Crafty — Hover Pack, Crash Pack and the Odd Sound
Two packs flown after the morning review. Pack 1 was a
deliberate steady hover; pack 2 was flown actively, crashed twice and needed a turtle flip. The
pilot also recorded flight audio because of an odd sound. The pilot calls this airframe the AIR75;
it reports craft_name = Crafty and Crafty’s DeviceUID, so it is the same drone.
The flash was full again: btfl_005 stops at 52 s with no clean end.
| Log | Pack | Armed | What happened |
|---|---|---|---|
btfl_001 | 1 | 203 s | Steady hover, one pack |
btfl_002 | 2 | 5 s | Short hop, bump at 4.5 s |
btfl_003 | 2 | 17 s | Turning flight, ends in a crash |
| 21 s gap | 2 | Same power cycle, nothing recorded; most likely the turtle flip | |
btfl_004 | 2 | 156 s | Active flight, ends in a crash (7.5 g) |
btfl_005 | 2 | 52 s+ | Rearmed 2.4 s later; logging stopped when the flash filled |
All five logs share one tune, flown in Horizon. Pack 1 bottomed at 3.52 V, pack 2 at 3.20 V.
The sound is a once-per-revolution vibration, loudest on motor 4
The recording lines up with the logs to within ~0.1 s, so each motor’s measured RPM can be matched to the audio. Crafty flies 3-blade props, so a healthy whoop sounds mostly like blade pass (~1 050–1 100 Hz) plus electrical whine at 6× rotation (~2 100 Hz). This recording is dominated by 1× rotation, ~335–370 Hz, the tone of an out-of-balance spinning part.
Audio tone prominence, dB above the surrounding noise, at each motor’s own harmonics:
| Harmonic | M1 | M2 | M3 | M4 |
|---|---|---|---|---|
1× (imbalance), btfl_004 | 14.4 | 12.8 | 12.8 | 17.3 |
1× (imbalance), btfl_005 | 18.8 | 14.5 | 13.6 | 19.5 |
3× (blade pass), btfl_004 | 4.1 | 6.1 | 3.6 | 4.5 |
The gyro agrees, and has history. Unfiltered roll and pitch gyro demodulated at each motor’s exact rotation phase in steady flight (background ~1–4):
| Log | M1 | M2 | M3 | M4 |
|---|---|---|---|---|
btfl_009 (12 Sep) | 8.6 | 4.1 | 8.1 | 12.6 |
btfl_028 (before the crash) | 6.9 | 3.9 | 8.0 | 16.5 |
btfl_036 (after the crash) | 6.4 | 4.9 | 5.5 | 27.3 |
btfl_039 | 6.8 | 3.1 | 9.9 | 41.7 |
btfl_001 (hover) | 3.5 | 6.4 | 3.8 | 34.2 |
btfl_004 | 7.8 | 2.8 | 7.0 | 27.7 |
btfl_005 | 14.7 | 3.9 | 4.8 | 23.3 |
In the hover log the strongest gyro peak between 250 and 450 Hz sits at 369–373 Hz, exactly motor 4’s rotation speed. Motor 4 is front left. Its imbalance roughly doubled at the morning crash and has not recovered, while its RPM per command is normal (1.005), so it is not dragging. Motors 2 and 3 have settled.
The rattle
The pilot describes a rattle, not a hum. In the 2.5–9 kHz band there is no steady clicking (about 30 separate clicks in 214 s, mostly handling), and click energy at each motor’s rotation rate sits only ~2 dB above background for all four. The roughness follows loudness (r = +0.60) and only faintly follows motor 4’s vibration (r = +0.14 with loudness removed). The recording is HE-AAC at 48 kbit/s, which smears clicks, so audio alone cannot locate it. The best fit is a loose part shaken by motor 4’s imbalance, such as a prop worn loose on its shaft.
The front-left motor is visibly tilted
On inspection motor 4 leans forward a little, which fits its imbalance doubling at the btfl_029
crash. A few degrees of lean tips only about 2 % of the craft’s weight sideways, and steady-hover
yaw I-term and diagonal mix wander between −1.7 % and +1.6 % with no step at the crash. The tilt
therefore matters through vibration and a possibly loose mount, not through a steady push.
In the hover, forward stick was held all the time
The FC’s own roll and pitch angle varied only ~1° in every 40 s block, yet holding that took continuous stick:
| Time | Pitch stick | Roll stick | FC pitch angle | FC roll angle |
|---|---|---|---|---|
| 4–12 s | +35 (7 %) | +32 | +1.6° | +1.5° |
| 52–60 s | +87 (17 %) | +36 | +4.6° | +1.7° |
| 140–148 s | +104 (21 %) | +66 | +5.7° | +3.2° |
| 196–204 s | +118 (24 %) | +48 | +6.9° | +2.3° |
On the ground the FC read level (pitch +0.1°), but by the end of the pack it reported +5 to +7° nose-down while a quarter of forward stick was held. Most likely the FC’s idea of level drifts nose-up in flight and was being corrected without noticing.
This is the third attitude-reference issue in two days, after the bad Crafty calibration and the Air65’s 6.5° offset. It differs by starting at zero and growing in the air. Vibration can bias an accelerometer and motor 4’s imbalance is the obvious candidate, so fix that first. Then hover a pack with light hands and read the attitude in the Configurator right after landing.
What the active pack says about the flying
- Very gentle attitude: under 11° of tilt 90 % of the time, under 15° 99 %. Throttle stayed between 1367 and 1465.
- Flat turns: yaw stick was in use 60–85 % of the time, but real bank in turns was 3.4° against 3.1° flying straight. The nose rotates while the craft keeps sliding.
- Both crashes were turns into something, not loss of control. In
btfl_004the craft was flying forward while yawing at 60–100 °/s, only 6° nose-down 50 ms before a 7.5 g impact. - Throttle does not follow tilt (correlation about zero in pack 2, +0.5 in the hover pack).
To practise: bank into turns with roll rather than steering on yaw alone, and leave more room before a turn.
Other observations
- Centre of gravity: front motors 7 % faster than rear, pitch I-term −17. Slightly nose-heavy.
- Current sensor: the 1320 A glitch recurs in
btfl_001,002and004. - Flash: 16 MB holds about seven minutes at this logging rate. Erase after every two packs.