RE: Optimum Prop Diameter

From: John Barker <john.barker783_at_ntlworld.com>
Date: Thu, 6 Sep 2007 15:37:15 +0100

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.

Certainly it is a good general rule that the bigger the prop the better but
I think Martin Simons does read more into the Froude, actuator disk,
approach to efficiency than is wise. It is true that the efficiency =
1(1+v/V) equation does give zero if the flying speed is zero but that is not
a point of great interest and then to extend that argument in the other
direction to say that fast aeroplanes like pylon racers are better off does
not seem to be
a reasonable assessment of cause and effect. Racing engines develop high
power by using high rpm and are forced to use small diameter propellers for
that reason. The inflow factors are usually much higher than on rubber
props.

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.

A further point is that if the diameter is increased it is usually necessary
to reduce the P/D ratio unless extra torque can be made available. Many
tests over the years have shewn that propellers with a low P/D ratio have
narrow peaks to their efficiency curves whereas higher P/D ratio have a
broader range of efficiency.

So, whilst I would not venture to tell anyone what size their propeller
should be I am not at all surprised if some of the experts who fly the
aeroplanes are using propellers below the maximum possible size!

John



[Non-text portions of this message have been removed]
Received on Thu Sep 06 2007 - 07:47:01 CEST