Dear fellow aircraft modelers!

I'm sure I'm not the only one here who's got a thing for biplanes. Lately I've had this idea to put one in my hangar.

My initial choice was Pitts. But after I gave it a little thought, considered all the complications (cowl, wings tips and tail) and settled for a Comet. Somehow, it seemed easier to construct.

Fk 12 Comet is a small biplane designed in Germany in 1997. It comes in a one-seat or two-seat modification. The aircraft is 5.5 m long, its wings span 6.7 m and the maximum takeoff weight is 450 kg. Here's a video to get you started:



Sadly, I couldn't find any design sketches around the Web. I came across this pic, though:

I resolved to build a model with a wingspan of 1.5 metres with its parts as folding and portable as possbile. It took me quite a while to sketch a draft in Kompas 3D software. After that I took to construction.

I built the inner frame out of 8 mm Styrofoam, like I usually do with my models.
Click the photo to zoom in.


Next thing I did was glue side frames onto the hull and then reinforced the front side with poplar strips (8x4 mm).

The strips are necessary for fixing the upper wing center section struts.

Moving on: I added a wiring duct and a cockpit with removable hatch. Then I glued 3x3 mm cross-section strips to the tail side to replicate the covering.


The tail and horizontal stabilizer were getting prepared at the same time as the fuselage. I bent a 6x3 mm poplar strip to for the outline. The struts are 3 mm Styrofoam. The strip was sturdy, so I had to incise the point of inflection, flex it and then glue it back





I glued two Ø3 mm carbon fiber tubes into the tail frame. They are necessary for further attaching the tail and horizontal stabilizer. The covering and ribs here are 1.5 mm Styrofoam. The rudder and elevator are crafted out of carbon fiber tubes, Styrofoam ribs and covering. I placed the hinges onto the tube right away (no photo of the hinges here, see the ailerons description below).

The upper wing center section ribs were made out of 3 mm plywood. 8 mm diameter tubes will serve as wing panel fixtures. Aluminium plates will fix the struts.


Center section is pasted over with 4 mm Styrofoam, 6 mm Styrofoam for fuselage.The part of fuselage, where the strips are glued on, is pasted over with 3 mm Styrofoam to replicate the covering flexure.

The upper center section was quite a headache to get right. This part turned out to be the most tricky, however, I was determined to follow the original construction. After all, it looks really gorgeous.

The struts here are made out of slightly flattened TV antenna tubes. The small screws, that hold these tubes, were found in a local mobile phone spare parts shop.

The tail rigging wire is just like on the original aircraft.

Now, the wings. I built the ribs along with strut fixtures all at once.

Ribs and struts are made of 3 mm plywood. I fit DIY rectangular M3 screws into the struts. After that, I glued two laps, forming a slot for the strut, onto the ribs and drilled a mount screw hole.

Pasted both sides of the struts with 2 mm poplar veneer, reduced the ribs' weight.

Struts are now done. They are fast and easy to attach, just tighten the four screws.

The wing consists of three spruce strips and Styrofoam ribs.

The first, fourth and tenth rib in the upper wing are made of 3 mm plywood. The tenth rib is the one we're already familiar with, the fourth one has a wing lock attached.

The lock is made of a plastic cutting board.

It's basic idea is quite simple, as you twist the screw the carbon fiber tubes clamp locking up the wing panel. If you need to remove the wing, just loosen the screw, the panel will come off effortlessly.

The bottom wing panels have another plywood rib for installing the aileron servo.

The wing is pasted with 4 mm Styrofoam. 2 mm thick Styrofoam is used on the part where fabric covering sag is replicated.

Now, on to the ailerons. They are based on carbon fiber tubes. The upper aileron tubes have a diameter of 3 mm, while the lower ones are 4 mm. The ribs are 1.5 mm balsa wood pieces, except for one which is aluminium and has a bracket for fixing the pushrod. Small glass fiber pieces, attached to the tubes, serve as hinges.

The ribs are secured on tube with CA glue. The aluminium rib is glued on with epoxy adhesive, it should keep firm upon force transmission.




Aileron attached to the wing.

I'd like to point out that the big number of ribs in wings and ailerons serves to better resemble the original wing covering. This photo illustrates texture on the wing and horizontal stabilizer.

The cowl is crafted out of Styrofoam, see the how-to here: EXTRA - my first experience+3
Jan 27, 2016, 10:37:26 PM | Roman Slavinskiy Doneck
Article http://www.rcfair.com/blogs/view_entry/13194/


Every single surface was emeried and covered with fabric to ensure integrity. I did the emerying and painting part in my garage. Frankly, building a 1.5 m wing span model in an appartment proved to be a questionable idea.

