Fiberglass Bloody Wonder, Version 2

[rcfred]

DETAILS

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A while back, I made an FT Bloody Wonder using foam and fiberglass (build thread here). Lately, I've been working on a new version of the airframe, this time with a design more tailored for composite construction. Major changes in this version:

Custom design
My original build was based off the Flite Test plans, with some minor modifications. This time around, I'm making a fresh set of plans, borrowing features from a few variants that others have built.

Hot wire cut wing
The foam core for the wing is cut from blocks of insulation foam using a hot wire. I had originally planned to do parts of the fuselage with the hot wire cutter also, but I scaled my plans back due to my current lack of skill.

Rudder
I'm adding a rudder to facilitate landings in crosswinds and to permit maneuvers like stall turns and snap rolls. I'm also moving the tail back a few inches to make the plane less short-coupled.

SPECS

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WINGSPAN​	30 in
FUSE​	28 in
CHANNELS​	4
WEIGHT​	700-800g (target)
ELECTRONICS​	LIGHT MOTOR - NTM 28-30 1200kv 68g 400w 28a PROP - 8x6 ESC - Turnigy Dlux 30a BATTERY - 4s 2200 SERVOS - 4x 9g HEAVY MOTOR - NTM 28-36 1000kv 30A PROP - 7x6 ESC - HobbyKing 40A BlueSeries BATTERY - 1300mAh 25C 6S SERVOS - 4x 9g
BUILD DIFFICULTY​	Beginner – You’ve never flown a plane before, let alone done a scratch build Novice – You’ve flown a plane but have no experience with scratch building Intermediate – You’ve flown and built a few FT style planes before Advanced – You’ve mastered the standard FT build style and are looking for a new challenge Expert – You’ve mastered all build techniques and aren’t afraid to improvise or tackle tedious builds
MATERIALS​	1 sheet DTFB 1 Foamular Project Panel, or about 4 square feet of 1-inch EPS foam insulation 1 BBQ skewer About 1 square yard 0.5 or 0.75 oz fiberglass cloth (Available on EBay) About 6 oz of laminating epoxy (Available from US Composites) About 8 oz of microballoons, also known as "glass bubbles" (Available on EBay or from US Composites) 4 yards of carbon fiber thread (optional)
TOOLS​	Hot wire cutter (See this article) Single-edge razor blades for cutting foam and fiberglass 1 oz epoxy mixing cups (Available from US Composites or you can use plastic souffle cups) Popsicle sticks for stirring epoxy (available at Dollar Tree) 3ml oral syringes for measuring resin and hardener (available from pharmacies, or search on eBay for "oral syringe 3ml") Plastic squeegee for spreading epoxy (available at Harbor Freight or at US Composites) Disposable latex or nitrile gloves Sandpaper in all grades from 60 to 320 grit Rosin paper for keeping epoxy off your work surface (available at Home Depot) (Optional) Dremel tool for cutting servo slots

PLANS

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Current BETA plans: View attachment fgbw_plans_v2.1.zip

Version 2.1 June 2015	Change log --First full prototype --Switched to box fuselage (inspired by the Bloody Wonder MkIII) --Added rudder --Lengthened fuselage
Version 2.0 January 2015	Change log --Initial design study --Hot wire wing and rear fuse

FAQ

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TBD

PICS

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VIDEO

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TBD


REVIEW

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The review is broken down into major characteristic groups that capture what most pilots will look to learn when researching a plane.

STALLS
TBD

WIND
TBD

LAUNCHING/LANDING
TBD

ACROBATICS
TBD

GLIDE SLOPE
TBD

LOOK AND FEEL
TBD


FINAL COMMENTS

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TBD

 

[rcfred]

Fuselage


The fuselage is based loosely off of the fuse of the Bloody Wonder MkIII. I used DTFB with the paper removed. I glued the pieces together, then sanded down the corners.

After that, I filled the pores in the foam with a mixture of epoxy and microballoons, sanded the surface smooth, and covered the bottom and sides of the fuse with fiberglass cloth. The top of the fuselage is not glassed at this point. The rear of the fuselage will be glued directly to the bottom of the tail. I will glass the front top after the wing is attached.
The fuselage sides are of double thickness for the first 2/3 or so, tapering to a single thickness at the tail.

In the middle of the fuselage is a saddle for the wings. I've cut some grooves in the saddle for the servo wires.

The rear of the fuselage curves up to meet with the tail. Gluing that curved piece in place was a bit tricky; I needed to pin the foam in place while the Gorilla Glue dried. I may add some tabs or something to the next version of the plans to make the tail self-jigging.

The rear of the fuselage slides into slots in the horizontal and vertical stabilizers.

 

[rcfred]

Wing

I built a hot wire rig to cut the components of the wing. The design is an amalgamation of various designs I found on different forums.

The electronics are a cheap brushed ESC attached to my servo tester. I put a 15 amp fuse on the battery leads to keep from burning down my house in the event of a short circuit.

The nichrome cutting wire is suspended between two all-thread posts. The ESC's output leads are wired to the base of the posts. Nuts on the all-thread allow for height adjustment, and rubber bands provide tension.


