After a quick review of propeller theory, it appears that a given propeller/engine combo should in fact turn about the same RPMs as density falls off, provided that the shaft losses are a similar percentage of total power. This is because as long as we stay at moderate reynolds number and mach number, torque required for a given RPM is roughly linear with density.


However, because the shaft losses stay the same at all altitudes, the available power is actually decreasing faster than the density, while the torque is falling off with density, which should result in a prop spinning slower at higher altitudes, if anything.