Sheeting Foam Wings

LAST EDITED ON Jun-23-04 AT 02:39 PM (CDT)
 
I'm down to 'put on the epoxy and scrape off the excess with a credit card'. How much epoxy should I scape off? Obviously there needs to be some to adhere the sheeting to the foam, but as little as possible to keep the weight down. How do I know what is right?

Speaking of epoxy...I have a LOT of 30 minute but can get some 2 hour. Which is best for this application?

I could have asked this on an R/C site but I chose to pick on you guys since you typically know a LOT more about building light, strong planes.

Thanks in advance!

Eugene S. Chernosky
AMA 639635

LAST EDITED ON Jun-23-04 AT 03:12 PM (CDT)
 
Forget the 30-minute epoxy. In fact, I wouldn't use the 2-hour stuff either, preferring overnight setting times for a job like this.

The general rule is to do as you suggest, spreading epoxy on the skins and then scraping it off, a credit card being a suitable tool.

The trick is in knowing how far to go.

It has been my experience that if you are using runny epoxy and do not go to extraordinary lengths in its removal, you've still got plenty of glue to make the bond. Remember that you're gluing to foam; how much better a bond do you need than the bond between beads of underlying foam?

Still, the real trick is to do some test panels. Short of rubbing the sheeting down with a towel, I do not believe you can get rid of enough epoxy to prevent gluing skin to foam.

At least you're going in the right direction. I am convinced that, properly done, this is the best and lightest glue to use in laying up foam wings.

I have been using West Systems epoxy and it is a slow cure (16-24) hrs. Can be picked up at West Marine boat places, also I have been putting a coat of Nitrate dope on the side of the skin next to the foam to help keep from too much epoxy from soaking in to the balsa.
When you dope the balsa skin let it dry for 2-3 days before puttimg down as the dope fumes can attack the foam..
When you scrape the skin hold it up to a light bulb and look for a "dull" film of epoxy..
Hope that this helps....

>I'm down to 'put on the epoxy and scrape off the excess with
>a credit card'. How much epoxy should I scape off?
>Obviously there needs to be some to adhere the sheeting to
>the foam, but as little as possible to keep the weight down.
> How do I know what is right?
>
>Speaking of epoxy...I have a LOT of 30 minute but can get
>some 2 hour. Which is best for this application?
>
>I could have asked this on an R/C site but I chose to pick
>on you guys since you typically know a LOT more about
>building light, strong planes.
>
>Thanks in advance!

Ron B.
F4Fguy

You can't scrape "too much" off.If you can see any glaze at all,from the epoxy,scrape some more.The real trick,if there is one, is to apply enough uniform pressure to get a good bond all over.If you have,or know anyone who has,a vacuum bagging setup,use it!* Even though you can only use minimal vacuum (7-10 in. hg) on the 1# bead foam,it will still be better than a stack of old RC Modeler mags (these are better than Flying Models or Stunt News since they have more B.S. and ads,which are heavy).
*My first bagged wing was done with a plastic garbage bag and a vacuum cleaner,noisy,but effective.(My wife was NOT amused.)

Re the epoxy,West is best,but any laminating epoxy is satisfactory.If you can't get a really slow cure epoxy,thin it about 10-15% with denatured alchohol(do NOT use anything else and only use the 90+%pure alchohol).This will both slow it down,and make it easier to spread really thinly.The alchohol will not attack the foam.By the way,never mix epoxy in a cup.Always mix it in an open tray or pan so that it has as little depth as possible.This keeps the exothermic heat from building up,and slows the reaction.

>
>Re the epoxy,West is best,but any laminating epoxy is
>satisfactory.If you can't get a really slow cure epoxy,thin
>it about 10-15% with denatured alchohol(do NOT use anything
>else and only use the 90+%pure alchohol).This will both slow
>it down,and make it easier to spread really thinly.The
>alchohol will not attack the foam.By the way,never mix epoxy
>in a cup.Always mix it in an open tray or pan so that it has
>as little depth as possible.This keeps the exothermic heat
>from building up,and slows the reaction.
>
>Ron B.

