Different fuel tank emptying rates in J430

[RooviatorJ]

We have a home built Jabiru 430, built in 2019.   We have noticed a very variable and extensive emptying rate of the tanks to the point that one may not empty at all after a couple of hours flying.  We have conducted a lot of ground tests and inspected flows from all the pipes down and up to the tanks, all appears  normal.  We have a 5 litre header tank fitted.  We cannot find an explanation for this, has anybody had a similar experience?

[rodgerc]

No experience with Jabiru, but I (and others) have noticed same in my RANS S-20. Assumed I’m flying slightly out of balance. 

[IBob]

I did some work on my Savannah to get more even fuel flows. Two things stood out:

1. The individual tank vents may deliver different pressures to the tanks in flight. The Savannah vents are rislan pipes sticking out under the wing, and normally cut at a 45deg angle. I was able to adjust these by altering the angle of the cut. A very small difference in pressure will make a big difference.
Cessna get round this by having just one vent, then cross-porting the upper tanks. Were I to build again, I would look at doing this.
2. Undulations in the fuel lines from the tanks. These can capture air at the high points, greatly impeding fuel flow. And this will change, giving varying results depending on the changing amount of trapped air. I went to some lengths to straighten my lines.
Note: I suggest you also check the filler cap seals: if they do not seal properly, then any pressure from the vent system will be lost, resulting in uneven tank pressure and fuel feed.
And finally, I am told by experienced pilots that most aircraft have some degree of uneven fuel feed.

[mkennard]

I drained the tanks in my 230D by removing the fuel hose from the mechanical fuel pump and draining into fuel cans and used the electric fuel pump to speed things up.

 

My left wing emptied first then the right wing.

 

During long flights one wing always drains faster than the other. Talking to other jab people they have the same issue. Wish we had a fuel slector sometimes.

[IBob]

MKennard, where two pipes are siamesed together, the flows collide. And depending on the dynamics of that, sometimes one flow beats the other one out, so the stable state is not flows combining, but one or the other 'winning' and excluding the other.
I would expect that effect to be far less, if at all, at low velocities. So it may be that draining the tanks as you describe is not what will happen at slower flow rates.

[RooviatorJ]

Thanks for all instructive replies.  So it seems that this may not be an unusual issue.  In summary, it may boil down to minor differences in vent pressures for either tank plus flow conflicts where down pipes join or meet in a header tank.  Interesting that Cessna have dealt with this issue long ago!  On further observation of our Jabiru, it seems that it is always the starboard tank has the priority to empty, sometimes there is little difference, sometimes a lot!   The assumption and the testing while safely on the ground is that even if one tank completely empties in flight, there should be nothing stopping the other one to take over once there is no "competition",  but it would be comforting to have a way of monitoring this when in the sky and maybe even over water!   On another forum, the issue of keeping the skid ball in the middle is also often mentioned, but other than this producing different air pressures on the wing-top stick vents, I cannot see that this would cause the big differences we have noticed, especially as on our Jab, there are built in tank vents linked down to the header tank and most of the time (at least) we fly with the ball centred!  I will update on here if we find anything else.

[IBob]

Hi Rooviator, I suggest you also check for fuel line undulations, and filler cap seals.

[RooviatorJ]

Thanks IBob,  Fuel lines all smooth and no undulations or sags.  We are currently looking at the filler caps and the stick vents that are part of their design.  Our flying coach has suggested there may be an issue with the lower disc component orientation,  difficult to explain in so many words, but if proven will elaborate later.

[IBob]

Tank flow conflicts are unlikely to be a factor at normal fuel feed rates.

[facthunter]

Skid ball off centre affects more than vents. Don't dismiss it, especially IF the effect changes with different pilots as happens with Gazelles.  Nev