this helicopter made an odd noise

Daveroo

Daveroo

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AS350B3 belonging to the Placer county Sheriff's office or PCSO..

rather than a rotor chop chop sound,or like the CHP's AS350B3 which has a deffenant chop sound to it...
this PCSO unit made a thrushing sound,you cant hear it until its literally on top of you
whats differant between them?...does this have some sort of silencing unit?..btw they are picking up two snipers in this pic,while searching for a shooter who was loose in my nieghborhood last week..
 
There is nothing really to special with it. I know with the H-60's they are louder at ground idle with no collective applied than they are with power applied in a hover/flying. I have always attributed it to sound pitches that can and cannot be heard by humans. The only tricks that I have personally seen to quiet down helicopter rotors is using special kind of tip caps, but even those have only provided a negligible improvement based on my ears. Those little AS helicopters are small with a high rotor RPM so is probably a combination of pitches you can't hear combined with it not needing to displace a lot of air to fly. Cause in my time the one thing I have noticed is the bigger the helicopter the louder and deeper it is. Then with the smaller ones they are usually quieter with a higher pitch.
 
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