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16ft Pink foam Sailplane

thenated0g

Drinker of coffee, Maker of things
Mentor
#1
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As usual, whenever any flitefest footage comes out i get excited and start imagining epic builds the next time it comes to our area. After seeing Lains pink paper airplane and all the cool Rumbuilder profile stuff it got me thinking about how big can you go with rigid insulation.

The main point i took away from both build styles is that big builds do not need to take a ton of time. I mean that in a good way, more in reference to my previous large builds that took months to accomplish (200% ft-guinea pig and ft22).

So with that in mind i started thinking about going really big with 2x sheets of pink foam on edge, like the FT style of 60" builds from 2x 30"x20" foam sheets, but in this case they would be 8 foot by 4 foot sheets!

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For the airfoil shape i am going to use a kfm3 airfoil. The reason i am leaning towards this is that it will make a much more solid shape, the steps will assist with keeping the foam more rigid. Since i am going to use 1" thick sheets, the front 50" of the wing will be 3" thick which will allow me to have a 3" deep plywood "stringer" (more on that later). From the 50% mark back to the 75% mark it will be 2" thick. And the back 25% will be 1" thick.

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In surfboard building there is a wooden stringer often installed that is in the very center of the build from the tail to the nose and as thick as the board. My first thought was to put an 8 foot stringer right on CG in the center of the wing with a shorter one towards the back. The stringer would be made from half inch plywood (as seen from above), would go all the way thru the wing top/bottom, 3 inches deep, and be 8 feet long from wing tip to wing tip, glued into the foam. But another issue related to this large wing is how to transport it. Even just one 8' section is going to be "fun" to transport, let alone 16'. So than i thought about cutting out the slot for the stringer but gluing it in place on site. The concern there is will the glue be set enough to fly it. So currently my thought is to insert a 7'ish foot stringer in each side of the wing and connect on site with pieces of wood screwed thru connecting each stringer together, as you can see in the picture above.

The next issue i was trying to think of was how to make such a long fuselage stay rigid. I had originally thought to make the entire plane out of pink foam. I could do a profile style tail boom with maybe some reinforcements here and there. But the more i thought about it i just didnt have confidence in the strength of this foam in this configuration. Its going to be almost 9 feet long nose to tail, but its going to be a lot narrower than the wing. It seemed much simpler to design it using carbon fiber tubes that interlock into each other. The picture above has these in black. The size is not to scale as i dont have the carbon in front of me, but i have some wound mandrel carbon tubes around 22mm OD and some perfectly matching tubes that slide right into them. I was going to use them for a large wing to split into pieces but ended up not using them. So there will be a center section that connects to the wings and a tail and nose section that slot in with this.

The tail section I will probably do a very light fiberglass job on the foam, mainly so i can epoxy in some carbon tow in areas to make the large surfaces a little stronger. The Tail boom itself will be bare carbon.

The nose section i will need to make a fuselage that attaches to the carbon tube so there is a place to carry the battery and electronics. The Motor mount will probably be a wooden block fiberglassed to the end of the carbon tube. Possibly the battery and electronics could just "hang" on the carbon tube. I will have to wait to see what it looks like. This is more about trying to get a 16' wingspan flying than a super pretty plane.

The prop pictured is a 16" prop on a 4020 emax motor. It puts out about 5kg/11lbs of thrust. The servos pictured are standard 51g servos. All parts pictured, including the stringer, are the real world dimensions.

Posting here for ideas. While i have built several large builds in the past im sure there are many things i am not thinking of.
Here is a comparison in size between a 60" wingspan guinea pig and 16 foot wingspan.

ps. It may initially seem crazy to put 100g of servos at the tail, but my personal experience with these large builds is that electronics weight does NOT scale up with the size of your plane. IE the parts weight SIGNIFICANTLY less than you think they will. I think it will be fine.

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SlingShot

Maneuvering With Purpose
#5
ps. It may initially seem crazy to put 100g of servos at the tail, but my personal experience with these large builds is that electronics weight does NOT scale up with the size of your plane. IE the parts weight SIGNIFICANTLY less than you think they will. I think it will be fine.
Cool. :cool: A big ship can carry a big battery load too. How long do you want to stay up?
 

kilroy07

Well-known member
#7
I am definitely interested in how this turns out. I have no experience with insulation board builds, but am curious about a big build such as this AND the stepped airfoil!

One question though (and I honestly don't have any idea) is 1/2" plywood really needed for your stringer?
I would think even 1/8" would be sufficient. I assume this is to counteract vertical movement of the wing
(I think the technical term is wagging...:LOL:)
 

thenated0g

Drinker of coffee, Maker of things
Mentor
#8
I am definitely interested in how this turns out. I have no experience with insulation board builds, but am curious about a big build such as this AND the stepped airfoil!

