Seradon

Four arms, two plates, one rail.

A quadcopter in X, arms on the 45 degree diagonals, 610 mm motor to motor. Each arm is a 300 mm carbon tube clamped 66 mm deep between two printed plates, and the 16 mm gap between those plates is the avionics bay. Under the bottom plate runs one dovetail rail. Whatever the aircraft carries slides onto it.

Everything hangs off the bottom plate.

Twenty one printed parts, ten lengths of carbon tube, the rest bought. Above the plates is the power deck: battery, straps and the GPS pylon, with the flight controller and power board in the cavity beneath. Below is the rail, the sled that rides it, and the module the sled carries. The airframe comes apart with a 2 mm and a 2.5 mm hex key. The module comes off with no key at all.

The interface was designed first.

One spine down the centreline with a 45 degree dovetail on each edge: 64 mm across the crown, 50 at the neck, 122 mm of engagement. A stop face at the front registers the module and carries the blind mate. A tongue at the back takes the safety pin. The profile has not changed since the first revision, so every module drawn for it still fits.

Twin rails do not survive contact with this airframe. On a 90 mm track they cross the arm diagonals exactly where the tube channels and clamp bolts live. The centreline is the only line through the plate that never crosses an arm.

Four moves, no tools.

Slide the sled on. A ball plunger clicks into a dimple in the crown and pulls the sled onto the stop face, so it registers itself. Throw one over centre lever: the cam pushes the moving jaw into the dovetail flank, closed ten degrees past dead centre, and the jaw cannot leave the sled even with the lever wide. Drop a 6 mm pin through the fork behind the tongue. It carries nothing unless the lever opens.

Power and data cross on their own.

Eight spring pins on a carrier set into the rail's front stop face, pointing aft, and a pad board in the sled's front wall to meet them. The carrier floats a millimetre on four elastomer pads behind a retaining plate, so a printed dovetail with half a millimetre of clearance cannot bind it. The pins touch 1.5 mm before the sled seats and sit compressed once it does. They never scrub, and no module is live while it is loose.

Everything lives in the gap.

A Matek H743-SLIM sits in the 16 mm cavity between the plates, rotated 45 degrees so its standoffs land on the X and Y axes and clear the arm tubes, with the USB-C edge facing out so it never comes apart to reflash. The power board sits behind it, the receiver in front. On top: the pack on two pads under two straps, and the GPS pylon aft of it with its puck 92 mm up.

Three hundred millimetres of carbon.

Each arm is a 16 mm carbon tube with a 1.5 mm wall, clamped between the plates and carrying a printed motor mount at its tip. The motor is a SunnySky V3508 on a 6 mm shaft, the propeller a 15 by 5.5 inch carbon two blade under an M6 nut. The ESC rides in a printed saddle mid arm, and the rear arms carry the antennas on clamps of their own. Going to 15 inch props is what lengthened the arms, and the longer lever is why the tube wall grew.

The module is the variable.

The search and rescue module is a 130 by 140 by 58 mm printed shell on the sled's six bolt pattern. Inside: a single board computer, a software defined radio, four Wi-Fi sniffers in pods on the underside and a five element microphone array in the floor, steered at the ground by delay and sum beamforming. Anything that fits the bolt pattern, draws 5 V and 12 V off the pogo block and keeps its centre of gravity near the centreline will fly.

The law enforcement module is a mechanical slot only. Cell site simulators are federally restricted, so the source in this project is a test transmitter under our own control.