Mr DJ
Posts: 1734
Score: 100
Joined: 5/11/2006
Last Login: 5/18/2013
From: Maryland,
MD, USA
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Randy M
After flying this morning, I had a change of mind--since I already possess 4 new OS .25 2-stroke, I'm gonna go with them and save me 4 x $180 on the OS .40s and 4 x $8 on the 4-bladed props. Even though they may load up by being inverted and vibrate like hell--the money outweighed them. Besides, I'm not too happy with the bird being made out of plastic, so why pour in more money.
Ok, to the blades: 2 going to 3--reduce the size by 1 and keep or even increase the pitch by 1. 10x7 2-blade equals 9x7 or 9x8 3-blade. As for 2-blades on 2 and 4-strokes--see below:
Mr DJ
Engine Size 2-Stroke -----------------------------------------------4-Stroke
.049-.051 5.5x4, 5.5x4.5, 6x3, 6x3.5, 6x4 -----------------------------N/A
.09 -.10 7x4, 7x5, 7x6 ------------------------------------------------N/A
.15 7x6, 8x4, 8x5, 8x6, 8x7 ----------------------------------------- N/A
.20 -.25 8x6, 8x7, 9x4, 9x5 ------------------------------------------ 9x4,9x5,9x6,9x7
.29 -.35 9x6, 9x7, 9x8, 9.5x6, 10x4, 10x5, 10x6 ---------------------- 9x6, 9x7, 10x4, 10x6
.40 9.5x6, 10x4, 10x5, 10x6, 10x8, 10x9 ---------------------------- 11x6,12x6
.45 -.50 10x8, 10x9, 11x4, 11x5, 11x6, 11x7, 11x7.5 ----------------- 10x7.5, 11x6, 12x5, 12x6 (2-blade), 9x7 (3-blade)
.60 11x5, 11x6, 11x7, 11x7.5, 11x8, 11x9 ----------------------------11x8,11x9,12x6,13x6
.71 -.80 12x6, 12x8, 13x6, 13x8, 13x10, 14x8---------------------------Sport: 11x8, 11x9, 12x7, 12x8, 12.5x6; -------------------------------------------------------------------------------------------------------Scale: 13.8x8, 14x7, 15x6, 16x6; -------------------------------------------------------------------------------------------------------------3-Blade: 11x7
.90 13x6, 13x8, 13x10, 14x6, 14x8 -----------------------------------11x9,12x8,14x6
1.08 14x8, 14x10, 15x8, 16x6---------------------------------------- N/A
1.20 14x8, 15x8, 16x6 -----------------------------------------------14x8,15x8,15x10,16x8
1.5 16x8, 16x10, 18x6, 18x8 ----------------------------------------16x8
1.8 18x8, 18x10, 20x6, 20x8 ----------------------------------------N/A
2.1 20x8, 20x10 ----------------------------------------------------N/A
2.7 -3.0 22x8, 22x10, 22x12, 24x8, 24x10, 24x12 ------------------- 18x10, 20x19
(These are not my words but expertly written):
What do the numbers mean?
The first number is the prop diameter. The second number is the prop pitch. This is the theoretical distance that the prop would move forward if rotated one revolution. If you were to mount your prop on a shaft and submerse it in jello, you could see this in action... A 10x6 prop is 10" in diameter, and in theory, the prop will move forward 6" in one revolution. The pitch can be thought of similar to the transmission in your car. A low pitch (like on a 10x4) is like low gear on your car--lower speed, but more power. A high pitch is more like a high gear--higher speed, but it takes longer to get to top speed, and you may lose power on the verticals.
Propeller Rules of Thumb
First, for two blade props, the "load" the prop will give the engine is approximately the diameter times pitch. For example, a 9x7 (load factor of 63) prop will approximately load the engine the same as a 10x6 (load factor of 60).
Second, to move to three blade props and maintain the same load, drop one inch in diameter.
Third, a larger prop with lower pitch will provide more thrust but lower top end speed. A smaller prop with high pitch will speed the plane up on the horizontal, but the thrust will be reduced. Think of a helicopter prop--they have a huge diameter, but a low pitch for maximum thrust, but they travel at low speeds.
Fourth, the fewer the blades, the more efficient the prop is. Sure, four bladed props look cool on your scale plane, but they are inefficient, especially in the RPM ranges that our models run. It is very popular in the racing circles to run single bladed props. YES! Single blades--they actually balance them with a lead weight.
Note that all of the different brands of propellers have different efficiencies and characteristics. For smaller planes (17" and under), APC seems to be the best by quite a bit. They have a good stiff blade that is very efficient. With the larger planes & props, downline braking is a factor to consider. Because of this, the wider blades are popular.
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RE: 101" C-130 arf - 
