Fuel tank problems.

I have an airplane that flies about 5 minutes on a full tank of fuel so I decided I need a larger fuel tank. The wedge tank is now about 4" from the engine to the LE, 1 1/8" thick top to bottom and about 2" wide. I made a new fuel tank that is 4" between the engine and LE, 1 1/8" thick and 3" wide. It is made of the same metal, same tubing, exact same design, only a little wider. The engine gives me all sorts of problems in flight with the new fuel tank like it is having a fuel shortage problem. I cleaned the needle valve, checked the fuel hoses, swapped the glow plug, it still won't run right. Just to rule out the needle valve or glow plug as being the problem I put the old fuel tank back on the airplane and it run excellent and flies great. I swapped the fuel tank to the new tank and again the engine does not want to run. The only difference is the width of the 2 fuel tnaks. I swapped the fuel tank again just to satisify myself that I am not nuts and the engine runs fine on the smaller tank but won't run on the wider tank. What in the world is the problem the only diffenence is width. I put a wood spacer behind the good fuel tank just to space it out to the same location as the wider fuel tank as an experement and now the engine acts like it is having a fuel shortage problem like it did with the wider fuel tank. Humm.........??? What is this telling me? The fuel now has to travel 1" farther to get to the engine. Is centrifical force the problem? Is the OS 40 LA having a problem sucking the fuel that extra 1" distance? I put a long flat fuel tank on the opposite side of the fuselage opposite the engine and the engine runs great. It looks strange with the fuel tank on the wrong side of the fuselage but it works. Maybe a flat fuel tank about 1" wide and 2" top to bottom and 4" from the engine to LE would work better??? A flat fuel tank that sticks out only 1" past the fuselage surface will have much less centrifical force for the fuel suction to over come. It seems to me if I can use centrifical force to put fuel into the engine instead of having to suck the fuel up against centrifical force it will work better.

Gary, I think you have hit the answer all by yourself. First off, is this a uniflow tank? If so that will help immensely. Still, there is a matter of suction and I don't know what venturi you are using. But the problem, as you have guessed, is centrifrugal force and the distance away from the engine.

In an upright or inverted situation, the tank is usually about 2 inches wide and the center of suction is centered, 1 inch away from the outside edge of the tank. On a profile using a 2 inch tank, the center of suction is 1 1/2 inches away from the outside of the tank. If you made the tank 1 inch wider, it is now 2 1/2 inches away from the center of suction.

Now add centrifrugal force. Say you are pulling 3 gs. On the inverted installation the center of suction is the equivilent of 3 inches away counting the centrifrugal force. But your new tank is the equivilent of 7 1/2 inches away. And remember, this is not constant. It varies up and down with cornering, altitude, etc. So it can vary from 2 1/2 to 7 1/2 during flight--or even more, and this does not take into account the distance it must pull up when you aim the nose up, or the added weight when you aim the nose down, or the fact that sometimes you will have even more than 3 gs, or...

I think you get the idea.

Ideally the tank should always be located as close behind the engine as possible. On a profile it would be better to deepen the tank top to bottom rather than widen it side to side. Then the venturi needs to be chosen to give adequate draw under the conditions in which you are flying. If you put a smaller venturi on the engine you could probably use the tank you have now. It would be better, however, to use a taller tank. Brodak makes both Oval profile tanks that are taller for profiles, as well as a very tall profile tank to alleviate the problem you are experiencing. It may well be that you will find your best results with this latter type of tank, whether you buy one ready built or make it yourself.

Make sure that you plumb it for uniflow, and adding muffler pressure could help the situation as well.