Put simply the prop blades are probably flexing in a twisting manner where the pitch is actually changing. One could actually call it Prop Flutter to give a better perspective on what is happening. Virtually ALL plastic props will flex to some degree with the thinner ones being the worse. Wood and carbon will flex too but to a lesser degree with wood being the more stable in model size. The real problem comes when one blade flexes differently from the other. This uneven flexing is an indication of too high an rpm for the prop in use and will eventually cause a failure. Prior to that it is introducing destructive vibrations into the airframe.
I'll bet you can take your prop and holding the tip, easily twist the blades back and forth? This type of prop relies on the centrifugal forces to stiffen the blade when in use. Helicopter rotor blades take this concept to extremes. But with our models it is possible to reach an rpm where there simply is not enough force to hold the blade in proper shape. The sounds you hear is probably the twisting action being out of control. Again, think flutter here. But at 10,000 rpm.
Another possibility for the sound would be the blade tips breaking the sound barrier. But for a 9 inch prop that would require an RPM in the range of 26,500. Possible but unlikely. And BTW, you do not want that to happen.
Last edited by Appowner; 11-04-2018 at 07:47 AM.