the hope that the air bubble will be gone



Your ideas?

rampil

Joined: 04 May 2007
Posts: 870

Posted: Tue Jun 20, 2017 5:10 am Post subject: Re: Variable Oil Pressure Reading

I could be wrong about this, but a small bubble is far more likely

to smooth out dynamic swings in pressure than to create them. Think

plumbing water hammer arrestor (a pipe length full of air).



Do these oil pressure changes depend on oil temperature (as they normally

do)?

_________________
Ira N224XS

BARRY CHECK 6

Joined: 15 Mar 2011
Posts: 738

Posted: Tue Jun 20, 2017 6:16 am Post subject: Variable Oil Pressure Reading

Ira:

You are correct, you are Wrong!

Air, Gases are COMPRESSIBLE!  Liquids are NON-COMPRESSIBLE.  

Liquids will transmit pressure equally in ALL directions and at the same pressure even through different/mixed size hoses.  Physics - Fluid Dynamics 101.

Air on the other hand is compressible and also expandable and will expand and compress CHANGING its volume and pressure - Boyle's Law also a part of Physics 101.

So, the air bubble will cause the transducer and/or the gauge to pulse.

Barry

On Tue, Jun 20, 2017 at 9:10 AM, rampil <ira.rampil(at)gmail.com (ira.rampil(at)gmail.com)> wrote:

 	  Quote: 	
		  --> RotaxEngines-List message posted by: "rampil" <ira.rampil(at)gmail.com (ira.rampil(at)gmail.com)>

 

 I could be wrong about this, but a small bubble is far more likely

 to smooth out dynamic swings in pressure than to create them. Think

 plumbing water hammer arrestor (a pipe length full of air).

 

 Do these oil pressure changes depend on oil temperature (as they normally

 do)?

 

 --------

 Ira N224XS

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=470266#470266

 

 

 

 

 

 

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rampil

Joined: 04 May 2007
Posts: 870

Posted: Tue Jun 20, 2017 4:57 pm Post subject: Re: Variable Oil Pressure Reading

Hey Barry,



You are right about the static situation, it's a bit different dynamically.

Back in the day when I used to teach about the fluid dynamics

of invasive arterial blood pressure, I would note that

the distortion or "ringing" in the system was related to the

resonance of the system which in turn is a function of the

compliance of the tubing plus air bubbles, and the inertance 

of the fluid column. When the driving frequency(pulsation of the

oil pump) gets close to the system's resonant frequency, the pressure

Oscillates wildly ( in a measurement system that  has adequate frequency

response).



I will generalize (and guess) a bit to say that

1) the driving frequency of the oil pump, a multiple of RPM

   is way higher than the responsiveness of mechanical

   Oil pressure gauges to track the waveform even though you

   would be able to see the ringing with an electronic transducer

    and an o-scope.

2) air (increasing compliance) decreases the resonant

    freq of the tubing system thus reducing the apparent

    Oscillation on the gauge by pushing the resonant frequency

     of the tubing well below the rpm harmonic.

_________________
Ira N224XS

JohnF

Joined: 13 May 2010
Posts: 124

Posted: Tue Jun 20, 2017 5:23 pm Post subject: Re: Variable Oil Pressure Reading

I am the OP....I suspect I have found the problem and will know tomorrow when I button up the plane and do a run up or perhaps a short flight. I will post my results. I doubt that it will involve anything very exotic, but we'll see.

JohnF

BARRY CHECK 6

Joined: 15 Mar 2011
Posts: 738

Posted: Tue Jun 20, 2017 6:12 pm Post subject: Variable Oil Pressure Reading

Ira:

Sorry to say teach, wrong guess!

Well, not a guess, just over thinking a simple situation.

What about open and closed end resonance of the oil lines?

What about cavitation cause by bubbles in a bend, or the reduced flow especially at 90 Deg bends.

If you want to prove my reasoning true or disprove yours just add a length of oil line.  That will change the 'frequency' as you stated and if you are correct.  The pulsing will go away.

But, if you are wrong and if the air bubble remains then the problem will continue and your theory is proven wrong.

Think of it this way:  Why does Rotax make a big deal about 'Burping' the system?  Because air bubbles cause all these types of problems.  If what you say is true, then there would be hundredths of plane owners complaining about the same problem.  Both with and without Rotax engines.

Get rid of the air and the problem goes with it!

K.I.S.S.  M.E.!

Barry

QA Engineer that punched holes in theories.

 



On Tue, Jun 20, 2017 at 8:57 PM, rampil <ira.rampil(at)gmail.com (ira.rampil(at)gmail.com)> wrote:

 	  Quote: 	
		  --> RotaxEngines-List message posted by: "rampil" <ira.rampil(at)gmail.com (ira.rampil(at)gmail.com)>

 

 Hey Barry,

 

 You are right about the static situation, it's a bit different dynamically.

 Back in the day when I used to teach about the fluid dynamics

 of invasive arterial blood pressure, I would note that

 the distortion or "ringing" in the system was related to the

 resonance of the system which in turn is a function of the

 compliance of the tubing plus air bubbles, and the inertance

 of the fluid column. When the driving frequency(pulsation of the

 oil pump) gets close to the system's resonant frequency, the pressure

 Oscillates wildly ( in a measurement system that  has adequate frequency

 response).

 

 I will generalize (and guess) a bit to say that

 1) the driving frequency of the oil pump, a multiple of RPM

    is way higher than the responsiveness of mechanical

    Oil pressure gauges to track the waveform even though you

    would be able to see the ringing with an electronic transducer

     and an o-scope.

