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sheet alu. skin over solid foam core?

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GESchwarz

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Please check out my thread in the design forum that I referenced above. All my testing experience shows that the flexible adhesives are superior to the brittle. The brittle is stronger, but the parts must be stiff; any flexing at the joint and it's all over for that joint...it will fracture like glass.

Silicone I believe has very poor adhesive qualities. Nothing I know of can beat that Partite Methacrylate product I identified in the other thread. Pro seal is also a pretty tough customer but is certainly nowhere near as strong as the methacrylate or epoxy. But because the Proseal is stronger than any of the foams, it is a suitable adhesive for that application.

One thing that I did just discover...I should have known it earlier, is that Proseal does not achieve full strength for several days, depending on temperature and humidity. Most of my tests where I compared Proseal to the stronger adhesives, the Proseal proved much weaker. Following the most recent tests with Proseal, Methacrylates and foam, I discovered that the Proseal can be very tough. I just have no data to describe how strong it really is.
 

Starman

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Yes I read that and adhesives look like a good idea that I might be able to use. I think I'll refind that and bookmark it so it doesn't scroll off into oblivion. I wasn't totally serious about silicone, just something flexible 'like' it as I assumed that even your flexible adhesives in the reprt were hard compared to silicone. It seems to me that not that much strength is required in the adhesive because the foam is so weak, flexible is definitely better for sticking aluminum to foam.

As far as the strength of silicone rubber is concerned, did you ever see those little sample cards that had a dab of silicone rubber stuck to it, the dab having a thin end you could grab. Well try as I might I found it impossible to break the little bit of silicone off of the card, and I work with my hands so they aren't weak. It seems to me that is way stronger than needed. Of course with silicone you have something that is impossible to apply fast and thin enough and is also too heavy I suppose ... but ... maybe it won't stick that well to aluminum ... but ... when I was building aluminum boats I had another builder tell me about some silicone sealer that I could use to seal a jet pump intake. He said the stuff was so strong that you couldn't get the pump off even if you removed all the bolts.
 

Starman

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Yes, but I think I'll stick to what's been tested already for starters if I end up using glues. I was just basically sayin', flexible is good, agreeing with you.

How flexible are those flexible glues compared to silicone?

I know, for example, that something like contact cement would be incredibly strong for gluing skins, but I'm pretty sure it would melt polyfoams. Also maybe it would get brittle over time.

Concerning the glue you recommend, how much would you estimate the cost at for enough glue to do a, say, 100 ft.sq. wing, both sides, 200 ft?, what is the working time?

If it works so well it could also be used to glue the foam to a tubular spar as well as the skin
 

Dana

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There are many contact cements that won't melt styrofoam (as well as many that will!); R/C'ers use them all the time on foam wing cores.

The adhesive subject interests me as well as I'm panning on aluminum capstrips over foam ribs for my new design.

-Dana

This is an Uzi. This is an Uzi on full auto. Any questions?
 

Starman

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With contact cement you only have one chance to get the alignment right when you put the pieces together :)

Also, I was thinking that if you had a solid foam wing the glue would probably weigh more than the skin. The idea of using foam cap strips sounds good for weight savings (thumbs up emoticon goes here)
 

GESchwarz

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Dana,

I don't understand the idea behind foam ribs and aluminum cap strips. Could you explain the rationale?

Starman,

Your use of words like Glue and Contact Cement scare me. Both of them typically air dry and therefore have no way of drying between foam or aluminum which are both non porous. Neither of these materials are ever used in aircraft structural applications. All of the products I've described, and that are used in aircraft structures are two-part compounds that do not depend on air, nor do they "dry", rather they "cure". Or am I missing something?

When I hear "glue", I think Elmers, or some other type of glue typically used to join wood and other natural materials.
 

Dana

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A foam slab with a cap strip is a way to make a very light, but still strong, rib. A while ago I had posted in this thread a comparison of some different rib constructions; the original author found that a wood capped foam rib was the strongest thing he had tested. I like the idea of an aluminum capstrip, though, since I'm not building a wood airplane. The Lazair ultralight had alum capped foam ribs, with a Tedlar (plastic film) covering. I'd rather use Stits covering, but MEK and polystyrene don't mix... I haven't looked into other alternatives yet.

Contact cements air dry, but before assembly. One dry, the adhesive sticks very agressively to itself, but to nothing else. I'm not sure I'd want to trust it on a man carrying aircraft without considerable testing, but as I said R/C'ers have used it for years.

-Dana

We wonder why the dogs always drink out of our toilets, but look at it from their point of view: why do humans keep peeing into their water bowls?
 

Dana

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It provides the strength... the foam just gives the shape and provides a shear web.

-Dana
 

Starman

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Dana,

I don't understand the idea behind foam ribs and aluminum cap strips. Could you explain the rationale?
Sorry, I used that term because I had just read it, but I meant foam ribs, not cap strips, which I think you mentioned earlier.

Starman,

Your use of words like Glue and Contact Cement scare me. Both of them typically air dry and therefore have no way of drying between foam or aluminum which are both non porous. Neither of these materials are ever used in aircraft structural applications. All of the products I've described, and that are used in aircraft structures are two-part compounds that do not depend on air, nor do they "dry", rather they "cure". Or am I missing something?

When I hear "glue", I think Elmers, or some other type of glue typically used to join wood and other natural materials.
By glue I meant any thing that is used to stick stuff together that starts out as a liquid, including epoxy, and silicone, which is an amazingly strong glue if you apply it right. Silicone does need air exposure to set up.

