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FTCC'18 Ohka Model 43(x) -- Team Motherless Child


Cardboard Boy
This cheater intake actually looks quite similar to the annular intake of my Messerschmitt so it should work quite well, if I remember correctly the wikipedia article claims a 4% decrease of airflow to the engine but 20% decrease in drag.

One thing I thought on doing with mine is cheater holes on the wings, this would actually require the EDF to work harder to suck the air all the way from the spar but apparently it was tried in a X plane as a way to reduce the boundary layer and decrease drag.

Anyways it's almost 3am and I just wanted to say: looks good!


Maneuvering With Purpose
We might want to verify the dimensions. I believe we will end up with a fatter fuse. It looks skinny to the naked eye. Do you have a hole in the nose? That will help. Also the canopy? Where did the leading edge of the fan wind up?
Unfortunately, you might not have a whole lot of options there. Even with a larger inlet at the nose, the electronics and battery have to be stored somewhere and that fuse does not have a lot of space. Take the 3D sublet and turn it 45* then add the aircraft intakes aft of the sublet. You should have enough airflow then.

You may have to use something other than foam for the vanes as it will be a structural necessity.


Elite member
@SlingShot - just saw your PMs on the dimensions - will have a look at these later

@Namactual - yep, there are not going to be any great options for inlets so if we want to do an EDF version we are going to have to compromise on some scale details. A pusher version would probably be fine.

This is a post maiden pic of the plane and a close up of the holes I had to cut for the EDF - not pretty!

2 - post maiden.JPG

3 - hacked in holes.JPG

But she flies!



Elite member
Thanks Guys!

You loved it! Tell the truth..... Flying it I mean....
Hand launching it into a knife edge was a challenge, but yep, it was not a bad little flyer. The EDF sounded a little different than usual, I am wondering if that it a sucking sound as it tries to get more air in. It could also be the outlet/exhaust which is really not ideal because I had to perform more hacking to allow the elevator to have some down throw but this meant adding a cross piece of foam that constricted the outlet.

All that being said, with some more work on the design to remove the hacks, I think this will be a nice plane to build and fly.



Maneuvering With Purpose
Thanks Guys!

Hand launching it into a knife edge was a challenge, but yep, it was not a bad little flyer. The EDF sounded a little different than usual, I am wondering if that it a sucking sound as it tries to get more air in. It could also be the outlet/exhaust which is really not ideal because I had to perform more hacking to allow the elevator to have some down throw but this meant adding a cross piece of foam that constricted the outlet.

All that being said, with some more work on the design to remove the hacks, I think this will be a nice plane to build and fly.

Hehehe.....it did look a little hairy on the takeoff. Nice recovery!


Building Fool-Flying Noob
I just re-watched the FT vid on the vampire (from Eflite or HK?). They solved the intake issue without ruining the scale look by adding gill slots on the bottom of the plane like a manta ray.

that looked amazing. I'm excited to see the final plan. and vid.


Elite member
I just re-watched the FT vid on the vampire (from Eflite or HK?). They solved the intake issue without ruining the scale look by adding gill slots on the bottom of the plane like a manta ray.
That's a great suggestion FoamyDM - Thanks!. I looked up the vid and the "gills" approach could work. Not 100% sure about putting them on the bottom as this is a belly lander - but definitely something to think about because even on the lower sides, they would not be so visible.


Edit - having the gills on the bottom didn't seem to bother Josh when he was landing on the grass so maybe I am overthinking the "sucking in grass" thing.


Elite member
Thanks again folks for the ideas on the intake for the EDF.

I will admit to being fascinated by the concept described by Namactual

You might try a 3D sublet and a reverse cone around the circumference of the fuse section just forward of the EDF. From a distance you wont even know its there, and it will have great airflow.
Doing some math on this, the fan swept area (FSA) of my 70mm EDF is around 4.75 square inches, lets call it 5 for now. So just from an FSA perspective (and ignoring the shape and aero components), I need 5 square inches of inlet (just over 100% FSA) to feed the fan.

The next version of the plane will have a fuse diameter of 4 inches based on the scaling work done by SlingShot.

So what kind of gap would you need between two cylindrical fuse sections to give you 5 square inches of inlet? Using a number of online calculators, a gap of just 1/4 inch produces an inlet surface area of 28 square inches! You gotta love Pi.
But the fuse is not actually circular but a 24 sided cylindrical prism which reduces the surface area calcs considerably. But even then, a 1/2 inch gap in the fuse would be more than 6 square inches of inlet area. So could I get away with a 1/2 inch gap in the fuse dedicated to an inlet? Rather than build a plane to test this, I built a test rig. I pulled the EDF out of the Ohka, created a crude inlet, and a short section of the 4 inch fuse.

