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Dec 12, 2011, 10:50 PM
just Some Useless Geek
That's not so bad if that's the same foam you used in the first post. But was that one- or two-sided? If you got that kind of stiffness with only one side of paper then the weight penalty is well worth it. If it took both sides to get that stiffness then I'd look for another solution.
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Dec 13, 2011, 12:48 PM
Registered User

finishing method


I have almost completed a 40" wingspan pusher scale Vampire Mk5. I flew it in the uncovered state last Friday and had a very good flight. I am now in the process of finishing this a/c. The fuse is built up using 3mm Depron skins. I have filled this with lightweight filler and started covering using silk on the bottom section of the fuse. So I can now give you an idea of weight addition for this step.
Weight of a/c prior to cover was 639gm. The silk bottom cover measured 19"x8" ( 1.05 sq ft) weighed 1.5gm. The original silk piece bought measured 36" by 40" and weighed 15 gm. After adding the silk using WBPU the a/c weighed 644gm.
The silk followed the fuse shape very well and blended onto the wings at the rear. I did not have to cut any silk off or slit fillets to make it fit. I am very impressed with the way it went on and the finish it has given before using more spackle. Silk is expensive- this full piece cost about $8 and so the fuse will cost about $1.50 to cover. The weight increase is very reasonable.
Dec 13, 2011, 12:55 PM
just Some Useless Geek
franciscan, that finishing sounds great. Do you have any feel for how much stiffness or ding protection was gained with the addition of the silk exoskeleton? Is there some way to measure that?
Dec 13, 2011, 02:00 PM
Space Coast USA
hoppy's Avatar
Quote:
Originally Posted by A Useless Geek
franciscan, that finishing sounds great. Do you have any feel for how much stiffness or ding protection was gained with the addition of the silk exoskeleton? Is there some way to measure that?
My suggestion is to try it out as the questions you are asking are very subjective and hard to answer without some expensive testing equipment.
5 min of work and 24 hours for drying will give you some test pieces to evaluate and will give you much more info in regards to how it works than what we can tell you here.

It will cost maybe $3 to buy a piece of $tree foam and a bottle of yellow carpenters glue and the paper is free. You can even try a piece of silk or any other material with the 50/50 glue mix and see how it works. The WBPU is going to be much more expensive just for trying a method out if you don't have any other use for it.

Give it a try -

As far as stiffness goes and one side vs two - If you want a piece to be still in only direction, you can just cover the side towards which the bending will occur. If you want it to resist bending in all directions, cover both sides. The test piece I used was covered on both sides.
The weight gain is really very small. The weight of a nickel(5g)/sqft for each side. Not much to pay for stiffness, ding resistance, dirt resistance/smooth surface/etc.
Dec 13, 2011, 03:04 PM
just Some Useless Geek
I have some Minwax Polycrylic left over from my HPR daze, flying I357s and J350s in a 2.56" body tube. Heh. Perhaps I can play with that a while and see what comes of it. If I get any kind of decent results I can post the numbers for all to see.
Dec 13, 2011, 04:59 PM
Space Coast USA
hoppy's Avatar
Post them regardless.
Even negative results are worthwhile as they tell us what doesn't work.
Dec 13, 2011, 05:44 PM
IAMCANADIAN
Grejen's Avatar
Glue vs WBPU?, newsprint vs coffee filters? Every time I read threads on this topic I get more confused and undecided.

I've made (hotwired/spackled/sanded) an EPS wing. It's a 36" span delta about 1" thick symetrical foil. The idea was that the covering would provide the strength as well as the finish so no spars would be required. I was planning to use WBPU on .75oz glass but maybe glue is stiffer? Certainly it's cheaper.

Has anyone tried the 50/50 glue with glass? The glass won't shrink but I know the glue does.