Contact cement must be air dried first and if needed, recoated, on both parts before use so that it is like scotch tape. In fact it is like scotch tape so the parts become like pieces of giant tape, and it's like sticking two pieces of tape together, and it is strong! It's why I mentioned the need to get the alignment exactly right before the pieces make contact. If not you end up with a lot of expensive junk on your hands

You, for example, could possibly use silicone to stick aluminum ribs to wing skins because it would have enough air exposure to 'dry' within the hollow wing. Actually silicone doesn't dry, it sets up.
 

GESchwarz

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One problem I see with tape-like and contact cement bonds is that any out-of-flat conditions on either part will result in a void in the bond line. Another concern is about how strong these joints are. Before you commit to it, I'd do some rigorous testing to compare against the joining methods your replacing. If you want to go with tapes, look into the 3M VHB tapes. Search the Design Forum for my thread on that subject.

All these materials we are talking about here are relatively weak compared to rivets. The way to compensate is to increase the bond width to compensate. But that's just my opinion and therefore I wouldn't bet my life on it. Reliability and durability are very important characteristics to investigate and prove out. There can be many gotchyas with these alternative joining methods. Gotchyas can and do kill. I prefer to go with materials that have been used by the big aerospace manufacturers.
 

Starman

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Thanks Gary, if I'm going to use adhesives I'll probably use the proven aircraft stuff, I was just sort of thinking out loud about some other possibilities that seemed promising.

Concerning the out of flat condition, if I was to use a contact type cement I would use the technique where you curve one surface away from the other at the point of initial contact and then roll it on, which is how you could do a wing skin on a foam wing.

How much would your favorite adhesive for this application cost to coat 400 sq.ft.? (both pieces of both sides of a 100 sq.ft. wing)
 

GESchwarz

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Coverage will have to be determined by experience with the exact materials and application techniques that you are using. Only you can figure that out. The common glues and adhesives that are ordinarily available at your corner Home Depot will probably have at least one failure mode that will be revealed at some point within the first little while after you've begun flying your new homebuilt. Vibration under load, resistance to gas, soak at temperature extremes, corrosion, biological attack, you name it, are all things that these products may not withstand for long. Aerospace products have been put through a full spectrum of qualification testing to prove that it can survive many possible conditions for extensive periods of time.
 

MalcolmW

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Hello, Starman, Gary S. (it has been a while);

I've followed your discussion about bonding an aluminum skin to rigid foam to produce a structural body. This requires some careful engineering to obtain the necessary stiffness and flexural strength (in a wing) to meet the performance requirements. It is complex enough to suggest working with a good engineer.

As for the bonding aspect, I must agree with Gary that silicone adhesives / caulks are not regarded nor do they perform as structural adhesives. Do review the long series of discussions on bonding aluminum led by Gary over the past year. His recommendation to use a methacrylate adhesive comes from testing and comparing with other adhesives and he must be commended for sharing the results with this panel.

I have worked with structural adhesives and know that both military and commercial aircraft use epoxy adhesives for structural aluminum bonding, however, there is a caveat: These adhesives are used in production facilities which may be difficult to duplicate in a home-built workshop.

Methacrylate adhesives are used for assembling aluminum truck bodies which see severe service conditions (corrosive, extreme temperatures and all kind of weather) and hold up with minimum maintenance. Polysulfides (ProSeal or equivalent) have been used to assemble aluminum aircraft and in marine applications where they provide a long service life. Both have much to commend them to a home-builder of aircraft.

As for aluminum to rigid foam adhesive bonding, the type of foam will affect the choice of adhesive. Remember, foam strength will be far less than the strength of any structural adhesive. So, the performance requirements of the adhesive chosen will not need to be as high as an aluminum-to-aluminum bond. Do NOT use solvent based adhesives for bonding foam - ever.

This opens the door to a number of different types of adhesive: Methacrylates (check Gary's work); polysulfides (aka ProSeal, also boat caulking material); and polyurethane elastomeric adhesives. There are contact adhesives that will also perform reasonably well in this role, however, unless tested for environmental durability, do not use.

As usual, I urge any bonding of aluminum be done with adequate surface preparation (see Gary's work) to ensure that whatever adhesive is chosen, that the aluminum surface is properly prepared to provide a suitable substrate for the adhesive.

As I've said in the past - always do some testing & fly safe.

MalcolmW
 

GESchwarz

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Hey Malcolm, yes it has been a while. You can go over to the design forum to see my results on foam testing if you haven't already. I have always appreciated your inputs and your wealth of experience on this subject.

Without guys like you, some of us just might get ourselves killed.

In my opinion and despite the quotation below, I think aviation is a death-defying act because it is terribly unforgiving.

"Aviation is not inherently dangerous, but to an even greater extent than the sea, it is terribly unforgiving of any carelessness, incapacity or neglect" — Capt. A.G. Lamplugh
 

heytwo

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Can you pressurize the skin with air ?

Can you force sound posts under skin , like a violin ?


Styrene + Toulene (aka Methyl-benzene).

I did some simple tubes with

polyester cloth and liquid styrene ( from foam ) .

It was impressively light weight and very strong/stiff .


Also wrapping packing tape around foam .

Makes it stronger . Plain syrofoam will compress
permanently if you dont wrap it into a cyclinder .

.020" 2024-T3 and 3/32 AD rivets is reliable .
now add tubes of plastic to take out oil canning .

Im designing a 450 HP pusher , using Yamaha engines .
1300 lbs empty . 2 wings , fold forward . or maybe
a P-38 w/ front engines . Cant find aluminum sheet !
Need .080" for spars ...

Where to buy aluminum ?
 
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