Scale Build-Off – 140" C-47 Foamy

[wilmracer]

I'm with willsonman on the epoxy+glass. If you really wanted to avoid using resin on the entire wing surface you could probably sand in a few very shallow channels on the wing surface and flush bond CF tow or strips using resin, then come back over the wing surface with glass and WBPU. The CF would (hopefully) bear the structural loads while the WPBU would be for ding protection and to give a good surface for finishing. At these sizes CF tow or stripping can get pretty expensive. If I was to go that route I'd probably do 2 "spars" of CF, so essentially 4x the wing length.

Something like this would work: http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXB965&P=ML

I expect that would be strong enough for scale flight.

 

[ScottyWarpNine]

I'm with willsonman on the epoxy+glass.

Epoxy it is then. I'll order some tomorrow, then hopefully by the time it gets here I'll have stuff that's ready to be glasses. I like the idea of the carbon tow as well. I may buy some of that in addition to the epoxy and glass.



Im working on carving a motor nacelle and getting ready to permanently fix the wing center section to the fuse. Tonight my friend is coming over so we can get the second wing panel shaped. Rudder servo and linkage has been planned out. I'm going to 3d print some servo mounts that I can epoxy in for the servos to screw into.
Oh, and what do you guys think? 1 layer of glass overall with 2 layers on important things like LE, control surfaces, etc? Or 2 layers over everything? Or more?

 

[wilmracer]

Honestly it depends on the weight of the glass you want to use. For the P-40 I used 2 layers of 0.52oz glass with the bias in the cloth run at 90*. That just means that the first layer ran from the front of the wing to the back (with the segments overlapping about 1") and the second layer ran from wingtip to wingtip.

That wing has considerable internal support from multiple full length basswood and CF spars. The glass there really isn't structural.

Someone else may have a suggestion based on more experience with "spar-less" wings. Depending on the weight I would probably opt for something heavier for at least the wing. I might do a layer of 2.75oz glass the length of the wing and top it off with a layer of 0.5 or 0.75oz glass. The lighter glass will be easier to fill the weave cleanly and get a good surface for finishing and the heavier glass should hold up to flight loads.

For fuse I'd probably just do 2 layers of .5oz with a bit more added at the wing saddle and landing gear locations.

 

[willsonman]

I might do a layer of 2.75oz glass the length of the wing and top it off with a layer of 0.5 or 0.75oz glass. The lighter glass will be easier to fill the weave cleanly and get a good surface for finishing and the heavier glass should hold up to flight loads.

For fuse I'd probably just do 2 layers of .5oz with a bit more added at the wing saddle and landing gear locations.

This would be my suggestion as well. Heavy stuff goes on first then the light stuff for better surface finishing. With all that area make sure you squeegee the crap out of it to fill ALL of the weave on the heavy glass layer. And, do not wait for it to cure before adding the light cloth. Layer it all at once so that it becomes a uniform sandwich.

 

[ScottyWarpNine]

Photoshoot in my driveway

I have almost all of the rest of the wing done. I still need to do a segment of the right wing and the tips, but this thing is starting to get pretty big, and a little hefty. It's not really heavy, but it weighs something now.

 

[wilmracer]

nice work! The weight on big stuff is deceiving. It starts to "feel" heavy even though you know it is light for its size.

 

[AkimboGlueGuns]

Awesome. The glassing and painting should really make it come together. Are you still going to use the foam cowlings?

 

[ScottyWarpNine]

Awesome. The glassing and painting should really make it come together. Are you still going to use the foam cowlings?

If you mean those foam engine pods in the last pictures, yes. I'm going to make the engine shroud out of just fiberglass, which will be attached with screws so it can be removed to access the motor if needed.

Anyway, how's this for a nose?


I attached a chunk of foam to the back and shaped it into a tail:


And now I'm starting to daunting task of planking the top of the fuselage. I can do the entire top half now because everything, the electronics, the servos, the batteries, the wires, will all be in the bottom half of the fuselage. I have a lot of work to do but I think once it starts to get covered and begins to look like an airplane things should start coming together pretty quickly. I need to keep reminding myself to focus on the big picture things. This is by no means going to be a scale airplane. It will look good in the air, and I'll try to put as many details as I can into with the little time I have, but I mainly want it to be ready to fly by flite test.

My 3d printer is making retract parts as I type this and I think I'm going to head to bed now and get an early start on my day off tommorow.

 

[willsonman]

Either way you are good. IF you need to install a push rod for the rudder after you sheet you are only a hot metal rod away. Heat it up and make a hole. Easy peasy. Looking good.

 

[ScottyWarpNine]

A work in progress.

I'm using this beaded insolation foam for skinning the fuselage.

Note the string. There is a slight twist in the tail that I'm trying to tweak back to true. If I can't make it right with the foam planking, I'll fix it with the glass.

I bought one last sheet of foam, enough for the horizontal stabilizer, second engine pod, and whatever else I'm forgetting.

 

[ScottyWarpNine]

 

[wilmracer]

Looking good! not sure how I missed your post yesterday. That is a TON of planking but it looks like it is coming together pretty quick. Still think it will be ready for FTFF?

 

[AkimboGlueGuns]

That looks awesome! Nice job on all that planking, I sure hope it pays off at FTFF.

 

[ScottyWarpNine]

Looking good! not sure how I missed your post yesterday. That is a TON of planking but it looks like it is coming together pretty quick. Still think it will be ready for FTFF?

