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Better tank vent


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Noticed this neat tank vent mod on Charles Mollison's Savannah. Those original dangling tubes from the tank vents are a problem sometimes. If one gets facing forward and the other to the rear and tanks are full, the rear facing tube can start pissing fuel and keep pissing. This neat cable clamp pins the tube down and keeps it facing forward. Just be VERY careful drilling that rivet hole, all too easy to drill into the tank....


Tank vent.jpg

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Hi JG. I converted my Savannah underwing breathers to forward facing ones, much like the one in your pic, but using a very small 90deg bend from a garden irrigation system.

On the first outing, with just the inner tanks valved on, I experienced a huge amount of crossfeed, one tank level going down at a rate, the other tank level going up, so turned back after only a few minutes.

On landing I found fuel stains on the wing top from the tank that was going up: the tank caps were screwed tight as usual, but the additional pressure in the system had caused that tank to bleed pressure at a minor imperfection (a slight dip)  in the lip of that tank.


As a result of that experience, and after some thought, I reverted to the standard underwing setup for three reasons:

1. I had no wish to experience that degree of crossfeed again.

2. I was concerned that the additional tank pressure could 'bug out' the tanks, with the possibility of damage to the wing structure. (Note that the pressure has nothing to do with the size of the aperture. Note also that very small pressures can generate huge forces over large areas, so for instance a controlled atmosphere coolstore for apples, which seals hermetically and is tested with a domestic vacuum cleaner, can be imploded if the vacuum cleaner is left on for too long. And a pressure of 1PSI in a Sav tank may generate something like 500lb of 'bug' between it's top and bottom surface.)

3. Cessna, who have been messing with this much longer than most of us, position their forward facing breather behind the strut attachment, not directly in the airflow.


The main problem with the basic Savannah setup is that the plastic underwing pipes do not sit straight, and can also rotate.
My intention now is to mount solid aluminium pipes down through through the wing,, with flexible pipes from the top of these to the caps. At the underwing I will cut these pipes at an angle into wind, which should give a small positive pressure. And the pipes will be held in some way to prevent rotation.




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