I designed the wing in three parts, thinking that it would be easier to cut that way. In retrospect, I didn't know much about what is easy and what is hard with these foam core wings. I'll probably switch to a two-part wing for the next iteration. Anyhow, there's a trapezoidal center section and two tapered outer sections.

On the left hand side of the above photo you can see the two ailerons I cut off the trailing edge. Those pieces were too irregularly shaped to use, so I made some new ailerons out of laminated foam board.


I decided to stick with a close approximation of the normal Bloody Wonder airfoil. I made templates in the shape of a Clark Y airfoil. I printed the templates out, glued the printer paper to some rosin paper, then saturated both layers of paper with laminating epoxy. I cut out the airfoil shapes with scissors before the epoxy had fully cured.

In keeping with the original Bloody Wonder design, the airfoil thickness remains constant across the wing. The thickness at the center of the wing is about 7% of the chord, while the outer edge is a little under 10%. The fatter airfoil at the wing tips gives the wings some aerodynamic washout, so I did not add any geometric washout; that is, both templates were mounted at the same angle. The initial shape of the foam cores was pretty bad, but after a few rounds of filling and sanding they kind of approximate the intended airfoil shape.


I used some short lengths of BBQ skewer as dowels to hold the pieces together while gluing.

A pleasant side effect of the choice of airfoil is that the bottom edge of the wing lays flat. This made clamping the wing pieces easy.


Once the glue had dried, I added some reinforcement to the wing in lieu of a spar. I used a dull pencil to make a series of grooves across the top and bottom of the wing.

I tacked some strands of carbon fiber into the grooves with a few drops of wood glue.

I saturated the carbon fiber thread with epoxy, then I leveled the surface with an epoxy/microballoons mixture and sanded everything smooth.

Those tiny strands added a surprising amount of strength for almost no weight. I probably could have used fiberglass instead of carbon fiber without adding much more weight, though.

I marked the locations of the aileron servos, then cut out servo pockets with my Dremel and a router attachment.

Before moving on, I dry-fit the wing to the fuselage and made sure that the servo leads had enough room.


Next I sheeted the wings with 0.75 oz fiberglass cloth. To make things a bit easier for myself, I used five pieces of cloth: Two pieces on the top (left and right); two pieces on the bottom; and a strip across the leading edge. I used some scrap foam to hold the wing vertical while the epoxy set on the leading edge.

The areas where the pieces of cloth overlap provide some additional reinforcement at the center of the wing and across the thickest portion of the airfoil. The resulting structure is quite stiff.

 

[rcfred]

Dry-fitting

I finished applying fiberglass to both sides of all the foam board components, so I was able to dry-fit the parts of the airframe.

The control surfaces are just Scotch taped in place here. Everything fits except the ailerons, which are about 3mm too long and 1mm too thick. I probably should have double-checked those dimensions before I glassed them. Shouldn't be too hard to fix, though.

Here's a photo of the new airframe next to Version 1:

 

[rcfred]

Shaping and Gluing

On my previous build I had used a household iron to round the leading edges of the fuselage. This time I used heat to round off and bevel all the foam board pieces. The fiberglass tries to return to its original form as it cools, so I had to alternate between the iron and a block of wood: Heat for 10 seconds with the iron, then remove the heat and press the block of wood up against the piece until the resin cools, then repeat. After a few rounds, you can make nice straight bevels:

and smooth trailing edges:

It took some practice to get the technique down, but I was moving along at a good clip by the time I got to the last piece.

Once all the edges were rounded, I double-checked the fit one more time, then glued the four main parts of the plane together with Gorilla Glue.


A laser measure comes in handy for making sure the plane stays straight as the glue dries.

 

[rcfred]

Fillets

I decided to mount the tail servos on either side of the vertical stabilizer. To cut down on drag, I made the fillets at the base of the stabilizer large enough to cover the cross-section of the servos. I started with a rough structure made from some strips of scrap foam.

Then I mixed some spackiling from microballoons and water-based polyurethane to fill in the gaps.

Using WBPU instead of epoxy worked well here. The polyurethane I was using is only about 25% solids, so the filler was porous and soft once the solvents had evaporated off. I was able to sand the fillet without worrying about sanding into the underlying fiberglass.

On the underside, I initially made a 1cmX1cm fillet from some strips of foam, hand-sanded into shape with a sanding drum:


When I was done sanding, I took a step back and thought, "that fillet looks kind of thick". I consulted my reference book and found that the fillet was indeed too large. The correct size for a fillet on a 90-degree joint is the minimum diameter that the fiberglass cloth can conform to. I ended up prying off most of the foam, sanding down the rest, and making a new, thinner fillet out of microballoons and WBPU.


I used wedge-shaped scraps from the trailing edge of the wing core to fill the gap at the rear of the wings.


I used the WBPU-microballoons mixture to fill the gaps on the top side of the wings.


Next I cut down some scraps of 0.75 oz cloth to make reinforcements for all the joints between glassed parts. I also put down a larger piece across the top of the front fuse, which was still bare foam at this point.


I wetted out the cloth with epoxy and dabbed up the excess with some paper towels to finish off the joints.

