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Frame Extension

Status: design phase

Requirements and constraints are documented (this section), the actual CAD work is still open — see CAD Models for the per-mount design requirements and Print Guide for the printing workflow. Nothing has been printed or flown yet.

The stock SpeedyBee BEE35 Pro is a bare 3.5" CineWhoop: it carries the FC stack, motors, ducts, the analog FPV camera and the battery — and nothing else. For the delivery mission, four additional components have to be attached to the frame, none of which have an off-the-shelf mount:

# Component Function Mounting requirement
1 MicroAir MTF-01P Optical flow + LiDAR — the only horizontal position source indoors Looking straight down, level, free optical path, ~2 cm ground clearance that must stay clear
2 Raspberry Pi AI Camera (IMX500) Detects the landing pad on-sensor Nadir (straight-down) view, props and frame outside the field of view
3 Drop mechanism + payload bay Releases the delivered object (servo mechanism) Holds a 9 g servo rigidly, payload centred under the frame
4 Raspberry Pi Zero 2 WH case Companion computer running the state machine Protected, accessible USB/GPIO, short cable runs to FC and camera

For attachment we have M3 standoffs and screws (M3x9 and M3x12) provided, intended for a platform that stacks onto the frame's existing M3 pattern. All custom parts will be 3D-printed (see Print Guide).

Why the mounts are on the critical path

The drone is currently not flightworthy (see the incident report 2026-08-21), but even once the FC repair is done, milestones cannot be flown without these mounts: the MTF-01P provides the EKF's velocity estimate, the AI camera provides the target, and the drop mechanism is the mission. The mounts are therefore Task 6 of the project plan and the main remaining hardware work besides the crash repair.

Hard constraints from the platform

Mass. A 3.5" CineWhoop with Gemfan D90-5 props and Emax Eco II 2004 motors has very little thrust margin once the 4S Li-Ion pack, the Pi, the camera, the sensor and a payload are aboard. Every printed part must be as light as rigidity allows — this drives the wall/infill choices in the Print Guide.

Centre of gravity. The initial setup guide already lists an off-centre CG as a classic cause of drift and yaw-on-takeoff. The payload bay and battery must stay centred; the Pi and sensor mounts should be placed to balance each other rather than all hanging off one end.

Sensor placement is a safety issue, not cosmetics. The crash analysis showed how unforgiving bad height/position sensing is on this aircraft:

MTF-01P: nothing may ever slide under it

The MTF-01P must keep a free optical path to the floor with roughly 2 cm of ground clearance kept permanently clear — no cable, strap, landing foot or payload may ever intrude, on the ground or in flight. The rangefinder reads down to a few centimetres (RNGFND1_MIN_CM = 1); an object under the lens produces a confidently wrong range and a wrong optical-flow scale. During the 2026-08-21 incident the MTF-01P itself performed flawlessly — the mount must not be the thing that breaks that.

Mounts must survive contact — the GPS mast did not

In the crash the GPS module was torn off, and its damaged wiring is the prime suspect for taking down the FC's single I2C bus (barometer and compass). Externally mounted electronics on this frame must either sit inside the frame's protective outline or break away without ripping their wiring out of the FC.

Downwash. The ducted props produce strong downwash that already disturbs the barometer near the ground (4–6.7 m baro spikes at ~15 % throttle in the crash logs). The camera and MTF-01P sit in this airflow; mounts should not add loose flaps, and cables need strain relief so they cannot flutter into a sensor's view.

The four mounts at a glance

flowchart TB
    subgraph top [Top platform on M3 standoffs]
        PI[Pi Zero 2 case]
    end
    subgraph frame [BEE35 Pro frame]
        FC[FC stack + battery - unchanged]
    end
    subgraph bottom [Underside, nadir-facing]
        MTF[MTF-01P mount<br/>level, ~2 cm clearance]
        CAM[AI camera mount<br/>props out of FOV]
        DROP[Payload bay + 9 g servo]
    end
    top --- frame --- bottom

The concrete dimensioning, the Tinkercad workflow and the pre-flight validation checklist for each mount are documented in CAD Models.

Open work

  • CAD models for all four mounts (requirements are fixed, geometry is not)
  • Test prints and fit checks (Print Guide)
  • Weigh the finished parts and re-check CG before the first flight
  • Verify sensor placement against the pre-flight checklist