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Thread: Homebuilt Fuel Tank ("Cell") Design and Fabrication

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    Registered User SVSUSteve's Avatar
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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    Quote Originally Posted by bmcj View Post
    Not as likely in this country. If they call it a medical cause, then it's all due to pilot incapacitation, whereas if they say it was the plane or some other external source, then they have an avenue to a lawsuit.
    Well, it happens all the time in the US. Probably at least half of the crashes that I deal with, I hear the family say something like "Dad was such a great pilot. He never would have stalled/flown into a mountain/etc" and start looking for an excuse. Often this message is put forth by the family's lawyer especially if the case involved non-family passengers to minimize legal liability. You put those two together (perceived liability and not wanting to admit that your dad/brother/mother/sister was, as a point of fact, a marginal pilot) and you hear this a lot more often than one would assume.

    In fact, in one deposition I gave for a case, I spent the better part of three hours being grilled over whether I could tell if the injuries to the pilot were inflicted peri-mortem or post-mortem. The defense was trying to argue that the pilot was already dead or close to it when all of the evidence (presented by two pathologists and myself) said he was as alive as you or I at the time of impact. This is one reason why I have come to despise most liability and personal injury attorneys.

    Even in an over-gross situation as you described, a good lawyer can spin a jury in the plaintiff's favor.
    Most judges honestly put their foot down on crap like that. The public only hears about the few times you get a judge with his or her head up their ass. Usually those are quickly reversed on appeal.
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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    I apologize for the minor thread drift, but on the subject of fuel tank safety considerations...

    Does anyone have an opinion on the Aeroquip breakaway fuel hose fittings?

    http://www.herberaircraft.com/pdf/Ot...ng Catalog.pdf

    They are light in weight, and combined with the recommended AE701 flexible fuel hoses with the stainless braided outer cover allows flexibility in case of structural deformation during a crash. This should go a long way toward preventing or at least drastically reducing fuel spillage during a crash. This does not however help if the fuel tanks themselves rupture!

    Using these fittings and hoses would make it a little safer though for a situation with fuel in the wings that must have fuel lines routed through the cockpit, correct?

    George

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    Registered User SVSUSteve's Avatar
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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    Quote Originally Posted by gschuld
    They are light in weight, and combined with the recommended AE701 flexible fuel hoses with the stainless braided outer cover allows flexibility in case of structural deformation during a crash. This should go a long way toward preventing or at least drastically reducing fuel spillage during a crash. This does not however help if the fuel tanks themselves rupture!
    Aeroquip has a good reputation with regards to fuel lines. I have never heard anything about their breakaway fuel valves (although these may be the valves currently called "Stratoflex" by Eaton since the catalog you posted appears to be rather old) but if they are of similar quality, I would definitely consider them.

    Quote Originally Posted by gschuld
    Using these fittings and hoses would make it a little safer though for a situation with fuel in the wings that must have fuel lines routed through the cockpit, correct?
    Right, but I would still put some structure around the lines to shield them further if absolutely necessary to route them through the cockpit (which is seldom necessary for anything other than convenience).
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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    I kinda like the idea of an integral fuel tank located between the fwd and aft spars, outboard of the fuselage at least three feet.

    The tank to be lined with a single ply of Kevlar cloth and an elastomeric material like Proseal. This liner would serve the purpose of preventing or at least restricting the flow of fuel from the tank in the event of a rupture of the aluminum tank itself.
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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    Quote Originally Posted by GESchwarz View Post
    I kinda like the idea of an integral fuel tank located between the fwd and aft spars, outboard of the fuselage at least three feet.

    The tank to be lined with a single ply of Kevlar cloth and an elastomeric material like Proseal. This liner would serve the purpose of preventing or at least restricting the flow of fuel from the tank in the event of a rupture of the aluminum tank itself.
    Do you mean "integral" to the wing (e.g the forward wall of the tank is the main spar, the rear wall of the tank is the rear spar) or integral to itself? Those parts are going to flex and bend a lot in a crash, I'd think you'd significantly reduce the chance of fuel leakage with a tank that was independent from the aircraft structure.

