Prop question

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I pretty much understand how and why props have different pitches. But how does a larger diameter prop affect your flight since your only changing by say 1/2" on a side?

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try calculating the swept area of the two prop sizes, and will become fairly obvious that the larger prop is covering a lot more area than the smaller one. In other words, the larger diameter is moving through a lot more air. More air, more opportunity to MOVE air and produce thrust.

quick and dirty, an 11 inch prop "disk" is 121% as large as a 10 inch prop

bob

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You can see what effect changes in prop size will do

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That 1/2" covers a relatively large area of the prop disc due to its distance from the center, and therefore accelerates a large amount of air. A prop can be ruined by removing a small percentage of its diameter and the airplane might not fly anymore. It's related to the efficiencies of larger, slower-turning props over short, high RPM ones. Higher RPM means more power is lost to drag, and drag increases by the square of the increase in speed.

Dan

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That's 21% larger not 121%

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Yeah but he said "as large", not "larger". So actually, you are both right. The disc is 121% as large, or 21% larger, than a 10" prop disc. I know...I know...just semantics, but if you're going to correct someone, at least correct them correctly! :)

Kev

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How about somebody explaining the effect of prop shape. What's the difference between a Master Airscrew Black prop Shape, APC, or Scimitar shape? Some have a thicker width of the blade, while others have very narrow tips, how does this effect things. An 11 inch MA may have 21% more area than a 10 inch MA, but in the shape of a APC, the tip is very narrow so eventhough it covers the same area does it provide the same amount of thrust?

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It all makes a lot of difference. So its all pretty meaningless really to just 'bang on a 11x6' and hope.

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he didn't say larger..........he said AS LARGE

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Wow! The OS .40LA RPM range is up to 16K. That would take about an 8X4 to get it near that!

I usually start at the middle of the "accepted" prop range and try different ones until I get the performance I want from the plane and engine combination.

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Narrow tips reduce drag caused by tip vortices and allow the rest of the blade to have a bit more power. Same effect that narrow wingtips have. Ever see a bird with square wingtips? Dan

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From what I was told by hobby services, the maximum RPM ratings for OS engines is a theoretical number for the engine running without a prop.

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Yup, Flickers.

Tip shape and airfoil design determine how efficient the tips are. Bird wings aren't very good for examples because most soaring birds, like vultures, do not have solid wingtips. Their wingtips are made up of many individual wingtips (each long feather),

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That is also the RPM they use for their HP rating. Sounds like they were full of it.

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Yup. But we won't go into that :-) Anyway, my point was theat blade geometry is about more than diameter and pitch.

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To me 121% as large is over twice as large. Use the numbers correctly and the English won't matter so much.

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The numbers were used correctly. Learn english and the numbers won't matter so much. ;^)

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learn to use the numbers correctly, and you will understand that 121% is just a bit more than 6/5 which is a LOOOOOOOOOOONG way from more than double.

Let me guess, you are actually an MBA somewhere right?

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Not only the size of the disk area comes into play, but several other factors as well.

The unless prop center is already there, so every fraction of an inch added to the tips has a great effect. The longer the prop, the more each fraction of an inch added to the tip will affect the function. Tip speed also comes into play, as the outermost disk area does far more work than the inner area. An 11" prop is waaaaay different than a 10" prop in several ways.

The most important thing about prop length is to choose a length that makes the engine run in its best RPM range.

Rule of thumb: Length is dictated by the engine, pitch is dictated by the model type.

Bob

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