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inverted oil pressure

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HodgeJW(at)aol.com
Guest

Posted: Wed Apr 23, 2008 6:20 pm Post subject: inverted oil pressure

Hey Guys,

  

 Was doing an outside loop today in the old 52 and noticed the prop sounded  funny and the airplane was losing energy way too fast. Looked at the oil  pressure and it was 0. Zip. Nothing. Turned said outside loop into a tail slide,  got her upright and the oil pressure returned to normal. Played around with a  bunch more maneuvers, and noticed that oil pressure was normal anytime I was  positive and zero anytime I was negative. Makes me think check valve balls in  inverted system. What do you guy think? Where do I start looking? Thanks.

  

 Jay



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Rob Rowe

Joined: 10 Jan 2006
Posts: 124
Location: Berkshire, UK

Posted: Thu Apr 24, 2008 4:04 am Post subject: Re: inverted oil pressure

FWIW - I'm not an engineer but from the oil schematics I wonder if the oil pick up gimbal assemblies inside the oil tank ares no longer functioning or if your crankcase vents are blocked (as they act as oil return lines when inverted) or the inverted operation vent (from the sump) is blocked.



While no expert I'd speculate that if the oil pressure drops very quickly then it might be gimbal related, if it's more progressive then possibly one of the vents.



Did this also occur when inverted straight & level (+1G) i.e. neg G may only be a symptom?



How much oil did you have in the tank pre & post flight?



Had you spent much time performing inverted maneuvers prior to the outside loop that brought the low oil pressure to your attention?



Presume we can discount a faulty oil pressure gauge by the simultaneous change in prop noise - if the prop speed controller is starved of oil the prop pitch reverts to coarse (counterweights overcome aerodynamic prop forces) to stop the engine overspeeding ... hence the change in prop sound & loss of performance.



brgds, Rob R

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dsavarese(at)elmore.rr.co
Guest

Posted: Thu Apr 24, 2008 4:54 am Post subject: inverted oil pressure

Could be an internal problem in the oil tank, as the  pickup in the oil tank is suppose to rotate freely from upright to inverted when  the airplane rolls inverted.  With the oil pressure dropping to zero during  inverted flight, I suspect the pickup in the oil tank may be stuck in the  right-side-up position.

 Dennis

  

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kregg(at)balancemyprop.co
Guest

Posted: Thu Apr 24, 2008 7:45 am Post subject: inverted oil pressure

Another place to check while you are at it, is the canister where the oil

temp probes are located.  There is about a 90deg fitting off the right side

as view from the front of the plane that runs from the oil tank.  This area

where the fitting connects to the oil temp probe housing is prone to cracks.

In some cases if this is cracked, it can case low oil pressure in different

"g's" environment allowing air in the system.



Kregg



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dsavarese(at)elmore.rr.co
Guest

Posted: Thu Apr 24, 2008 9:04 am Post subject: inverted oil pressure

If there is a crack as Kregg suggests, it is almost certain there will be 

evidence of oil leaking at the bottom of the canister.

Dennis



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Jay Hodge

Joined: 24 Apr 2008
Posts: 27

Posted: Thu Apr 24, 2008 10:06 am Post subject: Re: inverted oil pressure

Hey Guys,



Thanks for the info. Now I have a place to start looking. Someone asked whether the oil pressure dropped slowly or quickly. It went down to 0 immediately upon rolling inverted. Negative 1 g was enough to do it right now. Just the flight before, I was doing negative 4 no problems. Hopefully I will get a chance to look at it tomorrow and I'll let you know what I find.



Jay

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Jay Hodge

Joined: 24 Apr 2008
Posts: 27

Posted: Sat Apr 26, 2008 6:59 pm Post subject: Re: inverted oil pressure

Got a chance to look at the oil tank and flop system today and everything looks fine in there. No matter what attitude you put the tank in, the pick up goes to the bottom, no problem. I will take a closer look tomorrow. Does anybody know if this flop system has a bunch of seals in it? I was wondering if it did, and some seals were bad, could the pump have a hard time sucking the oil into the system? Kinda like trying to drink through a straw that has a big slit in it. 



Jay

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Jay Hodge

Joined: 24 Apr 2008
Posts: 27

Posted: Sun Apr 27, 2008 3:25 pm Post subject: Re: inverted oil pressure

Well, an update on the oil pressure problem. As Kregg suggested I looked at the oil pressure housing and sure enough, a rather large crack. So off to the local welder tomorrow. 



Jay

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dsavarese(at)elmore.rr.co
Guest

Posted: Sun Apr 27, 2008 5:53 pm Post subject: inverted oil pressure

FWIW Jay.  The way these canisters get cracked is by someone trying wrench 

open the petcock drain on the bottom of the main oil screen housing.  Many 

of the petcocks are so tight that when one tries to open it, lets say with a 

pair of channel lock pliers or vise grip pliers, it actually twists the 

housing and causes the canister to crack.

Dennis



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