Scale Build-Off – 140" C-47 Foamy

[earthsciteach]

Nice. I like your approach. Makes me wonder why I was so hung up on DTFB for my build.

I feel the pressure, too. The B-25 has not been touched in about a month.

 

[wilmracer]

Looking great! If you're worried about the weight I would bet you could carve out more lightening holes in the profile formers of the fuse. Once you sheet it the strength will be fine. It will fly great at 32ish pounds though.

 

[ScottyWarpNine]

Man it has been a while. Finals are over so now I have a little more time, but that time, divided between my couple different projects, is still hard to come by. So I won't make any promises, but I will try my best to have this beast completed by Flite Fest 2015. I just ordered some nichrome wire so I can build a giant hotwire setup to cut the wings. I plan on cutting each wing section as a whole piece which should speed things along nicely.

Stay tuned because when my wire gets here in two days things are gonna get rolling again!

 

[ScottyWarpNine]

We are less than two months away from Flite Fest 2015 and I need to get this done!

I built a big hotwire cutting bow to cut the center section of the wing. The outer sections will be cut using the same technique as the flying wings where one end of the wire is held at a fixed point and the other is moved around a plywood form.

The power supply is just a dimmer switch which feeds 120V into a transformer which steps it down to 24V. With this transformer it does not get hot enough though, and feeding 12V straight into the wire makes it glow red and go all floppy. I'm looking for a more suitable transformer in the 50-75V range, that way my dimmer switch adjustment will actually work like I wanted it to.

Now I need to track down some 3-4" foam. Or I could buy 2" foam and somehow glue it together in a way that the hot wire can still cut through cleanly. With any luck I will have a center section cut out by the end of tonight!

More updates to follow soon.

 

[AkimboGlueGuns]

Spray glue works well for laminating foam. Back when I did a sport air work shop we used 3M 77 spray to stick about 5 layers of 1 inch foam together then hot wire. So long as the glue won't eat the foam, this should work fine.

 

[Joker 53150]

I love how some of the "foamie" construction is reaching rediculous size like this. Push the envelope!

 

[willsonman]

Well, good luck with that. After the wire heats it will sag quite a bit at that length. Hope your cuts are uniform. Not poo-pooing, just stating the obvious from my years of experience working with foam.

 

[ScottyWarpNine]

Spray glue works well for laminating foam. Back when I did a sport air work shop we used 3M 77 spray to stick about 5 layers of 1 inch foam together then hot wire. So long as the glue won't eat the foam, this should work fine.

I was thinking of 3M 77 but I was concerned that the hot wire wouldn't cut it very well and might get hung up on it. I'll give it a shot.

Willsonman, I share your concern. I got it hot enough to cut the foam slowly, and the wire didn't seem to stretch any noticeable amount. I only need a little hotter to be able to cut through faster and with a good smooth finish so I'm hoping it will still retain enough tension, but we'll see how it goes.

 

[willsonman]

I was thinking of 3M 77 but I was concerned that the hot wire wouldn't cut it very well and might get hung up on it. I'll give it a shot.

Willsonman, I share your concern. I got it hot enough to cut the foam slowly, and the wire didn't seem to stretch any noticeable amount. I only need a little hotter to be able to cut through faster and with a good smooth finish so I'm hoping it will still retain enough tension, but we'll see how it goes.

Yes, 77 will work. Years ago they changed their formula to include acetone as a propellant. Use it lightly (say 12-18" from surface) and apply to both sides. Allow to air dry for at least 30 seconds to that you do not lock in the acetone. Then stick together. While you will encounter some resistance during the cut it will work. Use a slightly higher setting if you can.

Yeah, not saying it will not work... just how well it will work is a roll of the dice. I've seen bows that are long like this incorporate a hefty spring on the cutting wire to take up slack during the heating process. The problem is that it creates additional strain on the wire when it is being weakened by the heat... usually ends up snapping the wire during a cut and then the panel is ruined. You may get away with a couple of cuts but yeah... too unpredictable for me. I stick to multiple sectioned cuts so that I do not have to face this problem. Never wider than 24".

 

[ScottyWarpNine]

I stick to multiple sectioned cuts so that I do not have to face this problem. Never wider than 24".

