Why two seperate fule tanks?

[argoldman]

I also run with L, R, Both. My guess is that the L, R was just a simpler valve. But to prove to yourself of the potential danger of running in both on a low wing aircraft (no check valves---or no visual monitoring of fuel level), try the following. Take two glasses of water and fill them to the top with water. Stick a straw in each glass and drink from both glasses at the same time----no problem. Now, dump one of the glasses out and try it again (one straw in the glass of water and the other straw in the glass of air). You will find that as long as you are in both, that you won't be able to grab any fuel from the other tank----if one is empty.

 

That's what the sump tank is for.

 

beer through a straw?? only with ice in the glass.

[Drew Swenson]

Not sure what you are saying about the sump.

 

The purpose of the sump is to make the aircraft less sensitive to aircraft attitude. In the Varieze (no wing sumps), you could starve the engine if you bunted the nose over because all the gas would run to the front of the strake. In the Long/Cozy, it gets "trapped" in the sump.

 

Now if you are talking about feeding down to a common sump (mine are not), then I see what you are talking about. But you would still want a selector switch (and/or check valve) in case you had water contamination in one tank (flying in the rain with a bad seal) or in case one of the gas caps blew off.

[argoldman]

Not sure what you are saying about the sump.

 

The purpose of the sump is to make the aircraft less sensitive to aircraft attitude. In the Varieze (no wing sumps), you could starve the engine if you bunted the nose over because all the gas would run to the front of the strake. In the Long/Cozy, it gets "trapped" in the sump.

 

Now if you are talking about feeding down to a common sump (mine are not), then I see what you are talking about. But you would still want a selector switch (and/or check valve) in case you had water contamination in one tank (flying in the rain with a bad seal) or in case one of the gas caps blew off.

 

While that is certainly one of the functions of the sump, with an unvalved flow from two tanks, its main function is to prevent sucking air. As long as the sump pickup is covered by gasoline, the engine will get an uninterrupted supply of fuel, no matter what the balance of, or lack thereof fuel in the tanks

 

If one is flying, either in rain or not with a bad seal, I would think that the fault lies with the nut behind the control stick, not the design of the aircraftfuel system. In any case, the slug of water that unknowingly would ensue would probably prevent the aircraft from restarting.

 

However one could put a placard on the panel: "Do not use left tank while flying in or around rain"

 

One of the things that should be incorporated in the sump tank is a water sump (the first 1/2"), above which the fuel is picked up. This would obviate the necessity of a gascolator.

 

As far as the loss of a fuel cap, If this does happen, it will be shortly after takeoff. I was taught that part of my normal scan was to constantly monitor the fuel level. Perhaps that, like full stalls, is not taught in flight instruction, today. The scan just might encourage one to land.

 

Even though we have fuel totalizers, it is the responsibility of the pilot to know how much fuel (s)he has on board, and what's happening to it. We need to be pilots, not airplane drivers. If you want that, get a boat (preferably sail)