New member, smoke oil system question

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BJC

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the vent plumbing that escapes me.
Two vent holes in the tank, one in the top, and one in the bottom. Tubing from the bottom hole to a tee in the top hole. Tubing from the other side of the tee goes down below the bottom of the tank to overboard vent to atmosphere.

Edit: Looked at the photo: looks like your tank has all the fittings, with a small filler hole.

Do you have inverted oil for the engine? That system is slightly more complex.


BJC
 
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rv7charlie

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Look at fuel vents for acro a/c. The 1 & 2 seat RVs with wing tanks (low wing) have the vent line running from the high point in the tank, inboard into the fuselage, then up to around the top longeron (~20" vertical), then back down and out the belly. Spill is limited to the vertical up/down run that's even with the tank (~8-10" of -4 tubing), that can escape while the engine is sucking on fuel, and air's trying to go the other way through the vent.
 

Mjpierce1

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BJC, RV7 Charlie - I think I get it now. Connecting the upper and lower openings makes for an air/oil loop. When normal flight, the one of the upper tee legs is vented through the belly to atmosphere. The tee is key - one leg to atmosphere, one leg to upper port and one to lower port. I have a check ball in the upper port that when normal attitude the ball sits in a slotted opening allowing air to enter. When inverted gravity seats the ball in the plastic fitting stopping oil flow. Now with the tee fitting added, the atmosphere leg can vent to the lower (now on top) port which will have drained its small amount of oil.

Not the best explanation but I think I can make it work now. Thanks to all and anyone please chime in if I’m not understanding something correctly.
 

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Mjpierce1

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“copy a fuel system vent design” good idea. I’m looking for a drawing or clear explanation is a great idea. I’ll keep looking. I tried to find one on Vans Air Force but it alludes me. Maybe I need to do some more thread reading. There must be some Vans aircraft that routinely do acro and have a vent system that works well.
 

BJC

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Mjpierce1

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BJC - that makes sense to me, appears that Charlie thinks a bit complicated. Different opinions I suppose. Your set up appears solid.
 

rv7charlie

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I can cut/paste from the RV plans, but it's so simple that a drawing really isn't needed. Plug the bottom vent fitting. Connect the vent tube to the vent fitting that's normally 'up' in the a/c. Run the tubing *up* in the fuselage at least 10-12", or as high as you can comfortably run it at that location in the fuselage, then turn 180 degrees (doesn't have to be all in one bend) and go down and out the bottom of the fuselage. Routing can be pretty much whatever is convenient & easy to secure in the airframe.

No doubt BJC's design will work, and may well be the best design, fitted within the constraints of the Pitts airframe and the location of the tank. But I'm cheap & lazy; if I can eliminate a check valve, a 'T', 3 or 4 extra flair fittings, etc and have a functional system, I'll likely go for it.

The biggest difference between the system as installed in an RV and what you're doing, is that the flop tube is a couple of feet inboard (and ~6" below, in normal flight) the actual vent point in the tank, because it's a long, leading edge tank in the wing.
 

Mjpierce1

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I can cut/paste from the RV plans, but it's so simple that a drawing really isn't needed. Plug the bottom vent fitting. Connect the vent tube to the vent fitting that's normally 'up' in the a/c. Run the tubing *up* in the fuselage at least 10-12", or as high as you can comfortably run it at that location in the fuselage, then turn 180 degrees (doesn't have to be all in one bend) and go down and out the bottom of the fuselage. Routing can be pretty much whatever is convenient & easy to secure in the airframe.

No doubt BJC's design will work, and may well be the best design, fitted within the constraints of the Pitts airframe and the location of the tank. But I'm cheap & lazy; if I can eliminate a check valve, a 'T', 3 or 4 extra flair fittings, etc and have a functional system, I'll likely go for it.

The biggest difference between the system as installed in an RV and what you're doing, is that the flop tube is a couple of feet inboard (and ~6" below, in normal flight) the actual vent point in the tank, because it's a long, leading edge tank in the wing.
Charlie, thanks. But what I don’t understand, from where does the system get air while inverted? I’ve drawn the system and inverted the drawing. I see that while inverted gravity fills the vent line to the level of fuel in the tank. The air vent which used to be open to air is now blocked with fuel. Where does the system get its air? And yes, one may not be inverted very long, but what if one chooses to be?

I appreciate your input, really do. I’m just dense when it comes to this stuff. 😊
 

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rv7charlie

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If there was water in the tank instead of fuel, and you had a 20" column of water in the vent line, that would be 0.72 lbs [edit: per square inch] of force for the pump to overcome. [Actual pounds of force would be the cross sectional area of the tubing ID * 0.72.] And fuel (or oil) is much lighter than water, so....

The vent just 'blows bubbles' in the contents of the tank when pumping the contents while it's inverted. The pump has no problem whatsoever in drawing the liquid out of the tank, and air will just flow past, or push the liquid back into the tank, as it replaces the liquid being drawn out by the pump. For perspective, even a 5 psi fuel pump can begin to collapse an aluminum fuel tank, if you block the vent line while the engine is running. A 30-40 psi injection pump won't have any problems with flow from the tank.

You obviously don't want to put the flop tube's free end where it will be directly on the vent; then it will start to pump the bubbles while inverted.
 
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Mjpierce1

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It is standard on (probably) 1,000 plus Pitts.
Eliminate the check valve.

That is what I posted here New member, smoke oil system question


BJC
BJC - without the check valve in the upper vent (now lower whilst inverted) won’t the air line fill with fuel (to the level of the tank) thereby blocking air from entering the tank. I tried to draw the system without the check ball valve showing fuel levels. There may be something I am not understanding. Thanks!
 

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Mjpierce1

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If there was water in the tank instead of fuel, and you had a 20" column of water in the vent line, that would be 0.72 lbs of force for the pump to overcome. And fuel (or oil) is much lighter than water, so....

The vent just 'blows bubbles' in the contents of the tank when pumping the contents while it's inverted. The pump has no problem whatsoever in drawing the liquid out of the tank, and air will just flow past, or push the liquid back into the tank, as it replaces the liquid being drawn out by the pump. For perspective, even a 5 psi fuel pump can begin to collapse an aluminum fuel tank, if you block the vent line while the engine is running. A 30-40 psi injection pump won't have any problems with flow from the tank.

You obviously don't want to put the flop tube's free end where it will be directly on the vent; then it will start to pump the bubbles while inverted.
So when inverted, the pump pulls fuel from the flop tube in the tank as it always does, but also pulls air through (or past) the fuel in the vent line and in the tank? Blowing bubbles I guess is analogous to my blowing through a straw in my coke glass, it is just that air is being “sucked” by the fuel pump contrasted being “blown” by my mouth on the “air vent line” straw.
Thoughts? And thanks!
 

rv7charlie

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I guess is analogous to my blowing through a straw in my coke glass, it is just that air is being “sucked” by the fuel pump contrasted being “blown” by my mouth on the “air vent line” straw.
Exactly. I considered using the straw in a glass analogy when I wrote the post. When the absolute pressure in the tank falls below the absolute pressure of the atmosphere, the air will push into the tank.
 
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