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Fuel tank fwd support- one of life's little challenges :(

Desert Rat

Well Known Member
Well darn.

I bought a second hand almost completed wing kit to start my build journey. Fast forward a couple of years and I did the first temporary install of the wings on my fuselage.

Wings fit good, triangulated easily, incidence and sweep set, drilled/reamed the all important aft spar hole a few thousands undersize for a AN5 bolt. Life is good.

Today I started all the cleanup tasks you do after the wings are on, and very first thing when I started looking at locating the hole for the bolt that goes through the fwd tank attach bracket to the slotted steel flange coming off the fuselage, I realized that the guys I bought the wings from had helpfully pilot drilled a #30 hole in exactly the wrong spot on the starboard tank.

I can't center the bolt hole on this because it's way to far out of the slot for the bolt to give any meaningful support to the nose of the tank, and I can't use it for as a rivet hole for one leg of the nut plate because it's a #30 hole instead of a #40.

This last part is what really sucks because one leg of a k1000-428 nutplate lines up perfectly with it.

As I see it, my options are:

1- to upsize one leg of a nutplate for a #30 rivet, which is iffy because the call out is for a skinny nutplate, so that would put the big hole and little hole uncomfortably close together.

2- to clock the nutplate vertically instead of horizontally and fill the errant hole with a double flush rivet.

3- to pull the tank off, drill out the aluminum angle from the inner end and build a new one without the offending hole, with the pro seal mess and extra time involved.

4- vans suggestion (which they just came back with while I was typing this) fabricate a new bracket and extend the ears out from the fuselage a little bit more.

curious what opinions the collective might have...
 

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I'll take "what's behind door #4", Bob!

Well darn.

snip

4- vans suggestion (which they just came back with while I was typing this) fabricate a new bracket and extend the ears out from the fuselage a little bit more.

curious what opinions the collective might have...
 
#2 or #4 depending on whether you can get angle stock that is wide enough.
If doing #2 I think you can fill the hole with a normally set universal or countersunk rivet. It doesn't need to be double flush because there is no interference.
 
Attach bracket

Door #4.
Easy peasy.
Go to a metal supply and buy a piece of the same thickness and same alloy. Ask them to bend it where you need. They might charge a few bucks but it's worth it. They have the tools.
All you have to do is drill and cut.
 
Just to clarify, if we're talking about extending the ears, it's not angle stock. It's a bent piece of 4130 steel.

I guess my reservation about that solution is that the bolt imparts a vertical load on what's essentially a pivot point because that bolt doesn't get torqued down to create a lot of clamping force. If you make the fuselage bracket longer, even by a little bit, it's creating more of a bending moment into the fuselage structure. I realize it's not much, but it's something.

I don't know what sort of loads are carried up there, but it seems like it has to be significant, because the structure and hardware is pretty beefy.
 
Ah OK! I assumed it was made from angle stock, same as the RV-6. I've recently cut those parts from 2.5" x 2" x 3/16" aluminum angle.

Honestly I don't see why #2 would not work just fine.
 
#4

Ah OK! I assumed it was made from angle stock, same as the RV-6. I've recently cut those parts from 2.5" x 2" x 3/16" aluminum angle.

Honestly I don't see why #2 would not work just fine.

Honestly, if Vans said to do it a certain way, I print the e-mail, file it and go. Why question it?
 
Honestly, if Vans said to do it a certain way, I print the e-mail, file it and go. Why question it?

Maybe Vans did not think of all the options? If they said to NOT do something, then of course. There have been many many ideas from builders over the years that Vans have incorporated (generally) and I regularly (constructively) question Vans drawings because there can be errors and elements that will just not work as drawn.
 
Ah OK! I assumed it was made from angle stock, same as the RV-6. I've recently cut those parts from 2.5" x 2" x 3/16" aluminum angle.

Honestly I don't see why #2 would not work just fine.

Yep. Agree. Mine is a -6 with the aluminum angle, so different. No reason not to fill the errant hole with the double flush and orient the nutplate north to south. Easy to do. Make sue you drill that new hole correctly, or you’ll be using option #4. Remember, that bracket is loaded in vertical shear, protecting the positive and negative G loads on a full tank of fuel. Lateral shear loads should be close to zero, so don’t tighten that bolt too much.
 
