Agile 3" with HDZero — Build Guide

Summary¶

After following this guide, the builder has a flyable, Betaflight-configured Agile quad with correct wiring, tested motor directions, and at least one successful hover. Total active build time is approximately 4–6 hours. Total elapsed time from ordering to first flight is approximately 3 weeks — primarily waiting for AliExpress delivery, not active work. The 6sfull.cz components arrive in 2–3 days and can be inspected immediately. The frame can be printed the same evening the decision is made.

Concept¶

Read before you solder¶

The single most dangerous step in this build is connecting the HDZero ECO VTX to the wrong power pad. The ECO VTX is rated 1S–3S (maximum ~12.6 V). A 4S battery fully charged reads 16.8 V. Connecting VBAT directly to the VTX destroys it instantly and permanently. It is not a recoverable mistake. The VTX must be powered from the SpeedyBee's 5V BEC output — minimum 5V @ 2A, which the SpeedyBee F405 Mini provides at 5V @ 3A. This note appears more than once in this guide. That is intentional.

The five build phases¶

Agile breaks down into five phases: print, assemble, wire, configure, maiden. Each phase has a hard gate — do not move to the next phase until the gate is passed. Phase 3 (wiring) is where most first builds go wrong. Read → wire-gauge-selection before picking up a soldering iron.

Reference¶

Agile 3" with HDZero — build specification¶

Wire selection¶

At Agile power levels (35A ESC maximum, 4S) the motor phase wires are the highest current path. The EX1404 ships with phase wires pre-soldered at the correct gauge — do not replace them with thinner wire.

Failure modes and prevention¶

ESC burnout. Cause: motor stall (crash with motor still powered), or motor bearing seizure. Prevention: enable crash detection in Betaflight — it cuts motor output immediately on impact detection. Also enable DSHOT Bidirectional — stalled motor back-EMF is detectable. The SpeedyBee BLS 35A has no separate ESC board that can be replaced; the FC/ESC stack is one unit. A burned ESC means replacing the whole stack (~CZK 1,640).

VTX failure. Cause: overvoltage from incorrect VBAT connection (see Concept above), or impact damage. The ECO Bundle composite signal interface is significantly more crash-resistant than MIPI-based systems. Impact damage is usually to the camera mount, not the VTX board. A spare ECO Bundle costs CZK 1,769 from 6sfull.cz and ships in 2–3 days.

Motor failure. Cause: bearing wear, ingested debris, hard crash. EX1404 motors have 1.5 mm shafts — thinner than motors on larger builds and more susceptible to bending on direct impacts. A motor replacement requires unsoldering three phase wires and resoldering. 15 minutes with a clean iron.

Frame failure. Cause: hard crash, rock or hard surface impact on an arm. PETG-CF arms survive grass and soil impacts well. Rock impacts at speed will crack the arm. Reprint time: under two hours. Frame material cost: negligible. Keep the slicer profile saved and the filament in the drybox.

Loss of video. Cause: VTX or camera connector, or ECO VTX overheating. The ECO VTX has a 25 mW and 200 mW output mode. Use 25 mW for the practice space — it is sufficient at the distances involved and generates less heat. If video cuts out mid-flight, land immediately using line-of-sight. Do not attempt to fly blind to recover video.

Procedure¶

Phase 1 — Print the frame¶

Download the Whifflepick 3" V1 from Printables (model 1207344). Print in PETG-CF on a 0.4 mm hardened steel nozzle. Use the 0.20 QUALITY or 0.20 BALANCED profile — 0.10 FAST DETAIL adds print time without structural benefit.

Slicer settings — non-negotiable: - Perimeters: 3 - Top/bottom layers: 3 - Infill: 0% - Supports: none - Material: PETG-CF (0.4 mm hardened nozzle mandatory — PETG-CF is abrasive)

PETG without CF reinforcement is not recommended — it is too soft under 4S motor loads. PLA will delaminate on the first hard crash. PETG-CF is the minimum.

