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Sep 18, 2011, 07:24 AM
Ed @ Experimental Airlines
ExperimentalAir's Avatar

Wing joining

Originally Posted by Tsavah View Post
Not sure of the math, but if I connected three 20" wing sections together, I am pretty sure the wing would be 60" long. Maybe I am not reading this right, but that sounds like a glider wing to me. Epoxy is a great way to connect wings, but I would use a bit of glass clouth also, or at least some coffee filters to make the joints stronger by placing the reinforcement all around the joint (outer surface, flat, 2"-3" wide strip). I bet you are planning on doing that also.

Gorilla glue, or any other brand of urathane glue tends to foam up a bit, but with a little pressure applied until it sets, makes a strong joint and fills minor gaps well also. You do need to keep an eye on the expanding of the foam glue and wipe it down with alcohol on a paper towel more than a few times if it is getting out of the joint. Pressure on the joint is the key until the glue sets. I use clamps, sandbags, block weights, or whatever else I can to put pressure on the joint without warping, or smashing the parts. Normally the glue sets within 30-45 minutes, and cures in around 24. Once cured, the foam or wood joint should be plenty strong, but it is best to leave the joint stress free until cured.
The first thing that come to mind for that 20-20-20 wing would be a 1/4" or 3/8" dowel, 48" long, and call it good. If you constructed the wing with a channel, you could just tape the surfaces to complete the joint. If no channel, see advice above by Tsavah for more secure skin joints.

I don't recommend it, but in case you mean to join without spar, here is some info I learned by trial and error. Use the glass or paper reinforcement above. I would also take care to insert some relatively non-compressible insert across the joint. The glass or paper skin joiner is great but the wing surfaces might still "pinch". For example, if you take a piece of foamboard and just bend it until it creases, note how one paper surface creases in because the foam itself is compressible. In your wing, putting something under that potential crease point (at the joint) will help inhibit that. The more rigid the better.
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Sep 18, 2011, 07:27 AM
Watt Waster
Tsavah's Avatar

Forming a Simple Airfoil in Thin Foam Board

Originally Posted by Aber View Post
I like ... could be adapted to do a symetrical or semi-symetrical airfoil? ... How about tapered wings? I guess all it would really take would be to taper the thickness of the spacers.
Yes, it is a very good idea to remove the paper skins right away since the paper on Readi-Board tends to come off at the first hint of moisture. As to what type of shape the airfoil takes, it is all about forming the foam using a jig, heat and pressure. If you haven't seen a wing jig, it is little more than wing ribs in a box frame. Normally folks use thin plywood (1/4" or thinner) to make a half dozen or so wing ribs with a flat front, rear, and bottom. Each rib slides into a slot in the box frame so it will stay put during use and make it possible to have different rib shapes. The only parts above the box frame (which is around 1/2" tall) are the top part of the ribs, which is the part the thin foam bends over. The leading edge and trailing edge of the foam wing plank are clamped down with just enough pressure to ensure the plank will stay put in the desired curve. Normally a long, thin piece of soft wood is used to prevent the foam from becoming crushed or marked by the clamps or pressure bar screws. Now the jig/fixture is ready for the oven baking process for around 5-15 minutes. In my blog I discuss how easy it is to cold roll thin foam sheet into a tube, but a builder could also do a less aggressive cold roll so only a slight curve appears in a foam plank at the 1/3 point from the leading edge. I would include foam ribs shaped to the desired airfoil and glued equally spaced apart, like is done in most balsa build kits for wings with a specific airfoil design.

I am trying to remember who offers plans for the wing-jig/fixture, but off hand I am not recalling the website. I will need to look in my notes and post the link for the wing bake jig. A very simple hobby device for near instant wing planks out of Readi-Board foam core (paper removed) or fan fold foam (FFF) board. I suppose a person could use the same idea on thicker foam board, but I haven't seen this done. The reason I believe thicker foam board can be given a slight curve with a mild hump at the 1/3 point from the leading edge is because all foam boards I am aware of can be thermo-formed as most other un-foamed plastics can be. It comes down to pressure applied and the best heat level for a specific time period. In any case, after the proper heat has been applied for the required time period, the wing-jig/fixture is removed from the oven and placed on a cooling surface for around 15-20 minutes. Once the foam has cooled properly, the wing plank can be removed from the wing-jig/fixture and it will remember the shape of the airfoil made possible by the forming ribs. Obviously the curve can be exaggerated and the hump larger if desired once the required plywood ribs are placed into the wing-jig/fixture.

