Summary¶

Every drone frame has mechanical resonant frequencies — specific frequencies at which it vibrates with amplified amplitude when excited. When propeller and motor rotation frequencies sweep through a frame's resonant frequency, vibration amplitude spikes. This spike couples into the IMU as apparent rotation, causing the flight controller to command incorrect motor corrections that amplify the vibration further — a positive feedback loop. The RPM filter and notch filters in Betaflight and ArduPilot break this loop by removing the resonant frequency from the gyroscope signal before it reaches the PID controller. Understanding the resonance mechanism determines which filter to apply and where to set it.

Concept¶

Mechanical resonance in frames¶

Every physical structure has one or more natural (resonant) frequencies determined by its mass distribution and stiffness. When an external force oscillates at this frequency, the structure absorbs energy efficiently and vibration amplitude grows — the same principle as pushing a swing at its natural pendulum frequency. At other frequencies, energy is not absorbed efficiently and amplitude remains small.

For a drone frame, the resonant frequencies are typically in the range 80–400 Hz. A PETG frame with 3 mm CF rods has different resonant frequencies from a PC-CF frame — stiffer material shifts resonant frequencies upward. Adding mass (battery, payload) shifts them downward. The resonant frequencies of a specific build can only be measured, not reliably predicted from material properties alone.

The motor RPM sweep problem¶

Motor RPM is not constant — it varies with throttle from near-zero at idle to maximum at full throttle. The rotation frequency of the motor (Hz) equals RPM / 60. The propeller blade-pass frequency equals rotation frequency × blade count. These two frequencies and their harmonics (2×, 3×, 4× rotation frequency) sweep across the full frequency spectrum as the pilot changes throttle.

If any of these swept frequencies coincides with a frame resonant frequency, vibration spikes at that throttle position. On a blackbox log, this appears as a sudden increase in gyroscope noise at a specific throttle level — not a constant noise floor, but a throttle-dependent spike. This is the diagnostic signature of a resonance problem.

RPM filter vs static notch filter¶

Two filter approaches address resonance:

RPM filter (Betaflight, ArduPilot): tracks the motor rotation frequency in real time using bidirectional DShot RPM telemetry, and places a notch filter at exactly the rotation frequency and its harmonics as they move with throttle. The filter follows the resonance — wherever RPM goes, the notch goes. This is the most precise approach but requires bidirectional DShot. See → rpm-filter.

Static notch filter: a fixed-frequency notch placed at the measured frame resonant frequency. Does not track with RPM — effective only when the RPM sweep happens to coincide with the notch frequency. Useful for strong fixed- frequency resonances (e.g. a frame mode that is always excited regardless of motor speed) or as a backup when RPM telemetry is unavailable.

The IMU coupling mechanism¶

The IMU's accelerometers and gyroscopes are mounted on the flight controller PCB, which is mounted on the frame via vibration-dampening grommets. Despite the dampening, high-amplitude frame vibration at the resonant frequency partially couples into the IMU. The gyroscope reads it as angular rate — the same signal it reads from actual rotation. The PID controller cannot distinguish mechanical resonance vibration from commanded rotation and responds by modulating motor outputs at the resonant frequency, reinforcing the vibration. Without a notch filter, this loop can grow to the point where the motors oscillate audibly and the aircraft is uncontrollable.

Reference¶

Resonance diagnostic checklist:

Filter configuration starting points (Betaflight): ```

RPM filter (requires BiDi DShot)¶

rpm_filter_harmonics = 3 ; Filter fundamental + 2 harmonics rpm_filter_q = 500 ; Q factor: higher = narrower notch

Static notch (if RPM filter unavailable)¶

gyro_notch1_hz = 200 ; Measure frame resonance first gyro_notch1_cutoff = 170 ```

Procedure¶

Identify frame resonant frequency from blackbox¶

1. Fly a hover, slowly sweeping throttle from 20% to 80% and back.
2. Download blackbox log. Open in Betaflight Blackbox Explorer.
3. Enable spectrum analyser view. Look for frequency peaks that shift with throttle (motor harmonics) vs fixed-frequency peaks (frame modes).
4. The fixed-frequency peak is the frame resonant frequency.
5. Set a static notch at ±10% of that frequency, or confirm the RPM filter is covering the motor harmonic that excites it.

Rationale¶

Understanding resonance as the underlying cause — rather than treating filter settings as magic numbers — allows builders to diagnose novel problems. A new payload changes the frame mass distribution and shifts the resonant frequency. A damaged arm changes the stiffness. Both require re-evaluation of filter settings. Builders who understand the mechanism adjust correctly; builders who copied filter settings from a build video struggle to understand why the same settings do not work on their build.

Connections¶

requires: - vibration-isolation-theory - imu-gyroscope - rpm-filter related: - imu-filter-tuning - pid-tuning-rate-profile - propeller-balance - floating-motor-mounts - blackbox-analysis - dshot-protocol leads_to: - rpm-filter - imu-filter-tuning - pid-tuning-rate-profile