

11092006, 05:54 PM  #1 
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I am: Daniel
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Perplexed with Propellors!
Ok guys, now I am really confused with prop selection!
I am planning on buying a SuperTigre G51 engine, and I want a few spare propellors so that I still have a range of props ready for applications requiring either thrust or speed, or somewhere in between. So if I was to buy a few different props without any of them having overlapping performance (ie. what's the point of having a 108 and an 116 if they do the same job), what would they be? How does Propellor Load Factor come into play? The manufacturer suggests sizes 98, 106to8 and 115to6. If it helps to answer my question, I'd like think of this as if it were staircase, where one direction means thrust and the other means speed, and where one step can have several prop variations. thanks, daniel. 
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11092006, 06:48 PM  #3 
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Hijack?... sounded right on topic to me... but then, maybe I'm crazy?
The trouble with hard rules for props is things are variable. ...air moves, and changes... So the stairway analogy can't really work... if it did, the x and y axes would equate to torque and speed, I think... with torque being generally higher with a finer pitch, and speed generally coming with a higher pitch... Anyway, that seems a simple way to look at it... obviously you have to first be in the right ballpark for rpm with your engine/aircraft combo.... ie, not to allow so high an rpm as will damage your engine, as well as not to overload and prevent a decent rpm... Now, within that ballpark you can play with the diameter and pitch a bit (as well as number of blades) to find what you like best... Generally, a larger diameter and finer pitch will allow less speed, but make for better, more instantaneous acceleration. (as will more blades with a finer pitch) A courser pitch, usually accompanied by a smaller diameter so as not to reduce available rpm, and if it's not too course for the power available, ...will give more top end speed. (as do fewer blades) There are tradeoffs everywhere. 
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11092006, 06:54 PM  #4 
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Yes, an 11x6 and 10x8 might do the same job but you gain some clearance with the 10.
If I've only learned one thing it's that the mfg "recommended" props sometimes bear NO reality to what you can use in real life. example: recommended prop for my ST75 is 12x6 or 13x6. I'm currently running a 15x6 on it and it's wonderful. I even tried a 15x8 and NOTICED NO DIFFERENCE IN THE AIR. A lot of the time you take their recommendation with a pound of salt. It doesn't hurt to have a selection to choose from as what won't work on one engine may very well work perfectly on another. 
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11092006, 07:04 PM  #6 
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Here is what I found on another website. Might be a good starting point.
Prop Chart For TwoStroke Engines Alternate Propellers Starting Prop Engine Size 5.25 x 4, 5.5 x 4, 6 x 3.5, 6 x4 , 7 x 3 6 x 3 .049 7 x 3, 7 x 4.5, 7 x 5 7 x 4 .09 8 x 5, 8 x 6, 9 x 4 8 x 4 .15 8 x 5, 8 x 6, 9 x 5 9 x 4 .19  .25 9 x 7, 9.5 x 6, 10 x 5 9 x 6 .20  .30 9 x 7, 10 x 5, 11 x 4 10 x 6 .35  .36 9 x 8, 11 x 5 10 x 6 .40 10 x 6, 11 x 5, 11 x 6, 12 x 4 10 x 7 .45 10 x 8, 11 x 7, 12 x 4, 12 x 5 11 x 6 .50 11 x 7.5, 11 x 7.75, 11 x 8, 12 x 6 11 x 7 .60  .61 11 x 8, 12 x 8, 13 x 6, 14 x 4 12 x 6 .70 12 x 8, 14 x 4, 14 x 5 13 x 6 .78  .80 13 x 8, 15 x 6, 16 x 5 14 x 6 .90  .91 15 x 8, 18 x 5 16 x 6 1.08 16 x 10, 18 x 5, 18 x 6 16 x 8 1.20 18 x 8, 20 x 6 18 x 6 1.50 18 x 10, 20 x 6, 20 x 8, 22 x 6 18 x 8 1.80 18 x 10, 20 x 6, 20 x 10, 22 x 6 20 x 8 2.00 Prop Chart For FourStroke Engines Alternate Propellers Starting Prop Engine Size 9 x 5, 10 x 5 9 x 6 .20  .21 10 x 6, 10 x 7, 11 x 4, 11 x 5, 11 x 7, 11 x 7.5, 12 x 4, 12 x 5 11 x 6 .40 10 x 6, 10 x 7, 10 x 8, 11 x 7, 11 x 7.5, 12 x 4, 12 x 5, 12 x 6 11x6 .45  .48 11 x 7.5, 11 x 7.75, 11 x 8, 12 x 8, 13 x 5, 13 x 6, 14 x 5, 14 x 6 12 x 6 .60  .65 12 x 8, 13 x 8, 14 x 4, 14 x 6 13 x 6 .80 13 x 6, 14 x 8, 15 x 6, 16 x 6 14 x 6 .90 14 x 8, 15 x 6, 15 x 8, 16 x 8, 17 x 6, 18 x 5, 18 x 6 16 x 6 1.20 15 x 6, 15 x 8, 16 x 8, 18 x 6, 18 x 8, 20 x 6 18 x 6 1.60 18 x 12, 20 x 8, 20 x 10 18 x 10 2.40 18 x 10, 18 x 12, 20 x 10 20 x 8 2.70 18 x 12, 20 x 10 20 x 10 3.00
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11092006, 07:24 PM  #7 
RCC Supreme Contributor

Half the fun is experimenting..try a prop, fly, and see if you like what you get........
Blade shape. width, and material also come into play as well. Check out Bolly's web site...they have an excellent discussion on propellors....
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11092006, 07:51 PM  #8 
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rcpilot, you have stumbled onto my exact problem. Those are all recommend (or at least average) ranges for a starting prop.
Does prop load factor have anything to do with this issue? daniel. 
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11092006, 09:40 PM  #9  
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Quote:
The prop selection not only depends on an engine, it also depends on the type of plane and the flying you need. The best way to find the right one is putting it on and flying. So, I'd say take that information as a satrting point and make a judgement as to the type of plane and flying you intend to do and then narrow it down, then try those and stick with what you like. I'd give you an example. Bought a pattern plane with Ys140 and it had a 15x12 prop on it, I was having a tough time slowing it down on landings, everyone I asked was recommending 15x12 prop. From my past experience with a smaller airplane, I decided to try the 16x10 prop, dramatic change, less pitch less speed and the bigger disc like a bigger brake, made enough difference to land the plane nicely and not loose much performance in the air. Hope that helps a bit.
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11102006, 12:08 AM  #10 
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I came across this equation somewhere, and it seems to relate well to props.
Prop load can be estimated by squaring the diameter, multiplied by the pitch. 106 prop = 10x10x6=600 estimated load. 124 prop= 12x12x4=576. The above props, while wildly different in the air, represent similar loads to the engine. I don't mess with props too much, but I do have about 12 different sizes (and different brands). I often bring them to the field, and allow others to try them out if they think they have the wrong prop. I'm thankful I'm only using 4090 sized engines, as the props are pretty cheap. The guy's with the gasoline engines are spending big bucks on props, so experimenting is alot costlier.
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