My wing. It's about to get real!


I may be wrong, but I think find the center of pressure first, then make sure CG is in front of that. I think CP can be calculated like finding center of mass in geometry.
The first post shows a full plan w full dimensions, so I calculated it. It's a little hard to explain in one post, but I added up the area of each shape, multiplied each area by their individual positions of center relative to the front edge, added THAT up, and divided by total area. I get CP at 42.6 behind the the nose, so CG should be ahead of that.
Thanks everyone for chiming in! If I can get this plane to flite fest, it will have to fly with the kraken. That means driving ten hours instead of flying for two. Seems more than worth it!
So if CP is 42.5 behind LE, and CG should be ahead of that, where do you think that should be? Seems a little far back, I'm convinced that wings need to be ridiculously nose heavy to fly well. Shouldn't be hard in this case, as I have two motors hanging out way in front. That is why I went with tractor style, I was afraid of needing to put too much weight up front. I was hoping the shape of the wing would help get the CG requirement pushed a little further back so I had a better chance for a pusher. I like the way they sound! I'll see where it ends up. I have all the electronics, but forgot to order more silicone wire to make a wiring harness. Everyone seems to be out of silicone wire in red or black (RMRC, HK, Altitude) so I got tired of looking and just bought some blue and yellow wire. I'll have a pretty harness. Package is on it's way and I can finish it soon. I'll be sure to update and make a maiden video. I love a maiden video, so exciting.
Anyone have any ideas as to whether or not the control surfaces I intend to install will be up to the task of maneuvering this thing?
Anyone doing math for me is never spoiling anything!!!


Senior Member
So my go-to wing cg calculator got blocked by work, so it forced me to go to the next link, which was amazing! This one has the capacity to add different wing panels so you can adjust more than just root/tip chord!
It came up with almost the same number as mine (42.6 vs 40.3) but I included the ailerons in my calc, and it did not accommodate that shape.

But the suggestions of where the CG should be were @ 28 for 15% (it calls "beginner"), and @ 30 for 20% (general purpose?). That's only 2 mm difference, but sounds about the correct place. And as earthsciteach said generally too far back, I'm thinking that 28 is about the sweet spot.

Gut feeling on your control surfaces is that they'll be fine. Lots of distance to the CG should give pretty good authority, I think.


I'm getting closer! I've got servos installed, rx in place, making wiring harness for fpv stuffs. Just need to make harness for battery, and figure out a landing gear situation. I hope to get it in the air this weekend. Guy at my club has a quad with a gopro on a brushless gimbal, so I hope to get some good footage of the maiden!

Quick question, one of my servo extensions ended up at 1.5 meters or so, is that an issue? I wouldn't think so, but if anyone has any experience with a long ext, please feel free to reassure me. I made the extension, so there aren't a bunch of connectors, just one long wire.

Also, anyone have any ideas for the landing gear? I'm going to go stare at it some more and hope for inspiration. That oddly works. Just stare at it and let the mind go. Sometimes I only end up sitting there singing songs from Frozen in my head. Mostly the summer song by Olaf. Oh well, either way, a good way spend some time.


She flies this weekend! She will be called Mothra.
I recall an instruction in a great planes manual. "If you wish to photograph your model, do so before your first flight."


It was a smashing success.:D Smashing because some of it got a little smashed.:p Successful because there was little damage (nothing about five hours of reworking the engine mounts won't fix):rolleyes: and I learned a lot.

I set Mothra down on the runway and taxied her to and fro several times. The differential thrust really controlled the thing nicely on the ground. Those two DT 750 motors spool up the 1147's without issue very quickly, quite responsive. Had to be mindful that unlike a rudder, diff thrust works only when a prop is spinning! I may program another mix for ground control where the rudder will increase throttle when the stick is down. As it is now, the rudder decreases throttle on one motor. Anyway, it was pure excitement, my first twin experience. Sounds awesome.

