Hi Ramon,
The higher pitch propellor allows the airplane to travel at reasonable and constant speed while the motor turns a comparatively slow RPM, greatly reducing noise and increasing efficiency.

Modern generation motors in the 23cc class are optimized to turn, on an average, 8000rpm. To achieve proper speed on the upline at this RPM, the propellor needs more pitch. The large diameter, when idled down, especially by the YS 4-stroke motor, conversly creates a large disk that creates a wonderful brake when on the downline. Where a low pitched prop provides wonderful pulling power at slow speed, the upline speed of the airplane would be ungainly slow with a 8 or 10 pitch propellor turning only 8000.

Finding the correct propellor for a given airframe, motor, weight combination is more of an art and personal preference than an exact science, but recommendations from people flying particular combinations can get one really close if not right on. Once the perfect propellor is found, it may need a complete change to deal with different wind conditions (for me, more pitch in higher winds), fuel changes, and significant changes in barometric pressure can also create a desire to change pitch by .5". That said, I know plenty of pilots who fly one prop in all conditions and fly just superbly.



OK, that's my plain language explaination. I'm sure somebody here can dazzle you with brilliant lessons of prop efficiency and, as always, I would love to hear from some of the more experienced pilots.

Mark