Originally Posted by
currawong
The funny noise you hear as the lever is moved rearwards?
Is the propeller cavitating.
No it isn't. All you are hearing is the different vortex-shedding pattern as the blade's AoA and Cl change. It might even be the blade stalling, although I doubt it because (a) that would produce a huge increase in blade drag that would case a correspondingly large RPM drop, and (b) at the Re the blades run at I'm not convinced they actually can stall in the conventional sense.
Cavitation is something that only occurs in liquids. While air and water are both
fluids they do not behave identically in this respect. This can be easily seen by placing air into a cylinder with a close-fiting piston and then pulling the piston. The air inside the cylinder will drop in pressure and increase in volume. If you try the same thing with water you'll find that you can't actually pull the piston out AT ALL until you've applied enough force to overcome the surrounding air pressure. When you get to that point the piston pulls away from the water and leaves a vacuum* in the space - the water does NOT expand to fill the extra volume.
Air and water are both incompressible (Bernoulli), but air is elastic while water is not. Try filling a gass strut with water and you'll soon see the difference. That's also why Hydraulics are inherently safer than pneamatics. In high pressure pneumatic systems a heck of a lot of energy is stored in the pressurised gas, energy which leaps out and hurts people if there's a leak. But the fluid in a hydraulic system has NO stored energy, so a leak just goes "phut", dumpingt the overpressure with no great amount of movement.
PDR
* into which the water may boil, but only in certain ranges of temperature and pressure