Tank Width

here's a phenom for all those posters on
"all the variables" thread.

Why does a wider tank tend to go rich above 45 degrees?

I never really thought about this, but then Tom Hampshire made mention of it and I realized that my Giles, with a GRW hopper tank(about 2.8 in. wide) does go slightly rich up high and planes with narrower tanks don't.

What gives? I can't figure out what is going on. I will try a narrower tank and see if it fixes the sl. problem.

WAG: The angle allows for gravity to counter some of the centrifugal forces during flight. This combined with the fuel not trying to flee in a direction directly opposed to the direction we want the fuel to go may also have an effect. Could also consider any possible flight speed reductions due to increased drag from the tighter turning radius. Brett, any input?

>here's a phenom for all those posters on
>"all the variables" thread.
>
>Why does a wider tank tend to go rich above 45 degrees?
>
>I never really thought about this, but then Tom Hampshire
>made mention of it and I realized that my Giles, with a GRW
>hopper tank(about 2.8 in. wide) does go slightly rich up
>high and planes with narrower tanks don't.
>
>What gives? I can't figure out what is going on. I will
>try a narrower tank and see if it fixes the sl. problem.

It could be as simple a thing as to remember that the fuel will flow out to the wedge because of centrifrugal force. When level, the fuel is still at the same level as the engine. When overhead and running a wide tank, you are now placing the fuel above the engine, so it will flow easier, hence richer. It is just the opposite of when you point the nose up from level flight and the tank drops behind (under) the engine and leans it. This may be the theory behind the tal profile tanks which are not very wide, and have the pick up tube location (inside the tank) about even with the intake of the engine.

>It could be as simple a thing as to remember that the fuel
>will flow out to the wedge because of centrifrugal force.
>When level, the fuel is still at the same level as the
>engine. When overhead and running a wide tank, you are now
>placing the fuel above the engine, so it will flow easier,
>hence richer. It is just the opposite of when you point the
>nose up from level flight and the tank drops behind (under)
>the engine and leans it. This may be the theory behind the
>tal profile tanks which are not very wide, and have the pick
>up tube location (inside the tank) about even with the
>intake of the engine.

Note that flying "level" up high, though, the plane is in a constant "loop". This would actually tend to force the fuel towards the floor of the tank rather than to the wedge.

Phil, what would you mean by "wide"? In which direction(s)?

Who says this is a given? I run tanks up to 3" wide and 1 3/4" deep in short nose airplanes with no ill effects. I'd say the problem is elsewhere, venting, prop, engine wear or?

>Who says this is a given? I run tanks up to 3" wide and 1
>3/4" deep in short nose airplanes with no ill effects. I'd
>say the problem is elsewhere, venting, prop, engine wear or?
>
>Al

I agree, Al, that it doesn't have to be a given, and I think it would depend a lot on tension as well. With your tanks, because the wedge is so far "out" there, you are always pulling the fuel inboard a certain amount against centrifrugal (yes, I will use that word) force. The fuel must go to the end of the wedge at all times or we would run out of fuel. The loops we do (vertical and overhead eights, clover, etc) even up high will force the fuel "down" (towards the floor of the tank) while it is stil being thrown out by centrifrugal force. Probably the centrifrugal force effect cancels the added gravity effect and maybe my original comment doesn't deal with all forces.

But I agree that it should work, and you have shown in your installations that it does. There may be a slight difference in how you set your needle, buy you learn that very quickly. (A tank set on the inboard side of the fuselage on a profile will run differently than a tank set on the outboard side of the fuselage, but, again, we can adjust for it with the needle).

width is spanwise.

Who knows what it is? That is the question.

Current setup is a profile wiht a GRW 4.25 oz. uniflow chicken hopper tank. It starts slightly rich, then leans out, final third of the tank runs rich again. In the overhead eights the plane get richer. Same tank in a combat style plane runs steady 4 or 2, depending how you set the motor.

Tom mentioned the tank width as affecting the motor run and I hadn't really noticed it before.(most bladders are 8 in. spanwise!!) So chalk one up to Al that 3 in. wide tanks don't affect his planes. Anyone else have any data?