    Kevlar--I like the strength a lot, but wonder how it will hold up to fuel, and if it is compatible with Proseal. It is absorbent stuff. Even Kevlar body armor has to be replaced regularly due to contamination/weakening from sweat, body oils, etc. If you aren't going to have the inner layer be a true bladder (i.e. reduce in size as fuel is used in order to reduce the volume of flammable vapors in the tank), maybe consider putting the Kevlar/Proseal layer outside the AL tank to serve as the container in the event the AL tank is compromised.

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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    What about building a tank using Kevlar or carbon fiber with a flexible resin? That would make the tank flexible and crush/cut/burst resistant. The problem is figuring out what resin to use.

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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    What about building a tank using Kevlar or carbon fiber with a flexible resin? That would make the tank flexible and crush/cut/burst resistant. The problem is figuring out what resin to use.
    I would give Kevlar preference for the middle layers (to give "structure" to the tank for ease of handling) but on the inner and outer layers, I would use mil-spec 1050 denier double weave ballistic nylon as the primary fabric for cut or burst resistance. As for "resins", on the outer and inner (penetration and rupture prevention) layers it might be plausible to use an urethane elastomer similar to the sealants used for more traditional fuel tanks.

    Most "resins" as we think of them in composite aircraft is not the best idea if you're trying to produce a flexible tank. They are simply too rigid and prone to more or less brittle fracture or delamination. The other issue is whether you really want a truly 'flexible tank'. You really have to find a happy medium between a glorified water balloon (very flexible) and a rigid tank as we traditionally think of fuel storage. If you make the tank too flexible, it poses a greater risk of hydraulic blowout opposite the point of impact not to mention it would be a pain in the butt to install in the aircraft. We built a "bag" style tank of Kevlar, ballistic nylon and a for drop testing and it ruptured after a 15 ft drop and gave us a 60+ ft geyser from the tank's contents (in this case, it was water dyed to make it more visible). I should also point out that it was a major pain to hoist it and move it for the drop, akin to moving a 20 gallon lightly reinforced water balloon (or a two-man tent filled with water with the frame attached). A design using a more rigid design but built to crumple without rupturing was dropped from a height that exceeded even the US Army's somewhat rigorous drop testing standards and some of the more "inventive" torture tests we could come up with.
    Last edited by SVSUSteve; November 24th, 2012 at 02:21 PM. Reason: Typo
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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    Quote Originally Posted by SVSUSteve View Post
    We built a "bag" style tank of Kevlar, ballistic nylon and a for drop testing and it ruptured after a 15 ft drop and gave us a 60+ ft geyser from the tank's contents (in this case, it was water dyed to make it more visible).
    Way off topic but this reminded me to ask:

    Are you recording these tests with high speed camera? If so I'd be interested in what hardware you are using. I'm looking for an adequate slow motion digital camera for a reasonable (translation - as cheap as I can get it) price to record some similar testing.
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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    Are you recording these tests with high speed camera? If so I'd be interested in what hardware you are using. I'm looking for an adequate slow motion digital camera for a reasonable (translation - as cheap as I can get it) price to record some similar testing.
    We have recorded them but I don't know what camera was used specifically. An engineering student friend of mine (actually the husband of an ex-girlfriend I have remained friends with) provided the cameras and operated them. From the way he described them, it sounds like they were the property of the department he is affiliated with and he simply kissed the ass of the right folks to get permission to use them. I keep telling him he needs to kiss up some more to get me access to a wind tunnel (even if I have to nominally make the Praetorian's aerodynamic testing a student's project).

    However, I do believe one can rent the sort of technology you are looking for. That would honestly probably be the cheapest way to obtain this sort of thing under normal conditions. Another option might be looking into local colleges or universities with a film or audiovisual program to see if they might be willing to partner with you in exchange for the "hands on" experience for their students. Often they have some really nice technology at hand.

