terry_georgia said:
I wan't going to use a pump but rather just let gravity keep the two tanks level. The added weight is one line from the left tank to the tunnel, a check valve, an on/off valve and a "T" fitting. The added benifit is no longer having to switch tanks every 15-30 minutes. Just seems like a reasonable thing to think about doing.
At first glance and thought you are quite right, it's not much weight, but here's another way to look at it that represents the increased complexity and increased failure points.
1 "T" Fitting
1 B nut/flare & connectionon the fuel line that the "T" Fitting is inserted in.
1 B nut/flare & connection on the other side of the "T".
1 B nut/Flare and connection on the tank line to the T fitting
1 B nut/flare and connection on the line hooking to the "valve"
1 Valve
1 more B nut/flare on the other side of the valve
1 more B nut/flare on the line to the tank
1 Bulkhead fitting to plumb it back into the tank
1 "Check Valve" (no fittings if inserted directly onto the T or on/off valve).
This equates to 6 more connections in your fuel system, all inside the cockpit along with 2 additional valves (On/Off and "check"), a T fitting and a new tank connection.
That being said it's still not that simple. In fact, using gravity is an even less desirable way of doing it. Yes, there may be some plane somewhere that has done it for a million years, but a couple out of 4,000 flying RV's doesn't mean it's right or will work in your installation. It's designed the way it is not just because Van's did it that way, but because he did it that way for a reason.
Here's the problem. Unless the venting system of both tanks is 100% equal, the the laws of physics take over and one tank ends up being ever so slightly having an increased pressure over the other. What then happens is all the fuel will be pushed from one tank to the other side, until it's full, then maybe out the vent until the other tank is empty. This is a very well known and proven phenomenon on low wing airplanes. Unless the vents are interconnected, the plane is flying perfectly level, etc... you run the rist of all the fuel ending up in one tank. Heck, even a few select high wingers have this same problem. Some have solved this problem by interconnecting the vent lines, but then here you go with yet more plumbing to solve a problem that really isn't there until you create it.
I'm not knocking your desire to make the plane better, in fact I'd encourage it in other areas....just not the fuel system. Switching tanks 2-3 time per hour isn't that much of a pain in the rear. I still submit that if you build it light, build it simple, and build it fast you're better off in the end.
Once again, the above information is only my personal opinion and carries no more weight than any others so take it for what it's worth - Good luck!
Cheers,
Stein.