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Windsong (Skinned foam frame)

Boberticus

Active member
#1
An attempt at skinning foam-board like it was a balsa build.
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link to the original outerzone plans https://outerzone.co.uk/plan_details.asp?ID=5944

So ive been thinking about trying my hand at building and skinning a old school film skinned balsa plane, just for the heck of it, and have been looking through different outerzone plans trying to find something that piqued my interest, when @FoamyDM had to throw a wrench in my plans by showing what was capable with foamboard and some dollar tree cellowrap with his Skinimoa.

So I cruise by my local DTFB to pick up my weekly dosage of foamboard, look thorough the gift wrap, and not only did they have the clear cello-wrap Foamy couldn't find and the pink stuff he was using, but also some beautiful navy blue on one side and silver on the other, and a roll of gold and silver glitter. so now that my mind has been made up, I just needed a easy design to try out, and this one seems to fit the bill. nice smooth curves and a flat wing bottom to give myself some leeway with skinning something for the first time.

I converted the plans to 4.5mm DTFB, taking a few leeway decisions on how i was going to do the hatch and how the wing attaches. I also decided to fit the entire fuselage around a 8mm arrow shaft for strength, stiffness and peace of mind. not planning on doing the motor pod, I just want a proof of concept before trying my hand at something with an odd airfoil shape and interesting wing twist(like a Horton wing or a Prandtl-D)

Ive also been looking for some video tutorials, found a guy on you-tube showing how to cover really well using laminating film.

And someone using dollar store cellophane and spray adhesive

I've been cutting out wing offsets for two days straight, and will finish the last 4 and fit them and the the wing leading and trailing edges together tonight, with hopes to finish the fuselage tomorrow, and skin them both Saturday for a maiden this weekend. last weekend i tried out a few different glues, and have a good idea of what will work and what wont. probably not gonna be any Hot glue in this model, but then again tere wont be alot of foam touching foam.

wingspan will be 60 inches, with a overall length of 40 or so inches.

pictures once i have something to show later on tonight.
 

Attachments

CapnBry

Well-known member
#2
Wow I didn't know you could do skinning over DTFB, I thought for sure it would just melt when you tried to shrink the film or that it wouldn't be strong enough in the end. Looking forward to seeing how it works and what techniques you use to build it.
 

Boberticus

Active member
#5
Well, I certainly need a new razor blade. 48 offsets later
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I ended up cutting three wings worth of ribs at once, so that i can just build and skin, with some mess-up wiggle room.
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i fitted one half of the wing together, im sitting here contemplating finishing the entire wing and waiting the 24 hours for it to dry, or to treat this as my test panel and skin it. i really want to test skinning, but im beginning to think this just might work, and dont wanna lose steam after proving the concept to myself, so im going to finish up the other half of the wing today and let it dry to be skinned tomorrow. Im not stalling, your stalling
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Ive glue a strip of sandpaper onto a BBQ skewer, and sanded out a groove in the trailing edge for a bamboo skewer edge to give it strength against warping, and ive skewered about 80%of the wing through the middle for strength too. leading edge had packing tape applied before folding for strength and a "waterproofing" layer against the spray adhesive's solvent. Its pretty amazing how stiff it is for how light it is, even before the covering has been applied.
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total weight is right at 49 grams, including the full spar, looking at a full wing weighing in at 100-10 grams, extrapolating out further, i might be able to build the whole glider for less than 250g, so thats the new goal. the body will have less foam than the wings, but a carbon fiber arrow stiffener at 40g, and im planning on doubling up the tail surfaces, and i have literally no idea about ballast, so thats a loftly dumb goal, but i digress...

Pretty colors!!!! i bought enough of the blue to do a navy blue Corsair or a chrome Spitfire, so ill be using the clear and the glitter on this one. the glitter is sorta dazzeling, the blue is soo shiny, and the chrome backing is literally mirror-like if i can get it flat enough. . this might end up really cool looking.
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After reading through BobK's postings, i think im going to cut the wing profile out of a scrap piece of foam board, use that as a masking to spray adhesive onto the skin in the areas i need while keeping it mostly clear. im going to pre cut the sections of skin to be exact size + .5inch, and ill do the bottom of the wing first, trim the excess, then the top, while overlapping at least a half inch around the entire edge.. Im using literally the cheapest Iron you can get at wally world, probably on the lowest setting and going from there. this is going to be interesting

4d foamboard SU-26 @FoamyDM I meant to post this video in your minimoa thread, but watching this made me do a a bit of learning of skinning techniques to not sound dumb, and i quickly realized i wanted to do this myself. this guy is just laminating with German super77, but hes not doing the crazy complex curves and heatshrinking you were trying, whats intersting is he pretensions and tapes down the plastic so he can set it mostly wrinkle free. im pretty sure hes using shrink wrap for food storage, so a lot less shrink tension...Have you gotten anywhere with the body? and how does the wing weight and stiffness compare to a traditional "flight-test" wing?
 