I used a nitro enamel commonly available in hardware stores for painting. It comes in 0.9 litre tins. Lettering and numbers were stenciled through a plastic sheet with symbols plotter-cutted in it. Painting turned out to be a much more time-consuming activity compared to the process of building. I think it was worth it, though.

The plastic stencil should be removed as soon as the paint has dried to the touch. After using it several times the glue layer may remain on the model, in my case, possibly due to the heat. The temperature in my garage was 32 °C. If there's some glue left, treat it with white spirit (adhesive remover).


Oh, I almost forgot about the lower center section. I'll ditch the talk, just see the picture. It has landing gear strut attached to it, while it itself is attached with screws to the motor mount, which is embedded in fuselage.


I used heat gun on polystyrene to make wheel fairings.




The motor mount is carved out of 4 mm construction veneer, 6 mm for the front side where the motor is mounted. The motor mount has a battery compartment.

The motor mount is removable and, as I said earlier, held down with screws along with the lower center section.


Before I'd started painting, I made the upper side of the cowl removable for ensuring easy access to the motor and battery.



The cowl can be removed by loosening the screws that fix it to the motor mount.

There's a removable hatch under the cockpit where electronics and wiring can be accommodated.



Plastiс pin horns, available in stores, didn't quite fit in the concept of the model. A little rasp work, and these DIY aluminium beauties are ready to be mounted!

The pin horns go right onto the tube.

Since the tail and horizontal stabilizer are removable (also, to save the husstle with pushrods), I decided to install the servos inside the tail and horizontal stabilizer.




Here's the aileron drive connection. Pin horn and forks are duraluminium.

If you're following my story, you should remember that there was one aluminium rib in ailerons. It had a bracket for attaching the pushrod. It's time to build that pushrod.

It's made of beechwood, 6x3 mm, with aluminium tips. One tip is dead-fixed with glue, the other one is attached with thread and adjustable.

As soon as the pushrod is assembled, I shape it to the proper cross-section and paint it over. Then put it where it belongs.


And we're now down to the final part, the wing rigging wire. It took me some time to figure out how to make those bastards functional, not just fake. A couple hours spent with an electric drill and a thread file and we're done: behold these tension tips!

Now let's assemble. Put the cable (0.8 mm stainless steel wire) into the threaded tip and rivet it over like you would do to a nail head. Make sure the length of the cable under tension allows a few spare turns on the threading.



Not sure how to call little plastic part. It's there for a better resemblance.

These foam parts will make sure you carry and store wing panels and their fixing tubes safely.


A few pics of the almost-finished model.








Let's sum it up.

Weight: 2,400 g not including battery and ESC (with motor and servos installed)
Wingspan: 1,460 mm
Total wing area: 60 sq. dm
Horizontal stabilizer area: 8 sq. dm
Wing loading (flight weight of 2.8 kg): 46 g per dm

Not accounting for the electronics, building this model is dirt cheap. A 50 mm piece of polysterene foam, a scrape of lining cloth, three cans of paint (yellow, white and black). It's all about affection and a bit of handwork!

Sadly, it won't take flight soon. It is yet to be equipped, which is really challenging. There's no way to have a parcel delivered to Donetsk these days. Not to mention fierce militarymen with assault rifles who put the town under siege making a walk in the field a farily dangerous activity.

That's all I had to tell you about my Fk 12 Comet model.

There's another German aircraft that caught my eye, though:

But that's another story.

Thank you all for reading my reviews.

All of you who felt they were helpful, thanks a lot! I gotta say, it's really fun to build a model. It's exciting! Go for it, fly your airplanes high and stop making onetime crap.

Yours sincerely,
Roman

In response to public request, I attach files with the sketches:
Kompas 3D file (ver.13): (Fk12CometForRCFair.frw)
For those AutoCAD savvy: (Fk12CometForRCFair.dxf)
CorelDraw vector graphics for plotter-cutted stencils: (Fk12CometStencils.cdr)

Full-size bitmap graphics for those at odds with design and drafting tools (may take a bit to load): (Fk-12ColorScheme.jpg)
(Fk-12BasicDimensions.jpg) (Fk-12Wing.jpg) (Fk-12Fuselage.jpg) (Fk-12StabilizerAndMotorMount.jpg)
(Fk-12WingStrutAndFin.jpg) (Fk-12Support.jpg)

Have a great time building and flying!
This is the translated version. You can read the original Russian article here.