26-10-2016, 08:41 PM
Everything else being equal, a 3-blade prop will be louder than a 2-blade. There is more interference between the blades - dirty air - and one more blade contributing noise.
Clearly, since in this case the 3-bladed props are noticeably quieter, everything is not equal. It could be many things:
1) The 2-bladed prop may be badly unbalanced.
2) The shape of the blades may be somehow inherently loud.
3) RPM under load is significantly less with the 3-bladed props, because the motor is not able to cope. (Have you tested both setups with a power meter? Is the motor coming down hot after a flight?) Less RPM means less noise.
4) The lower tip speed of the smaller 3-bladed 9x4.5 prop is resulting in less noise. (The tip speed of the 9x4.5 prop is 81% that of the 10x4.5.)
5) The higher pitch of the 8x6 is significantly reducing RPM under load, and thereby the amount of noise.
6) The 2-bladed prop just happens to be hitting some sort of resonant frequency where the motor mount or the entire airframe are vibrating and generating noise.
Or some combination of these factors.
Clearly, since in this case the 3-bladed props are noticeably quieter, everything is not equal. It could be many things:
1) The 2-bladed prop may be badly unbalanced.
2) The shape of the blades may be somehow inherently loud.
3) RPM under load is significantly less with the 3-bladed props, because the motor is not able to cope. (Have you tested both setups with a power meter? Is the motor coming down hot after a flight?) Less RPM means less noise.
4) The lower tip speed of the smaller 3-bladed 9x4.5 prop is resulting in less noise. (The tip speed of the 9x4.5 prop is 81% that of the 10x4.5.)
5) The higher pitch of the 8x6 is significantly reducing RPM under load, and thereby the amount of noise.
6) The 2-bladed prop just happens to be hitting some sort of resonant frequency where the motor mount or the entire airframe are vibrating and generating noise.
Or some combination of these factors.