Any number of wing airfoil designs can be formed quickly in this manner and two shaped wing planks can be glued together, even with the difference between their shapes leaving an open space between them. The one part that is normally placed within this space is a wing spare and it may be tapered if the builder doesn't want a constant thickness the entire length. If you have seen the wings I made for the Funky Flyer project(s), I simply glued two halves of a wing together as a top and bottom half. The spar between them determined the thickness of the wing. After gluing the ledging edge to a balsa stick, and the spar at the 1/3 point, allowing the glue to set and cure, I placed weight (sandbags) on the back half until the glue had set and cured. By doing the assembly in this manner on a flat surface, the leading edge was well shaped, pulling the bottom half of the wing upward a little, curving the thin FFF board, but the rest of the bottom foam remained flat. If the bottom foam wing plank had of been heat formed on a wing-jig/fixture first, it could have had a curve upward (inward), or outward depending on my design and placement before gluing the assembly. Take a look at my blog for details and pictures for the Funky Flyer using the FFF board. I think you can understand the same process can be used with Readi-Board foam core material.

I am sure I need to say a few more words about the cold rolling method to form a wing airfoil. In this case I will assume the desired airfoil is a design that includes a bottom half that has an inward curve and leaves a space between the top and bottom wing plank. In one airfoil design the top wing plank has a hump at the 1/3 point from the leading edge, but the bottom curves inward around the half, or midway point of the wideth. I would cold roll the top half of the wing to form an upward curve at the 1/3 point from the leading edge by placing the wing plank upside down on a piece of foam rubber wider than the wing plank is long. Starting with minimal pressure, I would place the rolling tube at the 1/3 point and roll back and forth a short distance, but going to the leading edge with a bit more pressure with each pass until the desired curve was evident. The back half of the top wing plank would not be rolled since I would prefer it to be near flat, or flat after assembly. Next, I would turn my attention to the bottom half of the wing plank. To cold roll the bottom half I would need to place it on the foam rubber and roll it for a short distance starting in the center. Starting with minimal pressure, cold roll the center until it begins to show the desired curve and then flip it over to focus on the leading edge. Cold rolling only around 1/4 of the leading edge of the bottom wing plank will give it a nice upward curve in contrast to the rest of the wing plank. What will be produced will look like a lazy "S" with gentle curves. I would assemble the wing by first gluing the leading edges together as I did with the combat wing design using a hard balsa square stick. This is a series of butt, or edge to edge joints, so the only way to apply pressure while the glue sets is to use tape that is applied tightly to pull the parts together. The next step would be to shape and glue the spar in place.

Since we are talking lightweight foam wings, I tend to use 1/4" foam board (FFF) to make a spar, but I glue hard balsa sheet on one side or both sides of the foam spar depending on the results of a stress test. The stress test is done by placing enough weight on one end of the shaped foam and balsa spar calculated to be what the wing will feel at 4 or 6 G's, which means 4 or 6 times the weight of the finished and ready to fly model. One end of the spar is secured to an edge and the other end is allowed to hang free and is the end that gets the weight of a sandbag (ziplock freezer bag with sand inside). A simple loop of cord will allow the sandbag to hang off the end of the spar, or you can simply place the sandbag on top of the spar. If the spar fails under the load of the sandbag weight (equal to 4 or 6 times the weight of the model), I know the foam board spar needs to be designed stronger, and that often means hard balsa sheet on both sides of the foam board. I tend to use epoxy to glue the hard balsa sheet to the shaped foam spar and naturally the sheet balsa has the same shape. If I need hard balsa on both sides of the foam spar to pass the stress test, the end result will look like a balsa and foam centered sandwich, or one might call it a very long, and narrow simple sub sandwich with only one slice of meat. How thick the balsa skins are depends on the results of the stress test, but if the spar is at least 1/2" wide, it provides a much better glue area and joint for the wing skins. The key still remains to have a strong assembly with minimal weight, or one is forgetting the primary objective for TFPF's. When the bottom half of the wing is glued in place with the foam wing ribs that are shaped to preserve the airfoil design, the only work that remains is to connect the wing halves and tips. Hopefully I have provided enough words to discribe the wing assembly process and how flexible the design is with minimal complication.
Last edited by Tsavah; Sep 18, 2011 at 08:41 AM. Reason: spelling ...
Sep 18, 2011, 07:29 AM
Ed @ Experimental Airlines
ExperimentalAir's Avatar

Symmetrical airfoil

Originally Posted by Aber View Post

I wonder if this could be adapted to do a symetrical or semi-symetrical airfoil? What if after making the leading edge bend you were to cut away the paper on the lower side in front of the foam spacers. That way when you bend it over and glue down the top side it pulls up the bottom a bit. I guess you could work it into a curve before you start glueing the top.
I think you could do it just like you described. It might be tricky to replicate the curved lower surface between matched R/L wings, but I believe you could fashion a jig on top of your work surface to make a consistent up-curve for each wing while constructing. It would technically make a more efficient wing and it is enticing to try!
Sep 18, 2011, 09:51 AM
Registered User
'Thats HUGE' - Whoopz's Wife

Thats what my wife said when she saw this wing.

