It seems to me that if wanting to form a 28” pitch propeller on a 26” pitch
block that the blank would need to be moved inwards, not outwards, to use
the higher forming angles.
There are a couple of errors (misstypings?) in what Nick wrote. They
probably don’t affect the conclusions much but in case anyone is trying to
follow along. Nick wrote that Pitch = Diameter*2PI*arctan(blade angle) but
that should have read either Pitch = Pi*Diameter*tan(blade angle) if
dealing with the blade angle at the tip or Pitch = 2*Pi*radius*tan(blade
angle) where the blade angle is at any other radius than the tip.
Mark is right to mention the rapid angle change near the root and it is
interesting to look at this in relation to the common practice on indoor
propeller of only having a spar at the inner radii. Assume that the inner
0.2 of the propeller is spar and the actual blade is from the 0.2 radius to
the tip then it is possible to calculate the change in blade angle over the
length of the blade for a range of P/D ratios. I have done this below and
the angular change is in brackets after the P/D ratio:
P/D 1(40.25), 1.2(41.46), 1.4(41.80), 1.6(41.57), 1.8(40.94).
It will be noted that the change is very close to 40 degrees over a wide
range. Note that this is the change of angle from end to end of the blade,
the blade must still be set on the spar to get the correct pitch. However
it does indicate the blades formed on less than perfect moulds (or even
bottles!) can perform as well as they do.
John Barker - England
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Received on Thu Jul 03 2008 - 08:49:45 CEST
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:45 CET