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fulcona

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I’m building a fuel tank from quarter inch foam with vinyl ester resin on the inside (a tip from this forum) and E Z Poxy on the outside ( because I already have it). I’m hoping the “sandwich” along with the baffles will give me good ridgity and strength. A couple of places will be glass to glass. (Fuel cap area). My question is... will vinyl ester resin bond to cured E Z Poxy. (And vice versa) ? Thanks
 

Aerowerx

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Ask the manufacturers?

But done mention anything about aviation. If they ask, say its for a custom built off-road vehicle, which wouldn't exactly be lying would it?:)
 

BoKu

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I’m building a fuel tank from quarter inch foam with vinyl ester resin on the inside (a tip from this forum) and E Z Poxy on the outside ( because I already have it). I’m hoping the “sandwich” along with the baffles will give me good ridgity and strength. A couple of places will be glass to glass. (Fuel cap area). My question is... will vinyl ester resin bond to cured E Z Poxy. (And vice versa) ? Thanks
My experience is that polyester and vinylester don't bond very well to cured epoxy. Epoxy seems to bond better to cured vinylester and polyester. I'd suggest doing a couple of test coupons and proceeding accordingly.

As usual, good bondline prep is key. We usually scuff 60 grit until all the shiny is gone, clean away all sanding residue, and then wipe with acetone to degrease.

--Bob K.
 

wsimpso1

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Listen to Bob.

I like the idea of running all four permutations (E+E, E+V, V+E, and V+V), that way you get a good idea of how much strength you could have versus how much you choose to have.

What is making you concerned about rigidity and strength?

Billski
 

lr27

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Is there something thin that both bond well to that you could put in between?

Might be best to make the thin portion entirely of vinyl ester and overlap it onto the foam. The epoxy part could overlap it on the foam, where the bond wouldn't need to be as strong.

Are you going to add something to the vinylester and the epoxy to make them very slightly conductive? Might make things safer.
 

TFF

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Why not do inside and out in vinyl ester. Double containment but it can be bonded in place with epoxy. Not unless the outside epoxy is meant to be part of some other main structure.
 

fulcona

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Will the outer surface of the tank be structurally bonded to the airframe?


BJC
The tank will have wooden beams glassed onto the tank exterior. These beams will be bolted to the airframe.
 

fulcona

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Listen to Bob.

I like the idea of running all four permutations (E+E, E+V, V+E, and V+V), that way you get a good idea of how much strength you could have versus how much you choose to have.

What is making you concerned about rigidity and strength?

Billski
My only concern is that I want it to be strong. Am I missing something?
 

fulcona

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Why not do inside and out in vinyl ester. Double containment but it can be bonded in place with epoxy. Not unless the outside epoxy is meant to be part of some other main structure.
Yeah I’m leaning in the direction of using the same epoxy on both sides. Just wasn’t sure how the E Z Poxy would hold up to possible ethanol contamination. I was recently informed of coating products that would help this issue.
 

wsimpso1

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My only concern is that I want it to be strong. Am I missing something?
I did not set out to challenge you on the design of the tank. I did reply with the best answer I could about compatibility of epoxy and vinylester resin. You pressed my hot button on "strong" and designing stuff for airplanes. "Strong" and "Stiff" are not "Specifications". They are like Hot and Cold without using the temperature of the item and putting it in context.

Everything you put in an airplane should be strong enough to stand the loads put on it. Sometimes that is defined by flight g's or survivable limit crash g's or something else and the things that put loads on it under those g's. Making something strong enough without making it too heavy is a balancing act. In airplanes, Weight is the Enemy, so let's not make it much stronger than it needs to be.

Some items may be strong enough for primary loads but they vibrate excessively becoming likely for failures or not functioning correctly - in those cases, stiffness may be inadequate. If the item is not in resonance but you need less vibration amplitude, adding stiffness selectively may help. If the item is in resonance, you have to either isolate it or stiffen it up. Changing stiffnesses can help or hurt. Making something stiff enough without making it too heavy is a balancing act. In airplanes, Weight is the Enemy, so let's not make things much stiffer than it needs to be.

Let's back up a little. Do you have plans for this airplane and does this airplane have a good history? If yes, please stick to the plans. Odds are your airplane will fly sooner and safer if you do. If the bird has a bad history but a good history with known fixes, please stick to the plans for the known fixes, and for the same reasons as in the last sentence. If no to both questions, a different project should be considered. Once you start designing your own updates and improvements, you are doing airplane design. You had better have some suitable combination of experience and training to be an airplane designer, experimental fabricator, and test pilot. Otherwise, you are guessing, and guessing is bad.

Now onto "My only concern is that I want it to be strong. Am I missing something?" Almost certainly... but we do not know what. You are looking at the plane and have decided you need a fuel tank someplace. You have already decided upon some layer of composite held together by two different types of resin and a certain thickness of foam, all of which may or may not be up to the tasks of holding fuel while flying, landing, and presumably having an otherwise survivable emergency landing. How do you tell if it is OK? Up in the air, you should want to know it is OK.

You know the circumstances and sizes, you are designing it and connecting it. To design it to be adequately strong, you have two ways you might go:

You should know the weight of fuel and g's and loads and directions that can be applied to it. You should also know how much movement the mountings have relative to each other. And your design should cover all of the loads and deflections and vibratory inputs it sees;

An alternative is if somebody else solved the same problem adequately and at acceptable weight and you can copy their design, do so exactly. I call this "Monkey See, Monkey Do" engineering. If yours is much different, their design may or may not be adequate for your circumstances, and having it spray your airplane with fuel while in flight is bad news.

