Three bladed propellers on planes are more efficient but are less effective than four bladers as altitude increases.
If you want to have a prop-driven aircraft which can deal with both these competing demands, then today’s invention should help.
On the left, a twin bladed propeller is shown. On the right, it has rotated (and locked) two blades from behind the initial two, so that it now has four.
The same kind of arrangement could be made to work with 3->6 etc.
This switch between propeller numbers could be made to happen in flight.