wboayue / stable-flight / 38666987320

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push

github

Committed by web-flow

Commit Message

Tailsitter back ramp: sink guard, slow low-thrust sim ramp (#343) (#398)

The tailsitter back ramp's thrust now rises while the vehicle sinks, and
the sim tailsitter flies a slow, low-thrust back ramp that turns plane
speed into a slowdown, not a climb or a fall.

## Why

The climb after a back transition is mostly plane speed turning into
height: v²/2g ≈ 11 m from 15 m/s, matching #341's 10–12 m. Lowering
`TRAN_VT_THR` trims it. On a slow ramp, though, too little thrust stalls
the plane nose-high and it falls. In the sim (`airframes::tailsitter()`,
25°/s, done at 20°), thrust 0.55 or less fell 30 m into the ground. A
fixed ramp thrust had to be guessed per airframe, with a cliff just
below the right value. If it's a choice between climbing and falling,
the vehicle should climb.

## Change

- **eXplora:** `TRAN_VT_THR` 0.84 → 0.75, from the flights below.
- **Core:** the `ToCopter` thrust is `back_thrust`, raised by
`TRAN_VT_SINK_GN` (default 0.4) per m/s of sink past `TRAN_VT_SINK_LIM`
(default 1 m/s), capped at 1. Thrust saturation is flagged while it
acts. The guard math moves to `math::guard_sink`, shared with
`CopterGuidance`'s piloted sink guard.
- **Sim airframe:** back ramp 25°/s, done at 20°, `back_thrust` 0.6
(cruise 0.69, hover 0.91).
- **Sources** (cited in the `vtol` module docs):
- ArduPilot flies a fixed ramp throttle (`Q_TAILSIT_THR_VT`,
`tailsitter.cpp:137-142`, `:306-341`).
- PX4 runs its hover vertical-speed loop through the ramp
(`FlightTaskTransition.cpp:50-86`). The guard is the sink side of that
loop.

## Sim results

Measurements on `airframes::tailsitter()`, from a hover at 30 m and a
plane leg at 15 m/s, back request at 45 s:

| Ramp | Thrust | Guard | Speed at handover | Peak climb | Lowest |
Thrust at 0 after handover |
|---|---|---|---|---|---|---|
| 50°/s, done 45° (`main`) | 0.5 | – | 9.2 m/s | +7.5 m | −0.1 m | 0.82
s |
| 25°/s, done 20° | 0.6 | either | 2.5 m/s | **+9.5 m** | −1.2 m | none
|
| 25°/s, done 20° | 0.5 / 0... (continued)

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122 of 130 new or added lines in 6 files covered. (93.85%)

2 existing lines in 2 files now uncovered.

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