A perfectly balanced 3" frame.
Many of my frames come about 'cause the boys pester me about one thing or another, and this is another one of those. Alex loves flying his Shrieker around his house, but he doesn't like the imbalance of an underslung battery. He dealt with it by flying tiny 500mah packs, but he'd still rather have a 3" frame that has perfect CG on all three axes.
Perfect CG is something I've been thinking a lot about since I designed the Corgi. There's many reasons to get that CG right on the prop line, equidistant from all four motors. Rolls are tighter, motors work equally at all pitch angles, and sharp turns are cleaner. Corgi was my first crack at it, and though it flew very cleanly it was heavy and delicate. The Odonata is the 6" version of Corgi without those flaws, and the Zoompod, with a camera opposite the battery, is the simple X version of the same concept, and perhaps my best solution to this problem so far.
You can get away with an underslung battery more easily on larger frames, since the distance from the prop line to the battery is a smaller percentage of the quad's overall size. But on a tiny 3" frame, that distance is relatively much larger, so the imbalance is felt to a greater extent in flight.
With the goal in mind Alex and I spec'd the parts for this frame. We intended it to work with very specific components so that it could be as tight as possible. First of all, we're going with a pico fpv cam. Alex loves the crazy power to weight ratio of the Shrieker with a Bonka-style 500, so we're saving weight everywhere we can. Next, we're using the Furious FPV Piko BLX fc/pdb. It's only 27x27 so I can design a simpler frame, unlike the Shrieker, which takes a 36x36 board. And with the fc doubling as a pdb, we can slam this build as well. I'm also using Furious' mini rx to save room.
Variable cam tilt is always great, so I thought up a mount using the flexible TPU as a hinge, with a strap up top that hooks into the frame to change angles. I drew up this beautifully intricate part and started to print it, only to realize that it was for a pico cam, and WAY smaller than the 10" image on my computer screen. The delicate features turned out to be about the thickness of the filament extrusion, which meant the part came out like a blob.
So much for that. My pico cam lens is really fisheye, so I tried a fixed 30 degree mount, hoping it would work for high and low speeds. A few iterations later I got a snug press fit.
Now, I thought that all I had left to do was move the battery to the top and I'd be done, but boy was I wrong. Once the battery goes up on the prop line, well, the props actually have to clear the battery. So I thought I'd be clever and make it a long frame and run the battery ACROSS the top. It'd be strange looking but everything just fits so much better.
The front and back are TPU, and screws will run through the full length. There's only a 15mm gap.
I got the carbon in and rushed out frames to Alex and Patient 0 Chris. I built mine that night, and as I hovered it I realized that I failed to account for the battery leads when I laid out the props. A few chopped up leads later I messaged Alex and Chris and told them to not bother with their frames. Chris printed out the TPU parts, assembled the frame, and hung it up on the wall.
Ok, back to the drawing board. The battery's going lengthwise after all, so the fuselage will be lengthened. The motors have to spread out, so I went back to a square X layout. I was using a pigtail vtx smushed into the back, but with this newfound space I changed it out for a 90 degree sma vtx.
Normal straps are too big so I opted for two velcro straps to make sure the battery doesn't rotate into the props.
I is dum
I rebuilt it with red bottoms and Littlebee 20's from the bottom of my parts bin. It hovered ok but as soon as it touched down the motors would rev up again and it would shoot up. The only way to get it down was to disarm midair. I wondered if it had something to do with its extreme light weight (237g w/ battery), or maybe it was air mode making it freak out. I could fpv it fine but clearly something was wrong, and I was stumped.
Then Alex plugged it into his laptop and saw that the esc's were all over the place. Ooooh, those old Littlebees were from different batches, with different firmware... I was so fixated on air mode being the problem I neglected to check the esc's.
So, properly set up it flew much better (funny how that works). It's so light that it almost feels like a Tiny Whoop in the air. If you bank and yank on a turn it would practically stop dead in its tracks. It felt like flying in angle mode, where you have to keep pushing the nose down to make it go forward. After a minute or so of acclimation it was fine, so light and responsive and easy to maneuver.
I handed it to Mike (@control.fpv) and he tore it up at speeds I couldn't imagine, and as I watched along in my goggles it was apparent that 30 degrees just wasn't enough tilt to bring out Hedwig's full potential.
There was another problem. The Furious micro rx has just one antenna, which I ran under one of the rear arms. I didn't think I'd ever fly this little guy that far out, so I didn't think it would be an issue. But I got a few failsafe's so I added antenna tube holes to the butt.
Next I had to draw up a 45 degree nose. I could do a simple press fit nose at 30 degrees, but 45 degrees is too much of a turn for the camera to make in a small amount of space, and it risked ripping off the delicate wires coming out of the camera's bottom. I ended up with a drop-down style mount. It's crazy how intricate a silly little part can get, the whole thing can fit on a quarter.
The increased camera tilt really did fix the 'no inertia' problem – it's still light as hell, but at least now it's going faster, with more momentum, and it flew more like a 5" quad. I have a really wide angle camera, so it was still possible to hover and fly through branches at low speeds. Then I handed it to Mike and he really squeezed some speed out of it. This was his one and only battery on it, and you can see that he wasn't used to it for hard 180 snaps, but this should give you an idea of Hedwig's potential. Also, note that it was a really windy day, and tiny little Hedwig handled it pretty well.
And this is me flying Hedwig. There's a roll and a flip in there, I love how uneventful they are on quads that are perfectly balanced. The center of mass is simply rotating about itself, not traveling in a circle. Also, check out the attempted split s over a tree at 1:26. I chop power when I'm directly above the tree...and drop straight down towards the tree. It's a perfect illustration of how light it is, and how you have to stay on the throttle 'til you get where you want. With a 5" rig you can chop power 20 feet from the tree and have enough momentum to carry yourself up and over to the other side. Not so with Hedwig.
And here I am again a few days later, nothing special, just reveling in how smoothly Hedwig flies with a fresh set of props. It's so light that it really gets tossed around by propwash, so I try to keep it moving forward and execute turns smoothly without hard 180 snaps that back it into its propwash. I stayed on the power to complete the split s, but I leveled it off too early – you can stop a descent almost instantly with Hedwig.
Alex found a lot of the same things about Hedwig – that you have to stay on the throttle 'til you get where you want, that you can wait 'til the last second to pull out of a dive, etc. He built his with Furious' Innova stackable vtx, and dropped another 12 grams from his build. Since he doesn't need the space in back for a 90 degree vtx, he asked for another set of holes to make the fc more centralized, to make esc wiring cleaner. Hedwig's already too light for me, so I'm happy to leave an option for a simpler build using a 90 degree vtx.
Alex goes by 'Legacy', so this frame was called Legacy while I was working on it. At some point he made a joke about the 'angry 3 inch', which led me straight to Hedwig. Alex liked the name so much he didn't mind that it wasn't to be named after him. I mentioned the name to Pam, and her mind went to the owl from Harry Potter, which led me to...owl drag queen. Here's a working sketch, can't wait to see the sticker.