You'll have to check out our RVator Builders Group here in Wichita. We meet the 3rd Saturday of every month. Look for meeting notices on Facebook-
AirCapital RVators
 
If you make the fuselage bracket longer, even by a little bit, it's creating more of a bending moment into the fuselage structure. I realize it's not much, but it's something.

To answer this concern, some images of the structural arrangement here would be helpful. Would be good to see both the mounting bracket and its backup structure. Actual pics or snippets of the drawing. I doubt it is a problem, but lets see to be sure.
 
If you reorient the nutplate and drill another hole, what will the edge distance be to the existing hole (once the new one is drilled or reamed to final size for the bolt)? Looks awfully close to me...

I'd go with #4. Easiest and cleanest solution, IMO. Not to mention, it's what the Van's engineers recommended.
 
Option #5

This may be another option to consider: Rivet the plate nut to a round .025 doubler and then rivet the doubler to the bracket using the errant hole and another new hole on the other side. Clock the holes as required.
 
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I dunno, whenever I ask [a lawyer/a surgeon/a tax professional/Van's aeronautical engineers/any expert] what I should do, the very next thing I do is NOT do that, go to the internet, and get every opinion under the sun until I find one that I like because it's [easy/cheap/looks about right], and then I do that.

After all, they're just "so-called 'experts'", right? What do they know?
 
Hey guys-

After sleeping on it, I've decided to go with van's suggestion. Builder support confirmed with me this morning that an additional small (around .25" ) lever arm of extending the ears and notch on a new steel bracket outboard to match the existing erroneous pilot hole is not a factor worth considering.

As far as "easiest" The easiest and cheapest solution by far would have been to just drill out 1 ear of the nutplate to #30, use the existing hole to mount it and move the whole nutplate inboard by that amount. unfortunately, I couldn't get past the idea that I was unsure of the loads carried thru that area and vans felt the better option was to extend the steel bracket a bit.

Discovered that there's now a metal supermarket here in Wichita, so I called them up and am having them cut an oversized blank with a nice bend for the flange that I can final size here at home. Should be ready in a couple of days and we can put this one to bed.

For those that asked for pictures, I guess that I thought what's on the RV7 was a typical enough arrangement that everybody would know what I was talking about, but I'll try to explain;

I've already pulled the bracket off the offending side, but here's a couple of pictures of the other side to show how it goes together.

The steel bracket, primer yellow in the photo, is held on the fuselage via 2 AN3 bolts thru the skin into a beefy angle associated with one of the vertical members in the footwell area. The aluminum tab coming off the tank is fabricated from 1/8" angle that rivets thru the root rib nose into a stiffener plate inside the tank.

The tank side tab gets a nutplate on the front side for a AN4 bolt and the bolt goes in from the back thru the slot. It takes a big washer and only gets lightly snugged down, not torqued.

The idea is that it fully supports the inboard nose of the tank in the vertical vector, but if you hit something and bend the wing back, the tank side (bolt) can slide out of the groove on the fuselage side without tearing the end rib out of the tank, thus spewing fuel everywhere.

It's a clever design, but you can see how it gets complicated fast if everything isn't where it's supposed to be.

Thanks for all the input. It's always a pleasure to work through stuff like this with you guys.
 

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I dunno, whenever I ask [a lawyer/a surgeon/a tax professional/Van's aeronautical engineers/any expert] what I should do, the very next thing I do is NOT do that, go to the internet, and get every opinion under the sun until I find one that I like because it's [easy/cheap/looks about right], and then I do that.

After all, they're just "so-called 'experts'", right? What do they know?

pump the brakes there for a minute. I get your frustration at seeing what you describe, but this is not that.

1st of all, the Van's tech support guys are great at what they do, but it's not like they have some magical power to always be right with off the cuff answers. It seems likely that while they are going to give you a solution that they know will work, it doesn't guarantee that it's the only solution or even the best one. Just that it's one they have in their toolbox that they can hand to you efficiently and promptly.

As far as the Aeronautical Engineers- I have no doubt that tech support could have kicked this upstairs to engineering who could have then spent a week modeling this and figuring out if I could move one nutplate by 1/8" Is it reasonable to ask them to do that? Of course not.

