5.5 aux tank wood floor

Fuel tanks, systems and parts
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Nathan K. Hammond
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5.5 aux tank wood floor

Post by Nathan K. Hammond » Mon Jun 27, 2011 13:04

Do you really need the wood plate that goes under the tank? When I mock up the tank install; the tank is sitting proud of the wood floor by about 1/8"-3/16". Plus the way the tank is designed, the top rest on the upper cap strip.

Any idea's?

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Re: 5.5 aux tank wood floor

Post by DELCOM » Mon Jun 27, 2011 15:50

Nate

I found the same while installing my wing tank. It was either install and shim with something so that the wood would support at least some of the tank or no wood. I finally decided that no wood was needed. My IA was happy doing it this way. Glad to see you really moving on it again.

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Re: 5.5 aux tank wood floor

Post by MikeB » Mon Jun 27, 2011 18:12

Ditto!! Although I left the wood in for whatever reason. It's a pretty convoluted STC. When putting the ribs in, make sure you leave enough room between ribs for the fuel tanks. I didn't and wound up having to move the second rib over a bit. Also, try to figure out some way to brace the butt rib on the bottom as it's necessary to remove the usual brace (don't have the number in front of me) and tightening the fabric will tend to move the bottom inward no matter how careful you try to be. Don't ask me how I know :oops: . I never did figure out a good way to install the fuel valves according to their instructions and wound up putting them on the top of the cabin where they usually are on the L16's and others with installed wing fuel tanks. Just makes more sense to me as you're shutting off the fuel closer to the tank (in case of a leak) if nothing else. Also, I intend to vent the caps with a 'vent tube' rather than relying on the cap vent.

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Re: 5.5 aux tank wood floor

Post by kyleb » Mon Jun 27, 2011 21:00

Nathan K. Hammond wrote:Do you really need the wood plate that goes under the tank? When I mock up the tank install; the tank is sitting proud of the wood floor by about 1/8"-3/16". Plus the way the tank is designed, the top rest on the upper cap strip.

Any idea's?

nkh
Nate,

Can you clarify what you mean by "the top rests on the upper cap strip"? Does the STC call for a cap strip on the inboard segment of the spars, and the front and back flanges from the tank rest on those? That's not how I visualized it, but I haven't looked at the paperwork in a while... I may have to do that tonight.

The board *might* be the solution to the issue Mike mentioned. It would transfer some of the fabric load from the butt rib to the second rib, stiffening things up a bit, but there must be a better way.

Kyle
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Re: 5.5 aux tank wood floor

Post by MikeB » Mon Jun 27, 2011 21:53

Kyle,
If I remember correctly there are brackets that fit around the spars and attach the front and rear of the fuel tank. In addition the fuel tank flanges fit over the butt rib and the next outboard rib. Actually the board transfers pressure from the butt rib to the next rib when tightening and tends to distort that rib 'slightly'. Some kind of a brace from the butt rib to the spar would help, Unfortunetely, there isn't room for one between the tank and the spar (as was original brace).

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Re: 5.5 aux tank wood floor

Post by Nathan K. Hammond » Mon Jun 27, 2011 22:23

Thanks guys. Think I'll leave it out; if it doesn't hold the tank, and doesn't keep the rib in shape, I can't see a reason for it.

Kyle.... here's the general idea. I don't have the "Z" braces in place but you can get the idea.

Image

Image

Image



This last pic you can see the gap between the tank and wood.

Image



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Re: 5.5 aux tank wood floor

Post by kyleb » Mon Jun 27, 2011 22:57

Mike & Nate,

Between Mike's post, Nate's pictures, and taking a look at the STC, I understand.

On the butt rib flex issue, I wonder if you could take a piece of aluminum angle and rivet it to the flange of the butt rib so the turned up angle on the flange rests against the inboard wall of the tank? That would brace the butt rib and keep it from flexing. It would also let you omit the board, which doesn't seem to have much of a purpose.
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Re: 5.5 aux tank wood floor

Post by Paul Agaliotis » Mon Jun 27, 2011 23:45

Citabria uses a couple of wood blocks screwed to the butt rib vertical separations.
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Re: 5.5 aux tank wood floor

Post by Nathan K. Hammond » Tue Jun 28, 2011 01:04

Paul Agaliotis wrote:Citabria uses a couple of wood blocks screwed to the butt rib vertical separations. Paul
That's what I'm thinking of doing.