Print time at 0.20 is approximately 90–120 minutes. Store the filament in a drybox during printing — PETG-CF absorbs moisture and prints poorly when damp.

Gate: Frame printed and post-inspected. Check motor mount holes for clean geometry. The 20×20 stack mount area must be flat. Verify M3 hole diameters accept bolts without force. If geometry is off, re-print — do not file.

Phase 2 — Assemble the airframe¶

You will need: M3 standoffs (4×), M2 screws for motor mounts, M3 nylon nuts or lock nuts for stack, threadlocker (medium, not high-strength).

Motor installation. The EX1404 motors use a standard 9N12P 14×4 mm stator with 1.5 mm shaft. Mount pattern fits the Whifflepick motor mount directly. Torque the M2 motor screws to finger-tight plus a quarter turn — no more. The frame material at motor mounts is the thinnest point; over-torquing cracks the mount. Use medium threadlocker on all motor screws. Do not use high-strength threadlocker — you will need to remove these motors after crashes.

Motor direction for Agile (standard Betaflight quad layout): front-left CCW, front-right CW, rear-left CW, rear-right CCW. Do not install props during assembly. Props go on last, after motor direction is confirmed in Betaflight.

Stack installation. Install the SpeedyBee F405 Mini FC + ESC stack using M3 nylon standoffs. Anti-vibration grommets are preferred if supplied — they reduce IMU noise. Do not overtighten. The stack must be able to flex slightly on the standoffs. The USB-C port must face a direction accessible from outside the frame with props removed. Check access before bolting down.

Phase 3 — Wiring¶

This is the critical phase. Read → wire-gauge-selection before starting. Read the HDZero ECO VTX power warning above again.

Solder in this order — do not skip ahead:

1. Motor phase wires to ESC. Polarity does not matter for brushless motors — motor direction is changed in Betaflight (or by swapping any two phase wires). Keep wire runs short — no more than 30 mm of exposed wire between motor and ESC pad.
2. Battery lead. Solder a short 20 AWG pigtail to the ESC battery pads with XT30 female connector. Wire length from ESC pad to connector body: 40–60 mm. No longer. Every additional centimetre of battery lead increases the parasitic inductance that causes voltage spikes on hard throttle chops. The ESC stack ships with a capacitor — verify it is soldered to the ESC power pads, not on a wire pigtail.
3. VTX power (CRITICAL). Identify the 5V BEC output pad on the SpeedyBee F405 Mini FC — labelled 5V. Do NOT use the pad labelled VBAT or VTX_PWR unless the SpeedyBee documentation explicitly confirms that pad is voltage-regulated to 5V. Solder 24 AWG red wire from FC 5V pad to HDZero ECO VTX power input, black wire from FC GND to VTX GND. The ECO VTX is rated 1S–3S. 4S fully charged = 16.8 V. VBAT on 4S destroys the ECO VTX immediately. Power it from 5V BEC only.
4. VTX data. The HDZero ECO uses a composite video signal — no MIPI cable. Connect the video data wire from the ECO camera to the ECO VTX per the bundle's wiring diagram. This is the primary reason the ECO Bundle was selected for Agile: the MIPI cable is fragile and difficult to resolder after a crash; the composite interface survives crashes.
5. ELRS receiver. The EP1 Dual connects to the FC via UART. Solder to an available UART TX/RX pad pair on the FC, plus 5V and GND. Note which UART number — you will need it in Betaflight. Mount the two antennas at 90° to each other for diversity. Route antenna leads away from motor phase wires.

Post-solder inspection. Before any power-on: visually inspect every solder joint. Look for bridges between adjacent pads, cold joints (dull, granular surface), wire insulation melted against the PCB, motor phase wires not fully tinned at the ESC pad. Do not power on until all joints pass visual inspection.

Phase 4 — Betaflight configuration¶

Connect the SpeedyBee to Betaflight Configurator via USB-C. Do not connect the battery until motor direction is confirmed in Betaflight with props off.