They are devoting resources to my question as appropriate to the significance of the deviation. I'm good with that. I've explored other options and have elected to go with their suggestion because as I have previously said, there's obviously a lot going on in this little corner of the airplane and I don't see another solution that GUARANTEES the same design margins.

This isn't because any of them are obviously bad suggestions, but rather because I lack the resources to verify design margins at my home garage/airplane factory.

As far as the rest of your comment. I'm not offended by what you said, and welcome your input. I've seen the same uncomfortable tendency for people to sometimes keep asking until they get an answer they like from somebody who may or may not be qualified to give it. But, I've been in the aerospace business in one way or the other for over 30 years now and want to make it crystal clear that I'm not one to take the easy way out over the right way. Hence my lengthy reply.
 
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pump the brakes there for a minute. I get your frustration at seeing what you describe, but this is not that.

1st of all, the Van's tech support guys are great at what they do, but it's not like they have some magical power to always be right with off the cuff answers. It seems likely that while they are going to give you a solution that they know will work, it doesn't guarantee that it's the only solution or even the best one. Just that it's one they have in their toolbox that they can hand to you efficiently and promptly.

As far as the Aeronautical Engineers- I have no doubt that tech support could have kicked this upstairs to engineering who could have then spent a week modeling this and figuring out if I could move one nutplate by 1/8" Is it reasonable to ask them to do that? Of course not.

They are devoting resources to my question as appropriate to the significance of the deviation. I'm good with that. I've explored other options and have elected to go with their suggestion because as I have previously said, there's obviously a lot going on in this little corner of the airplane and I don't see another solution that GUARANTEES the same design margins.

This isn't because any of them are obviously bad suggestions, but rather because I lack the resources to verify design margins at my home garage/airplane factory.

As far as the rest of your comment. I'm not offended by what you said, and welcome your input. I've seen the same uncomfortable tendency for people to sometimes keep asking until they get an answer they like from somebody who may or may not be qualified to give it. But, I've been in the aerospace business in one way or the other for over 30 years now and want to make it crystal clear that I'm not one to take the easy way out over the right way. Hence my lengthy reply.

I appreciate your response, and I didn't mean to imply that you, personally, were avoiding adopting the solution provided by Van's and purposefully looking for an alternative for any reason, only that I've seen it over and over and over here and elsewhere. And thanks for the pleasant response, I realize what I wrote could seem harsh; I just don't want to see anyone "blow off" answers they don't like, implement a (worse) solution because they like it and TLAR, and end up in a world of hurt when things go bad.

Like you, I've been in aerospace for over 30 years now, so I tend to be *very* conservative in solutions, analysis, etc. Glad you're of the right mind, to do the jobs right, and not just the easiest or cheapest way!

Best of luck as you proceed...let us know how it turns out!
 
Likewise. I 100% agree that especially on the inter web sometimes the intent of a comment and how it's taken don't always mesh.

I also have concerns when I see someone tinkering around with something when they may or may not understand the downstream ramifications. Especially if it leads to "so and so did it, so it must be okay."

thanks for keeping me honest in my comments and communications.

Terry
 
That fuselage bracket is STEEL? On the -8, it’s aluminum.. are you sure it’s steel? Is that confirmed that it’s supposed to be steel in the plans? Seems odd to me..
 
That fuselage bracket is STEEL? On the -8, it’s aluminum.. are you sure it’s steel? Is that confirmed that it’s supposed to be steel in the plans? Seems odd to me..

The bracket on the -7/9 looks wider than the one on the -6 (2.5"), which the builder also fabricates from raw aluminum angle. The wider bracket is presumably steel because angle stock that wide is not available, or not strong enough. It also has a big hole in it for the vent line to pass through!
 
That fuselage bracket is STEEL? On the -8, it’s aluminum.. are you sure it’s steel? Is that confirmed that it’s supposed to be steel in the plans? Seems odd to me..

It's a vans part, not something you crank out in the garage. it's 4130 steel 0.063 thick. The gap between the tank end rib and the side of the fuselage is quite large on the 7. The long leg of the bracket is about 3 7/8"
 
Work effort

It will be interesting to see how much effort it takes to make and fit the new part. Hopefully you will get it right on the first go!
 
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