ISO 5109:2023

Overview - ISO 5109:2023 (Evaluation method for resonance frequency of multi‑copter UA)

ISO 5109:2023 defines a practical, test‑based method to evaluate and avoid resonance in multi‑copter unmanned aircraft (UA) by measuring rotor and body vibration frequencies. Applicable to multi‑copter UAs weighing less than 150 kg, the standard correlates rotor excitation (rotational frequencies and thrust) with the UA’s natural (modal) frequencies and gives guidance on test setups, measurement techniques and evaluation criteria to prevent resonance‑induced damage.

Key topics and technical requirements

• Scope and applicability: Multi‑copter UA under 150 kg; focuses on resonance caused by coincidence of rotor rotational frequency and body natural frequency.
• Test methods: Two primary approaches depending on UA size:
  • Impact/hammering method for small UAs (suspended on wire or supported on soft sponge) using an impact hammer, accelerometers and FFT analysis to capture natural frequencies and mode shapes.
  • Excitation table/vibrator method for larger UAs, following IEC vibration test practices (IEC 60068‑2‑6, IEC 60068‑2‑64) to sweep and measure frequency response.
• Rotor and thrust measurement: Measure rotor speed and thrust force using tachometers or microphones (microphone preferred for very high‑speed small motors) and load cells to determine the operating frequency range and arming frequency.
• Modal analysis: Use accelerometers, multi‑point measurements and frequency response functions (FRFs) to determine natural frequencies, damping and mode shapes. FFT and signal analysis tools are used for processing.
• Design factors: The standard highlights the principal design drivers for natural frequency - Young’s modulus, second moment of inertia, mass distribution, and arm length - and provides a cantilever‑arm formula to assess their influence.
• Resonance evaluation: Compare the UA natural frequencies with the operating (rotor) frequency range; if overlap exists, take corrective measures such as changing structural design or altering motor/rotor specifications. Margin determination is left to designer experience or project requirements.

Practical applications and who uses this standard

• UA designers and structural engineers: to validate designs and avoid resonance during design iterations.
• Test engineers and labs: to perform vibration and rotor tests, modal analysis and certification testing.
• Manufacturers and integrators: to choose motors/propellers and structural materials that avoid operational resonance.
• Regulatory and safety assessors: to assess structural robustness and conformity with vibration safety best practices.
• Researchers: studying aero‑elastic and vibration interactions in multi‑copters.

Related standards

• IEC 60068‑2‑6 (vibration - sinusoidal)
• IEC 60068‑2‑64 (vibration - broadband random)
• ASTM E1876 (support system guidance referenced for small‑article testing)

Keywords: ISO 5109:2023, multi‑copter, unmanned aircraft, resonance frequency, natural frequency, rotor speed, vibration testing, modal analysis, thrust measurement.