Flight modes

Summary¶

Flight modes are a spectrum of automation, not discrete categories. Rate mode (manual) corrects rotation rate only — the pilot controls attitude directly. Angle mode adds accelerometer levelling — the drone returns to level when sticks are released. GPS position hold adds GNSS — the drone holds its position against wind. Each mode depends on additional sensors; losing a sensor drops the drone to the next less-automated mode. A pilot who understands this dependency chain responds correctly to failures. A pilot who does not may fight the wrong problem.

Concept¶

Rate mode (manual / acro)¶

The flight controller corrects rotation rate using the gyroscope alone. It applies corrections to hold the commanded rotation rate — but it does not level the drone when sticks are centred. If the pilot releases the sticks at 30° of roll, the drone stays at 30° of roll. The pilot controls attitude directly and must actively return to level.

Rate mode gives maximum responsiveness and authority — corrections are direct with no additional processing. It requires the most pilot skill. libdrone is not optimised for rate-mode flying (the PID baseline is tuned for GPS-assisted mapping), but rate mode is the fallback when higher-level modes fail.

Angle mode (self-levelling)¶

The flight controller fuses gyroscope and accelerometer data to maintain an attitude estimate. When sticks are centred, the drone levels itself. Maximum tilt angle is limited — the pilot cannot roll past a configured maximum (typically 45°). This makes the drone significantly easier to fly than rate mode.

Angle mode requires the accelerometer. If the accelerometer is producing incorrect data (vibration-contaminated, miscalibrated, temperature-drifted), angle mode will not hold level correctly. In practice, on a well-built libdrone with good vibration isolation, angle mode is stable and reliable.

GPS position hold¶

The flight controller adds an outer loop using GNSS position. When sticks are released, the drone holds its current position against wind and drift. The GPS position loop runs at 10 Hz (one update per 100 ms) — significantly slower than the inner PID loop (8 kHz). The outer loop feeds position corrections as attitude setpoints to the inner loops.

GPS position hold requires: accelerometer (for attitude estimate), barometer (for altitude hold), GPS fix (≥ 8 satellites recommended), and compass (for heading-lock position hold in wind).

Losing GPS drops the drone to angle mode — it will hold attitude but drift with wind. The pilot must apply corrections manually. This is not an emergency; it is the designed fallback.

The sensor dependency chain¶

``` GPS position hold └── requires: GPS fix + compass + barometer + accelerometer + gyroscope

Angle mode └── requires: accelerometer + gyroscope

Rate mode └── requires: gyroscope only ```

At each level, losing the required sensor drops to the level below. A pilot who feels the drone suddenly start drifting should immediately consider whether GPS was lost (switch to angle mode, maintain orientation manually) rather than fighting the drift with GPS stick corrections that are no longer having effect.

Reference¶

Mode comparison¶

Mode switching on libdrone TX16S¶

libdrone does not use a dedicated flight-mode switch for Rate/Angle/GPS. In Betaflight, the mode stack is:

• Default: GPS position hold when GPS fix ≥ 8 sats
• GPS lost: Angle mode automatically
• Arm switch off: Disarmed

The pilot does not manually select between angle and GPS hold — the firmware transitions automatically based on GPS fix quality. The Low Speed mode switch (SD) changes the PID profile (throttle scale), not the flight mode.

Procedure¶

Responding to GPS loss in flight¶

1. OSD will show GPS fix loss (satellite count drops to 0 or flashes).
2. Drone transitions to angle mode — will now drift with wind.
3. Do not release sticks. Apply corrections to maintain position manually.
4. If GPS recovers within 5–10 seconds, position hold will reactivate.
5. If GPS does not recover: fly home in angle mode using VLOS. Keep airspeed low to reduce drift.
6. If drone is at risk (obstacles, loss of orientation): activate GPS Rescue manual trigger (SE switch) only if GPS fix was present when armed — GPS Rescue requires a recorded home point.

Rationale¶

Why libdrone does not expose a manual rate-mode switch¶

Rate mode requires active attitude control at all times. On a payload-carrying mapping drone flown by operators with varying skill levels, accidental mode switches into rate mode over an urban area are a safety risk. The firmware stack is configured to keep the drone in the most automated mode for which sensor conditions are met. Operators who specifically require rate mode for freestyle or testing can enable it via a dedicated Betaflight mode switch, but it is not in the default configuration.

Connections¶

requires: - closed-loop-control - gnss-gps - imu-gyroscope related: - betaflight-gps-rescue - piloting-progression - inertia-and-stopping - ardupilot-flight-modes leads_to: - piloting-progression - ardupilot-flight-modes