Filter Tuning¶
Filters keep vibration out of the control loop. Too little filtering lets vibration through to the servos -- buzzing, heat, and a tune you can't push. Too much adds delay -- a soft helicopter that can't take high gains. The aim is filtering just strong enough to remove the peaks.
Start with the RPM filter at Medium, one lowpass filter around 100 Hz and the dynamic filter with a few notches -- the defaults. Then check with a log.
1. A test flight¶
Take off at the highest headspeed you'll use and hover for a minute or two, then fly a little forward flight. If you see or hear fast vibration on the first hover, land at once and investigate before going on.
Log at a high rate (1 kHz or more), with Raw Gyro and Gyro ticked on the Blackbox tab.
2. Read the spectrum¶
In the Blackbox Explorer:
- Find a steady stretch at constant headspeed. Mark its start with I and its end with O.
- Show the filtered gyro for roll, pitch and yaw, and open the spectrum (frequency analyser).
A well-filtered helicopter shows large values at the far left (the helicopter's real movement), low values everywhere else, and no sharp peaks -- maybe some small bumps between 40 and 80 Hz.
3. Identify the peaks¶
Divide each peak's frequency by the main rotor and tail rotor frequencies:
main rotor frequency = headspeed ÷ 60 (2400 rpm → 40 Hz)
tail rotor frequency = main rotor frequency × tail gear ratio
| Peak at | Is | Do |
|---|---|---|
| A whole multiple of the main rotor frequency | A main rotor harmonic | Raise the RPM filter strength, or add that harmonic with custom notches. |
| A multiple of the tail rotor frequency | A tail rotor harmonic | As above, for the tail. |
| The motor frequency | Motor vibration | Make sure the motor notch is on (Medium and High have it). |
| A fixed frequency that doesn't move with headspeed | A frame resonance -- skids, fin, boom, belt | Check the helicopter mechanically first. If it can't be fixed, add a notch filter centred on it. |
A broad bump at the 1× or 2× main rotor frequency usually means blade tracking or balance is off. Check the blades before adding filters.
4. Adjust¶
- Widen a notch by lowering its Q -- but not below 2.0, which adds a lot of delay.
- The lowpass filter shouldn't go below about 60 Hz; below 80 Hz only if you really need it. Lower it only if there's a broad "grass" of noise at 60-80 Hz, especially during manoeuvres.
- With RPM filtering working well, the dynamic filter needs only 2-4 notches.
3D flying¶
3D raises vibration, especially at the main rotor's 2× harmonic. Check a log of 3D flight too. Peaks that only appear there may need a wider notch (lower Q) on the 2× harmonic. A tall "grass" floor is usually fine.
Overspeed and autorotation¶
The RPM filter follows the motor RPM. When a one-way bearing lets the rotor turn faster than the motor -- in an autorotation, or the governor easing off in a dive -- the notches are briefly in the wrong place. The dynamic filter covers that gap; keep a couple of its notches on.