I've had bad experiences using alcohol for thinning epoxy in boat building applications (West brand epoxy, in that case). It interfers with the cure and the final hardness and strength. If you have a little bit of cured epoxy left in your mixing cup after it cures, check the hardness: It will probably be "rubbery" if you add very much alcohol it won't cure at all... The best commonly available epoxy thinner I've found is Xylene. However Xylene will attack the foam. It's available at hardware stores like Home Depot etc. Also, trying to thin with an more than about 10% by volume of anything is very risky. And, don't expect any of the "15 minute" epoxies to have the strength of the slow cure types thinned or not. If you want thick epoxy use the good stuff and thicken it with fillers like phenolic micro ballons... If you put your mixing cup in a bowl of ice and stir frequently it will slow the thermal reaction in the mixing cup but the cold epoxy will not wet out as well as warm epoxy. "West" brand is good and readily availble but IMHO the best room temperature cure epoxy available to us casual users is "MGS" brand epoxy available from Aircraft Spruce etc. It can be spread very thin, has a good pot life and working life, actually spreads better when slightly warm and does not need to be thinned.

One thing I've wondered about for foam wing sheeting is: does epoxy make a "stiffer" wing than the urethane glue? I haven't tried it but there has been some interesting discussions about it on this forum.

with any epoxy: measure carefully and mix well

>>
>>Re the epoxy,West is best,but any laminating epoxy is
>>satisfactory.If you can't get a really slow cure epoxy,thin
>>it about 10-15% with denatured alchohol(do NOT use anything
>>else and only use the 90+%pure alchohol).This will both slow
>>it down,and make it easier to spread really thinly.The
>>alchohol will not attack the foam.By the way,never mix epoxy
>>in a cup.Always mix it in an open tray or pan so that it has
>>as little depth as possible.This keeps the exothermic heat
>>from building up,and slows the reaction.
>>
>>Ron B.
>
>I've had bad experiences using alcohol for thinning epoxy in
>boat building applications (West brand epoxy, in that case).
>It interfers with the cure and the final hardness and
>strength. If you have a little bit of cured epoxy left in
>your mixing cup after it cures, check the hardness: It will
>probably be "rubbery" if you add very much alcohol it won't
>cure at all... The best commonly available epoxy thinner
>I've found is Xylene. However Xylene will attack the foam.
>It's available at hardware stores like Home Depot etc. Also,
>trying to thin with an more than about 10% by volume of
>anything is very risky. And, don't expect any of the "15
>minute" epoxies to have the strength of the slow cure types
>thinned or not. If you want thick epoxy use the good stuff
>and thicken it with fillers like phenolic micro ballons...
>If you put your mixing cup in a bowl of ice and stir
>frequently it will slow the thermal reaction in the mixing
>cup but the cold epoxy will not wet out as well as warm
>epoxy. "West" brand is good and readily availble but IMHO
>the best room temperature cure epoxy available to us casual
>users is "MGS" brand epoxy available from Aircraft Spruce
>etc. It can be spread very thin, has a good pot life and
>working life, actually spreads better when slightly warm and
>does not need to be thinned.
>
>One thing I've wondered about for foam wing sheeting is:
>does epoxy make a "stiffer" wing than the urethane glue? I
>haven't tried it but there has been some interesting
>discussions about it on this forum.
>
>with any epoxy: measure carefully and mix well :D

Ron B.
F4Fguy

Ron:

A couple of points:

It's true,too much thinning will retard the cure,or in some cases prevent it.What makes it "rubbery" though is water.To avoid lowering the hardness you must use at least 98% pure alchohol and never more than 10%.You must also use the slowest epoxy you can find.The faster the cure the less time available for the alky to flash off.Oddly,heating helps,apparently lowering the vis enough to encourage the evaporation process.Within limits,this loss of hardness isn't a problem.

As to strength and stiffness,I'm convinced that,in the case of foam wing sheeting,extreme strength is not required ,and stiffness can actually be a deficit.The strength of a foam wing is primarily derived from it's skin.The purpose of the foam core is to keep the skins apart and to keep them from buckling .The aim is to keep the skins in "pure" tension or compression without allowing any bending other than that caused in the entire structure by the linear stress in the skins.Some flexibility in the bond area will distribute the stress across the bond line without fracturing the foam or breaking the bond.A very hard,stiff bond line will be more subject to local fracture leading to separation and eventual failure of the entire wing due to buckling of the skin on the compression side.Any strength exeeding that of the skin or the foam is unnecessary,especially with the 1# bead foam usually used.This can change with some of the more structurally adequate extruded foams.A sidelight of this is that any dings ,dents or other damage to the skin of a foam wing should be fixed immediately,as they produce discontinuities which can lead to buckling due to localized stress.