    You would be amazed what you can get access to simply by asking nicely in a lot of cases. Our two series of drop tests were conducted using either a crane or a fire department ladder truck. In the former case, the fact that we wanted to break something ended up with the company agreeing to do it for a minimal fee and the cost of a good lunch so long as we agreed to do the test on their property (to prevent the need to drive the crane elsewhere, thereby saving fuel costs). The volunteer fire department offered their services simply for a small donation ($100) and a lunch of pizza from one of the better places in the area. The announcement that we were going to break stuff for the test and that there would be food present resulted in us having all the manpower and equipment we could need.
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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    Ok, my last response to this topic. Just few things.

    First, when I asked about size of your fuel tank, it wasn't actually only about crash safety issue (as it's not problem at all, if designed correctly) I don't know how many changes do you want to do compared to the original design. But for such a small airplane like Sone*x you may or may not have a problem with your C.G, if you decide to mount the fuel tank behind your back rather that in front of you, especially if it was designed that way. Of course you could redesign it... This is another reason, why the best place for the fuel is in the wings, as close to C.G as possible. For some ideas take a look at Midget Mustang (later they introduced also wet wing option), and Rv's family, but all those planes have fuel tanks front to the spar.

    Resins, ethanol, etc.

    I really don't know why someone would like to use epoxies, for their separate fuel tank instead of Vinyl esters resins. I could understand it, if someone was going to build composite wet wings, all those issues with second bonding of Vinyl esters, etc. but for separate fuel tanks, I really don't get it. Vinyl ester resins are much better for chemical applications, than epoxies (unless we are talking about exotics one, that I haven't heard of), not only because I'm saying it, Orion was saying it also.
    Quote Originally Posted by Orion
    In my opinion, the best resin choice for fiberglass fuel tanks is Vinylester.
    If it's still not enough, take a look at those specifications, (ethanol page 13) DERAKANE Chemical Resistance guide pdf free ebook download from k.b5z.net

    One of the best resin, that you can actually get is Derakane 470-300. There were some issues earlier with vinylester resins combined with carbon fibers etc. But it seems, that there is not a big problem any more (based on Orion post).

    Some papers
    Mechanical behavior and damage evolution in E-glass vinyl ester and carbon composites subjected to static and blast loads

    there are also methods of additional surface preparation

    CARBON FIBER-VINYL ESTER INTERFACIAL ADHESION IMPROVEMENT BY THE USE OF A REACTIVE EPOXY COATING

    etc

    Another advantage of VE is HDT (high distortion temperature), even for room cured resin, for some +100 C deg. So in short, if you put a composite fuel cell in an aluminum wing, you could paint that wing with whatever color, without worrying that it might become really hot inside of it (wing).

    Of course it's really depend from what type of fuel you are going to use. Someone might say, epoxy resin and proper sealant. But even good sealant can't give you guarantee. Take a look here, Auto fuel troubles I've seen more cases like that, but unfortunately forum I was seeing it, is gone, so I can't give links for it.

    Just in case if you decide to build ballistic proof tanks Woven fabrics for ballistic protection

    Testing.

    Assuming that you are in very early stage at the moment (still deciding what to build), probably I won't be far from true, if I say that you won't fly your airplane in the next 4-5 years. Make tests now. Call guys at Ashland (derakane producers), ask them what is the best option for your application. Buy samples of resin (even here I can buy 1kg without any problem at all). Make specimens, and put them into a jar (turn jar upside down, to be sure is properly sealed, and nothing is going to vapor quickly) with different petrols (you could replace petrol every couple months or so, to make those test even more realistic). If you make samples big enough, you could cut them in half, put one half into jar with petrol, other into for example a cardboard box (hide it from UV). And compare it after a year, two, etc. Thanks to that you will have first hand experience, even before making molds for your tank.

