Replies to This Discussion

Permalink Reply by John Marzulli on September 3, 2013 at 11:18pm

Did you drill the additional hole in the cap?

I had problems with cross-flow and uneven fuel flow for a while.

A series of modifications were made, which in turn worked but I am not sure which one (if any) was the fix.

1) Duel, separate gascolators.

2) Andair "L\R\B\Off" switch.

3) Duel fuel filters.

Even with the fuel selector on "BOTH" I no longer see un-even fuel feeds. My suspicion is that it may be something to do with the "T" junction of the gascolator.

Good luck!

Permalink Reply by Dr. Edward L. Olds III on September 4, 2013 at 9:34am

John,

What hole?  I drilled the one in the filler neck, is there another?

Ed

Permalink Reply by Jonathan Porter on September 4, 2013 at 2:36am

OK, so we have four tanks on our 701.  They are never at the same levels when we dip them - but we believed that fluids always found the same level... so we thought that 'perhaps the bottom of the fuel tanks are at different levels, giving different readings'.  Then, we set about drawing a string line above the aircraft in the hangar and measuring the levels of the tanks in relation to the horizontal line (with a string spirit level on it) ABOVE the tanks.

Here we discovered that the levels were actually a lot closer than we realised.  It is fairly easy for us to believe that we have built our aircraft perfectly symmetrical and that the planets surface, including our hangar floor, is perfectly flat.  Then we believe that all and any discrepancy must from the fuel flow, and ignore the fact that fluids actually do find their own levels.

A quick bit of math.... let us say that the distance between the two tanks is about 3metres (10feet).  An error of build, floor, etc of 2degrees would make a fluid level difference of over 100mm in fluid height!  That is 4inches for the imperialists!  Likewise, an error of 1 degree would make a 50mm or 2" fluid level difference.  We were getting regular funnies - upto 30mm or 1.25" differnece on the outboards - and after we did our 'overhead string reference line' we actually found that our levels, in relation to the planet and gravity, were a lot closer than we thought - less than 3mm or 1/8" (that calculates to about a 0.06 degree error over 3m), which we put down to our testing method and measuring equipment!  FLUDS REALLY DO FIND THEIR OWN LEVEL - but you have to measure them from the planet datum, not from the bottom of the fuel tanks!  

We put down some of the tank level differences to the floor, some to the wing angle difference at build (even half a degree between the two wings would create a 25mm or 1" level difference) and then also the way the tanks sit in the wings - they have some play, and the cork thickness, the tank itself has variations in production. 

We actually link all four tanks together via a header tank, and normally fly less than full tanks with all tanks open - so we get FOUR different level readings.... the variations are pretty constant, meaning that the laws of fluid dynamics and fluids finding their own levels are still alive and kicking!

For the record, we have also found that as one tank lowers, the fluid pressure in the fuller or higher tank tends to feed.  Now, we have a header tank and so we don't mind even having one tank empty and the others full - and have flown all the possible tests on that - but that is with a header tank!  You could try some ground running with one tank full and one empty to see what happens - I guess it will function fine, since air is less dense than fuel... but test it - that is what we are all about here!

Permalink Reply by Ken Ryan on September 4, 2013 at 9:25am

Really interesting Jonathan. It seems to me that if the problem really was tank venting, then simply switching the caps would transfer the problem to the other tank. I like your analysis.

Permalink Reply by Dr. Edward L. Olds III on September 4, 2013 at 9:36am

Ken,

I have switched tanks in every combination.  Nothing seems to work?

Ed

Permalink Reply by Ken Ryan on September 4, 2013 at 10:41am

You could try draining fuel at the gascolator and monitoring the flow. If it flows evenly, that would indicate something about flight (attitude or forces) is causing the uneven draining. Could be a trim issue.

Permalink Reply by Dr. Edward L. Olds III on September 4, 2013 at 1:10pm

Ken,

That's a good thought.  I will try that when I get the tanks near dry.  

Ed

Permalink Reply by Dr. Edward L. Olds III on September 4, 2013 at 9:36am

Jon,

I follow the math and the theories of physics, but my difference in flow is "huge".  Such that I'll get 9 gal. in one and 2 in the other?  I don't think that is measurement error, I think it is not flowing?

Ed

Permalink Reply by Cliff Stripling on September 4, 2013 at 7:13am

I've seen so many posts about this it must be a common problem.  My gut feeling is it has to do with tank venting as well as bubbles at restrictions and junctions like "T"s, nipple connections, etc.  I anticipate it will happen with my plane as well so...  I plan to install a header tank not only to give me peace of mind about wing tank fuel level differentials but also to give me peace of mind on approach to landing or descending when the nose is pointed down and the possibility of un-porting the pick up tubes in a partially filled wing tank is a very distinct possibility.  I think it is absolutely necessary to drill the tank caps and add the "J bent" tubes so air can easily enter as well as exit the tanks when vapor pressure builds in the heat.  The fuel caps supplied by Zenith will allow air to enter when sucking (requires some vacuum) but will not allow air to exit. That is good to prevent fuel spillage if you have auxiliary fuel tank venting, but I won't have any wing tank vents except for those "drilled" caps vents.

Permalink Reply by Dr. Edward L. Olds III on September 4, 2013 at 9:37am

Cliff,

I appreciate the response.  My next call is to Zenith.

Ed

Permalink Reply by John Austin on September 4, 2013 at 7:20am

Ed,

Did I understand correctly you drilled a vent hole in the right tank neck? (Doesn't that leak a lot when the tank is full?) Seems to me that if you then attempt to "pressurize" the tank with the Vagabond-style cap, you'll just lose the pressure through the neck vent (unless you plugged it back closed?)?

FWIW, my tanks are not interconnected except by the Andair valve when on "BOTH." They have always fed a little differently (as nearly every high wing airplane does!), but maybe only 2 or 3 gallons when one tank has reached "0" or in other words, after burning nearly 20 gallons of fuel.

I had the Zenith caps originally which are vented without restriction (they leak!) and replaced them with Cessna caps with one-way vent check valves (they don't leak!). It made no difference in the differential feeding. However, in my case, since the differential takes a while to become apparent, it's easy to just select the tank that's got more fuel and burn it down even again.

John

N750A

Permalink Reply by Dr. Edward L. Olds III on September 4, 2013 at 9:40am

John,

You suggest something that I did not do, and that is plugging the hole in the filler neck that I drilled.  I will try that. I hate the thought of having to add a second fuel system to utilize a different Andair valve with "BOTH".  I may borrow some Cessna caps and see if they work?

Ed

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