Fuel tank modification advice?

Scott Kelley · #1 07-31-2018, 06:02 PM

Due to limited time (I need to get the plane back together by Friday for my CI), I need to modify my tank in order to install the float type sending units that I picked up from Van's yesterday. Problem is, the builder of my RV6 built it with capacitive sending units in mind, and didn't leave a large enough hole for the float type sending unit to fit through.

https://drive.google.com/open?id=1Jm...hEAFTcD0LEMniu

It's easy enough to open the hole - I can make a guide plate for a hole saw that attaches to the flange. My concern with that method is to do so without getting aluminum swarf in the tank, or to be able to know that it is completely cleaned out after the cut is done.

So I'm looking for thoughts / ideas of how to open up this hole and avoid swarf in the tank.

Thanks

David Paule · #2 07-31-2018, 06:35 PM

I bet there's an easier way, but I happen to know of one rather labor-intensive method of doing this, not that you'll want to do it this way.

1. Remove the tank from the wing. And to make it easier, here's the description of one I did.

2. Remove the root rig from the tank.

3. Make the change and clean the rib up afterwards.

4. Reinstall the rib, checking first to be sure that the rest of the tank is okay.

Dave

Ralph Inkster · #3 07-31-2018, 06:54 PM

Tank sealant takes time to cure, Friday is pushing it.
Float senders are mounted 1/2 way up on the side of the tank, the old location will likely have to be sealed and a new (about) 5" access hole created at the proper level. Refer to the plans for access cover layout, doubler & nutplates installation layout, sender mount layout, note there is a left & right float assembly, the bolt pattern is different.
This is not a small job. Good luck.

Mel · #4 07-31-2018, 07:32 PM

Never "RUSH" a modification to an airframe; especially a fuel tank.

Why is Friday a deadline?

Sam Buchanan · #5 07-31-2018, 08:24 PM

Quote:

Originally Posted by Scott Kelley

Due to limited time (I need to get the plane back together by Friday for my CI),

Forget "Friday" (at least for this week.....).

To do this job right you are going to need a new root rib for each tank.

lr172 · #6 07-31-2018, 11:21 PM

I suspect you should be able to get a standard fuel sender in there. The hole looks just about big enough, though you may need to enlarge it a bit. The hole pattern likely won't align though. In case you haven't noticed yet, the holes on the SW sender are not equally spaced. One hole is offset a bit to clock the orientation.

Probably easiest to drill out the rivets and make a new plate for the new sender. Should be very little debris in the tank afterward and the resulting hole will be big enough to be able to clean it out. You can use closed end pull rivets to put the new plate on, like those shown in your picture.

With the sender that low, you will need a custom bend in the arm that will take a bit of trial and error, but doable. Likely a straight arm with one bend right at the sender.

for tight fitting items like the new plate you would be making, I have successfully used anaerobic flange sealant. It cures in an hour or so. My senders are installed with gaskets and flange sealant. No leaks in three years and none expected. It is routinely used to seal crankcases on old Porsche boxer engines and I used it to seal the case halves on my 320. Also no leaks in 600 hours.

Larry

Scott Kelley · #7 08-01-2018, 12:10 AM

Thanks for the feedback.

I don't think the vertical position of the flange would be a problem - that should only affect the linearity of the readout. But the bolt pattern difference that Ralph pointed out, on top of the too-small opening makes the floats even that much less attractive an option. I had been contemplating using the blower side of a vacuum cleaner in the fuel filler opening to blow the shavings out while opening the hole with a hole-saw...

The sending unit flanges are on the back of the tanks in the second bay - it sounds like you guys are assuming that they are in the first rib - true?

It's looking more and more like overnighting a couple new capacitive probes is the best option.

I have thought about making capacitive plates and installing them through the access plates in the inboard rib.

https://drive.google.com/open?id=1dc...PGNFPPmGITQYnN

My question there of course was how much of a PITA would it be to get it off with all that Pro Seal on it. David doesn't make that sound like a walk in the park, but given plenty of time, and copious amounts of stripper it doesn't seem like too big a deal. The stripper I have, the time I don't (Condition Inspection is Friday, and planning to fly next week).

Mel · #8 08-01-2018, 05:45 AM

Quote:

Originally Posted by Scott Kelley

The stripper I have, the time I don't (Condition Inspection is Friday, and planning to fly next week).

Not sure I understand this. Your condition inspection expires on the last day of the 12th month after it was last completed. i.e. If it was signed off on 03 AUG 2017, it is good until 31 AUG 2018.

Ralph Inkster · #9 08-01-2018, 11:49 AM

Do you have a set of plans to look at? Do you have flop tube pick-ups being the reason the old sender was mounted on the second bay in the back- this would rule out the possibility of mounting the new float sender on the inboard rib.

Going from relative accurate capacitance gauge readings- to float readings (sender properly located mid point) that normally register the bottom 2/3 of the tank- to a low mounted float sender arrangement that would now take that accuracy down to the last 1/4 of the tank wouldn't be desirable.

Reason for the larger access hole and cover plate as depicted in the plans is so you can get the tools required to install nutplate fasteners as needed, rivet squeezers won't fit thru a small hole, don't even think of rivnuts. Also for clean up the aluminum shavings have a tendancy to stick to any exposed proseal in the tank, so just vacuuming won't do it, you need a hole big enough to get your hand in there to work around.

Scott Kelley · #10 08-01-2018, 01:38 PM

Quote:

Originally Posted by Mel

Not sure I understand this. Your condition inspection expires on the last day of the 12th month after it was last completed. i.e. If it was signed off on 03 AUG 2017, it is good until 31 AUG 2018.

And once it has expired, the deadline becomes the last day that you can schedule with your A&P before you want to fly...

Ralph - yes, it has a flop tube on one side. Sorry, I should have mentioned in the OP that the sending units are mounted at the rear of the tank in the 2nd bay - I guess the photo was taken from too close to make that clear.

I think the mechanical sending units would work OK - they have a sensing range of about 70 degrees, and the full range could be achieved at the lower mounting point - just less linear, with greater sensitivity at full than at empty. But if I were going to spend a lot of effort, rather than deal with the hassle of opening the hole and re-positioning the screw holes, I would spend that time making capacitive plates to install in the inboard and outboard bays and calibrating for high accuracy. But I'm going to just go for greatest expediency (and expenditure) and overnight ship a couple new capacitive sending units from AS.