Two AIs, and a Hollow Idea

FPVCycle has a thing for dental nomenclature. The Incisor frame. The Molar, Glide Floss, and Toothpick Frames. So when a new motor concept emerged with a notably hollow center stator - wide on the outside, empty in the middle - the name wrote itself. The Cavity.

The Cavity 4012 started as a question, how do you make a motor for a corporate client that doesn't want to pay for R&D. AI is the answer...but also not. AI loves to conjure all sorts of random stuff but we need it to work within the real constraints of how brushless motors are actually manufactured. Existing winding techniques, standard stator lamination processes, proven bell construction. The AI's job wasn't to invent a new motor; it was to find the best motor that could be built with the methods that already exist. Think of it less as "AI-designed" and more as AI-optimized within human-defined real-world constraints - the same way a good engineer would approach it, just faster and with better math at 2 AM.

The clients need was an efficient motor for running 9"-10" props that still has good performance. 3215 is the workhorse for 9"-10" on 6S, but its geometry is fundamentally a tall, narrow stator - efficient at filling a spec sheet, less efficient at actually producing torque. So keep the same general stator volume but redistributed it outward, the way physics actually prefers? Anyway, the AI, both of them, said move it to a 40* diameter with 32 length poles. (In fact it's a bit shorter than 32 length...for physics reasons we cannot comprehend but AI can in a very much 'trust me bro' style.)

Given the limitations of manufacturing and how the proportions of the stator could be distributed, the 40*09 stator was evaluated first and found technically sufficient. The AI pushed to 40*12 for a meaningful performance gain with a negligible efficiency penalty - the kind of judgment call that takes a human engineer an afternoon of spreadsheet wrestling and takes an AI about four seconds. We agreed and here we are. There were a few other requirements for the customer such as a 'sealed' motor but otherwise, this thing performed so well that we decided to carry a small batch ourselves to see what others have to say.

Why 40mm* Diameter matters

The 3215 stator is 32mm across and 15mm tall. The Cavity 4012 is 40mm across and 12mm tall. On paper, these look like a mild reshuffle. In practice, they represent a fundamentally different approach to making torque.

Torque in a brushless motor is generated at the air gap between stator and rotor - and the further that gap sits from the center axis, the more leverage it has. Going from a 16mm radius to a 20mm radius increases that moment arm by 25%. Every amp of current you push through the windings now does more rotational work than it did in the 3215. The total stator volume of the 4012 is roughly 25% greater than the 3215, which means more copper, more magnetic material, and a wider thermal footprint to spread heat across.

Meanwhile, the shorter 12mm stack height means each individual winding turn is physically shorter - less copper per turn, lower resistance per turn, less heat generated per amp. The motor is simultaneously bigger where it counts and leaner where it doesn't. That's the whole trick.

What the Math Says. What the Field Confirmed.

At 1020KV on 6S (22.2V nominal), the Cavity 4012 spins to roughly 22,600 RPM unloaded. Under a real 9" or 10" prop at flight throttle, loaded RPM settles in the 18,000–20,000 range - right in the efficiency band where props of this diameter do their best work. Peak thrust sits between 2.5 and 3.5kg per motor depending on prop pitch, making this perfectly matched to builds in the 5–7kg AUW class. (this 'math' section was entirely written by AI, not real world verified)

The AI predicted this. Testing confirmed it as best we could. We were quiet impressed at just how accurate the AI's were.

The Sealed Bell — Because Nobody Wants a Stick in Their Motor

Motors don't often fail from dirt and debris - but they can, and it's always annoying when they do. The Cavity's bell housing is machined with a near-zero gap between bell and base. Sticks, gravel, and miscellaneous field debris simply can't find a way in.

Motor weight is ~125g, comparable to a 3215

The top of the bell carries an integrated impeller - essentially a structural centrifugal fan built into the motor. As the motor spins, it draws air upward through intake holes in the finned aluminum base, pulls it across the stator and windings, and expels it radially at the top. The base fins act as a heatsink, pulling stator heat out before it accumulates.

Rain? To get inside, water would have to travel a rather convoluted path, against the direction of high-velocity airflow. FPV motors are naturally waterproof due to enamel coated winds but it's still inconvenient to get liquid inside the bell.

The 3215 Went to the Dentist and Came Back Better

The Cavity 4012 isn't a replacement for the 3215 in the sense of fitting the same hole. It is what a 3215 would be if someone had asked, from the start, "what shape should this physics actually be?" and then had the manufacturing infrastructure to build the answer. optimized stator volume. More torque per amp. Lower winding resistance. Active impeller cooling. A sealed bell that laughs at crash debris. Similar in weight to a 3215, and a name that makes everyone who hears it do a small double-take.

Designed by humans. Optimized by AI. Tested in the real world. Filed under: things that probably shouldn't work this well but do.