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Tank Baffle change

NorthernRV4

Well Known Member
Since I'm nearing completion of my tail kit I've been reviewing the build manual and plans for the next stage of construction, my wings. A little background to my project. Kit #658 circa 1986 was purchased from EAA105 who sold on behalf of an estate. Some components had been lost over the years and I purchased new parts from Vans to replace them. I have the factory built Phlogiston Spar option and the wings are completely untouched.

As always, a problem when mixing new and old parts you run into design changes that create a lot of head scratching. I realized this weekend that the wing tank baffle (T-402) had been changed from a forward facing flange to an aft facing flange that slips over the Spar Web Doubler (W-406B) as per the new drawing 17A





My kit came with drawing 17 so the tanks are the old style where the baffle faces forward. I can see that with the new design the forward row of screws through the tank skin and the baffle to the spar doubler flange. This seems be a more secure method of attaching the tanks.


My plans show the forward row of screws that attach the tank skin to the main spar flange only.



My question is whether there are known issues with the older design. From what I understand, if I wanted to change to the new design I'd have to change the forward spar doubler to the new dimension part which would allow the tank baffle to fit over it's flanges. This would require disassembling the main spar and retrofitting the new spar web doubler. I wouldn't want to do this unless it was absolutely necessary. I plan to call Van's on Monday but I'm sure there is great experience to glean here on VAF?
 
I knew it wasn't just me out there!

...Okay, so here is the deal on the tanks. I had the exact same scenario that you mentioned, and I too talked to Van's.

My wing kit was a second hand, incomplete kit, that had basically nothing done to it, other than a lot of pieces lost. I had to buy so many new parts, that only part original are the spars and some of the ribs.

Feel free to take a look at my buildlog, as this procedure was fairly well documented there too:

So, the older kits had a forward facing baffle, and the tank ribs were recessed at the root to accept the baffle flange. The new baffles have the aft facing flange, which moves the web of the baffle forward about a half an inch or so. This is no good, because now your tank ribs will stick forward a half inch of the outboard leading edge of the wing. So, you guessed it, ALL NEW TANK RIBS. But the joys of mixing new and old are just getting started. The height of the new baffle, is exactly the same height as the spar. (In other words, you cannot step the flanges over the spar flange per the drawing, or you will be too tall.

Here is the HYBRID solution of new and old that I used, and was given the thumbs up from Vans:

The new baffle flanges are roughly 1" wide. Remove half of that width, so the baffle flanged are now ~.5" As the baffle and spar flanges are the same height, you will make a butt joint of these two sections. The tank skin will fit really nice over the spar flange this way. There are still the SAME number of screws attaching the tank to the wing. It's a lot of work, but a good method of using new tank parts on the older spars.

Feel free to PM me if you want to chat further on this. I'll see what I have for pictures.

Gregg
 
Thanks Greg, I was reading through your Kitlog yesterday, a great resource so thank you for well documenting your build. What wasn't clear to me was if there were any issues with building the tanks the "old" way. I noticed the new tank baffle is the exact same height as it's always been, 7 25/32" but the flange has been increased from 9/16" to 1". I had expected that if I was to build the tanks the "old" way I could simply trim this down and face the flange forward. I have all the original tank ribs so no worries there. Going this route means the only material attaching the tank to the spar will be the skin. Granted there would be the same number of fasteners but I was worried there was a reason it was changed. Cracking around the screw holes or elongation of the holes possibly??
 
Baffeled

Has the discussion of putting a step down to allow the end of the baffle to slide into the ID of the spar flange. Something a bead roller with step dies may do. I have to ask or I will not learn..
 
I kinda thought the same thing, that or add a strip to the inside of the baffle flange that extends back and inside the spar web doubler. There are a few areas where one could run into trouble though.

1. The tank skin is dimpled for the #8 machine screws that attach it to the spar doubler which would make it near impossible to slip the flange of the spar web doubler in the tiny gap.

2. The plate nuts would then become part of the tank baffle web instead of being attached to the spar. Obviously this is not how Van designed it so proper stress analysis has not been considered.

For these reasons I've decided to build the tanks the old way. I called builder support but they couldn't tell me why the design had been changed other than likely to make it easier to build.
 
I'm nearing the stage on my left wing where I'll have to start fitting the leading edge skin and assembling fuel tanks concurrently to match. I had been thinking I would modify the tank baffle and build the tanks as per my original plans but this means using AK-42H (sealed CS 3/32" pull rivet) to attach the wing skin to the baffle. This won't have quite as nice a finished look and I wonder whether these will leak over time. Since you can't put a dab of sealant on the tail after setting the rivet there seems like it would be a path for fuel to leak out between the rivet and the dimpled skin.

