Putting on Tank Baffle

Maybe I am over thinking this, but I thought I would ask and I don't really have a second chance at this one.

I am getting close to putting on the back baffle of the fuel tank and I have a question about the sealant. Should the sealant coat the entire inside flange of the ribs? In the plans, it says to coat the inside edge of the flange on the end of each rib and a heavy glob at the corners of end ribs. Then a thin smear of sealant around each rivet hole.

It sounds like not all of the end flange of the ribs a get a coating of sealant.

It also sounds like the Zee brackets only get a smear of sealant over the holes and not the entire mated surface. True?

Thanks
ken

The design of the parts only accounts for a very thin build up of sealant.
If you were to have a thick buildup between the ribs and baffle, and then the baffle and z brackets, it can push everything fwd from the intended position.
In a severe case, it can effect the alignment of the tank skin attach screw holes.

Keep in mind that the goal is only to keep fuel from leaking out, not to glue the parts together.

I spread a thin layer of sealant all along the rib flange. I don't think it needs to be thick but you do want to get good coverage. The note in the instructions is addressing the small hole that is present due to the flange bend relief. It's no big deal just glop some in those corners. The critical part for me was the amount of sealant that I put down for the final bead. I think the instructions over state the caution that is required with regard to the size of the bead of sealant. Don't minimize this too much like I did on my first tank.

This worked for me on my -7. I can't imagine the -14 being much different. I sealed the z-brackets the same way, then covered the outer heads "just in case".

Miles-

A picture is worth a thousand words and in this case answered another question I had. I noticed that you bent up the vent line up to get it as close to the top of the tank as possible. This was another question I was wondering about.

As for the interior ribs, it looks like you really only put adhesive where the rivets are placed and not along the entire edge of the flange.

I take it that the blue tape is to keep sealant from getting everywhere.

Is it necessary to put a bead around the Z-brackets after they are in place?

Thanks for the help.

Ken

Question about the bulkhead union for the vent line:

Is the nut for the bulkhead union torqued down to a certain in-lbs? Or do we just put sealant on the threads tighten and put sealant over the bolt?

Thanks
ken

Stockmanreef said:

Miles-

A picture is worth a thousand words and in this case answered another question I had. I noticed that you bent up the vent line up to get it as close to the top of the tank as possible. This was another question I was wondering about.

As for the interior ribs, it looks like you really only put adhesive where the rivets are placed and not along the entire edge of the flange.

I take it that the blue tape is to keep sealant from getting everywhere.

Is it necessary to put a bead around the Z-brackets after they are in place?

Thanks for the help.

Ken

Click to expand...

Ken,

I REALLY apologize for not replying sooner; somehow I totally missed your follow up questions.

That photo doesn't show the interior ribs well. I did put a continuous fillet of sealant along the entire length of the edge of the flange. What you may be seeing is where I encapsulated the rivet heads.

I laid down that line of masking tape with the baffle dry installed. I placed the edge of the tape about 1/32 to 1/16 aft of the aft edge of the baffle flange so I could run a narrow fillet along that edge, then pull the tape to leave a clean line and remove any sealant that might interfere with the wing spar. After riveting, I mounted the tank to the wing to make sure it cured in exactly the right shape.

Between the baffle and z-brackets I used only a circle of sealant around each rivet hole, same as on the aft rib flanges, then encapsulated the outer heads after pulling the rivets. This photo shows both the narrow fillet along the baffle flange, as well as the circles of sealant between the z-brackets and baffles.


I hope I'm not too late with this reply...

Stockmanreef said:

Question about the bulkhead union for the vent line:

Is the nut for the bulkhead union torqued down to a certain in-lbs? Or do we just put sealant on the threads tighten and put sealant over the bolt?

Thanks
ken

Click to expand...

Those fittings are designed to seal with dry threads. You can either use a crows foot and torque wrench, or tighten a certain number of flats after hand tight. I did the latter, then coated the joint with sealant to "safety" it in place. I don't recall how many flats to tighten, but I'm sure there's a link to a reference somewhere here on VAF.

Thanks for the reply. Not too late. I do wish that I used the tape trick on the outer ribs. I ended up with a bit bit of excess on the skin and have to remove some of the dried sealant so the tank will fit in place.

I was also debating priming the Z-brackets. Did you prime the entire bracket or did you leave the part that connects to the tank unprimed?

Ken

Stockmanreef said:

...I was also debating priming the Z-brackets. Did you prime the entire bracket or did you leave the part that connects to the tank unprimed?

Ken

Click to expand...

Ken, I primed all surfaces of the z-brackets, but before installation I scuffed and cleaned a circular patch around each rivet hole on the side facing the baffle. That way I didn't leave any unprimed non-alclad surfaces exposed.

Mount tank

Ken
Miles makes an important point. Let the tank cure for a few days then put some wax paper on the spar cap and mount the tank. It doesn't have to be all the bolts and screws. It allows the spar and tank to form the proseal where it might contact. It only needs to be there for a couple days and the wax paper will keep it from sticking or peeling proseal off the tank. It's a really cool trick.
On the 7 wing the spar cap is really close to the tank skin/baffle joint. Even though I tried to keep those fillets neat, there was still a line when I removed the tank.
When it's done, set them aside for at least a week. Resist the urge to test until the proseal is fully cured.

I already scuffed up the entire area under the z-Brackets on the baffle. Maybe I will just prime the back of the baffle. The sealant should stick to the primer (2 part epoxy).

In the mean time I will just move on to the ailerons and flaps. I want to get everything to the point of priming on the wings. I would like to do one big primer session and be done with it until I start the empennage.

cheers and thanks for the help.

ken