 

[rcfred]

Painting

Hello again, everyone! I’ve been a bit preoccupied with our new baby for the past few months, and this build has slowed down quite a bit as a result. I have managed to sneak in bits of work here and there, though, and the model is nearing completion.

Before painting, I filled the weave of the fiberglass reinforcements using a mixture of water-based polyurethane and microballoons, tinted with a drop of blue acrylic paint so I could see what I was doing.


I used Minwax Polycrylic, which is only about 30% solids. The resulting resin-microballoons mixture was pretty porous when the WBPU set, which made it easy to sand but difficult to sand smooth. To get a smoother base coat, I mixed latex primer with microballoons and painted the entire plane using a brush.


Then I sanded everything back with fine grit paper.


Around this point, someone sat on the elevator and rudder. Oops. I cut out and glassed some new control surfaces. On the replacement control surfaces, I tried out a different finishing recipe which gave better results with significantly less effort. Immediately after applying the fiberglass cloth, I poured some microballoons on top of the wet cloth. Then I smoothed out the layer of microballoons with a plastic squeegee. The microballoons pulled excess resin out of the cloth and formed a layer that was easy to sand back once the resin set. I put down one more coat of WBPU + microballoons to fill the remaining imperfections in the surface, then sanded.



Instead of primer, I used a very thin layer of epoxy to seal the surface of the WBPU/microballoons mixture. Then I painted the parts. Here’s a closeup of the joint between elevator and horizontal stabilizer. The elevator, finished with the new method, has a noticeably smoother finish.



The white base coat is Minwax self-priming spray paint left over from a previous build.


The black paint is Minwax oil-based paint for metal, applied with a brush. I picked up a gently-used quart for $2 at the Home Depot paint department.



The red paint is gloss acrylic paint from Walmart, applied with a brush. After applying masking tape, I put down a thin coat of white to prevent the red from tunneling underneath the masking tape. At first I used spray paint, but then I discovered that conventional oil-based Minwax paint (left over from painting the railings of our stairs) is a very close match in color. So I switched to just using a brush for the white.



I needed three thin coats of red to get consistent coverage. The paint dried quickly, so I didn't need to wait long between coats.



Once the paint had dried, I peeled off the masking tape to reveal the completed paint job.


Still a bit rough around the edges, but it came out better than my previous model.

 

[rcfred]

Final Setup and Initial Flights

I used half-inch Blenderm tape ("hinge tape" on HobbyKing) for all the hinges. I've had some trouble with the Blenderm tape coming off on Version 1, so I reinforced the high-stress parts of each hinge with Extreme packing tape. The stuff doesn't look as nice as the Blenderm, but it has a very strong adhesive.


All control rods are Dubro 2-56 threaded rods with nylon clevises. That's 2.2mm for you metric types. Stiff control rods are a must when building from fiberglass.


I used Flite Test plywood control horns, but in retrospect I probably should have made my own. The FT control horns just don't have enough plywood left around the holes when you ream them out to 2-56. The control horn on the elevator cracked due to the lack of wood and a less-than-ideal angle. I cut it off and replaced it with a control horn I made from a hotel key card.


I designed the fuselage to extend below the base of the power pod so that I can mount the speed controller on the bottom of the pod.

Note the popsicle stick that holds the front of the power pod firmly in place.

Having the speed controller on the outside leaves plenty of room inside for the battery.

There are cooling vents in the firewall for the battery; and I glued about 3/4 inch of EPP foam at the front of the power pod to prevent the battery from impaling itself on the motor's shaft. The firewall is 1/8-inch plywood glassed with 1.5-oz cloth.



Initial Test Flights
I miscalculated the CG rather badly and ended up with the center of gravity more than an inch too far back. The maiden flight was scary, but I managed to get the model down it one piece. I moved the battery all the way forward and tried again. Still tail-heavy, but I was able to make a circuit of the field and land a second time.
I moved the mount for the power pod to the very front of the fuselage (the firewall had been about 3/4 inch behind the front of the fuse), mounted a 2200 mAh 4S battery all the way forward, and went out to the field again. This time the plane was a bit nose-heavy. I landed and moved the pack about 3/8 of an inch back, then did a fourth flight. This time the CG was about right. I was able to make nice controlled figure-eights across the field and a gentle landing. I haven't been able to fit in any additional flights since. Once I do manage to get out and fly again, I'll get the plane properly trimmed up, then try swapping in my 6S power setup.

 

[PBernett]

Nice build and documentation on your techniques. Your plane should be pretty much indestructible. I have a 36" pink foam wing that I built with no spar and just used fiberglass tape taped on the top and bottom along the length and then covered the wing with packing tape. I have used it on a plane powered by a .15 glow engine. Plenty of power. That wing has been through a lot including crashes and it still is in tact. My point is that if you are looking to reduce weight you may be able to do so and still have a strong enough wing. I'm not sure if your fiberglass is heavier than packing tape. It may not be and therefore you have a considerably stronger wing in the same weight range. You may want to experiment with wings now that you have your hot wire apparatus in place and try building a lighter wing and see how it holds up. I really like that you are experimenting. Keep it up that is how we all learn.