Tank venting "isobaric bush-o-lator" question

All,

In order to resolve engine run issues on a model with a standard vented (non-uniflow) tank, I'm going to try this :-
http://www.aeromaniacs.com/busholator.htm

My questions is - can the forward facing hole in the tube linking the tank vents be anywhere with respect to the tank & engine, or should it be roughly aligned with the centre line of the tank & engine?

thanks,
Col

I use the equivelent vent fitting for my planes. Lets see if I can find the pic.

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>I use the equivelent vent fitting for my planes. Lets see if
>I can find the pic.
>
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>
>[photo not recovered: 29267.jpg]
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What Phillip has here is what is called a Tinny (or Tinney sp?) vented tank. It is a standard vent with one vent then blocked off and the other used for venting in flight. About the only advantage to this is to prevent siphoning during the flight. Forward facing the tubes makes no difference in the run except to add positive air pressure.

And, as others have mentioned, with this system there can be a slight between upright and inverted at first (the tube when immersed acting somewhat like a uniflow, and when not as standard vent), but as the fuel is burned off, centrifrugal force will push the fuel to the outside and leave the vent open upright or inverted.

On what was described in the web page with a fuel line connecting the two and a hole in the middle does nothing! It will not act like a uniflow, although it may eliminate siphoning if that is a problem. It does nothing to alter the run, however, since both vents are still open to the atmosphere and operating independently. Just because they have a common air source will not affect anything in the run (other than, perhaps, to eliminate siphoning if that is a problem. Angling the tubes forward or cutting them off at a forty-five degree angle into the airstream should serve just as well.)

I don't see the big deal here. You could do almost the same by blocking off one vent, and putting a piece of fuel tubing on the other with a 45 degree cut facing into the breeze.

But why bother? Conventional setups will go rich on takeoff and lean out during the flight--how much depends on the tank width.

When blocking 1 vent, there could be a difference of engine regime upright >< inverted at the beginning of the flight. A soon as the open tube gets uncovered by fuel this no longer plays.
FWIW
Paul

Once you take off the fuel all moves to the outside wall of the tank, and as air comes in, the fuel surface is nearly vertical--there is no top and bottom surface, just an inside one. So top and bottom vents are irrelevant for control line setups. Both are in air on the inside of the tank. So it doesn't make a difference--except if you want two openings into the ram air to pressurize the tank more.

Indeed. It might however give a problem during the reverse wingover. After that you should be ok.

>Once you take off the fuel all moves to the outside wall of
>the tank, and as air comes in, the fuel surface is nearly
>vertical--there is no top and bottom surface, just an inside
>one. So top and bottom vents are irrelevant for control line
>setups. Both are in air on the inside of the tank. So it
>doesn't make a difference--except if you want two openings
>into the ram air to pressurize the tank more.

Not quite. There will be about a 3 to 1 difference, meaning the fuel will angle out at about a 60 degree angle. You will never get a vertical wall as such. So there will have to be some burn off before the single vent is open to the outside air both upright and inverted.

Add to that the fuel sloshing. Just going upright to inverteed sloshes it some. Doing a loop adds more "gravity", and out loops can be severe enough that the vertical pressure can be greater than the outward pressure. That, again, changes the angle of the fuel.

But fuel "sloshing", that's an issue that can make a big difference. On some planes, because the nose is angled out, the fuel will actually move forward, resulting in the tube being covered for a longer period of time. This is why, on an open vent, it is best to have two vents (which the original concept retains). It is also why, on a uniflow tank, the vent needs to be as close to the pick-up as possible.