I think this is my plan for joining and reinforcing the wings

Going to butt the pieces together. Put a strip of tape across the seam on the bottom. Fold the piece back. Spread the epoxy on. Fold the pieces back out and let the epoxy set.

Do that for both sides of the wing. Once the epoxy sets, I'll remove the tape from one inch on each side of the seam on both sides. The glass that area of the wing for some added strength.

After that I'll add control surfaces. I know the original how-to basically just used the what was already on the wing. But since I'm going with three wing sections I decided that I want my ailerons to be all one continuous piece for each aileron.

I'm taking pictures of the build. If it successful I'll post it up. Between Ed's videos and the pictures w descriptions people should be able to knock the entire build out in less than an hour minus the glass time. After a did the first section it only took me 10 to 15 minutes to build the other two sections of wing.

Think I need to get a scale because thi
Sep 18, 2011, 10:17 AM
Registered User
BatterysIncluded's Avatar
ExperimentalAir -

Thanks for posting this method. I have plans on trying this with a few builds that I am planning for this winter. It seems that this wing would be well suited to aerial photography and could replace some of the solid foam standard flat bottomed "Clark Y type" wings that are popular in those aircraft. Also, dihedral can be formed by cutting the joining edges of the wing on a table saw with a few degrees tilt to each side so that when you join the wings it will form the dihedral. For reinforcement of the joints you could use strapping tape and hot glue. I'll try to find a pic.

For those wanting to build a tapered wing, there is a thread in the Flying Wings forum that uses a similar construction technique with bluecore forming the tapered wing. You could take the best of both techniques and combine them easily to form a tapered wing.
Bluecore Wing Build Thread

Strapping tape and hot glue used to reinforce wing joints (applied to top and bottom of wing):

Last edited by BatterysIncluded; Sep 18, 2011 at 10:22 AM. Reason: Added link
Sep 18, 2011, 10:29 AM
Registered User
I've also found that using Sketchup, importing a wing file (there is a ruby airfoil .dat plugin to import airfoils - can be found in the "Using Sketchup for modelling" thread) and using the unfold plugin, one can create templates, that would later fold in the correct shape...
Sep 18, 2011, 02:14 PM
"Use the foam, Luke!"
SpaceDrydock-22's Avatar

Depron Aero

Hi, Depron Aero is new light weight Depron modelling foam. More than 15% lighter than Classic Depron. It can be bend and twist without tape or heat and it did not break like the classic Depron.

Sep 18, 2011, 02:20 PM
Registered User
stonerscolony's Avatar
hehe nice, I already have a depron stock but I've bookmarked that thanks

Also I totally forgot about the classic depron bending technique of using a blunt edge to compress the foam like when making control surfaces without tape or hinges. This will also allow a bend.
Sep 18, 2011, 02:45 PM
Registered User
Maybe if you rolled the bending area a bit before bending it would be better..
Sep 18, 2011, 06:21 PM
Registered User
stonerscolony's Avatar
not a bad idea. I'm making 2 wings each 60CM span with a 18CM cord.
Sep 18, 2011, 09:26 PM
Ed @ Experimental Airlines
ExperimentalAir's Avatar
Parts 5 & 6 on wing spars and joiners now posted on YouTube.

There are a million valid ways to spar and join wings. These are some methods that have worked well and simply for me.

A single wooden dowel is probably the most simple and economical spar. Really can't go wrong.

I have a healthy supply of carbon arrow shafts from Cabelas ("Hunter" model), which are cheap, joinable, very light, and very stiff. They can also be had from most decent sporting good stores and some big-box retailers. $5-6 each is about par. Don't bother with the professional hunting arrow shafts. If you find arrow shafts with fletching and nocks (most do) just slice off the fletching with a couple of passes with a utility knife and twist out the nock with pliers, leaving just the carbon shaft. Let me know if anyone has questions.

Looking forward to some other guys' build pix and videos.


the Armin Wing: part 5 - Spar Selection and Planning (5 min 26 sec)

the Armin Wing: part 6 - Wing Joining Concepts (6 min 26 sec)
Sep 18, 2011, 09:56 PM
Registered User
IdahoAir's Avatar
Great videos Ed! Right now, I'm messing about with using this technique to build a 2m sailplane wing. It's not a plank wing and it looks promising if I can get the chord and thickness tapers right ...

Sep 18, 2011, 10:02 PM
Registered User
Like always...Good job man.

Question is...Are you ready for a cult like following?

Arrows can be had at walmart for 3 dollars a piece. Both CF and Aluminum. The neat think is the CF ones fit snuggly inside of the Aluminum shafts.
Sep 18, 2011, 10:59 PM
Registered User
Here is a thread where I have posted a picture of my progress so far.

Didn't want to post the same picture in two differant posts. I have more detailed pictures of wing that I will post with explanations once it is finished.

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