The center section is about 36" wide, so I'm actually going to make the bow a little smaller. The outer panels are about 55", so that may end up being an issue. If this doesn't work I'll try a few smaller sections.

 

[AkimboGlueGuns]

You could just do smaller sections (24") with the hot wire then laminate them together with some carbon for extra strength.

 

[wilmracer]

Looking forward to seeing more progress on this one.

 

[ScottyWarpNine]

This is what failure looks like:

The wing is just over 3" thick and now I'm out of foam. In hindsight, I shouldn't have chopped up all my 1/2" foam for this, I should have just bought thicker sheets to laminate together. So now an entire 1/2" sheet that I could have used for planking is ruined, and I'm still going to have to buy thicker foam for making the wings. So that's a fail. Maybe next time I'll think a little harder before cutting...

I guess I can use those for the stabilizers or something.

 

[ScottyWarpNine]

I am absolutely exhausted, but I finally have the start of a wing. I went and bought a sheet of 1.5" foam. I was able to cut it into (this time measured) usable chunks that fit in my hatchback. I glued two layers of 1.5" foam and one layer of .5" foam to get the thickness required to make the wing.

Then, I traced the wing cross section onto some balsa and cut out two patterns and sanded them side by side to make them as identical as possible. If I were to do it again I would not use balsa. It is easy to work with, but if you use the hot wire against the grain of the balsa it easily digs in. I would use some kind of ply.

Here are some pictures:

Its not the prettiest thing in the world. The wire did sag at that length, big time. The trailing edge was extremely hard to do with the stretched wire. I think I may be able to make it work though. I can fill and sand it till I'm happy, and it will be covered in fiberglass anyway. If all else fails, I have enough foam to make a second attempt, this time doing two 18" sections and this time using ply patterns.

Until then, this is what I have. Now I need some sleep before I can think about it some more.

 

[FeWolf]

God I remember my Dad jumping out one of those and me watching at the drop zone!

 

[willsonman]

Ok, so with the templates you can use balsa or something of the like if you also treat the edge with something smooth. Have a gander at my article here:

http://flitetest.com/articles/how-to-make-inexpensive-hot-wire-templates

I've done this with large and small templates and it works quite well. You can use ply or formica.... all expensive and labor intensive for a technique that is supposed to be fast and easy. Also, if this is your first time hot wire cutting I suggest you try smaller pieces just to get the feel of how you need to manipulate the bow and get the right technique for a perfect cut every time.

 

[AkimboGlueGuns]

Looks good. Once it's glassed it will work just fine.

 

[ScottyWarpNine]

I just spent a lot of money, but I think I got all of the electronics I will need. 4 Heavy duty servos for Ailerons, Rudder, and Elevator, and one extra just in case one is defective or breaks, 2 standard servos for retracts, 2 100A ESCs, supplying power to two C5065 370(?)KV motors, and two Master Airscrew 16x10 3 bladed props (counter rotating, unlike the real thing. I didn't want to deal with correcting thrust angles). I also bought a 20A Castle Switching BEC to power everything and two ($$$) Multistar 10,000mAh 6S batteries, among other things. If spending almost an entire paycheck on this thing doesn't motivate me to finish it, I don't know what will. I'm picking up some spackle on my way home, and I still need to order fiberglass supplies, but that will probably wait a couple weeks because I trust Aircraft Spruce's shipping times. I wanted to get all of my hobbyking and ebay ordering done now so I know they will get here in time.

 

[Joker 53150]

For the spackle, I found that thinning it with a little water made it MUCH easier to work with, and it filled all the pinholes in the foam much more easily. It takes only a few drops of water to thin 1 or 2 cubic inches of spackle, so go very light with the water at first if you try it. I now thin spackle as well as balsa filler in most applications. On larger surfaces the spackle goes on nicely with a plastic trowel.

 

[ScottyWarpNine]

For the spackle, I found that thinning it with a little water made it MUCH easier to work with, and it filled all the pinholes in the foam much more easily. It takes only a few drops of water to thin 1 or 2 cubic inches of spackle, so go very light with the water at first if you try it. I now thin spackle as well as balsa filler in most applications. On larger surfaces the spackle goes on nicely with a plastic trowel.

How structural is the Spackle? Do you think I Will I be able to fill in the large dips on my wing center section? Im sure it would be quite brittle, but I wonder if it would matter under the fiberglass.