Back to fundamentals:

I do not need the answers to the following questions. You do.

What is your basis for the materials and sizing of the tank, it connections, mountings, etc?
Does the design pass analysis or are they identical to somebody else's successful solution?
That somebody else's solution, did it have the same g levels and fuel tank sizes and vibration inputs and mountings as your airplane?

If it has all of those, terrific, you might have a solution for your airplane. But if it is flimsy compared to the others or greatly different in circumstances, you have some real engineering in front of you to know it is strong enough and stiff enough.

Billski
 
Last edited:

davidb

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I’m building a fuel tank from quarter inch foam with vinyl ester resin on the inside (a tip from this forum) and E Z Poxy on the outside ( because I already have it). I’m hoping the “sandwich” along with the baffles will give me good ridgity and strength. A couple of places will be glass to glass. (Fuel cap area). My question is... will vinyl ester resin bond to cured E Z Poxy. (And vice versa) ? Thanks
Take a look at the q & a section of E Z Poxy from the AS&S online catalog. Unlike a lot of epoxies, it apparently has very good resistance to ethanol. Vinyl ester is good stuff but it is not as user friendly as epoxy in that pot life and cure time can vary greatly with temperature and activator amount. You’d want to figure out that before starting a big layup. Not a big issue if your shop is temperature controlled.

Saving a few dollars “because I already have it” mentality can cause a lot of pain and suffering. Why venture into the unknown? Use one or the other for the entire tank.
 

fulcona

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Take a look at the q & a section of E Z Poxy from the AS&S online catalog. Unlike a lot of epoxies, it apparently has very good resistance to ethanol. Vinyl ester is good stuff but it is not as user friendly as epoxy in that pot life and cure time can vary greatly with temperature and activator amount. You’d want to figure out that before starting a big layup. Not a big issue if your shop is temperature controlled.

Saving a few dollars “because I already have it” mentality can cause a lot of pain and suffering. Why venture into the unknown? Use one or the other for the entire tank.
Good suggestion. I tend to try to use as few words as necessary to get my point across so my original post probably prompted more questions than the one I was looking an answer for. ( which in this case is a good thing ) What I should have said is “because I already have it, and I know it will work for the intended application.” And believe me..I have no problem spending whatever is necessary to make sure I have a safe and adequate fuel tank. Thanks
 

wsimpso1

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One of the questions that might be asked is if vinylester will result is less rigidity or strength than with epoxy. Properly done, Vinylester is a very close second to epoxy, will produce roughly equivalent strength and stiffness.

Second question that might be asked is that quarter inch PVC foam with 2 BID on each side is pretty standard laminate, and the starting point for lots of structures, including fuel tanks... with either epoxy or vinylester, I would still do the design checks to make sure it has enough structure for its job.

Vinylester can be fussy. Good education and practice is important with it.

Billski
 

fulcona

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One of the questions that might be asked is if vinylester will result is less rigidity or strength than with epoxy. Properly done, Vinylester is a very close second to epoxy, will produce roughly equivalent strength and stiffness.

Second question that might be asked is that quarter inch PVC foam with 2 BID on each side is pretty standard laminate, and the starting point for lots of structures, including fuel tanks... with either epoxy or vinylester, I would still do the design checks to make sure it has enough structure for its job.

Vinylester can be fussy. Good education and practice is important with it.

Billski
Thanks. From all the suggestions, I think I will go with E Z Poxy on both the interior and exterior. It’s what I have the most experience with. ( I have built another composite aircraft in the past). And to go along with your other post, I had thought about most of the concerns you expressed except for things like vibration analysis, etc., which I don’t have the resources to accomplish. I try to always build which has already been done with successful designs. The fuel tank, unfortunately, one area where I will be most “experimental” in that I want a tank that is fairly easily removable. So other than that feature, it would closely resemble a KR2 tank. I am using the KR2 plans, articles from Tony Bingelis and, of course, great advice from this forum for guidance. Thanks
 

wsimpso1

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Thanks. From all the suggestions, I think I will go with E Z Poxy on both the interior and exterior. It’s what I have the most experience with. ( I have built another composite aircraft in the past). And to go along with your other post, I had thought about most of the concerns you expressed except for things like vibration analysis, etc., which I don’t have the resources to accomplish. I try to always build which has already been done with successful designs. The fuel tank, unfortunately, one area where I will be most “experimental” in that I want a tank that is fairly easily removable. So other than that feature, it would closely resemble a KR2 tank. I am using the KR2 plans, articles from Tony Bingelis and, of course, great advice from this forum for guidance. Thanks
If this is a relatively slow bird with the tank inside the fuselage and of the size of the KR2 and Tony's designs, you are reducing risks. Precautions to take:
Prevent the tank from serving as an airframe stiffener unless it is as stout as the fuselage - Tony's book shows a number of mounting methods that do this;
Keep an eye on the tank for vigorous flexing, drumming and oilcanning. They are the likely way that vibration or airframe deflections would show up in the tank and cause fuel leaks.

Billski
 

fulcona

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Messages
29
If this is a relatively slow bird with the tank inside the fuselage and of the size of the KR2 and Tony's designs, you are reducing risks. Precautions to take:
Prevent the tank from serving as an airframe stiffener unless it is as stout as the fuselage - Tony's book shows a number of mounting methods that do this;
Keep an eye on the tank for vigorous flexing, drumming and oilcanning. They are the likely way that vibration or airframe deflections would show up in the tank and cause fuel leaks.

Billski
Gotcha
 
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