So, the taxiing reached a crescendo and she jumped up a foot or two and really seemed like she wanted to be in the air. Now, an experienced pilot would have done something different but I was indecisive as to whether or not I wanted to keep her up. After a moment of vacillation, I settled on bringing her back down. I must have allocated a bit too much brain power to decision making, as pilot error became a huge factor at this point. I slammed her back to terra firma. The shape of this wing puts the elevons way back and they have great influence. There's a name for that cycle of over-correction, the up down thing, but I forget exactly what that word is. Any way, hit the gear a little too hard, gear folds a little, unbeknownst to me, the prop hits the ground. Mothra took a bounce back up and I figured that would be a good time to keep her there, so I just "poured the coals on." This is about when the heretofore unbeknownst prop-ground incident entered into its new category: the beknownst prop-ground incident. Half of an 11" prop at full gas is mildly unbalanced and proclaimed as much by tearing off the nacelle.

Here's the great part, from about twenty feet up, Mothra leveled off, dead stick, and glided so sweetly into the tall grass adjacent the runway. Thank you engineer for the cg suggestion, 30 was quite nice! Only damage was the torn nacelle, and two bent gear.

Since both nacelles were compromised, I am refitting them in a more secure manner. I'm also going to braze a support or two on the gear and perhaps add some shock absorption.

Going to dial down throws and ramp up that expo a bit.

What I learned is that designing your own airplane is not always fun in the classic sense of the word, but really a great challenge, gratifying and quite exciting. Quite enjoyable. And I do think she might actually fly, certainly well enough to keep the concept and refine the design, I should think. I also learned to have realistic expectations. I was not attached to the idea that it had to fly great, there's no reason my first design should! I expected to learn from whatever happened and i did. It was awesome. Sorry, no video, I had no camera operator. I should have asked my three year old to try, it would have been interesting.

So thanks for reading, not sure if anyone actually cares about this, I know I like to read about everyone's builds and maidens, so maybe others do? Can't wait to try again. I'll take comfort in the knowledge that I have some building to do this week. I do enjoy the building. I have to get better at drafting plans, cutting on the fly, marking the foam board and trying to figure out how to get all the b and a folds measured up correctly to make the boxes for the nacelles/pods, cooks my brain. Practice will bring about myelination of my brain's foam board circuit. Neurons that fire together, wire together. That's my hope, anyway!
Ramble ramble ramble! Here's a picture of Mothra ready to go. I will need to decorate her if she flies well.

Question: I'm thinking of setting the nacelles into the wing, right above where they currently reside, glued to the bottom inside of the wing. This would allow a better bond as it will be supported by the spar and by the cross section of the airfoil. So the motors would wind up a bit higher, is this ok? Any reason that would change things for worse?
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Senior Member
Congrats on your "smashing success"! I'm certainly glad your CG worked out!

I think your nacelles are great as they are; simple and sort of modular.
About moving your nacelles up into the wing, I can't really picture what you're describing exactly.
Maybe you mean, cut some section of your wing out to allow the pod to raise up and tie into the spar somehow; that concerns me for the structure of the wing. Every place there's a cut or kink in your wing, it generally makes an area where forces accumulate more, thus more places to break on an impact.
Or maybe you mean making the back end of the pods have a cut-out in the shape of your airfoil, so that the pod is sort of mid-wing. Structurally, I think this is better than cutting a place into the wing, because you get to keep the continuous paper/foam. You'd also get more area to glue the pod to the wing surface. However, if one of the pods does break, I think repairs would be more complicated.

About A/B folds, I'm gonna sketch something up for you real quick, should make measuring easy.


Senior Member
So, if you know the OUTER dimensions of the box/tube/U-channel/L-section, measure those lines out first.
Then, determine if you want the MIDDLE section on the INSIDE or the OUTSIDE of the box (I can never remember A- or B-style folds.)
If you want the middle section on the INSIDE, measure the thickness of your material and add that line on the Inside piece (shown in red). Vice-versa for outside (in blue).

Hope that helps!


