Just a few questions: How could it be that at 7.9V, the Kama Flex spins virtually at its rated speed?
Secondly, it seems what you are saying regarding the Kama Flex is that because it has relatively high pitched blades, it is not all that well suited for high resistance. Is that a general rule?
I just checked, and my "L" Kama Flex is 2000RPM, 27dBA. Thank you for pointing out my error; I've corrected the first post (1500RPM is an "SL").
Are you at all familiar with the variable-pitch propellers on small commuter prop-jet aircraft? They begin the takeoff at lowest pitch, and increase the pitch gradually as the aircraft speeds up. At cruise, the prop is at its highest pitch. This acts as a continuously-variable "gearbox", keeping the engine running at a constant, optimum speed.
For a given airflow, the lower the pitch, the higher the needed RPM and so the higher the noise. If there were not some sort of tradeoff, everybody would build high-pitch fans to keep the noise level down. The tradeoff is, for high-resistance airflows, "low gear" on the "transmission" is best, and that means a low pitch and high RPM.
Examples: The Yate Loon and GW NCB 120mm fans are long-time favorites here at SPCR; they are the two lowest-pitch fans I know of. A very recent favorite is the Slipstream, which is the highest pitch fan I've used. I believe it works best in low-resistance airflows, but its 9 blades seem to make it handle high-resistance better than the 7-blade high-pitch Noctua fan, which is notorious for lousy performance in high-resistance situations.