Fuel tank question

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Pops

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I have built 6 fiberglass fuel tanks in the past. I used fiber glass on each side of a 1/2 urethane foam core.
Question. What is the pro's and con's of using CF instead of the heavier fiberglass ?
I have been very happy with the FG tanks except for the weight. I know I can make a lighter weight tank from CB, but what are the negatives other than cost?
 

Geraldc

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I used kevlar and fibreglass.Here the Kevlar is the same price here as Carbon but is better for impact.
Or you could use Kevlar and Carbon fibre or Kevlar and Fibreglass.
 

Pops

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I'm looking for the lightest weigh possible for a small 8 gal fuel tank. Lighter than if I welded one from aluminum.
Trying to same every ounce that I can.
One thing that I like about a fiberglass tank is being able to see the fuel in the tank as your fuel tank gauge like in the picture.

In the past I have made a couple of fiberglass/Kevlar canoes .
 

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Pops

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Can see the fuel level and defects through glass. I think Kevlar is best in a crash,
Yes, that is the reason you use a layer of Kevlar in a canoe. I used to do whitewater canoeing and the Kevlar is better in keeping the canoe together in one piece when hitting a large rock. But , then I went to the lighter aluminum canoe. Everything is a compromise.

With a small fuel tank of 8 gal or less, the weight of the fuel is less and would help in not having a ruptured fuel tank in a crash. In the past I used 2 layers of 6 ounce FG on the outside of the foam and on the inside of the foam. Believe I could reduce the foam to 1/4" instead of the 1/2" thickness for such a small fuel tank.
Your thoughts ?
 
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blane.c

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I'm just asking the question, isn't it common to make fiberglass fuel tanks out of one of the "Lestors" resin and isn't carbon fiber allergic to the "Lestors"? So can you use a "epoxy" resin that is fuel proof?
 

Geraldc

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You can leave a window in the kevlar with glass .Like on the old Ducati 750ss.
 

Voidhawk9

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So can you use a "epoxy" resin that is fuel proof?
The great majority of the Rutan canards and I assume many other composite types do. The key seems to be a good post-cure; if the epoxy isn't fully cured it is vulnerable. Blowing warm air into the tank to raise the temp for an adequate period will do the trick.
There are also some epoxies that seem to be more forgiving when used in fuel tanks, I believe Ez-87 is a favourite in this case.
 

Pops

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The great majority of the Rutan canards and I assume many other composite types do. The key seems to be a good post-cure; if the epoxy isn't fully cured it is vulnerable. Blowing warm air into the tank to raise the temp for an adequate period will do the trick.
There are also some epoxies that seem to be more forgiving when used in fuel tanks, I believe Ez-87 is a favourite in this case.
I use EZ-87 for making my FG Tanks.
 

wsimpso1

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I designed my fiberglass 9.5 gallon tank using 8.8 oz BID cloth, 1/4" foam, and baffles to meet 14 psi pressurization (for failure mode management - scenario is transfer pump can make 7 psi, return line to wings becomes blocked, tank is pressurized to 7 psi, FOS of 2 requires 14 psi). Baffles were the light way in my tank to make the structure work in this scenario. Your mileage will vary.

Graphite and 1/4" foam should be fine for structure behavior. Going to 1/4 foam will save somewhere around 0.4 pounds. I am guessing you are either going to lighter cloth than common 6 ounce cloth or going single ply for this tank or you won't save any weight in the facings. Vacuum bagging any of it? This is turning into knowledge transfer in the opposite direction from your intent - my apologies on that.

My plan for my tank is to male mold with an open wet layup against the mold and cure for a fuel proof layer, then vacuum bag the cores and outside skin in an attempt at minimizing resin weight. Then destroy the mold to free the tank, tape in vacuum bagged baffles, flange transfer for the lid, and bond on a lid made the same way. Your thoughts on this fabrication? Will it seal OK, or should I go open wet on outside too?

PM me if you want to talk details on fabrication and an opportunity for me to visit you shop.

Billski
 
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TFF

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I would be worried about pinholes with a single layer of cloth on the inside. I would think two layers of 3.5 oz, at least on the inside would give the best chance of sealing. Maybe one 3.5 is strong enough with a face of 1 oz glass. That would have to be determined. Outside with one 6 oz. What is the weight estimate for the aluminum tank?
 

Pops

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The last FG tank that I built as in the pictures of the SSSC I didn't use any foam. I used FG only . Also, I didn't vacuum bag it. I made the parts of the tank on a plate of glass and made a resin rich layup and covered the top with piece of plywood and weights. Worked good. Except for a slight leak from a trapped air bubble on the edge of the bottom of the left tank. Made the patch and no more problems. The top of the FG tank was the structure top of the wing. I didn't paint the top of the tank so light would make it easier to see the fuel level.
After flying with these wings for a few years, I made a new set of long range with with 2-- 8 gal aluminum fuel tanks. The first set of wings just had 2- 4 gal wing tanks. I also increased the strength of the wings for the extra weight of the fuel. These changes increased the EW of the aircraft 20 lbs. From 450 lbs to 485 lbs, with the other weight increase of the change from 5" wheels and tires to larger 6" wheels and 800x 6 tires and also adding the die springs in the LG strut over the bungees.


I keep saying no more airplane projects. But, it going to be hard not to build something after the JMR is flying. Dallas and I have been selling most of our larger tools, ( 4' Rolls, 4' and 8' brakes, shears, etc ) Still have a 12" and 4' shear and a 12" and 30" brake and a 30" bar folder.
I have never stopped building or restoring airplanes since I started building the KR-2 in I think, 1974. Dallas is about the same, I think he started building a Baby Ace about the same time. Both just getting old and tired. My mind says build and my body says take a nap :)
 
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