TooJung2Die

Well-known member
#6
I haven't tried the metallic gift wrap yet, only the transparent cellophane. That'll look amazing. Almost any aerosol adhesive will work. I use a cheap clothes iron for gift wrapping foam. Heat setting is between "synthetic" and "silk". Here are a couple of Adam's foam board wings with the paper removed and gift wrap applied. The gift wrap will shrink with heat; just not as much as ordinary heat shrink covering films. When you apply it to one side like these wings the foam board curls up like a potato chip. You have to flatten out the curl over a rounded edge if you only cover one side.

SE5 wings.jpg

Jon
 

Boberticus

Active member
#7
Got some work done on this a couple weekends back, man do i need to take more pics mid process I used ELMERS spray adhesive, it didn't melt if I backed of by a couple feet and just let the rubber get to the exposed foam
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The wing covered great, but the tail is just, meh.

The tail i peeled paper from both sides, then sprayed and covered it. I tried to skin both sides at once by sticking one side then rolling over to cover the other, ended up a bit wrinkly. Control surfaces were pre scored, then covered, then both skin and foam were ironed for the bevel. That part worked great at least.
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The airfoil shape was very easy to cover with the flat bottom, fully assembled, but one half at a time so i could cut four sections of skin and get it covered with no seams. I ended up using exacly one roll on all four pieces with like a inch left over.

I didnt peel any paper from the ribs or leading edge, I was afraid of weakening the wing.
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I pre cut sections to cover the wing, about 3/4 a inch longer than i needed it on both sides. I taped the far side of the long edge of the pre cut skin down to the table, then sprayed the wing ribs and the skin, the pulled the skin taunt and set the front edge down, then just let it sit flat down on the table. The spray adheesive stuck just enough to then let me fold the extra skin over, then stuck it down better with the iron. Then the top was done, it was actually a bit easier because it rolled on because of the curve.
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I ended up with a over lap of about a inch in the middle, and a quarter inch or so on the wingtips. but i want to cut a nice 6 inch wide piece of skin and reinforce the middle, and cut the wingtips flush and then skin the ends.
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I totally made a mistake on the plans, the leading edge piece needs to be wider by a single thickness of foam on the top side, but you cant even see the gap it leaves once the skin is on.

It's, soooo shiney, the pics dont even do it justice. Its not even in direct daylight, just so sparkly.

I think next time i want to slot the ribs a bit differently, should be a bit easier to get properly glued up, and maybe stronger too.
 

Boberticus

Active member
#8
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Got the fuselage finished up, of course with no build pics. Ended up cutting stringers to holds the skin in place, used a three pack of lazer color tape I found at the dollar tree to color the stringers. Re did the horizontal tail piece, it looks a bit better and has more control surface area
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Skin is clear to show its just foam, its pretty cool to be able to see through it. It was much more difficult covering the compound shape vs the wing, and to be honest im a little disappointed in myself.
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I'm about 90 percent sure it'll fly, but, im also thinking I have a strong enough motor on it to fold the wings in a decent dive pullout... The wing, flimsy is not the word, but its definitely not stiff. Gliders are supposed to have flexible wings, right? Ignore that motor...

I'm honestly worried that tension between the wing tip and root will just tear the foam apart, as there isn't much in the wing countering that specific force. It holds the weight of the plane plus a pound or so, so level flight should be ok. Planning a wing around a stiffer spar would be a good idea, the bamboo sticks are unglued and planning the wing around a carbon fiber arrow shaft would help too

The fuselage is much more stiff because of the arrow shaft, but the slim stringers are pretty flexible too and didn't take the iron well. That being said, every other place I peeled the paper and skinned with cellophane are incredibly durable, a bit more flexible than paper, but doesnt dent or wrinkle as easily, and seems a scoche lighter to boot. The tail section I didnt peel, and its much stronger than regular dtfb foam.

The cellophane seems to be a good insulator, if your iron is hot enough to melt and deform foam, your hot enough to melt the cellophane, and lower heat gets absorbed by the cellophane and it tends to shrink before the heat starts messing with the foam. That being said, the stringers were flexible enough to just be bent out of shape with the weight of the iron, and small enough to absorb the heat and stay in that unwanted shape. Not heat shrunk but heat formed. I think thicker or doubled stringers and a lighter hand would help

I did a test piece of the ft corsair cowl and it honestly works better than paper for "master series" bending/curving techniques. You can even do inside curves, although the cellophane wrinkles, its uniform and still curves nicely. I need to take some pics of that.

Last night, I got all the control surfaces limited out, and my expo figured out, unplugged and got all my batteries charged up, and planned on a early maiden this morning.
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This morning, I get to work, (which has a nice field out back I have permission to fly on) get a jogging glide test in and it's nice, no trim needed, plug it in, and the horizontal stab servo just starts going nuts. All over the place, jittering to a stop, sometimes going fully out to the end of travel, just chaos. So, no maiden and no epic midflight unplanned deconstuslction today, getting in to change that servo will be a PITA, gotta cut into and re-skin that section. Might be a chance to cut off the entire fuselage skin and retry, its hard to take photos of clear plastic buts its a poor covering job at best
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