RE: Optimum Prop Diameter

From: John Barker <john.barker783_at_ntlworld.com>
Date: Fri, 7 Sep 2007 11:20:07 +0100

quote<Why is it that a larger prop will have a higher drag?>endquote

Thank you for the question even though it embarrasses me! I assumed the
larger diameter propeller would have more drag and I still feel that that is
the case but of proof I have not a shred. Looking at the matter from the
point of view of the aspect ratio of the blades does not seem very easy.
The usual way of doing propeller calculations using the vortex theory
approach uses wing section coefficients for infinite aspect ratio and blade
aspect ratio is not used in the calculations. Solidity does not appear to
be a substitute because, say, two wide blades or four narrow blades of half
the width would have the same solidity.

I suppose it would be possible to run through the design procedure for two
propellers of different diameter, giving the same thrust and finding if
there was any difference in the power required - which I think is, in
effect, your ‘torquenal’ drag approach.

However I think once one started looking into this one would realize that
different diameter propellers may work better with different rubber cross
sections at different rotational speeds and … and …and. It may be quicker
to make a few props.

John


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Received on Fri Sep 07 2007 - 03:28:59 CEST