I designed the Krieger around 5" props, and as soon as I showed the boys the sketch Ryan Gury asked for a 6" version. Simple, just extend the arms. Naturally, the thought of a 4" version immediately entered my mind–it'd be just as easy to shorten the arms. But Krieger's pod is disproportionately large and overbuilt for a 4". It'd be lazy to shorten the arms and call it a Kreiger 175.

See, the Krieger 200 is just 20mm larger than the minimum motor layout of 180mm for 5" props. A Kreiger 175, however, would be 31mm over the minimum of 144mm. I'd like to make each size as small and agile as possible, but there's no working around Krieger's pod. The 36x36mm FC is a big square right in the middle of the layout and you just can't avoid it. The thought of a 4" Krieger lingered but I was starting to think it was an unsolvable problem – a Krieger 175 is just an unacceptable compromise.

Meanwhile, thanks to my newfound Tweaker notoriety, I was offered the chance to sponsor team Australia in a race against New Zealand. The course will be a tight indoor track so it seemed like a good venue for Tweakers, but I thought I might as well go all in and design something that will really give them an unfair advantage.

Then I had my eureka moment, a revelation so dumb and simple it's embarrassing. TURN THE FC 45 DEGREES. Now it fits much better between the props, and I can easily reduce the layout to 150mm, just 6mm bigger than the minimum. Now we're talkin'!

I used standoffs to go up a level, and then made another 45 degree turn for a tilting cam mount. Here's what it looks like.

The FPV cam mount was a little tricky since the top plate is pretty small, and it has to share space with a vtx. I ended up making it specific to the Fat Shark 600/700 TVL, using its two rear screws to mount it to the plate. If you want to use another cam you'll have to improvise. The friction lock cam plate on the Krieger works really well, so I copied the hole/slot pattern.

Prototypes were made (that phrasing is meant to be foreshadowing). As I built some things dawned on me. One, wow, this build is super tight. The props are really close to the FC, and any stray wires risked being cut. I'm used to the odd balance lead getting chopped up in midair, but now there are esc signal leads and vtx wires in harm's way.

Two, with the vertical risers being simple standoffs, I was hoping I could slam the build down as far as the components allowed, but I forgot that the corners of my top plate overlap the props, so I could go no lower than 35mm. This bothers me a great deal 'cause a clean builder can reduce the stack to 30 or even 25mm, and reduce the moment of inertia for pitch and roll.

The build was otherwise fine and the proto flew as expected, like a tiny Krieger. Still, I knew this was a pretty bad miscalculation. The top plate simply can't overlap the props.

I sat down to redesign it and just stared at the computer screen. Without the overhang there wasn't much room for a cam. My FPVA sounding board suggested a simple fix – keep the overhang and extend the arms, but I wanted to keep it small for maximum control authority. The only solution was to REALLY go back to the roots of the quadcopter and make it a plus quad.

I asked around to see if any one had ever flown a plus, with no luck. Everyone was concerned that a plus may have some adverse flight characteristics, but I couldn't think of any reasons why. Pitch and roll are driven by two motors instead of four on a plus, but they're moving half the mass as well. Besides, in most cases you're blending so many controls at once I find it hard to believe there'd be any difference between an x and a plus. If anything, the plus' greater division of labor in the motor mix might make it less likely for one of them to be maxed out in a sharp snap and cause the quad to wash out.

The only concern left was whether a plus quad would look so lame that no one would buy one regardless of how well it flew. Luckily the first mockup looked totally badass so I plunged ahead.

I try not to have a 'signature style' in my designs, the way that some makers have a consistent look. I prefer to have each application dictate the lines. With its right angles and straight lines Krieger naturally took on a very mechanical look. But since Alex (@luckylegacy13) had dubbed this design 'Mitsuko' I decided to make it more flowing and organic. I took the basic Sketchup layout into Illustrator and converted the outlines to Bezier curves. It was a nice way to eliminate stress risers while giving it a whole different identity.

I also found a Hobby King VTX with a 90 degree SMA that'll fit flush against the bottom of the top plate, so that opened up some room in the pod.

And here she is built up.

Once I took her up in the air I forgot she was a plus. Rolls and flips were crazy fast, at the same .8 rate I use for my x quads. Once I got comfortable it just ripped, with the same slippery quickness as Krieger. Since the plus was a pretty risky format I didn't ask the wrecking crew to build any. Instead I brought it out and had them test fly mine, and they loved it. 

I found the Fatshark cam to be increasingly unreliable, as well as terrible at handling light changes, so I abandoned the rear mount and reverted to a more universal cam mount. I drew up the revision and ordered new parts, then...I got arrested for FPV'ing, and the lone prototype was confiscated. This sucks big time, as I'll be without it for at least 45 days, and it was a really nice/difficult build, with a de-pinned X4r and a Dodo. 

I built up another one, this time with Tiger 2204/2300's, and I went through many revisions of the pod. I was having a hard time finding a spot for the horizontal standoff – I kept putting it too low, where it couldn't exert enough pressure on the cam plate to keep it from slipping midair. If I put it up above the cam plate then the lines of the pod was ruined. I finally settled on putting the standoff in a bump that interrupted the upper curve of the pod.

I also found a better VTX with a 90 degree SMA from FPV Ninjas, a unit which seemed to have the same guts as ReadymadeRC's Cricket and Rotorgeeks' FX799T. Unfortunately it seemed to be a bit of an afterthought as many components protruded above the mounting line, so I redrew the top plate just to accommodate it.

5" Mitsuko

Concurrent to all this I started working on a 5" version of Mitsuko as well. It started as a very literal scaling up of Mitsuko 4, with a proportionally larger center pod that would allow for mounting VTX's vertically next to the FC stack. Once I built up the prototype I realized that the pod was hideously large, ugly and not aero. 

I redrew it to use the same pod as Mitsuko 4, and drew up an X version as well. 

The 4" version launched first while the two 5" versions continue testing. We want to see if we can tell the difference between the plus and the x, but I have a feeling I will release both versions, hopefully soon.

Naming

For those of you who don't watch Archer, Mitsuko is Krieger's virtual girlfriend. Here she is:

And finally, here's the production version of the 4" Mitsuko. The cam plate had a little wiggle in it, so I put the little locking tab on the opposite side, away from the standoff, to better resist the locking force from the standoff. Now it's rock solid.

Back to the 5"

I'd been flying the 5" + version for a while, and I finally got around to building an x.

The two don't fly terribly differently, except for the fact that the x got Tiger F40 2300kv's and the + got Cobra 2204 1960kv's, which makes the X much peppier. The X, having the standoffs at the corners, has to be about 11mm larger at 195mm, but it has a couple advantages over the +. Wiring is less exposed to the props, and its shape is cleaner and simpler. So as of now I'm leaning towards releasing just the x and calling it Mixuko.

3 Weeks Later

I really grew to love this baby. It's fast like the Krieger but with better agility and acceleration due to its lighter weight. It's the ultimate race machine, spec'd right in the sweet spot for power/weight/battery. You can go for a crazy light setup with a 1300 or opt for a smoother feel with an 1800. X quads are all the rage now and I couldn't be happier with my second entry in the field. The FC stack isn't structural so it doesn't get ripped apart in a crash, and the cam plate is rock solid AND adjustable, unlike the soft metal bracket so many frames use.

Design-wise, I think it's the prettiest thing I've done, which is ironic as it arose almost as an afterthought to the 4" plus. With its greater size the curves went from cute to elegant, and I stumbled onto a little beauty.

Check out team pilot Ken Loo's writeup about Mixuko.