ExpressLRS: what it is, what 4.x brings, and why we use it
RadioMaster XR4 — ExpressLRS receiver on our build
RadioMaster TX16S MK3 MAX — radio with internal ELRS Gemini-X
The TX16S was on the bench, the XR4 next to it, and somewhere someone had written “ELRS 3.x works too”. Technically true. For our Gemini-X hardware we did not want that. First understand what ExpressLRS actually is — then flash.
Hands-on setup (EU/NL, ArduPilot): ELRS guide. Process: step 4 — radio.
What is ExpressLRS?
ELRS is an RC protocol + firmware for transmitters and receivers. It carries sticks and switches over the air. It can return telemetry. It is not a flight controller. It is the reliable remote link under ArduPilot.
On our stack:
TX16S (internal ELRS) ←→ air (2.4 / Gemini) ←→ XR4 ←→ Pixhawk TELEM2 (CRSF)
See ports & wiring.
Why ELRS — and why 4.x
We choose ELRS because it speaks CRSF on a real UART. Not SBUS on a timer input. That fits the scarce ports on the Pixhawk 6X Mini. LD-06 already owns TELEM1; RC needs TELEM2.
Our hardware is Gemini-X (2× LR1121). For that we want ELRS 4.1.x, not 3.x “because it works”. 4.0 was the big jump for LR1121 / Gemini; 4.1 cleans up dynamic power and binding phrase.
MAVLink-over-ELRS waits until after stable CRSF flights. Checklist is ready: MAVLink + QGC via backpack. First CRSF + Yaapu. One problem at a time.
Flash = put firmware on the device. TX and RX to the same major (no 3↔4 mix).
What we get from that
- TX16S + XR4 Gemini — dual-band redundancy (2.4 + Sub-G).
- CRSF on TELEM2 — next to LD-06 on TELEM1.
- 4.x dynamic power + EU CE — strong link without living on FCC powers.
- Telemetry / Yaapu — battery and link on the radio.
- Later: MAVLink-over-ELRS + QGC on the MacBook via TX16S backpack — checklist.
- One update path via firmware.
What we do (update short)
- Props off; note binding phrase.
- Update ExpressLRS Configurator (we use 1.8.3).
- Flash TX first → ELRS 4.1.x, regulatory EU CE LBT (+ EU868 for Sub-G).
- Flash XR4 to the same 4.x + phrase.
- Power-cycle; check Lua + sticks in QGroundControl.
Details: ELRS guide.
The TX16S is handset + ELRS TX + UI (EdgeTX, Lua, Yaapu). The XR4 is the airborne side. ArduPilot decides what sticks mean.
Reference — what can it do?
| Capability | Short |
|---|---|
| Low latency | Packet rates into the hundreds of Hz |
| Long range | Lower rates + good antennas (within legal power) |
| Telemetry | LQ, RSSI/SNR, battery; with ArduPilot also Yaapu |
| Binding phrase | Shared secret in TX and RX |
| Dynamic power | Raises power only when the link needs it |
| Gemini / dual-band | 2.4 + Sub-G together (LR1121) |
| MAVLink over ELRS | Optional GCS on the same link |
| WiFi update | Flash RX/TX via browser / Configurator |
Reference — firmware 3 vs 4
We target ELRS 4.1.x (see firmware). 3.x still gets maintenance.
| Topic | 3.x | 4.x (our pick) |
|---|---|---|
| LR1121 packet modes | Fewer | Up to ~20 modes; bands separated |
| Dynamic power (LR1121) | Often stuck high | Smarter → aims ~99% LQ |
| Gemini telemetry | Redundant packets | Doubled telemetry bandwidth |
| MAVLink | More manual | Better integration |
| RX/TX sync | More manual | Auto True Diversity ↔ Gemini |
| WebUI | Older | Modern |
| Compatibility | 3↔3 | Same major on TX and RX (no 3↔4 mix) |
Reference — features since the early days
Rough timeline (not exhaustive):
- Basic link — LoRa/FHSS, rates, bind, CRSF to the FC.
- Telemetry & Lua — linkstats; ExpressLRS Lua in EdgeTX.
- Dynamic power & EU LBT — legal/efficient power for CE 2.4.
- WiFi flashing & Configurator — updates without a programmer.
- Diversity / Gemini — dual paths.
- LR1121 dual-band — 2.4 + Sub-G (our TX16S / XR4).
- MAVLink-over-ELRS — QGC without a second radio (checklist).
- 4.0 platform — more modes, smarter power, fatter Gemini telemetry.
- 4.1 — faster dynamic-power backoff, bindphrase from handset, fewer false “Telemetry Lost”.
Role of the TX16S
| Role | Meaning |
|---|---|
| EdgeTX handset | Model, mixes, flight-mode switches, safety |
| ELRS transmitter | Internal Gemini-X (2× LR1121) |
| Config point | ExpressLRS Lua: rate, power, bind, band |
| Telemetry screen | LQ/RSSI + optional Yaapu |
| Flash host | USB → EdgeTX passthrough |
Read next
- ELRS guide — flash, EU, params
- XR4 · TX16S
- Ports & wiring · Firmware
- Step 4 — radio
Sources: ExpressLRS docs · ELRS 4.0.0 · ELRS 4.1.0 · Oscar Liang · ArduPilot CRSF
Choice log
| What | ExpressLRS as RC stack (explain post) |
| Why | Understand 4.x + Gemini before the technical guide |
| Our pick | ELRS 4.1.x on TX16S Gemini-X + XR4 · CRSF · EU CE/EU868 |
Photos