So two thoughts:

1. Keep calm and build on.

2. replace the tank ribs and build with a rear facing flange baffle. Alternatively to buying all new ribs, I could trim the original internal 3 ribs and replace the flange with a length of .032 angle?
 
Keep calm and build on. The hole in the rivet head can be filled and smoothed out to where you won't know the difference. Remember- K.I.S.S.
 
more than the appearance of the rivet head is my (unfounded?) concern of future leaking. How likely is it for fuel to weep between the rivet and the skin? I imagine most of the sealant would be forced out. I'm interested to know if anyone has any direct experience with using these rivets and whether the old way of building the tanks was more prone to leaking. Van changed the design for likely good reason. Was leaking part of that reason?
 
The self sealing rivets I believe are used to close the tanks up even in the new design, and they don't seem to be leaking. Dip the rivet in proseal and indont think you will have any issues.
 
Progress continues on my port wing. All wing skins are now fitted/drilled to the ribs and spars. Working on fitting up the tank now. I have decided to build my tanks with the forward facing rear baffle design. Ribs and baffle are all fit and in place and just playing with the spacer blocks between the forward spar doubler and baffle to get the ribs in the right fore/aft position. The skin is fitting pretty nicely so far with the leading edge in quite a straight line.

Port wing by James Soutar, on Flickr

a few questions.

1. skin gaps, I fit the leading edge skin to the main skins with no gap. I considered leaving a small (.016") gap for paint but thought I can easily add that if needed. Now I've been reading that gaps tend to close when riveting so I'm concerned there may be some binding.

2. I'm fitting the fuel tank skins and I know I need to leave a gap to make R&R easier. 1/32" seems to be the norm, some say more. Advice?

3. Edge rolling. I have the Cleaveland edge rolling tool and wondering if builders roll the joint line of the main/leading edge skins. If so, is this best done before dimpling the rivet lines?

4. I don't currently have a #19 pilot counter sink cutter so I planned to drill/cs the spar flange using a #30 then drill the hole #19 after countersinking. Does this sound reasonable? Any issues with doing it this way?
 
......

4. I don't currently have a #19 pilot counter sink cutter so I planned to drill/cs the spar flange using a #30 then drill the hole #19 after countersinking. Does this sound reasonable? Any issues with doing it this way?

Drill it #19 and add the countersink later when it's assembled.

The pilot portion of a #30 countersink will center nicely in the #8 nutplate.

Mount the tank with #8 round head screws and countersink one location at a time and replace with a flat head screw.
 
"Mount the tank with #8 round head screws and countersink one location at a time and replace with a flat head screw"

Do you mean to drill the spar #19, rivet the nut plates but not countersink the hole? That would mean the skin would have to not be dimpled until each screw location is countersunk. What is the reason behind this?
 
"Mount the tank with #8 round head screws and countersink one location at a time and replace with a flat head screw"

Do you mean to drill the spar #19, rivet the nut plates but not countersink the hole? That would mean the skin would have to not be dimpled until each screw location is countersunk. What is the reason behind this?

I was mentioning the forward row of screws that have countersinks in the tank skin/spar.

The aft row of screws with the dimpled skin gets the spar side countersunk before the tank is mounted. the same "use the nuplate to center the pilot" technique will work, just do it before tank mounting.
 
tankyou

after reading the fella from across the pond thread I read there has been 3 tank versions for the 4 tank. does any one know why?? is variant 1 better than 2 or is 3 better than 2. or is vans just keeping us lemmings on our toes in the puzzle factory. I haven't researched but are there any reports justifying the changes is really the question. I am making mistakes and going fourward. 4ward I mean. thanks vaf
 
I have repaired leaks in RV-4 tanks twice. Both were the oldest tank design, and I think this one is prone to leaking at the top of the baffle because of the lack of support.
The Tank Baffle Overlapping the Spar Flange version looks like better support for the fuel tank, so that the weight of the tank is less likely to peal the top tank skin away from the pro-sealed skin / baffle.
The tank I most recently repaired is leaking again now from the top, as it did before. It seeps when the tank is holding more than 11 gallons, leaving it's blue stain to indicate where to repair.
The RV-8 (and -7) use Z brackets along the back of the tank baffle, with plate nuts. These are bolted to the spar web, giving much better support to the tank.
 
