Propeller flex

[RC4ZEKE]

I have been trying to figure out when a prop (GWS) flexes, what it is that causes it to produce a loud hum-like sound. If i'm not mistaking, the blades are flexing forward and also flattening out increasing the pitch, could be wrong :rolleyes:. Could someone tell me exactly why this produces that loud hum. Thanks.

[globemaster3c17]

Every prop makes noise. The majority of noise produced by full-scale piston planes comes from the prop, not the engine. Same thing with the rotor on a helicopter. A lot of the noise comes from the tip speed. Depending on prop diameter and RPM, the tips could be near mach 1. If they exceed mach 1, they get even louder. There is a lot of turbulent flow going on at the tips, and turbulence is noisy.
Yes, the blades will flex some as the blades are producing thrust, but the resistance to forward motion is at the hub. When the blades flatten out, they reduce pitch, not increase it. Airfoils have a reduce-pitch moment, in this case it makes the blades reduce their pitch some.

[RC4ZEKE]

But when I use an APC prop instead of the GWS prop spinning at the same rpm, I don't get that loud hum sound. Thanks, for your reply.

[globemaster3c17]

That's probably due to a difference in blade design. The APC prop is probably designed so that there is less tip vorticies, so there is less noise.

[BMatthews]

Either less tip votices or less stalling of the blades. I've done a fair amount of static thrust testing of propellers and found that some howl like crazy and others just produce a nice zzzzzz sound. I chalked it up to some being stalled while in the static test while others were not. The bad sound usually showed up on props with higher pitches or thinner blades. The GWS are about as thin as you'll find while the APC's have a better airfoil to the blades.

Most likely and depending on which prop sizes you're using it's a combination of both.

[RC4ZEKE]

Thanks guys, I'm pretty sure I understand now.