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docs: add more fixed-wing runway takeoff docs, especially for wheel control

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Signed-off-by: Silvan <silvan@auterion.com>
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Silvan
2026-08-07 13:32:31 +02:00
committed by Silvan Fuhrer
parent 1c033aae05
commit 1becdf229f
2 changed files with 97 additions and 2 deletions
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docs/assets/flight_modes/fw_runway_takeoff_wheel_steering.svg
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<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 720 482" width="720" height="482" font-family="Helvetica, Arial, sans-serif">
<rect width="720" height="482" fill="#ffffff"/>
<!-- ================= Panel 1: centered and aligned ================= -->
<text x="20" y="28" font-size="14" font-weight="bold" fill="#212529">1 — Centered and aligned: the ground roll follows the runway</text>
<rect x="60" y="48" width="600" height="120" rx="6" fill="#f1f0ec" stroke="#ced4da"/>
<text x="72" y="68" font-size="11" fill="#868e96">Runway</text>
<line x1="80" y1="118" x2="645" y2="118" stroke="#adb5bd" stroke-width="2" stroke-dasharray="14 10"/>
<text x="95" y="142" font-size="11" fill="#868e96">Runway centerline</text>
<circle cx="600" cy="118" r="7" fill="#ffffff" stroke="#4c6ef5" stroke-width="2"/>
<line x1="600" y1="127" x2="600" y2="174" stroke="#4c6ef5" stroke-width="1" stroke-dasharray="3 3"/>
<text x="563" y="190" font-size="11" fill="#4c6ef5">Takeoff waypoint</text>
<!-- vehicle at the start of the takeoff, aligned with the runway -->
<path d="M 118 118 L 100 107 L 106 118 L 100 129 Z" fill="#fff4e6" stroke="#d9480f" stroke-width="2"/>
<text x="86" y="100" font-size="11" fill="#868e96">Start</text>
<!-- commanded ground roll direction -->
<line x1="128" y1="118" x2="575" y2="118" stroke="#d9480f" stroke-width="2.5"/>
<path d="M 575 112 L 589 118 L 575 124 Z" fill="#d9480f"/>
<text x="170" y="96" font-size="12" fill="#d9480f">Yaw setpoint = bearing to the takeoff waypoint</text>
<!-- ================= Panel 2: offset from the centerline ================= -->
<text x="20" y="238" font-size="14" font-weight="bold" fill="#212529">2 — Offset from the centerline: the ground roll cuts across the runway</text>
<rect x="60" y="258" width="600" height="162" rx="6" fill="#f1f0ec" stroke="#ced4da"/>
<text x="72" y="278" font-size="11" fill="#868e96">Runway</text>
<line x1="80" y1="310" x2="645" y2="310" stroke="#adb5bd" stroke-width="2" stroke-dasharray="14 10"/>
<text x="95" y="334" font-size="11" fill="#868e96">Runway centerline</text>
<circle cx="600" cy="310" r="7" fill="#ffffff" stroke="#4c6ef5" stroke-width="2"/>
<line x1="600" y1="319" x2="600" y2="428" stroke="#4c6ef5" stroke-width="1" stroke-dasharray="3 3"/>
<text x="563" y="444" font-size="11" fill="#4c6ef5">Takeoff waypoint</text>
<!-- vehicle at the start of the takeoff, offset from the centerline -->
<path d="M 145 372 L 124 363 L 131 373 L 126 385 Z" fill="#fff4e6" stroke="#d9480f" stroke-width="2"/>
<text x="72" y="394" font-size="11" fill="#868e96">Start, off the</text>
<text x="72" y="406" font-size="11" fill="#868e96">centerline</text>
<!-- direction parallel to the runway, for reference -->
<line x1="155" y1="376" x2="298" y2="376" stroke="#868e96" stroke-width="1.5" stroke-dasharray="6 5"/>
<path d="M 298 372.5 L 308 376 L 298 379.5 Z" fill="#868e96"/>
<text x="196" y="393" font-size="11" fill="#868e96">runway direction</text>
<!-- commanded ground roll direction -->
<line x1="155" y1="371" x2="573" y2="314.6" stroke="#d9480f" stroke-width="2.5"/>
<path d="M 572.9 320.8 L 586 313 L 571.3 309 Z" fill="#d9480f"/>
<text x="170" y="296" font-size="12" fill="#d9480f">Ground roll: straight line towards the takeoff waypoint</text>
</svg>
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docs/en/flight_modes_fw/takeoff.md
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@@ -135,9 +135,38 @@ The _runway takeoff mode_ has the following phases:
2. **Clamped to runway**: Pitch fixed, no roll and takeoff path controlled until the rotation airspeed ([RWTO_ROT_AIRSPD](../advanced_config/parameter_reference.md#RWTO_ROT_AIRSPD)) is reached. The operator is able to nudge the vehicle left/right via yaw stick.
3. **Climbout**: Increase pitch setpoint and climb to takeoff altitude. To prevent wingstrikes, the controller will keep the roll setpoint locked to 0 when close to the ground, and then gradually allow more roll while climbing. It is based on the vehicle geometry as configured in [FW_WING_SPAN](#FW_WING_SPAN) and [FW_WING_HEIGHT](#FW_WING_HEIGHT).