I'm nearing the stage on my left wing where I'll have to start fitting the leading edge skin and assembling fuel tanks concurrently to match. I had been thinking I would modify the tank baffle and build the tanks as per my original plans but this means using AK-42H (sealed CS 3/32" pull rivet) to attach the wing skin to the baffle. This won't have quite as nice a finished look and I wonder whether these will leak over time. Since you can't put a dab of sealant on the tail after setting the rivet there seems like it would be a path for fuel to leak out between the rivet and the dimpled skin.

So two thoughts:

1. Keep calm and build on.

2. replace the tank ribs and build with a rear facing flange baffle. Alternatively to buying all new ribs, I could trim the original internal 3 ribs and replace the flange with a length of .032 angle?

Is that the correct rivet type?

http://fasteners.oemfast.com/item/all-categories/pop-closed-end-rivets/ak42h
 
My plans don't make mention of the forward row of #8 screw holes that attach to the forward spar doubler. Only the rear row of screws is shown as dimple countersunk so I assumed all #8 screw locations were dimpled. The newer plans for the -4 tank with the aft facing baffle (the one that slips over the forward spar doubler) does show to machine CS the forward row but in this case you have two layers of .032" material, the tank skin and the doubler. I will only have just the tank skin so machine countersinking is out of the question. In fact, I was playing with some scrap .032 tonight to test the depth required for machine CS for the AK-42H pull rivets (yes, I've double checked that) called out on my original plans and you simply can't machine CS for a 1/8" rivet in .032" Not sure why I thought I could. I think builders of this tank design back in the day must have dimpled all of the rivet holes for rear baffle. Must be a lot of fun installing that baffle with proseal.

As for the changes in design, I did call Vans and the change predated the gentleman I spoke to who had been there quite a long time. All he could say about why the design might have changed was that it was likely to make the tank easier to build... with that, I can certainly agree. It does however make sense that the tension on the upper row of rivets due to the weight of the fuel would serve to "peel" the two surfaces apart and could be more leak prone.

The trouble is that I don't have the option of buying new parts and building completely the new way because the forward spar doubler is the exact same height as the tank baffle so they don't nest together. My only option is a hybrid between the old and new designs. Gregg Brightwell did a fine job building his this way so I may have to consider ordering new parts.
 
4 tanks

I'm sizing up my phlog spars now and they are the older style . I haven't devoted much thought to it but I think there is a way to modify the t-402, or perhaps make a new t402 and put a step in the flange that was made long enough to slip inside the spar but I would have to test this theory first of course. pg 17a detail a shows what we do not have but offers the possibility of switching. I don't know yet of a "that's it" I will get back to you in a few weeks as its my busy season in san diego. stay cool, I will find a solution
 
I've decided to build the tank with the aft facing baffle (not shown yet below). This will allow the baffle to be installed in the position and method the new tank is built but it must be trimmed down to about 1/2" flange that will butt into the forward spar doubler. This means the tank attach screws will be dimple countersunk for -8 screws. The forward spar doubler (.040) needs to be either machine countersunk or dimpled to allow the tank skin dimple to nest into it. The trouble is in deciding between two methods.

1. Dimpling .040 is not something I have done and it's my understanding that .032 is the thickest alum that can be properly dimpled.

2. Machine countersinking the narrow 1/2" flange puts the screw hole very close to the edge. While there is a nutplate on the inside surface and the attach rivets are 1/4" from the edge, the screw hole leaves only 3/32" or so of alum. Is this an issue?

example:
Untitled by James Soutar, on Flickr

Options:

1. Dimple the doubler flange for -8 and clean up depth by machine countersink. This seems to me to be the strongest option.

2. Use -6 screws for the forward row of tank attach screws which would require smaller countersunk holes. Surely this affects the strength but is it still enough?

3. Machine CS the doubler flange and add a strip of .032-.040 to the inside of the doubler flange then attach the nut plates over that. This would give more depth for the -8 countersunk hole but the diameter of the hole in the doubler would be the same. Not sure I'm really accomplishing anything with this additional material.
 
Do you have any .040 scrap you can try it on? I'm pretty sure I've dimpled .040 in the past, but can't swear to it. The joint will obviously be stronger if you can dimple. Even a partial dimple + finish with the c/s would still leave more metal.

If you don't have any scrap to experiment on, PM me your email address & I'll drill/dimple some scrap & send you a pic.

Charlie
 
I think I do have some .040" somewhere. I'll try it and as you say, even if I have to partially c/s to finish it off it will be stronger.
 
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