Originally Posted by
AAKEE
Did I miss the posts where we were discussing the noice in the earlier posted blancolirio ’vid where there was a noice that probably was not very long from 5Hz just before the helicopter desintegrated or was this not discussed yet?
There was a note in the VH Blades info letter about the tailboom having a 5Hz self resonance.
A coincidence?
I don't think you have missed any posts directly about that. Juan Browne (Blancolirio) has released three videos related to the accident, the latest being only the overall footage with audio seemingly aligned with video footage. He doesn't actually suggest the audio is 5Hz (or any other frequency for that matter), rather in the preamble to the video he indicates "The RPM matches that of the main rotor of the Bell 206L".
I have briefly tried to work out the 'frequency' of that audio but my mobile phone based app (Physics Toolbox - Tone Detector or Spectrum Analyzer) struggles to determine a primary frequency. No surprise really because the sound would be composed of multiple frequency components and the frequencies certainly slow down over time in the audio simply by listening to it. A better option to try to match the frequency might be to use something like the Physics Toolbox Tone Generator and play with the frequency and waveform. But using that app and plugging in a 5 Hz frequency shows that the 'apparent' frequency heard by (my) ears varies whether I set a square waveform, triangle waveform or sawtooth waveform with the latter seeming like a much lower frequency than the others. Using a 5 Hz sine wave is below my hearing range these days!
Given the 206L main rotor speed is around 395 RPM at 100% RRPM (strange, the Bell 206L Flight Manual doesn't explicitly seem to state that, rather gives all information only as %), then a full revolution has a frequency of around 6.6 Hz (395/60). But since there are a pair or main rotor blades, the blade passing frequency, which is apparently what we would pick up with our ears, is twice that, or ~13.2 Hz (395x2/60). If I understand the Noda-Matic suspension system on the 206L correctly, it is designed to reduce vibrations from the rotor system being transmitted into the fuselage at this frequency (many Rotorheads refer to this as "2 per rev").
If a pitch link gave way and one blade was at a crazy angle of attack, then I think audio might pick up a 1 per rev (6.6 Hz) underlying frequency component. If the helicopter was flying normally or if the rotor was shaking the Noda-Matic suspension to bits at the frequency it is tuned to, then an underlying frequency more like ~13.2 Hz should be heard in the audio.
If there are Rotorheads that have experience in acoustic analysis, would welcome their feedback.