• Ports tab: enable Serial RX on the UART wired to the EP1; enable HDZero on the UART wired to the ECO VTX (or the dedicated VTX pad if the SpeedyBee provides one).
• Configuration tab: Receiver = Serial (CRSF/ELRS); enable Airmode and Anti-gravity; enable accelerometer for self-levelling practice modes.
• ESC/Motor tab: Protocol = DSHOT300; enable DSHOT Bidirectional for RPM filter; verify each motor spins the correct direction individually using the motor test (props off, 3–5% throttle maximum) — CCW front-left and rear-right, CW front-right and rear-left. Use the direction toggle if a motor is wrong, not a phase-wire swap at this stage.
• Failsafe: set to Drop, not Angle or GPS Rescue — Agile has no GPS. Drop disarms the motors immediately on signal loss.
• Rates: start with Betaflight defaults. Do not copy another build's rates onto Agile — rates are tuned per airframe.

Phase 5 — Props and maiden flight¶

Simulator prerequisite. Do not skip this gate. A minimum of 10 hours in an FPV simulator (Velocidrone or Liftoff) using the actual transmitter and goggles is the recommended threshold before maiden flight. A pilot who cannot fly a consistent circuit in the simulator will crash Agile on the first battery; a pilot who can will crash it on the third — a much better outcome.

Prop installation. Install props last, after motor direction is verified in Betaflight. HQProp T3X3X3 are polycarbonate 3-inch 3-blade props, 1.5 mm bore matching the EX1404 shaft exactly. Match prop rotation (marked CW or CCW) to the motor rotation confirmed in Betaflight — a mismatched prop generates lift in reverse and destabilises the quad on arming. Tighten to firm finger-tight plus one quarter turn; self-locking prop nuts are preferred.

Pre-flight check. Props installed correctly with no shaft play, battery connector mated but not yet connected, motor screws tight, no loose wires in a prop arc, VTX power confirmed on the 5V wire (not VBAT) one last time. Connect battery, verify HDZero Goggles receive video, arm only after a confirmed video link.

Maiden flight. Phase 1: hover at 0.5 m for 30 seconds, no obstacles. Land, inspect all motor mounts for movement and feel each motor — warm is acceptable, hot is not (indicates wrong prop direction or mechanical binding; resolve before continuing). Phase 2: slow circuits at low altitude, one obstacle at a time, no aggressive manoeuvres until 5+ flights completed. Agile is 4S — significantly more authority than a 1S/2S tinywhoop; respect the gap between simulator muscle memory and real hardware.

Rationale¶

Agile was built rather than bought (a BNF tinywhoop such as the Happymodel M8 HDZero) specifically for outdoor pilot practice in variable wind conditions. A tinywhoop at 80 mm wheelbase and 33 g is dominated by wind above 15 km/h and provides no crash resistance in grass. Agile at 130 mm wheelbase with 3-inch props and 4S power handles outdoor conditions while remaining buildable, repairable, and cheap enough to crash without concern.

HDZero ECO was chosen over analog video for ecosystem coherence with Cinematic: the same HDZero Goggle 2 works for both, so ELRS binding, EdgeTX model handling, and HDZero goggle operation all transfer directly between the two builds — see agile-variant. Buying a separate analog VTX and receiver module would have cost roughly the same as the ECO Bundle while creating a second ecosystem with no skill transfer value. Betaflight rate-profile and PID intuition built on Agile is manual-acro specific and does not transfer to ArduPilot's flight modes on Cinematic — that's a genuinely different skill, not a shortcut.

The Whifflepick 3" frame was chosen over the PecaJosef 3" because it is actively maintained, requires no supports (fast reprints), and uses PETG-CF natively.

Connections¶

requires: - agile-variant - [[print-profiles]] - wire-gauge-selection - betaflight-setup - lipo-batteries related: - electronic-speed-controllers - brushless-motors - elrs-protocol - digital-fpv - capacitor-placement-emc - pid-tuning-rate-profile - emergency-procedures leads_to: - betaflight-setup