One question though (and I honestly don't have any idea) is 1/2" plywood really needed for your stringer?
I would think even 1/8" would be sufficient. I assume this is to counteract vertical movement of the wing
(I think the technical term is wagging...:LOL:)
Good Point. My friend and i were talking about that last night and he had suggested the same thing, going smaller, somewhere around 1/4 thick. The stringer is the same thing as the spar that you put in the FT planes. It is to stop the wing from flexing and ultimately failing by snapping in half. This can be helped by not doing any severe pull ups or dives while flying as well. I need to compare 1/4 and 1/8 and see how strong it feels. Its really hard to visualize it without having it in person as 16 feet is crazy big.
 

thenated0g

Drinker of coffee, Maker of things
Mentor
#9
Just thought of a reason i wanted a little thicker material. When the wings join there will be bolts or screws going thru and all that force will be on them. I wanted a little more "meat" in that area so they dont rip out. I will have to look at it and see.
 

wilmracer

I build things that fly (sometimes)
Mentor
#10
I think 1/4" would probably be fine IF you taper the spars and add some lightening holes in the central areas where it gets close to 3" thick. Forces on the spar decrease as you get to the wingtips and you can probably get away with less meat out there.

Definitely think ahead on how you're going to transport this thing. A 2 piece wing will keep things simple but those are 8' wings to lug around and the tail will likely need to break down as well.

Good luck! I'm excited to see how it works out!
 

kilroy07

Well-known member
#11
Yea, I can see needing a little more meat in certain areas. (I just thought 1/2" seemed a little "excessive") :LOL:

With the plywood sandwiched between the foam (to stop it from flexing fore and aft) it should be quite resistant to flexing in the vertical plane since you have it on edge (and a 3" tall section), but 1/8" would be a bit thin to anchor the pieces together.

I am struggling with a design myself for detachable wings... (I just built a Kraken, and it either has a wingtip hanging out the back of my Avalanche or I have to put the rear seats down...) LOL
 

thenated0g

Drinker of coffee, Maker of things
Mentor
#12
I think 1/4" would probably be fine IF you taper the spars and add some lightening holes in the central areas where it gets close to 3" thick. Forces on the spar decrease as you get to the wingtips and you can probably get away with less meat out there.

Definitely think ahead on how you're going to transport this thing. A 2 piece wing will keep things simple but those are 8' wings to lug around and the tail will likely need to break down as well.

Good luck! I'm excited to see how it works out!
My buddy lets me borrow his covered trailer. It will hold a sheet of plywood with a little extra room so i am building it with that in mind. The wings will be the biggest section at 8' or 96 inches. the other sections will be in the 36-40" range i think. But yeah i have learned the hard way to previously to build a large scale plane around the vehicles you have access to lol. The 90" wing i built for last FF was touching the windshield and the back door of my 4 door jeep. That was fun :)
 

thenated0g

Drinker of coffee, Maker of things
Mentor
#13
Yea, I can see needing a little more meat in certain areas. (I just thought 1/2" seemed a little "excessive") :LOL:

With the plywood sandwiched between the foam (to stop it from flexing fore and aft) it should be quite resistant to flexing in the vertical plane since you have it on edge (and a 3" tall section), but 1/8" would be a bit thin to anchor the pieces together.

I am struggling with a design myself for detachable wings... (I just built a Kraken, and it either has a wingtip hanging out the back of my Avalanche or I have to put the rear seats down...) LOL
Yes welcome to the struggle ha! I have been going over this on and off for a couple years. Single piece wings in my opinion are just so much easier, but at this scale you dont have a choice really. Some really great break apart wing designs can be seen in the high end slope soaring glass ships. Google some "f3f" glass planes or build threads on rcgroups. Pretty cool how they break apart and handle an amazing amount of G's.
 

thenated0g

Drinker of coffee, Maker of things
Mentor
#14
Adjusted the design slightly. Smaller horizontal stabilizer/elevator and not as deep wing root, reduced by about 10 inches. Carbon rods are to scale now and i have decided to go with 1/8th ply as suggested and i will double that up as it gets closer to center where the main loads will be and maybe even trip that right where the wing connecting bolts will go thru. The 4 squares with holes represent some thin wood that will be glued to the top. There will be a plate that will join across the wings and bolt thru connecting the wing to the fuselage.

I was modeling this plane after a scale sailplane shape like the ash 31 and asw28. The previous wing was just way too deep and the horizontal stab too large.

This change reduced the amount of foam as well, down to 2 sheets of $22 foam. The current wing area is 2,568 square inches.

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Bricks

Well-known member
#15
Just throwing this out there, you could use arrow shafts with a dowel and epoxy as joiners, this would give rigidity in all directions. Many of our sporting good stores have a bin of arrow shafts and can be bought cheaply.
 

thenated0g

Drinker of coffee, Maker of things
Mentor
#18
Still planning to build it and have been thinking about it off and on. Just havent had a chance to borrow a trailer and get over the the town that has a home depot.