I agree with most of your observations,though I mix in a cup to get better intimacy of the components,then transfer to a flat tray to avoid the exothermic heat buildup.

All depends if you do it in vacuum or not. We use vacuum bags and balsa sheet is dry, it is hard to know which side has the epoxy.

Use slow thin epoxy for laminating. Not onlny because of slow curing, but also the result is much harder as the slow epoxy cures to very hard stuf, while quick stays soft. Such wing has better torsional rigidity.

Use dope before applying epoxy, it saves weight. We use only 16g epoxy on whole wing - 4 grams per sheet. - but once again - in vacuum bag full time of curing on pump.

Hi!

I'm with F4F on this one.

Just take your American Express card (And only EA. since they have proven to be absolutely the BEST tool for this job!) and after spreading the epoxy evenly onto the sheeting, scrape all of it off. Whatever is left is just the right amount.

Thats what Bobby Hunt told me eons ago and that method will produce a good result. You can easily see which side has the glue on it and if youre wrong, well, you won't make that mistake again!

Seriously now... It's really hard to screw up.

Have fun!

Bigs

" What constitutes the dignity of a craft is that it creates a fellowship, that it binds men together and fashions for them a common language. For there is but one problem - the problem of human relations."

Good info guys...thanks!

Now...you only put the epoxy on the balsa sheeting or on the foam, too?

I have some old 'el cheapo' lacquer that I could thin down a lot to use for a sealer on the sheeting.

The epoxy will probably end up being some 2 hour stuff that has been thinned with denatured alcohol. I haven't had much luck finding a cure time longer than that without spending big-bucks to get a LOT more than I need for this project.

Eugene S. Chernosky
AMA 639635

>Good info guys...thanks!
>
>Now...you only put the epoxy on the balsa sheeting or on the
>foam, too?
>
>I have some old 'el cheapo' lacquer that I could thin down a
>lot to use for a sealer on the sheeting.
>
>The epoxy will probably end up being some 2 hour stuff that
>has been thinned with denatured alcohol. I haven't had much
>luck finding a cure time longer than that without spending
>big-bucks to get a LOT more than I need for this project.

Epoxy glue only on the skin, in your case balsa sheeting.

As even you don't know what you've got for sealing the sheeting, I would advise against it.

Again I say, even more emphatically this time as you insist on using what I view as an unsuitable epoxy, best be doing some test panels before getting in over your head.

In particular, read the quoted weights from Igor and then see what they actually mean on a smaller scale. You are going to be very surprised at how little glue is required, *if you do it right*. (And I ain't never seen anyone do this job perfectly the first time through.)

LAST EDITED ON Jun-24-04 AT 03:32 PM (CDT)
 
>>Good info guys...thanks!
>>
>>Now...you only put the epoxy on the balsa sheeting or on the
>>foam, too?
>>
>>I have some old 'el cheapo' lacquer that I could thin down a
>>lot to use for a sealer on the sheeting.
>>
>>The epoxy will probably end up being some 2 hour stuff that
>>has been thinned with denatured alcohol. I haven't had much
>>luck finding a cure time longer than that without spending
>>big-bucks to get a LOT more than I need for this project.
>
>
>Epoxy glue only on the skin, in your case balsa sheeting.
>
>As even you don't know what you've got for sealing the
>sheeting, I would advise against it.
>
>Again I say, even more emphatically this time as you insist
>on using what I view as an unsuitable epoxy, best be doing
>some test panels before getting in over your head.
>
>In particular, read the quoted weights from Igor and then
>see what they actually mean on a smaller scale. You are
>going to be very surprised at how little glue is required,
>*if you do it right*. (And I ain't never seen anyone do this
>job perfectly the first time through.)
>
>Dan
>

Ron B.
F4Fguy

Dan:

While you and I never disagree,well hardly ever,well occaisionally,well sometimes,well a lot,I do disagree on this one.Laminating epoxy is best,hands down,no argument.However the advantage is in it's handling,not it's suitability as a bonding agent.Strength is not an issue here.It matters little if ultimate strength is greater with one epoxy or another.As long as its as good as the foam that's all you need.Cure time is about the same for any of the over 15 minute epoxies(about 8-12 hr.),the only variable is pot life,or working time(not necessarily the same thing).The open time really only needs to be long enough to apply the epoxy,position the skin and apply the pressure required to get intimate contact between the core and skin.The hardware store epoxies do tend to be loaded, to some extent with plasticisers.In this application,this is a benifit,giving a certain amount of flexibility to the bond line.This flexibility won't,in any material way,effect the wing stiffness,but could increase the fatigue life of the structure.