I had a friend come over and help me with the planking. He built a cedar strip kayak, so I figured he could teach me a thing or two. I got the entire top half planked so far, now I'm working on getting the wiring and linkages in the bottom of the fuselage all done so I can plank that too.

As for being done for Flite Fest, its going to come down to the last minute, but I'm still optimistic.

 

[ScottyWarpNine]

Sorry for the double post, for some reason I can't edit and add to my last post. The text box just turns grey and doesn't show my text.

Anyway, just wanted to let you guys know where I'm at right now with all this. I just fiber glassed the the inner surfaces of the rudder and elevators. You know, the part inside the hinge. Is there are name for that? The part where the bevel would be on a FT model. Anyway, did that. The resin I got is quite thin so it wet the cloth pretty easily. I'm using my lighter cloth for that because the heavy stuff won't go cleanly around a 90 degree corner without leaving an air bubble.

I think I have the rudder hinge finally sorted out. The hinge pin will be epoxied to the rudder surface itself so that when the pin rotates, so does the surface. The pin goes into the bottom of the fuselage and will have a control horn (actually a steerable nosewheel arm, if you want to get technical) which will have linkages hooking up to the servo about 2 inches away. That is the only control that is totally enclosed in the fuselage, so I'm trying to make it as robust and reliable as possible.

Next step is to install the elevator servos and glass the rudder and elevators.

I made a little revision to the wing design. The center section will also include half of the first segment of each wing panel as well. I decided to do that so that the dihedral didn't begin right at the joint. Now the dihedral is built into the center section so it can be glassed as a single part. The outer wing panels are about a foot shorter because of this. Everything should still be perfectly portable.

I know, boring text post, no pictures. Maybe tomorrow evening I'll have something worthwhile to show off with pictures. I guess you'll just have to wait and see

 

[wilmracer]

Text is fine as long as it is progress

I'm just hoping I beat you to the field and can get some video or pictures of you pulling up with this monster strapped to your car

 

[ScottyWarpNine]

Text is fine as long as it is progress

I'm just hoping I beat you to the field and can get some video or pictures of you pulling up with this monster strapped to your car

Lol, well I'm getting there sometime Wednesday. And I know I'm going to disappoint a lot of people with this news, but I'm not taking the fiat. A pop up camper became available to me so I'm borrowing a pickup truck. The C-47 should fit comfortable in the bed. I will defiantly stick it on the fiat and take some pictures when it's done though!

I watched the latest Flite Test video about the giant cargo plane and that inspired me to do two things; come up with a servo power distribution solution, and remember to glue my hinges!

This is the design I came up with for the distribution board. It basically uses two common power busses to supply power from the BEC to the servos and receiver.

all wires to the receiver can keep their positive and negative wires because it is just a single BEC, unlike using multiple ESC's where you need to snip one of the positive wires.

 

[Craftydan]

Scotty,

If you put your BEC connections on either end of your power bus, you can cut the power line midway and split your load intentionally between the servos. even if you don't , you'll reduce the current flow (and accompanying voltage drop) as you move down the bus since the sources are distributed more evenly among the sinks. Also keep in mind, your RX still needs power and ground, and depending on what you've picked, you might want it as far up on the food line as it can get -- some RXs can get finicky if the voltage gets lower than they care for.

Shameless plug:

you might even consider building a pair of these -- http://flitetest.com/articles/simplified-twin-control-harness-for-the-kraken.

I'm assuming you're already routing high voltage power from a battery bay in the middle to ESCs out on the wings, so if you build two of these, they would route the signals to the RX with a single servo wire per wing, and the fuselage electronics would get power from the BEC in there.

 

[ScottyWarpNine]

Scotty,

If you put your BEC connections on either end of your power bus, you can cut the power line midway and split your load intentionally between the servos. even if you don't , you'll reduce the current flow (and accompanying voltage drop) as you move down the bus since the sources are distributed more evenly among the sinks. Also keep in mind, your RX still needs power and ground, and depending on what you've picked, you might want it as far up on the food line as it can get -- some RXs can get finicky if the voltage gets lower than they care for.

Shameless plug:

you might even consider building a pair of these -- http://flitetest.com/articles/simplified-twin-control-harness-for-the-kraken.

I'm assuming you're already routing high voltage power from a battery bay in the middle to ESCs out on the wings, so if you build two of these, they would route the signals to the RX with a single servo wire per wing, and the fuselage electronics would get power from the BEC in there.

All very good ideas. That article is very interesting. I think I want to try to keep all of the electronics as modular as possible though instead of having a built up harness to try to route through the wing tubes. I can easily implement the split load though. I had never considered that.

 

[Craftydan]

It's up to you. The harness would actually reduce the wires being routed through your wing tubes, and the pig-tail design is intentionally there so only the extension is routed, then the short harness is plugged in at the end, joining everything up. Everything still remains perfectly modular.

In this particular case, since you've also got retracts in the wings, you'd need to add on another row of pins and another wire to match . . . or build the whole extension out of a four conductor strip of ribbon cable, and provide servo power locally from the ESC

A distributed harness would work best with a small bay at the Nacelle for everything to join at. If you've built one to house the ESC behind the motor, that starts making more sense. Otherwise you get to feed servo extensions and power cables into your wing tube (granted you've got a bigger than average wing tube). Three extension runs + power cables isn't unreasonable, but you've got other options.