I did try WBPU and tissue (came with an old Guillows kit) on a regular wing - 48"X6", hotwired EPS. Applied WBPU, then the dry tissue, then more WBPU to soak the tissue then squeeged about half of it back out. Covered both sides at once and jigged in some washout while it was drying. It held shape but there were some wrinkles. When it dried it was smoother and sanded out OK.
Last edited by Grejen; Dec 13, 2011 at 05:54 PM.
Dec 13, 2011, 06:11 PM
Space Coast USA
hoppy's Avatar
I tried .75oz glass/WBPU and as far as providing strength, I'd give it a 2 out of 10. I was able to snap the foam in 1/2 with little trouble.
However, there's nothing like a couple of bench tests to make your mind up on what to use. Try comparing a 2x4" piece covered with paper/50-50 glue and another piece of foam covered with 0.75oz glass and 50-50 glue. Squeegee out as much glue as you can with gentle strokes toward an edge. If you decide to try it, let us know what you think of the two methods.
Last edited by hoppy; Dec 13, 2011 at 09:57 PM.
Dec 13, 2011, 08:25 PM
RMS
RMS
Registered User
RMS's Avatar
Is this Tulle fabric anything like Sig Koverall? I would love to find a cheap substitute.
Dec 13, 2011, 08:50 PM
Vertical approach specialist
potshot's Avatar
Quote:
Originally Posted by RMS
Is this Tulle fabric anything like Sig Koverall? I would love to find a cheap substitute.
Don't know what the Sig product is like, but the tulle is very cheap, and you can get it at any fabric store, many Wal-Marts, etc.
Dec 13, 2011, 09:14 PM
low tech high tech
vtdiy's Avatar
Quote:
Originally Posted by A Useless Geek
hoppy, et al: We appreciate you guys doing research on what makes foam stiffer, stronger, etc. Knowing the after-process weight of parts is good, but we also need to know the before process mass to have a comparison.

So, if we can know what this stuff weighs once any particular finishing material is applied then we have a basis of comparison. Knowing what hoppy started with versus what he ended up with lets us know what we can expect. I'd like to be able to calculate ahead of time what a plane will weigh so I know what motor to use, what servos, what battery size [/shovelmode].
You can't just predict weight based on glue and paper choices. Various papers have different thicknesses, finishes, fillers, and absorption rates,for the final finish materials. When you add the filler coats and paint coat, you can add as much weight as the paper and glue sometimes, depending on the paper thickness and absorption, glue, etc. Your methods and desired finish will be different than hoppy's

If you want to predict weights, you need to not only glue paper down, but finish to whatever level you like, using whatever method you like (sanding between coats?, how much?, polycrylic? talcum?, acrylic finish?, enamel?, lacquer? number of coats? etc.) and do that the same for two samples you want to compare.

Comparing your results with different kinds of coverings to hoppy's won't mean much unless you finish each, the same way, with the same materials to the same degree of opacity and shine. Then the practical weight difference can be seen.

Two guys finishing the same tissue covered balsa model, using the same finish, dope, can come out with greatly different model weights.
Dec 13, 2011, 09:32 PM
low tech high tech
vtdiy's Avatar
Quote:
Originally Posted by Grejen
Glue vs WBPU?, newsprint vs coffee filters? Every time I read threads on this topic I get more confused and undecided.

I've made (hotwired/spackled/sanded) an EPS wing. It's a 36" span delta about 1" thick symetrical foil. The idea was that the covering would provide the strength as well as the finish so no spars would be required. I was planning to use WBPU on .75oz glass but maybe glue is stiffer? Certainly it's cheaper.

Has anyone tried the 50/50 glue with glass? The glass won't shrink but I know the glue does.

I did try WBPU and tissue (came with an old Guillows kit) on a regular wing - 48"X6", hotwired EPS. Applied WBPU, then the dry tissue, then more WBPU to soak the tissue then squeeged about half of it back out. Covered both sides at once and jigged in some washout while it was drying. It held shape but there were some wrinkles. When it dried it was smoother and sanded out OK.
Stiffness and strength are not necessarily the same thing. I routinely fly 36" span conventional planes with hot-wired wings and a 1" strip of strapping tape on the lower surface as the only "spar", with NO covering.