 2) air (increasing compliance) decreases the resonant

     freq of the tubing system thus reducing the apparent

     Oscillation on the gauge by pushing the resonant frequency

      of the tubing well below the rpm harmonic.

 

 --------

 Ira N224XS

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=470292#470292

 

 

 

 

 

 

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rampil

Joined: 04 May 2007
Posts: 870

Posted: Wed Jun 21, 2017 4:40 am Post subject: Re: Variable Oil Pressure Reading

Hi Barry!



Cavitation requires massive pressure changes usually caused by flow in 

hydrodynamic systems, high speed flow and shear. There is essentially no

flow in a dead end tube to a pressure diaphragm. If there is a bubble, 

maybe you will get a bit of to-and-fro, but not the kind of shear to create 

cavitation. Same reason bends really don't affect pressure transducer lines.

Small pressure changes like 0 to 60 psi would have to occur with huge 

acceleration to cavitate oil, a fluid designed to have high surface tension.

Adding a length of tubing increases inertance and a wee bit of resistance.

Burping these engines is all about refilling the reservoir and preventing

entrained bubbles from blocking oil channels in the block which would 

depress lubrication and heat transfer. Nothing to do with resonance 

phenomena.



--

If you can't measure something accurately, you can't understand it:

paraphrased from Lord Kelvin

For the curious, check out the electrical analog, a pi filter here at

https://en.wikipedia.org/wiki/Capacitor-input_filter

_________________
Ira N224XS

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Wed Jun 21, 2017 5:05 am Post subject: Re: Variable Oil Pressure Readin

Hi John,

You didn't say if you did a full oil purge? If you didn't you still need to do one before a flight. Yes air in the system can cause pressure swings and sometimes after a purge, but should go away.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

JohnF

Joined: 13 May 2010
Posts: 124

Posted: Wed Jun 21, 2017 9:20 am Post subject: Re: Variable Oil Pressure Reading

Thanks for your suggestions/ideas. As I suspected last evening at home while thinking over the problem it struck me that when doing the air purge after replacing fuel and oil hoses that upon reflection that evening that I had "pumped" a lot of oil out of the engine while doing the purging. I had originally put in 3-liters of new Aeroshell Sport Plus 4, but I suspected I had emptied too much while purging.



This morning I refilled the oil to the top of the flats on the oil dip stick, purged the oil system again, buttoned up the plane, and ran it up.



One thing I wanted to do is monitor the operation of the Thermostasis unit. It worked perfectly, and got me up to 50-degrees C in a few minutes, high enough to apply takeoff power if need be.



The oil pressure was rock solid, moving from 66psi/81-degrees F, to 55psi-122-degrees F in those 3 or 4 minutes.



Problem was low oil quantity...now fixed. Reminds me  what a mechanic once told me: If you have a problem, look at the last thing you did.



Seems good advise still.

Roger Lee

Joined: 10 Jan 2006
Posts: 1464
Location: Tucson, Az.

Posted: Wed Jun 21, 2017 12:55 pm Post subject: Re: Variable Oil Pressure Reading

Hi John,

Glad it worked out. I usually put in 3.5 liters for a good oil purge. One thing about the oil purge is you don't need a lot of oil to come out. A small quantity is fine. You're just trying to make sure the lifters have oil and there are no huge air voids in the engine. It doesn't take that much excess oil to do that. If you run low on oil during a purge you can pump air into the system and cause those air voids.

_________________
Roger Lee
Tucson, Az.
Rotax Instructor & Rotax IRC
Light Sport Repairman
Home 520-574-1080 TRY HOME FIRST
Cell 520-349-7056

BARRY CHECK 6

Joined: 15 Mar 2011
Posts: 738

Posted: Mon Jun 26, 2017 8:58 am Post subject: Variable Oil Pressure Reading

Once again John...

K.I.S.S.  M.E.

You can argue theory all you want but it ALWAYS come down to what works!

Barry

On Wed, Jun 21, 2017 at 1:20 PM, JohnF <n29cx(at)ridgeviewtel.us (n29cx(at)ridgeviewtel.us)> wrote:

 	  Quote: 	
		  --> RotaxEngines-List message posted by: "JohnF" <n29cx(at)ridgeviewtel.us (n29cx(at)ridgeviewtel.us)>

 

 Thanks for your suggestions/ideas. As I suspected last evening at home while thinking over the problem it struck me that when doing the air purge after replacing fuel and oil hoses that upon reflection that evening that I had "pumped" a lot of oil out of the engine while doing the purging. I had originally put in 3-liters of new Aeroshell Sport Plus 4, but I suspected I had emptied too much while purging.

 

 This morning I refilled the oil to the top of the flats on the oil dip stick, purged the oil system again, buttoned up the plane, and ran it up.

 

 One thing I wanted to do is monitor the operation of the Thermostasis unit. It worked perfectly, and got me up to 50-degrees C in a few minutes, high enough to apply takeoff power if need be.

 

 The oil pressure was rock solid, moving from 66psi/81-degrees F, to 55psi-122-degrees F in those 3 or 4 minutes.

 

 Problem was low oil quantity...now fixed. Reminds me  what a mechanic once told me: If you have a problem, look at the last thing you did.

 

 Seems good advise still.

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=470333#470333

 

 

 

 

 

 

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 eferrer" target="_blank">http://forums.matronics.com

 ====================================

 WIKI -

 errer" target="_blank">http://wiki.matronics.com

 ====================================

 b Site -

           -Matt Dralle, List Admin.

 rel="noreferrer" target="_blank">http://www.matronics.com/contribution

 ====================================

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