Propeller Sizing

Dear Expert, Propeller sizing help needed!
I bought a Hobby Shack Cessna 150 many years ago along with an OS MAX 10 and an OS MAX III 15.
I recently retired and built it with the OS MAX III 15 on board. The absolute distance from the prop center to the ground is 6 3/4".

The specs of the model are:
SPAN: 45.5 inches
AREA: 315 Sq.inches
WEIGHT: 23 TO 38oz
ENGINE SIZES: SINGLE CHANNEL .051 - .09
TWO CHANNEL .09 - -.15
3 & 4 CHANNEL .15 - .19
FREE FLIGHT .049 - .10

The final weight is 38.25 oz. empty and a 2 oz fuel tank. I fired up the OS MAX 10, on a board, and it started and ran very well. I intend to run the OS MAX III 15 in the model with some taxi-ing tests for break in.
I am leary about my ability to determine a proper range of propellers for this setup, any help would be appreciated.

My expertise is not great but I fly an EASY SPORT 60 with an ENYA 80 4 stroke about once or twice a week, and almost every morning I fly GWS Pico Moth and Cub interspersed with the new Apprentice.

Thank You in advance, Don Hopkins (AMA 612370)

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Propeller Sizing

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Aug 30, 2008

Prop sizes
by: Pete

Hello Don, I'm not sure about the 'Expert' label but I'll try and give you an answer... ;-)
A large part of selecting which prop to use is to see what the manufacturers themselves suggest for that particular engine, otherwise it's a bit of trial and error. But, the general range of prop sizes for a 15 engine is from 7x4 through to 8x6, or thereabouts.

The 2nd number of the prop size is the pitch, and represents how many inches that prop will move forward through the air per revolution, in a theoretically perfect world. Because your plane isn't a racer, you don't need to worry about high speed too much, so personally I'd choose a 4 rather than a 6.
As for diameter, I'd probably go for an 8 over a 7 but again you'll have to experiment and see which performs better. The engine will have slightly higher revs with a 7 and scream a bit louder, but the difference won't be large.

Both 7" and 8" props will still leave plenty of ground clearance on your plane.

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