A prop size and pitch is determined by the particular model and desired flying style, with the additional requirement that the maximum rpm should remain reasonable. I prefer to keep the maximum rpm in the 8500 to
9500 range, some people prefer a higher rpm but greater than 10,000 is probably not good. The rpm with a given prop will depend on the fuel used and brand of prop. A larger diameter or pitch can be used with
fuel that has more nitro. Also some models fly better with bigger diameter and lower pitch , and other models are better with more pitch and less diameter; you have to experiment to determine what best suits
you. Use the max rpm as a guide with values about 8500-9500 being satisfactory. Try these sizes as starting points, all are APC brand. If you use high-nitro content fuel you will probably need to use a larger
size prop to limit the maximum rpm to about 9500. (Note: The Saito literature that accompanies most engines states a maximum rpm value that is quite a bit higher than my suggested 9500 rpm maximum value.
I guess that the higher rpm numbers quoted by Saito are ‘not too exceed values’, and not actual recommended operating rpm). The props sizes below are for general sports type flying; if you are into
so-called “3D” flying you will probably want to use a prop with a 4 or 5 or 6 inch pitch and a larger diameter to get plenty of thrust at low speeds
- Saito-30 9x7, 10x6
- Saito-40a 10x7, 11x6
- Saito-50, 56 11x8, 12x6, 12x8
- Saito-62a 12x8, 13x6
- Saito-65 13x6, 13x8
- Saito-72 12x8, 13x6, 13x8
- Saito-80 12x9, 13x6, 13x8
- Saito-82a, 13x8, 14x6, 14x8
- Saito-FG14a 13x8, 14x6, 14x7
- Saito-91 13x8, 14x6, 14x7, 14x8 **(see note below)
- Saito-100 14x8, 15x6
- Saito-115 15x7, 15x8
- Saito-120, 125a 14x10,15x8, 16x6
- FG-20 15x7, 15x8, 16x6
- Saito-150 16x8 or 17x8N
- Saito-180 16x10, 17x8N, 17x8, 17x10N, 18x8
- Saito-220 18x8, 19x8, 20x8
- FG-36 17x8, 18x8
** My favorite prop for the Saito-91 is the APC 14x7 however most distributors only carry the 14x6 and 14x8. So if you want to try an APC 14x7 you will probably have to buy it direct from APC. The APC 14x8
prop seems too big for the Saito-91 unless you have lots of nitro in the fuel.
Three or Four-bladed Props ? To use a three or four-bladed prop you will have to use a smaller diameter and/or pitch
compared to a two-blade prop. You will need to experiment with different brands and sizes to get an appropriate full-throttle rpm. I have no experience or suggestions to help with this.
The prop must be well balanced. Fox sells metric prop reamers (a 7 & 8mm version, a 10mm version, and a 10 &12mm version). Horizon Hobbies sells a 10mm prop reamer. You will need the Fox 7,8mm reamer for
the 50,56,65,80,91,100, 125 engines (the prop shaft is 8mm diameter). You will need a 10mm reamer for the 120, 150,180,220 engines; although the thread size is 8x1.25 mm for these engines, the base shaft is a
smooth 10mm diameter.
If you are using a spinner with your Saito-220 don’t ream out the prop to 10mm because the length of the
smooth shaft may not be long enough to extend through the prop. First
ream out the prop to 8mm, then use
a 10mm reamer just a little on the back-side of the prop to get it to fit.
EZ Decathalon: Saito-80 with APC 12x9 prop. For reasons of ground clearance a 12 inch prop was used. This engine is also OK with a 13x6 or 13x8 prop.