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I'm designing a copter and trying to decide if the propellers should be ducted or open axial flow. I've read some theory on ducted and open air flow but I can't find any where that compares the two. I would prefer to use ducted over open for safety (ie put a safety guard round the propellers so they can't be run into with fingers etc). So are there any major advantages or considerations I should take into account, with ducted vs open axial flow before ploughing ignorantly ahead with one or the other type of flow?

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I think that ducted propellers are more efficient. Propellers are basically just spinning wings that produce lift (thrust). Winglets are sometimes put on airplanes to reduce induced drag. Basically high pressure air from the bottom of the wing spills over to the (low pressure) top. Putting the winglets reduces this effect. I think that using ducted propellers will give a similar advantage. – OSE May 17 at 16:06

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Actually, ducted fan propeller are much less efficient than open blade propeller soif efficiency is your priority, open propeller is your option

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Can you give an explanation for this or a reference? I am interested to know which one is more efficient. – OSE May 17 at 17:07
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I would also expect that a ducted fan would be more efficient, since it would have less loss due to radial wind. I also did a quick search, and only was able to find some information on wikipedia (ducted fan) in which it is stated that there are situation in which they are more efficient, but also situation in which they are less efficient. It also links to an interesting report in the reference, which they are experimentally compared. – fibonatic May 18 at 19:42

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