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Sep 18, 2019, 10:30 PM
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Build Log

Big Foamy Bomber!


Hey folks,

Last year at the NEAT Fair we kicked off the first Battle of the Tents (or BOTT for short). It was a friendly competition between the various clubs attending the show, and the Pigs that Fly won the initial award. On my way home I thought our group (i.e. the Big Top/Killer Rabbits) needed some additional "firepower" to have fun with our neighbors. That idea rolled around in my head until earlier this summer, when I realized that it was time to put the plan into action, and thus the Big Foamy Bomber (BFB) project kicked off.

The idea was as follows: (A) build something sizable to drop lots of stuff on our friends, and (B) keep it simple (well, relatively so). I made a few decisions up front that drove the overall design:
- build the airframe from 1/2" pink foam
- keep the wing layout as simple as possible
- use parts I already had, or could be printed on my 3D printer.

The wing was the challenge - I was trying to do this project in three months or less, so designing and building a traditional built-up wing wih a Clark-Y was going to likely blow my time budget. So I decided to try something non-traditional thus I went with a KFm-3 airfoil. I haven't seen this used for large planes (might have missed some examples) but I figured this would be a good experiment. I end up with approximately a 10% thickness airfoil section at the root (18" root). BTW I might have forgotten to mention the wingspan of the plane is 8' 9" (each wing panel is 4' long).

Earlier this year I picked up a Shapeoko XXL for my shop, and I decided to use it for this project. I modeled the plane in Fusion 360 and used the CNC to kit the fuselage, tail section, and wing control surfaces. Since the wings were essentially 4' long tapered sections of foam I opted to cut those out with a razor and straight edge.

Here's a shot of the plane from Fusion - I'll start showing the build in the next day or so. Thanks!

Rob
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Sep 19, 2019, 10:14 PM
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Starting with the wings...


One of my original goals was to build the entire airframe from two 4'x8' sheets of 1/2" pink foam. Ultimately it took an additional 1/2 sheet but I was close

To maximize the use of the sheet I opted for 4' wing halves. The wings are a double-taper with an 18" root and 12" tip. With the KFm-3 airfoil the second layer was 75% of the bottom (13.5" root/9" tip), with the top layer 50% (9"root/6" tip). The 25% of the bottom section that wasn't covered was used for the ailerons and flaps (20" ailerons/16" flaps) with the remainder left in place at the root. I used a 1/2" router bit to cut a 3/8" deep slot in the top and bottom wing sections which I used to locate top and bottom spruce spars.

To join the wing halves I used a piece of 3/4" x 36" aluminum tubing from Home Depot. After a little pondering about what to do about a tube socket I decided to print them out of ABS. The bonus of taking this approach was that I could make them rectangular and fill the gap between the spars so everything would be tied together. Since my printer prints to a max of 9" in height I had to make each wing socket half in two pieces.

Before joining the whole assembly together I had to do two other things:
- add hard points for the control surfaces
- incorporate motor mounts.

In order to hinge the surfaces I added 1/2" thick balsa to the trailing edge of the wing and leading edge of the control surfaces (removing the equivalent amount of foam in the process). I used traditional pinned hinges for the assembly - this foamy wasn't getting tape hinges

I happened to have a pair of E-Flite Power 60s in the shop meant for another project, so I "borrowed" them for this endeavor. I also had a pair of counter-rotating 15x7" three bladed props which would do the trick nicely. With this I located the motors inboard of the root so that I had about 2" of clearance from the fuselage, and then designed up a set of motor mounts that would be sandwiched between the bottom and middle layers of the wing. I added a bottom plate to allow me to bolt everything together, plus act as the landing gear mounts. I used 1/8" maple ply and CNC'ed them with my Shapeoko.

Before gluing everything together I located where the aileron and flap servos would be located, then added cut-outs to the bottom of the wing for them. I also added an additional slot behind the cutouts through which I would run the servo leads. To make things easier later I ran string from the servo openings to the root so that I could pull the leads.

With all the parts now ready and fitted, I first used Gorilla glue to glue the motor mount to the wing bottom (using the landing gear mount to clamp the assembly), then I used Loctite High Performance spray adhesive to join the KFm airfoil sections together. I found that this particular spray adhesive wouldn't eat the pink foam as long as you didn't lay it on too thick. With the wing sections now joined I used Gorilla glue again to secure the wing tube socket and spruce spars.

The last thing I did with the wings was design up a set of cowlings for the motors and mounts. I also drew up a set of nacelles for the wings but ran out of time before I could finish and print those, but the cowlings were printed using natural PLA.
Sep 20, 2019, 10:47 PM
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Kitting the fuse and tail...


My Shapeoko XXL will fit material up to 32"x30" in size, so I cut each 4'x8' sheet of foam into six (6) pieces at 32"x24" each. I then used the Manufacture option in Fusion 360 to lay out the parts for the fuse and tail to each piece of stock and generated the toolpaths. BTW I totally understand that it's quite likely that it would have been faster to do this all by hand, but the accuracy of the CNC (plus the cool factor of watching it work) made it all worthwhile.

Since many of the pieces were longer than my stock I broke those components down into subcomponents and keyed them so that they'd fit together nicely. BTW I finally found a good use for my old Monokote Woodpecker - I used it to make holes in the foam where the joints would be. I figured that way the Gorilla glue would make a better bond versus just having the flat sides mating with each other.

