Originally posted by RCBOZO
I am unsure about exactly what determines what size prop best suits each plane.
I read somewhere that if you multiply diameter by the pitch you get a certain number and the higher the number the more speed?
Presuming that I am somewhat correct. A 9x6 prop would be very close in performance to a 11x5?
Could someone please enlighten me?
A good rule of thumb is called prop loading. For example lets assume the manufacturer recommends a 10-6 (10" diameter by 6" pitch) the 6" is the theoretical distance the plane would move forward in one complete revolution of the prop. Multiply the diameter by the pitch (10 x 6 = 60) so your prop load is = 60.
If you want to go to a larger diameter prop (for example if you have a large cowl to get the prop beyond for more efficient air flow) lets say you use a 12" prop at 6" pitch and your prop loading would be 72 (12 x 6). This would load the engine too much so lower the pitch. A 12 x 5 = 60 which would be the same loading as the 10 X 6 and a better choice. A 12 x 4 would be a loading of 48 (which seems low but may actually work best).
Your example:
9x6 = 54 (faster)
11x5 = 55 (more torque)
is actually an almost equal loading match. The higher number has nothing to do with speed (prop loading number) as the higher pitch lower diameter prop combination will pull the plane faster. Think of this as gears in your car. Lower ones to get going and higher ones to cruise. Variable pitch props anyone?
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