1 - all components.JPG

Here is a close up of the "inlet"

2 - exposed NAMA duct.JPG

and here is a pic of the assembled "device" almost fully open.

3 - open duct.JPG

and almost fully closed.

4 - closed duct.JPG

I will run up the EDF to measure the static thrust, then slide the wall of the "fuse" downward to reduce the inlet size, and run up the EDF and measure again. I will repeat this until I start to see a drop in static thrust. I will assume that, because the exhaust tube is > 100% FSA and is not very long, any drop I see in static thrust is a sign that I am starting to starve the EDF. If I can get this pretty short (1/2 - 1 inch) then I think that this is actually a practical solution without too much impact to the looks of the plane - and it would be cool!.

If it works, I am going to call it the NAMA duct!

Anyway, it was late-ish when I got this finished so there will be no running the EDF at full throttle until tomorrow.

Will report back then.

LOL @ NAMA duct!

I can not take credit for that though. I am sure someone invented it ages ago and called it something else. Besides, if you attach my name to it wit will probably explode...


It looks great man, eagerly awaiting test results!


Elite member
So the results are in. I ran two test sessions - one set with the duct pictured earlier (which I labeled "Complex") and the other was just a simple "+" shape, without the additional foam fillets (labeled "Simple"). Each data point plotted is the average of 3 wide open throttle runs with 2 x 2200 mAh 4S batteries in parallel.

Static Thrust results.jpg

A few things to notice.
- Without any ducting (>= 3 inches) the static thrust was lower than that observed at 2 inches
- The complex duct starts to lose what I would call significant thrust below 1.5 inches
- The simple duct starts to lose thrust below 1 inch
- At 39 oz static thrust, I am getting approx 15 oz more thrust than I measured in the model maidened on the weekend. So if I can fix the ducting and set up a proper thrust tube this thing might just rip!

I am leaning towards the simple "+" shape version because I can close the inlet to 1 inch and its a little simpler for building. I need to check that it can be built strong enough to hold the plane together. But if I need the extra strength, I know that the complex duct will work at 1.5 inches.

Awesome test analysis! Man, what I would give to have those numbers!
I have to admit, I am a bit jealous.

Hmm, I wonder if I can shoehorn a few 70mm's...


Elite member
Build Post

The final version of the plans are attached (Rev 6) and include the Sketchup file, the DoubleCAD file, and both a tiled and full PDF files. Note that the tiled version is optimized for the least number of page joins so use this instead of the full version if you are printing on regular paper.

View attachment Ohka 43 Rev 6.zip

Generating the plans, cutting the pieces, and making the parts

Printed out the PDF and joined the tiled pages
3 - plans.JPG

Then cut out all of the pieces except the wing
4 - pieces.JPG

The HS is reinforced with a thin piece of paint stirrer embedded in the underside
5 - HS part 1.JPG
6 - HS part 2.JPG
7 - HS part 3.JPG

EDF holder ring - note cut out the hole and test fit the EDF before completing the piece which is a little fragile
8 - EDF holder ring.JPG

One of the inlet ducts (for decorative purposes only)
9 - ducts.JPG
10 - ducts.JPG

The canopy
11 - canopy.JPG
12 canopy folded.JPG

The NAMA duct (1 inch inlet and 1 inch either side to hold the fuse. Will add a wooden reinforcement later

Assembling the nose. All of the fuse pieces are assembled the same way. The paper where the foam is removed is where the opposite edge of the piece will be hot-glued. I tried a number of techniques for this and I think this is the best because it lets the piece fold as if there was a score cut in that position. This means that the join will fold with the same flexibility as the rest of the sections. Each of the score cuts are opened and a thin smear of gorilla glue is added (much like the hot glue reinforcement of a control surface hinge). Finally the piece is folded and hot-glued.
13 - fuse part.JPG
14 - test fold.JPG
15 GG on.JPG
16 - folded.JPG

Assembled pieces on parade. Note that I cut "frames" for some of the fuse pieces to ensure they maintained shape while the gorilla glue dried.
All parts .JPG

Assembling the Pieces

Nose pieces hot glued together. Fitment is okay. These will also be put aside as this nose will be the hatch (I think)
18 - nose.JPG

Front fuse pieces glued together - these fit together pretty well.
19 front fuse.JPG

EDF holder ring glued on and thrust tube taped on - making sure that the thrust tube is straight and parallel to the EDF
20 - EDF and TT.JPG

I had to notch the EDF facing side of the NAMA duct to accommodate the nose cone on this EDF
21 - modded NAMA.JPG

Test fit the NAMA duct and add a hole for EDF wires and the elevator servo wire. The blue marks are where the rear of the canopy will sit so the wires will be hidden by the canopy.
22 - wire holes.JPG