I do agree that "sealing" is not only unnecessary,but probably detrimental.The dope will penetrate far more than the epoxy and add more weight than it saves.The epoxy will also do a far better job of sealing the wood against the possibility of finishing materials "seeping through" than will a coat of thinned nitrate(which which will make matters worse,not better by promoting a solvent path through the wood).

All the foregoing presupposes the use of epoxy.If you use a bag,polyurethane is the latest and greatest.Just dampen everything,skim on a coat of Gorilla glue or the like,squeegee it off,slap the skins on and bag it.I don't recommend this without bagging,as I don't believe you can get enough pressure to get a smooth skin otherwise.

Ron,

We disagree less than you may have surmised. My objection to the epoxy quoted as being "2-hour" is an unknown. I really do not like dealing with unknowns when performing a job such as skinning a foam wing.

Thus one must make the assumption this glue is the normal hobby-shop stuff which is far from "runny." Even when spread over the skins, we must wonder if the required amount of glue will actually be removed or merely spread around, some of it migrating into the sheeting where it does nothing but add weight.

OR...the non-runny epoxy is recognized as a potential problem, the recourse being to thin it too much. Granted, this is a little hard to do.

I regard balsa skins glued to foam with the best epoxies and the best techniques to be a superior method of fabricating wings for CL Stunt models. It is, however, an area where only one or two seemingly small deviations from accepted practices can easily result in a worthless sandwich, one which cannot be eaten.

I used a method shown by Windy using a 3M spray on glue. It was so simple it's scary. Based on how easy it was, I'd never mess with epoxy for such a step.

But, I'm pretty darn lazy

TomK,

If you let a gram scale assign your work product a grade, you might reconsider your position on this. I've done it both ways.

>TomK,
>
>If you let a gram scale assign your work product a grade,
>you might reconsider your position on this. I've done it
>both ways.
>
>Dan

True, but as stated, I'm lazy

I remember having to find a very specific type of the 3M stuff. Didn't know that it has gone bye-bye.

>I used a method shown by Windy using a 3M spray on glue.

TomK

There was a discussion about this previously, there were several comments that the formula of the glue has been changed and will attack the foam. That would mean that the process is no longer viable for our use. Before doing a wing I'd definately do a test section to be sure.

The 3M spray-on adhesive is no longer available. Walter Umland has found a replacement - check out his website. I've made a lot of balsa sheeted foam wings for RC sailplanes. The spray ons are the easiest to work with. Vacuum bagging is essential!

And lighter too!

>And lighter too!

Ron B.
F4Fguy

Either you aren't using enough spray adhesive,or too much epoxy.

I'm an R/C sail plane flyer as well and have covered many a foam wing with 1/16" balsa for my sail planes. However, I have always used Sig Core Bond. The advantages are: cheaper, easier to use, readly available at the local hoppy shop, clean up with water (when wet), use what you need, and I believe has about the same strenght as epoxy. I do you a foam wing press to insure a 100% contact and it is a lot easire to use and make than a vacuum bag machine. Photos and directions on how to use a foam wing press available. Vern Sundstrom:)

>I'm down to 'put on the epoxy and scrape off the excess with
>a credit card'. How much epoxy should I scape off?
>Obviously there needs to be some to adhere the sheeting to
>the foam, but as little as possible to keep the weight down.
> How do I know what is right?
>
>Speaking of epoxy...I have a LOT of 30 minute but can get
>some 2 hour. Which is best for this application?

Why not skip the epoxy and use Titebond Polyurethane glue. Musch easier to use and WAY more forgiving (lighter too).

>Why not skip the epoxy and use Titebond Polyurethane glue.
>Musch easier to use and WAY more forgiving (lighter too).

Ron B.
F4Fguy

No argument!Check the last sentence in post #8 of this string.