RE: Optimum Prop Diameter

From: Tapio Linkosalo <tapio.linkosalo_at_helsinki.fi>
Date: Thu, 6 Sep 2007 20:05:22 +0300 (EEST)

On Thu, 6 Sep 2007, John Barker wrote:

> I was reluctant to quote theory on such a specialized subject as indoor
> propellers but now that Tapio has opened the door perhaps a few extra
> comments would be in order.

Well, I'm glad I did, to get your informative reply. A number of good
points and remarks. I have one question, though:

> Most texts on propellers (right back to Froude’s original!) acknowledge that
> the actuator disk theory ignores the effect of drag on the propeller blades
> and, as everyone knows, drag is not a thing to be ignored at low Reynolds
> Numbers. There will be a point when increasing diameter will increase blade
> drag to such an extent that efficiency falls.

Why is it that a larger prop will have a higher drag? If longer blades are
narrower, and thus of the same area, then the skin drag should be the
same, while for a higher aspect ratio blade the induced drag should
actually be smaller. Or should we consider "rotational" (vs. linear) drag
here, where also the moment arm = prop diameter affects "torquenal" drag
of the prop?



-Tapio-
Received on Thu Sep 06 2007 - 10:05:43 CEST