#vayu

I wanted to copy a file off the flight controller's SD card and get the same bytes back. Small files worked; anything past a kilobyte stalled or came back subtly wrong, non-deterministically. This is the hunt that followed — an isolation ladder that exonerated the SD card, a plausible theory I ruled out by measuring instead of guessing, and two bugs with the exact same shape: a synchronous primitive racing an asynchronous event that only collides under a preemptive scheduler. Eight fixes later, the round-trip is byte-perfect to 64 KB.

We had logged the flight controller on the bench before — but always down a USB cable, which is exactly what kept it pinned to the bench. A new UDP/WiFi telemetry bridge cut that cable and let the airframe move freely on a rig for the first time. This is a frame-by-frame read of that first untethered log: the transport, an attitude loop that shakes itself apart under its own feedback, and the sensors and telemetry bugs underneath.

vsim_d is the physics daemon that flies Vayu's real firmware in software. A field report from the seam where Newton–Euler rigid-body mechanics meets RK4, quaternion drift, impulse contact, and a hard-real-time loop — including the cleanest way I know to turn a triangle mesh into an inertia tensor, and the bugs where the physics was right but the numerics (or the systems) lied.

We built a closed-loop PID autotuner that flies its own experiments in simulation and searches the gain space — eight optimizers, a chatter-aware cost, and two failure modes that looked solved but weren't. A deep dive into the system, the algorithms, and the fixes.