### Wheel Controller {#wheel_controller}
The wheel controller steers the vehicle on the ground using the steerable nose/tail wheel.
It is enabled with [FW_W_EN](#FW_W_EN) and only runs in automatic modes during runway takeoff and during the landing rollout.
Whenever it is not active (in all manual modes, or if it is disabled), the yaw stick is mapped directly to the wheel.
The controller is cascaded:
1. **Heading controller**: The heading (yaw) error is converted into a yaw rate setpoint with a fixed time constant of 0.1s, limited to [FW_W_RMAX](#FW_W_RMAX).
2. **Rate controller**: A P-I-FF controller ([FW_WR_P](#FW_WR_P), [FW_WR_I](#FW_WR_I), [FW_WR_FF](#FW_WR_FF), integrator limited by [FW_WR_IMAX](#FW_WR_IMAX)) turns the yaw rate setpoint into a normalized wheel steering command in the range [-1, 1].
The heading setpoint during the ground roll is the bearing from the takeoff position to the takeoff waypoint, so the nose is kept aligned with the intended runway direction.
If no navigation reference is available (takeoff without takeoff waypoint), and during the landing rollout, the controller instead holds the heading that was captured at the moment the wheel steering engaged.
![Wheel steering heading setpoint during the ground roll](../../assets/flight_modes/fw_runway_takeoff_wheel_steering.svg)
Operator yaw stick input is added on top of the controller output to "nudge" the vehicle (enabled with [RWTO_NUDGE](#RWTO_NUDGE) for takeoff, and [FW_LND_NUDGE](../advanced_config/parameter_reference.md#FW_LND_NUDGE) for landing).
::: info
For a smooth takeoff, the runway wheel controller possibly needs to be tuned.
It consists of a rate controller (P-I-FF-controller with the parameters [FW_WR_P](../advanced_config/parameter_reference.md#FW_WR_P), [FW_WR_I](../advanced_config/parameter_reference.md#FW_WR_I), [FW_WR_FF](../advanced_config/parameter_reference.md#FW_WR_FF)).
The gains are scaled with groundspeed (using [FW_AIRSPD_STALL](../advanced_config/parameter_reference.md#FW_AIRSPD_STALL) as the reference speed) so that the steering becomes less aggressive as the vehicle accelerates.
This means the controller should be tuned at low speed, around stall airspeed.
:::
For a smooth takeoff the wheel controller usually needs to be tuned:
1. Start with [FW_WR_P](#FW_WR_P) and [FW_WR_FF](#FW_WR_FF) only (set [FW_WR_I](#FW_WR_I) to 0) and taxi/roll the vehicle in _Mission mode_ with active Takeoff waypoint item at low speed.
2. Increase the gains until the vehicle tracks the runway heading without noticeable oscillation of the nose.
3. Add [FW_WR_I](#FW_WR_I) to remove a remaining constant heading offset (e.g. caused by wheel misalignment or crosswind).
::: warning
Excessive gains lead to oscillations of the nose during the ground roll, which can quickly become dangerous at higher groundspeed.
Always test with increasing speed step by step.
:::
### Parameters (Runway Takeoff)
@@ -157,6 +186,19 @@ Runway takeoff is affected by the following parameters:
| <a id="FW_WING_SPAN"></a>[FW_WING_SPAN](../advanced_config/parameter_reference.md#FW_WING_SPAN) | The wingspan of the vehicle. Used to prevent wingstrikes. |
| <a id="FW_WING_HEIGHT"></a>[FW_WING_HEIGHT](../advanced_config/parameter_reference.md#FW_WING_HEIGHT) | The height of the wings above ground (ground clearance). Used to prevent wingstrikes. |
### Parameters (Wheel Controller)
The [wheel controller](#wheel_controller) is affected by the following parameters:
| Parameter | Description |
| ----------------------------------------------------------------------------------------- | -------------------------------------------------------------------- |
| [FW_W_EN](#FW_W_EN) | Enable wheel controller |
| <a id="FW_W_RMAX"></a>[FW_W_RMAX](../advanced_config/parameter_reference.md#FW_W_RMAX) | Maximum yaw rate setpoint the heading controller outputs |
| <a id="FW_WR_P"></a>[FW_WR_P](../advanced_config/parameter_reference.md#FW_WR_P) | Yaw rate controller proportional gain |
| <a id="FW_WR_I"></a>[FW_WR_I](../advanced_config/parameter_reference.md#FW_WR_I) | Yaw rate controller integrator gain (trims a constant heading error) |
| <a id="FW_WR_IMAX"></a>[FW_WR_IMAX](../advanced_config/parameter_reference.md#FW_WR_IMAX) | Yaw rate controller integrator limit |
| <a id="FW_WR_FF"></a>[FW_WR_FF](../advanced_config/parameter_reference.md#FW_WR_FF) | Yaw rate controller feed forward gain |
## See Also
- [Takeoff Mode (MC)](../flight_modes_mc/takeoff.md)