A notch added to the EDF holder ring to allow a wood spar reinforcement to pass by the EDF
23 - slot for wood.JPG

Used the NAMA duct to position the EDF into the fuse. Then holding the fuse tightly so that the EDF does not move, remove the NAMA duct and add glue to EDF holder to secure in place
24 fitting EDF for gluing.JPG

Cut a 6 inch long, 1/4 inch wide piece from a paint stirrer and glued to the top edge of the NAMA duct. Note that the EDF side needed to be trimmed to fit past the EDF.
25 - reinforce NAMA.JPG

Hot glued the NAMA duct in place into the rear fuse section
26 NAMA Installed.JPG

Joined the two fuse sections by gluing the NAMA duct into the front section of the fuse. NOTE - this was a mistake I discovered later that there was a slight rotational misalignment between the front and rear fuse sections. This meant that the wing and tail feathers were not 100% parallel to each other. For future builds I suggest the order of operations be
(1) Install NAMA duct into rear fuse
(2) Add tailfeathers to the rear fuse
(3) Install wing
(4) Join the front and rear sections via the NAMA duct watching the alignment of wing and tailfeathers and add a little twist if needed.
27 Two halves joined.JPG

Installed the tail cone. Note the deformed/creased foam panel. The "frame" I made to hold the shape of this piece while the gorilla glue set was too tight and I over compressed the piece to get the frame on.
28 tail cone.JPG

"The end is in sight!"
29 I can see the end.JPG

Added tail feathers.
30 - tail feathers.JPG

Time to start building the wing. Cut out the wing part. The "spar" is just a single piece of 1/2 inch wide foam - the strength will come from a wooden insert, nnerdnic style. This will help make a nice thin wing.
31 wing part .JPG

Flip the wing piece over and added a strip of tape along the leading edge..
32 wing LE tape.JPG

Then flipped it back over and "beveled" the leading edge. I stopped using a blade to bevel these edges. Instead, I just crush the bevel in place with the blunt end of a barbecue skewer. Then I glued the foam spar in place.
33 crush bevel.JPG

Now I offer up a paint stirrer to the wing, pressing it against the spar and aligning the end with the root of the wing. This allows me to mark a trench that will accept a wooden reinforcement cut from the paint stirrer.
34 sizing wing wood.JPG

Score cut the outline of the wooden reinforcement and then started to pull out the foam with a BBQ skewer.
35 wing trench.JPG

Insert the paint stirrer into the trench and mark it along its length at the height of the foam spar. Then cut this piece off (using a blade and a straight edge, repeatedly score the wood until the piece you want breaks off).
36 mark wood for cutting.JPG

Put hot glue into the trench and along the edge of the foam spar adjacent to the trench. Insert the wood reinforcement pushing it into the trench and up against the side of the foam spar.
37 wing wood installed.JPG

Create two pockets for the aileron servo and glue the servo into the upper wing surface pocket. Test fold the wing to make sure the pockets align. Also, don't forget to add your servo extension.
38 servo pocket.JPG

Add hot glue to the top of the foam and wood spars, into the lower wing surface servo pocket, and along the trailing edge of the lower wing surface. Fold it over and hold down hard across the spar and trailing edge until the glue sets. Airfoil is not brilliant, but not too bad.
39 wing profile.JPG

Repeat for the second half of the wing. Cut two 1 inch strips of foam and insert under each wing tip to set their height when you glue the two halves together - this will set the dihedral for the wing. When the glue is set, fill in any open spaces with glue and then add a strip of extreme packing tape to the seams, top and bottom. Note that I did not cut or add a spar to join the wing halves the way nnerdnic does. For this plane, although it should be reasonably quick, I don't think it needs this additional strength.
40 wing complete.JPG

This is the wing loosely installed into the fuse - looks like a plane! The NAMA duct is not really that conspicuous.
41 getting there.JPG

Finishing Touches

Installed the elevator servo by pocketing through both layers of foam (stabilized and fuse section)
42 elevator servo.JPG

Extended the motor wires and fed then through the fuse towards the front of the plane
43 wires.JPG

Decided not to go with the nose hatch. I had moved the EDF unit forward about 1/2 inch in this version and there should be no problem balancing her by placing the battery towards but not actually in the nose. This is a very simple hatch - just cut it out. The gorilla glue makes the piece rigid so it holds its shape without any more reinforcement
44 hatch.JPG

Scaled up the canopy slightly and re-printed and re-made the part. To fit to the fuse you need to bevel all of the edges and keep doing this until you are happy with the fit. Same approach is used for the decorative ducts.
45 new canopy.JPG
46 canopy bevels.JPG
47 duct bevels.JPG

And here she is, pretty much done. Just control rods and some tidying up of the wiring.
48 Pretty much done.JPG

And here she is, painted up.
52 - prepped for maidern.JPG

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