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Unusable fuel with a flop tube ?

7

777drvr

Well Known Member
Hi guys,

Just doing more work on my recently purchased RV-4 in preparation for my import inspection.

I did a new weight and balance and determined that I have 3.66 gal or 22 lbs of unusable fuel in the tank equipped with a flop tube . The other normal pick up tank has almost no unusable . Seems like a lot . Just wondering what others have found.

This was tested by seeing when the boost pump stopped supplying fuel and then draining and measuring the remainder from the quick drain.

Thanks,
Marc
 
That is a lot

I have almost no unuseable fuel in either tank, (one normal, one flop tube). You need to check and make sure the flop tube is not stuck in a bad position. If it is, I'd be careful when going xc. You won't be able to count on that gas. I always use the fuel in my flop tube tank first. That way I know I got it all.
 
Flop Tube

SB 06-2-23 issue with the pick up tube not being safety wired and acually came off leaving just about the amount of fuel you mentioned in the tank. Or like the previous post it could be stuck. I would copy and paste it but my computer is having some PDF issues.

Mark
 
Thanks guys ! The former owner said that he had recently complied with that service bulletin . He had gone into the tanks to remove the sloshing compound once used in the tanks and had resealed them . At the same time he did the SB . Hmmm....

Marc
 
You might have a problam!

Mine has flop tubes on both sides, I can use all 16 gallons on each side. You are loosing 30 minuets, that last 4 gallons is your reserve, I would fix it.

On the other hand you might get a different result when flying, you could run each tank dry in flight to check just how much you can use in the real world, this is how I tested mine.

Russ
 
I reviewed the plans for the flop tube configuration and see that there is no reason for that much unusable fuel . I wonder how flexible the tube is and if it's possible that it 's bent out towards the tip right now ? I'll go to the hangar and do more trouble shooting today.

Marc
 
YEP! That comes under a "Safety of Flight" issue!

There's definitely a problem. That needs to be looked into immediately.
 
I have a suggestion...

...if you are going to pull a tank. You may want to make a "tank jig" to hold the tank after you remove it from the a/c. You may be able to find a builder that has already constructed one and doesn't need it.

If you need (want) to construct one yourself, you can use 1/2 inch plywood and cut out the profile of the tank, leading edge facing down. Remove a wing tip and scribe the leading edge profile on the plywood. Make two of these form shapes, and screw / attach to another piece of plywood or board.

Space the tank jig forms far enough apart to securely hold the tank, and locate the jig cutout forms so that they are located on a tank rib. This will provide a secure method to work on the tanks without the worry of having a tank tip over or move around while it is being worked on.
 
Yanked the tank today and this is what I found. A bolt was used on the root rib to plug a hole and it made a perfect hanger for the flop tube . Somehow the flop tube got up there and the rubbery texture of the Proseal made it stay up there . It's a bit of a fluke for the clunk to get up there but it did ! I cut the bolt down and tested all possible attitudes to make sure it can't hang up again .

I'm glad I found a smoking gun and can now get every last bit of go juice !

Marc

4124122128_dfbea1beae_b.jpg


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Remnants of the sloshing compound that was once popular . It's been taken out of the tanks for the most part but there still is a little evidence here and there .

Marc
 
fuel tank

Nice pics Marc... you going to remove the flop tube or the bolt or both? Amazing what you find when trying to solve these issues. Did Bill have any other things to say? Any comments on weight? PM if you like, or call.
 
Hi Dave,

I just cut the bolt down and the flop tube no longer hangs . I'm sticking with the flop tube . I'll give you a call !

Marc
 
How old is the flop tube ? If it's over 1/2 to 3/4 the life span, now would be a good time to replace it with a new one.
 
I probably should have but it flopped so nicely that I didn't see the need . I thought they got stiff with age . Is there another failure mode ?

Marc
 
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