On the other hand, I do like covering foam with paper for other reasons, and it certainly adds lots of stiffness. But that stiffness wasn't necessarily the "strength" the wing needed to begin with -- it already had it.
Dec 14, 2011, 08:09 AM
Addicted to building...
Freddie B's Avatar
Vtdiy brings up some very valid points. Through experience you will even find the same builder will add different amounts of weight to his project using covering materials and liquid finishes. This can result in one wing half weighing more than the other. (not good). However, not to fear! With a bit of practice and paying attention a builder learns to get fairly consistent results.

Materials dp vary, some from batch to batch from a manufacturer (foam and papers are good examples) so results can vary. Best for each of us to test our own methods, materials, and results.

On the subject of WBPU or Polycrilic coatings on fiberglass and cloth. You get some ding resistance, but not much. Foam can still be compressed from heavy fingers and will break in a good crash, or crease. It's all easy to repair, and the final finish is terrific. The water borne finish is too soft. Compare it to epoxy resins which make for a very hard coating and good abrasion resistance. Weight can be kept low using epoxy too (very good removal of excess). But epoxy is very poor for paper coatings (soaks in too much weight).

I agree white glue is stiffer than the WBPU and PC coatings, but I use all of the discussed methods in various areas of my builds depending on what I'm building and the results I'm after. Large airplanes are the best subjects for all of these treatments, but then micro planes are so light they don't get much damage even in a crash. So again it's a judgement issue.

You can double an aircrafts weight with a coating/paint treatment, or add almost nothing with careful, minimal coats, sanding, airbrushing, etc. This is one of the building skills that is the most challenging yet holds the most intrigue for me as I spend the rest of my life trying to perfect the techniques and results.

Fred
Dec 14, 2011, 11:42 AM
Registered User

silk covering


My application of silk has dried out and I have put one coat of filler on the silk. This was a mixture of WBPU with about an equal amount of a mixture of Talcum powder and lightweight filler. This gave a nice liquid creamy mixture and this has now dried. I have sanded this with wet and dry and it has resulted in a very smooth finish. The bottom foam fuselage seems a lot stiffer to me than the top, which has not received this treatment. This is a subjective conclusion but I am very satisfied with the results. After painting I will put a thin coat of epoxy or tape on the under bearing area of the fuse to protect it when landing. For dings and dents I have always used brown paper covering put on wet with 50/50 white glue, typically on wing leading edges. This is however much heavier than silk or I imagine paper. My thunder and lightning has more than a 100 flights in very tough field conditions and my wing leading edges are still very good.
The Vampire has a wing loading of about 13oz/ft2 and on its maiden I found the final glide was very good with motor off and so I am not too worried about a few more gms in final finish.
Dec 14, 2011, 11:58 AM
Registered User
Here are some of the weights I've recorded for various coverings and materials (YMMV). Sometimes I'll use several materials on the same airplane in different areas. When I use paper, I apply it wet with WBPU. Generally, I'll use FG on compound curves (fuselage), paper on wings. Lately, I've been using liquid sheeting II (Styrospray 1000) for certain things with some success.

I'm building scale models, not flat foamies and tend to favor durability and quality of painted finish over absolute lightest covering.

Units are gram/square foot, one surface covered unless otherwise stated, covering ready to paint.

The "light" brown paper is very similar to newsprint, the "heavy" is more like a grocery bag.

covering g/ftsq
1/16" Balsa 21
FFF` 19
DT Foam 12

Brown paper (heavy)+wbpu 20
(light) brown+ Poly 11
FG 1/2 oz + WBPU 8.0
FG 1/2 oz +epoxy 6.9
fiberglass 1 oz +epoxy 10.8
FG 1.5oz epoxy 15
FFF, 2 layers 3/4oz FG 37.3
Doculam 4.7
Towercote 6

FFF + 1/2oz +epoxy 25.9
fff +tape 28
LSII 3 coats 6
LSII 2 coats over balsa 4
Birch Veneer 31


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