    It really amaze me, that some people here and not only here first builds things next, test them. Personally I have two rules, first I always calculate/test everything before, not after. Thanks to that I'm able to build things cheaper and better, just recent tool (3 shafts [4140 heat treated] driven system, 1.1KW, 340-350 kg, tube roller bender, 25-30 pages of hand calculations and some FEA) that I had to build for myself. Total cost of completed tool with cover, painted, etc 1700-1800 euro, for very same (slightly thinner shafts) tool I would have to pay here 5000 euro, and just for comparison local guys asked me about 1000 euro, just for bending a couple of pipes, I laughed straight at them.
    Second, well I'll keep for myself.

    Seb
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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    Just in case if you decide to build ballistic proof tanks Woven fabrics for ballistic protection
    LOL Don't tempt me. Although, impregnating ballistic fabrics often severely reduces or complete removes their ballistic protective properties.

    That said we've joked about making some more tank wall samples in a year or so (once we have a little more time to spare) and using them for target practice with a .40 S&W chambered pistol just to see how well they would perform (to use the American colloquial expression "just for ****s and giggles"). Given some of the properties with regards to puncture even by a sharp object of the elastomer we used in our current design, I would honestly not be shocked if the bullets would not penetrate.
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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    Quote Originally Posted by SVSUSteve View Post
    LOL Don't tempt me.
    It's not as crazy as it might seems, have you seen this thread Bullet Holes

    Quote Originally Posted by SVSUSteve View Post
    Although, impregnating ballistic fabrics often severely reduces or complete removes their ballistic protective properties.
    Do you have any paper on it? I've seen something contrary to that.

    This study demonstrates that the ballistic penetration resistance of kevlar, nylon, and ramie fabrics is enhanced by impregnation of the fabric with the compatible resin. Impregnated resin fabrics composites are shown to provide superior ballistic protection as compared with samples stacks of neat fabrics.
    source http://isjd.pdii.lipi.go.id/admin/ju..._0126-1533.pdf

    I have more papers that claims same.

    Seb
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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    20 Litre Jerry Can | Liquid Containment Bladders & Liners – Fuel, Water, Petrol, Diesel, Water Storage
    http://www.safebladders.net/products.html


    tons of links. a wet wing has really its issues due to the flexing. the area where the rubber bladder is located i would laminate with thin weave kevlar to avoid the carbon to puncture the bladder
    Last edited by berridos; November 25th, 2012 at 06:44 AM.

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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    Kevlar is a downright bad idea for any tank. Fuel will always creep through and trash any organic fiber (like Kevlar) in a short while, carbon and glass will do just fine there.

    For energy absorption, "brittleness" or "rigidity" are meaningless terms. What is relevant is how much energy (impact due to crash loads) you can absord. A corrugated tank with carbon/epoxy is pretty optimal for that, energy absorption per pound. Rubber and similar materials are absolutely horrible at absorbing energy.
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    Re: Homebuilt Fuel Tank ("Cell") Design and Fabrication

    I have more papers that claims same.
    I was going with what I was told by friends who work for the US Army's soldier protection lab. They are the guys that mess around with body armor and vehicle protection so I would assume they know a thing or two about it. It depends largely on what you are impregnating the fabric with and according to the folks I talked to most of the epoxies we use traditionally in homebuilt aircraft it is likely you'll reduce the penetration resistance because you're hindering the ability of the fabric to respond dynamically ("flex") to the impact. However, I am always willing to have my mind changed so thanks for the paper!

    A corrugated tank with carbon/epoxy is pretty optimal for that, energy absorption per pound.
    Go take a look at some of the TU-Delfft studies on sine-wave/corrugated beams for energy absorption and think about how that would behave for a tank. It fails catastrophically and would probably not be a great idea for a wall where you want to keep something in or out. For a subfloor, it's sometimes hard to beat.

    Kevlar is a downright bad idea for any tank. Fuel will always creep through and trash any organic fiber (like Kevlar) in a short while, carbon and glass will do just fine there.
    I would argue that S-glass and ballistic nylon would be a much better choice than CF. In a fuel tank you're not looking so much for energy absorption as much as you are trying to minimize anything penetrating it or tearing it.
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