Re: Optimum Prop Diameter

From: <RLBailey_at_care4free.net>
Date: Tue, 11 Sep 2007 18:19:59 +0100

John

Maybe it's time to tout your prop picker theory as published in 2000 F/F Sympo.

Bob

  ----- Original Message -----
  From: John Barker
  To: Indoor_Construction_at_yahoogroups.com
  Sent: Thursday, September 06, 2007 3:37 PM
  Subject: RE: [Indoor_Construction] Optimum Prop Diameter


  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

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Received on Tue Sep 11 2007 - 11:24:51 CEST