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Re: 5.5 aux tank wood floor

Post by MikeB » Tue Jun 28, 2011 08:59

As mentioned I don't really see much of a purpose for the wood 'floor'. My problem is I tend to do stuff according to the STC (other than the fuel valve location) even when it doesn't make sense. Bracing the butt rib some way certainly makes sense and I'd be a lot of smarter :roll: next time.

You'd think with the number of these tanks kits they've sold over the years they would have considered some improvements and better instructions, but then maybe the STC certification process doesn't lend itself well to changes.

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Re: 5.5 aux tank wood floor

Post by Dennis » Tue Jun 28, 2011 11:36

Concerning the wood floor, I noticed the same gap. But, like Mike, I installed it anyway but agree that I can't see much purpose for it. I called Wag about it and was told a gap between the take and the bottom board was normal. So, why the heck is it there. Maybe to keep the correct spacing between the butt rib and the next inboard rib?

I also had a problem with rib spacing on one wing. I agree that you need to make sure you do your "set up" before covering. I thought this STC was a "cut fabric, install tank, plump and recover". I just do not see it as such. On one rib, after relocating it, I attached an "L" shaped support for the board.

Also, fit on the top side was not good. Thanks to Paul's advise, I used a body hammer to fit on the top side.

I also agree about the clearance from the tank to the bolt heads. Not much there plus, on the main spar mounting bracker, the bolts must be back to front.

I also agree about the piss poor location of the fuel valve. I attached mine to the upper front sissy bars with Adels. Looks nice and is functional plus its easy to see the position of the handle.

By STC, the new wing tank fuel lines are connected at the primer line. Good Lord, it would take all day to drain one wing tank into the header tank. So I welded an aluminum bung into bottom of each side of the header tank. Prior to welding the bung into place, I welded an internal 4" alum pipe to the bungs to make the incoming fuel exit the pipe above the bottom of the header tank. (Visualize a fountain inside the header tank.) That way, if I get a leak in the incoming fuel line, all the fuel in the header tank will not drain out.

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Re: 5.5 aux tank wood floor

Post by MikeB » Tue Jun 28, 2011 13:45

As long as we're beating a 'dead horse to death', as I mentioned before I had to move the outboard second rib over to get enough clearance for the tank to fit. Part of the problem was because of the weld overlap on the butt end of the tank which wouldn't let the tank sit down. I filed the weld down some but finally decided I needed to move the rib :evil: . Suppose I could have 'modified' the butt rib but thought it best to try to do things right. This worked "OK" at best because you can only move the rib so far due to the trailing edge fit. It just seems to me this system is not too well thought out and needs several trial runs like Nathan is doing before making the final installation. Dennis gave me several ideas when I was in the process of installing the tanks which certainly helped some but as an old 'technical writer' I think I could have written better instructions to make installation easier.

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Re: 5.5 aux tank wood floor

Post by Nathan K. Hammond » Fri Jul 01, 2011 23:35

Spoke with the IA and he nixed my idea to omit the wood floor. His reasoning; during a crash, it may keep the tank from coming out the bottom of the wing and rupturing. Now my thought is to shim the floor along the capstrip or fill the gap with felt. Maybe I'll just let it be.

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Re: 5.5 aux tank wood floor

Post by MikeB » Sat Jul 02, 2011 08:59

Guess that makes sense. As mechanics we tend to look at something and think "Why did they do that?". and if it doesn't pass the common sense test (in our minds) we want to make a modification. Anyway, try to figure out some way to prevent the bottom of the butt rib from bowing in as it will push against the board which will tend to bow the second rib. You'd think that big husky butt rib would stay straight but not the case.

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Re: 5.5 aux tank wood floor

Post by kyleb » Sat Jul 02, 2011 13:08

Nathan K. Hammond wrote:Spoke with the IA and he nixed my idea to omit the wood floor. His reasoning; during a crash, it may keep the tank from coming out the bottom of the wing and rupturing. Now my thought is to shim the floor along the capstrip or fill the gap with felt. Maybe I'll just let it be.

nkh
Won't the flanges that rest over the spars hold the tank? How were the original tanks supported? I know (actually, I *think* - which is dangerous) they didn't have a plywood floor under 'em, so what was preventing original wing tanks from departing through the bottom of the wing in a crash?

I tend to believe the floor is of dubious value, but it is part of the STC, so it is probably best to include it.

All that said, I'd like to see your installation before you cover the wings. If there is an opportunity, drop me a line...
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