I agree with engineer 100% in terms of cutting into the wing. It might be tricky to get the shape right, but you could still center the pods on the wing by simply mating them to the top and bottom surface of the wing. You would have to glue the nacelles together vertically. Then, you can reinforce them with a piece of birch ply on which you mount the motor.


Splendid, thanks everyone.
I decided drawing it out would help me visualize the measurements for the folds, thanks for doing it for me! In the future I will look at that instead of furrowing my brow trying to keep one image in my mind while I mentally fold another image onto it. Too many images for me!
I decided on putting the nacelles in the same spot, and just made them wider for a little more support. I agree, cutting into the wing is silly. It would have been strong, as I would have made sort of half- ribs in the shape of the airfoil to glue to the side of the nacelle and keep the airfoil shape. Strong isn't always good, though, right? If something did take it off, it would have taken some wing with it. As it is, the things will just rip off and be easily fixed. I love help. I probably would have tried it out just because I thought it would look cool.
Alright then, back to it, she's already halfway there. Can't wait for next weekend!

This thing is going to be a blast at flite fest! I'm definitely going to light this thing up. Apparently, there will be more than a few Krakens being released there. I hope Mothra and the Krakens can get along, she will be outnumbered. I am more than a little bit excited for flite test flite fest. The festival of flight.


An update on the state of Mothra:

So the first time I flew her, things went less than smoothly. However, the rebuilt nacelles from that incident proved to be better suited to a gentle crash. The second flight was pretty good. I got her in the air, but the throws were anemic, and I did not dial in enough reflex on the elevons. I had to pull way back on the stick for her to fly level. Cg seemed like it could be moved back. Also, it lacked power, though that could have been due to one of the props coming lose, darn those dt750 threaded rods. I managed to land her clean though. Just to be sure, I ordered a 4S 4000mah from HK. I hope the motors handle it well. I should probably prop down from the 1047's. The setup draws 50 total amps static at WOT. Pushing past the spec'd 18 for these motors, not sure if that's ok for an occasional pulse, I rarely peg the throttle. Anyone have experience with dt750 on 4s? What size prop do you use, and what's the best way to mount them? I currently use washers and stop nuts.

So, with 4S installed, cg back a skoosh to 30ishmm, more reflex, and some proper throws, I sent her back up. Amazing, she trimmed out pretty quickly, and was quite stable. The differential thrust really gets her around a corner in dazzling fashion. I was so thrilled. Then I stuffed her into the ground in a shallow dive from perhaps a stall when I was trying to land her. This is where I was quite pleased with the new nacelles setup. I opted for a little box inside of a big box, with the end sticking out about 5 inches or so. Held in with two BBQ skewers, these took the brunt of the force. The one slid into the other as the skewers pushed through the foamboard. One skewer broke. All that absorbed a lot of shock, and the nacelles stayed in place, all I had to do was reposition the inner box and remount the gear. I don't even need to reposition the skewers as they still keep the thrust from pulling the box out, they only got jammed backwards.

I programmed in a crazy full throttle differential thrust mix for a spin maneuver but was too chicken to try it today. She flew great, though I have to work on those landings. Still, minimal damage, the motor mounts break out easily too, thanks to the little screws I used. Weak points are good. This was all a happy accident, I'm pleased.

I will get some video soon, I'm always too excited to remember at least to hand someone my iphone to get some footage. Anyway, thanks for that cg suggestion again, saved me a real headache!

Anyone has a thought about the dt750 on 4s, please let me know. Also, does any one know if you can outfit a wing like this with any sort of flaps or something to allow a slower landing? I feel like she has to come in a tad hot. Also how do you mount props to the silly threaded rods? Why on Earth did they make 'em like that?

Designing and building my own wing has been ridiculously fun.
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Foam Addict

Squirrel member
I must have gotten a really good DT-750, I ran mine on an 11x7 on 4 cells for 3 months!

It was only just warm during the winter, but I'd recommend a 10x5 or 10x6 prop on 4 cells in the summer and lower altitudes.

4 mm lock nuts and washers work very well.