Where to put center tank fittings on 3 tank system
 

I am re-doing a fuel system setup on an F15, it is using a 3 tank setup, and I have a question I am asking for advice on. The two saddle tanks will each have their own vent to the outside, the clunk lines of the saddle tanks will be plumbed directly to their own fitting into the center tank,, I know to keep all lines exact equal length. My question is, where is the best place to put those fittings in the center tank, I believe I have enough room to put 90 degree fittings on the top of the center tank, but if not would it be a problem if I put them on the upper edge of the front of the tank. I have included a rough diagram of what I am talking about, I have always been told they need to be high in the center tank to assure that tank gets completely full before filling the saddle tanks as I fuel the plane. Where is the best place to put those fittings, Advice/Experience is appreciated. thanks
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I always plumb to the top of the 3rd tank.

Imagine there were no saddle tanks and you were installing dual vents in the center tank. That's where your saddles should feed in.

When adding saddle or extra tanks and I can't put the vent fitting(s) in the main tank at the very top, I run the main tank's vent tube(s) in the stopper up to to top of the main tank internally. You are right. The vent(s) has to be at the top of the Main Tank.

When running multiple tanks, depending on the size of the turbine and where it's fuel pump is located (e.g. Like an RXI with the pump in the turbine) I had to run high flow fittings and lines for even my P140 RXI. It has saddle tanks and the pump is in the turbine. It simply would not spool up w/o high-flow fittings and fuel lines, etc. With a plastic UAT and 5/32 fittings, I could see the UAT collapse significantly. I have seen other planes with "solid" UATs and small fittings, where they just flame out at full throttle. The size of the vent fitting(s) and line(s) are often over looked. Your set up for vents should be good. Make sure the vent lines are equal in length also. I have found that the larger the vent(s) the better.

The turbine's inlet line size is a good indicator of needed fuel fitting and line size. (e.g. K160 (external pump) will run on 4mm line and a P140RXI and 160RXI (internal pumps) require a 6mm line, etc.) The entire fuel system capacity is limited by its smallest fitting/line.

Hysol the 90s into the highest point. I do the same.

If is difficult to fit from the top just do from the front top of all the tanks
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Don't use two vents! Tee the saddle tank vents together with an oversized tee, then use a larger diameter hose to the external vent.

With two vents it's possible for fuel to exit one vent and siphon the opposite saddle tank empty. This happened to a friend of mine. The plane sat for 5 - 10 minutes after filling. He took off and a couple of minutes into the flight, as soon as the center tank emptied, he ran out of fuel and crashed.

Also if you ever use a taxi tank, you'd need two of them with dual vents.

this has also happened to me. I am just more careful now about not over filling it.

the plane he is building is a big F15 with monster fuel tanks. No need for an overflow tank on this plane. I would still use 2 vents just to help keep the suction low. I always use 2 vents and the big BVM fittings with the single outlet that screws into the fitting.

make sure every length of the fuel line to each of the "outer" tanks is equal, down to the clunk line.

The is only the picture where the new vents goes the main fuel fittings are replaced with single aluminum bvm style fittings, sorry for not showing the finished product

Thanks for the input fellas, I'm sure they can go into the very top of the tank,,
and I will be using a 2 vent system with hoses exact lengths, and paying attention when filling.