Once all the parts were cut out and joined together I snapped together the assembly to see how it worked out. This is where the CNC really showed its stuff. I was able to assemble the entire fuse and tail group together before ever applying glue - cool!

Rob
Sep 20, 2019, 11:33 PM
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Fuselage assembly


With the fuse kit done I moved back outside to glue it together. As you've noticed this is a pretty boxy design. Honestly, I was trying to keep this simple (haha) and also that gave me a larger cargo capacity. Speaking of the bomb bay, the bay is 18" long in case anyone was curious...

The fuse is in two parts - the front section where there wings mount, and a tapered rear section. Each section was assembled separately then joined. To help tie the front and rear sections together I added a keel in the rear that plugged into the rear bomb bay bulkhead on the front section. Once the two sections were glued together I sanded the seams then applied fiber-reinforced tape over all the joints plus down the sides to give it a bit more stiffness.

While I was using the Shapeoko I made up a plug-in nose with cutouts so that I could make it look like a gun nose. I'll readily admit that this front end was less than aerodynamic - my plan at this moment is to make up a new nose section this winter. But with the clock ticking I had to keep moving...

At this point I broke out the scale to see how much the empty foam-only airframe weighed. I was shooting for a final AUW of 12-13lbs, which with 9 square feet of wing area would give me a wing loading around 23 oz/ft2. At this point the empty airframe weighted 7lb 6oz, so I think I've got a shot at getting close to my final target weight.

Rob
Sep 20, 2019, 11:55 PM
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Ply reinforcements


With the fuse and tail assembled (with hard points added to the stab and vertical fins and rudders) I turned to adding plywood reinforcements in key areas. I CNC'ed a set of plywood reinforcements for the fuse where the wing attaches, the tail saddle, and the top of the stab to mount the elevator servo and mounting bolts (the tail is removable via three 1/4" nylon screws). The fuselage sides and reinforcements have machined openings for a wing tube bushing, an alignment bushing for the wing, and two holes for the servo and motor leads. With the fuselage reinforced and the wing tube bushings in place I realized that I needed a spacer between the bushing to keep the fuselage sides from flexing, so I printed up a new section of tube sleeve to span the opening.

Rob
Sep 24, 2019, 02:23 PM
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radfordc's Avatar
Nice project! Big foamies are fun. Here is my 8 ft span Fokker Triplane with KFM-2 wings.
Latest blog entry: Assembling a Spandau kit.
Sep 24, 2019, 03:57 PM
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Originally Posted by radfordc
Nice project! Big foamies are fun. Here is my 8 ft span Fokker Triplane with KFM-2 wings.
Very cool! The pink foam does seem to lend itself nicely to larger models (I just wish I had more space). Nice to see the KFm-2 airfoil worked for your application.

BTW do you have a build thread on Fokker - I have a friend who'd be very interested in this

Rob
Sep 24, 2019, 07:01 PM
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Here's the built thread.... https://www.rcgroups.com/forums/show...Board-Triplane
Latest blog entry: Assembling a Spandau kit.
Sep 27, 2019, 07:13 AM
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Rob, does the machine fly? Paul
Sep 27, 2019, 07:20 AM
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Petefoss's Avatar
It not only flies very well, it dropped a ton of stuff at the NEAT Fair!
Sep 27, 2019, 10:32 AM
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I love your use of 3D printing in this design. I built a 110" span foam Beaver and used 3D printed cowl, dummy engine, tail wheel steering arm, flap and aileron hinges, landing gear mounting brackets, and servo mounts.

I will probably use your wing mount technique someday. Thanks.
Latest blog entry: Assembling a Spandau kit.
Sep 27, 2019, 02:03 PM
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Quote:
Originally Posted by radfordc
I love your use of 3D printing in this design. I built a 110" span foam Beaver and used 3D printed cowl, dummy engine, tail wheel steering arm, flap and aileron hinges, landing gear mounting brackets, and servo mounts.

I will probably use your wing mount technique someday. Thanks.
Thanks a bunch. I'm a big fan of integrating 3D printed parts into my designs. I was pretty happy with the control horns - they worked very well - they aren't very scale but if you want I'd be happy to share The same thing goes for the wing mounting parts.

Rob
Last edited by stangrob; Sep 27, 2019 at 03:03 PM.
Sep 27, 2019, 02:18 PM
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Originally Posted by FlyingW
Rob, does the machine fly? Paul
Well, I was going to wait until I finished up the build thread but since you asked nicely (and Pete chimed in )...

The plane flew very well, both loaded and unloaded. The plane came in at 19lbs (without payload), but with 3200W of power it flew with authority (170w/lb). The KFm-3 airfoil did a nice job. You have to fly it like a real airplane - coordinated rudder in the turns with power. If you try to yank and bank it like a small sport plane it will want to drop a wingtip in those turns but if you drop the nose and give it power it recovers right away. And as you'll see in the final pics this version isn't the most aerodynamic (the nose is flat) but the advantage of that is it bleeds off speed quickly, which makes for nice short, steep landings. The plane has flaps but we haven't had to use them yet.

Here's a quick video that Antslake took of the plane on one of its runs at NEAT - I think it's pretty cool...

Rob

Have you ever wondered what it's like to be bombed with whiskey, literally? (0 min 46 sec)
Sep 27, 2019, 06:29 PM
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FlyingW's Avatar
That's brilliant. Let's hope next year the ground forces do not deploy some kind of foam flak anti-aircraft countermeasure.

Nice job Rob.


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