wboayue / stable-flight / 38573515281

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Sim: plane steps, coordinated turn, stall (#348) (#385)

Plane step, turn, and stall tests on `airframes::plane()`, with the
tuning they need. Closes #348.

## Tuning (`airframes::plane()`)

- Each rate loop's feedforward is worked out from the model
(`rate_feedforward`): the surface command per rad/s that balances its
rate damping at trim. Roll 0.72, pitch 0.46, yaw 1.80, against PX4's
0.5/0.5/0.3. PX4's yaw FF left the rudder to feedback, so turns slipped
by 2.9–3.8°.
- Pitch rate I 1.5 (PX4's is 0.1). A held pitch at constant throttle
trades airspeed, so the elevator keeps moving after the step; with PX4's
I the attitude droops back 17–19% and doesn't settle.
- Everything else stays at core's defaults.

## Tests

| Test | Band | Source | Measured |
|---|---|---|---|
| `plane_step`: roll ±30°, pitch ±10° (`AbsoluteAttitude`, from level on
the line) | rise ≤ 2.2τ (0.88 s) | PX4 `FW_R_TC`/`FW_P_TC` 0.4 s,
`fw_att_control_params.c:56,71` | roll 0.67 s, pitch 0.56 s |
| | overshoot ≤ 5% | copter spec (ArduPilot `AGGR/2`), borrowed: neither
plane autotune has a target | 0 |
| | settled in 4 s | probe's 10% band | roll 0.80 s, pitch 0.70 s |
| | clipped ≤ 22 ms | copter angle steps' allowance | 0 |
| `plane_turn`: loiter, held 30° and 45° bank | \|β\| ≤ 2.5° | measured
+ margin | 0.9°, 0.9°, 1.4° |
| | loiter altitude ±1 m | ArduPilot `arduplane.py:2727` | < 0.01 m |
| | loiter off-circle ≤ 5 m | measured + margin | 2.8 m |
| `plane_stall`: pitch-up rate 1 rad/s at zero throttle for 4 s, wings
level and rolling 0.3 rad/s | α past the stall, states finite, off the
ground | | α 146° (tail slide) |
| | flying again (α < stall, airspeed ≥ min) ≤ 2 s after the entry |
measured + margin | 0.6 s |
| | altitude lost ≤ 8 m, body rate ≤ 3.5 rad/s | measured + margin | 3.7
m, 1.8 rad/s |

The plan had a banked-entry row. At line speed the elevator can't pull α
past about 6° against pitch damping, so a pull in a held bank only
tightens the turn and never stalls. A st... (continued)

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