BB Gravity fuel feed

[Bennie Vorster]

Have flown another hour this morning and the VW is only performing better and better.

I did notice though that the two fuel tanks does not keep the same level at all. After filling both, one burned 5L and the other one burned 15L . What will be the cause of this and what will happen if the one goes empty before the other. There is a combined little tank under the dash.

[Morph]

Are you leaving both fuel taps open? If so one of the tanks has a higher pressure than the other which is resulting in the fuel pushing from one, via the header tank (The little one under the dash) to the other. The pressure of the tank is caused by the forward bent tubes on the fuel cap.. They must both face forward perfectly. If they face backwards the tank will get an air lock and you will have a fuel starvation problem.

Typically you take off and land with both tanks open. Once in the air you close one for a period, then open it and close the other. On a long trip you would do it say every 30 minutes. Shorter trips use one or the other depending on how far you are flying.

How long did you fly for? was it possible you used 20 liters? if not maybe you have a leak on the one side.

[Mogas]

Bennie, mine also always uses more fuel from the left side in flight, only notice it on long sectors though.
I do as Morph says, manage the fuel as required. Just make absolutely sure you have at least one valve open and both on for takeoff and landing.
I have heard of many other BBs using more from one side. I know that the fuel will flow between tanks on the ground if wings not level, so I can only think that the plane is not being flown absolutely level.
No, the engine will not starve if one side is run dry but the other tank has fuel in it, valve open ofcourse! Consider the header tank as the main tank feeding the engine and each wing tank feeding the header tank.

[justin.schoeman]

Does your VW have fuel injection? If so, it will have a fuel return line which will empty into one tank, so this tank will always have a little more fuel in.

In theory, it won't be a problem if one of the tanks runs dry (because of the header tank), but it is probably better to manage the fuel levels, so you don't get caught out if flying out of balance.

It may also be an idea to disconnect the hoses from the header tank, and do some basic flow rate checks from each tank to make sure that one isn't restricted or air-locking...

-justin

[Bennie Vorster]

Does your VW have fuel injection? If so, it will have a fuel return line which will empty into one tank, so this tank will always have a little more fuel in.

-justin

A Ha Justin I think you might be hitting the nial on the head here. I'll check on that

[Boet]

For some or other reason the BB does not drain both tanks at the same rate. One ALWAYS drains faster than the other. Do not ask me why. I do not know. Perhaps it is just the nature of the beast.

[Bennie Vorster]

I have an idea to run a connecting hose between the two tanks by T-ing it in just where the fuel hoses comes from out of the wings into the cabin. That should sort out the levelling problem in both the tanks. I am also thinking of adding a piece of see threw hose in the feeding line from the tanks to under the dash board. In that way I will easily be able to see when the BB is running out of fuel and still have the header tank with seven / L of fuel to do a precautionary landing.

[Morph]

Bennie, having the header tank is creating a t-piece between the two tanks. See attached pics.

The red line is the line you want to put in. However if both taps are open then there already is a T between the tanks via the header tank. If the plane is standing level and you open both taps the fuel levels will equalise.

I too like the idea of see though tube just from the tap to the header tank. Gr8-dad has these. The only thing is when you first switch open the tap you will notice some air bubbles going down the tube. If you see this is the air you get a bit of a skrik. However the header tank traps the air and ensure just fuel goes to the motor.

[Mogas]

Morph said it right, wanted to myself but was just too lazy
This uneven feed is no big deal and I am 100% sure it is caused by wings not level either on the ground or in flight. Easy to manage it in flight, just feed from the higher one. This sort of fuel management is standard practice for bliks anyway. On the ground I make sure wings level with both tanks open before I put it to bed. Get there in the morning and the fuel is perfectly balanced.

[Dobbs]

It is important to remember that the header tank cannot bleed itself of air, so important to remember to confirm full header tank as part of pre flight.

[Bennie Vorster]

It is important to remember that the header tank cannot bleed itself of air, so important to remember to confirm full header tank as part of pre flight.

No place to check that on my plane.

[Morph]

It is important to remember that the header tank cannot bleed itself of air, so important to remember to confirm full header tank as part of pre flight.

That's cool if you have a translucent tank like the new BB's have. Some of the older planes have metal tanks

Once bled it should not easily get an air lock into it unless you run dry and it suck air from the main tanks

Bennie, it should have a bleed valve on the top of the tank for air. It will look similar to the drains on the wings.

[Slabfish]

As a matter of interest,we had the same case of the twing tanks running uneven. What solved the problem was to set the one vent pipe(of the tank that emtyed first) on top of the wing slightly on an angle to get less possetive pressure while flying,it solved the problem!

[Mogas]

In order to explore my theory about fuel draining across from the high wing to the low, I temporarily stuck one of those cheap string levels onto my spar carry through so that it is visible in flight.
Been flying fairly long sectors lately, 40 minutes+ so good time to try this. The verdict is I have consistently been flying left wing slightly high despite flying the turn co-ordinator ball in the middle. I proved this by looking at the little level in flight which seems much more sensitive than the turn-co. Also using the level it is easy to set the wings nice and level in the cruise by fiddling with ailerons and rudder.
Guess what........nice even fuel feed. Also usefull for leveling the wings on the ground when out of the hangar.

[Morph]

How this for a theory

You all know that the BB pulls left during takeoff and less so during flight. This is due to the rotating effects of the prop. To compensate you need to apply right rudder or have a rudder tab that does the same.

Lets assume the prop is turning clockwise when viewed from the cockpit, i.e the blade moves down on the RHS and up on the LHS. The downward moving blade will provide a higher pressure on top of the right wing while the upward moving prop on the left side will result in a higher pressure on the bottom of the left wing. This naturally results in the plane wanting to rotate to the left if not compensated for.

Since the fuel breathers are close to the fuse the tank on the right wing will experience this higher pressure than the left and thus it will be emptied quicker

What do you think?