LeRoy
As you suspect you cannot use the the pitch gauge in the same way for different diameters. Leaving aside all sorts of 'what ifs' which can come in on such an imprecise term as 'pitch' I will say that the basic formula is as follows:
tan(theta) = (P/D)/pi(r/R) that is the pitch/diameter ratio divided by pi and divided by r/R.
It is much more simple and informative to talk about pitch/diameter ratio than pitch alone. Pi is the familiar 3.142 and r/R is the radius at which you are measuring the angle divided by the radius of the whole propeller. Now there is one important place to measure propeller blade angle and that is near the three quarter radius point. This is because nearly all the work is done near there and also many propellers are terribly inaccurate as one goes along the blade. For this reason it is better to calculate the blade angle required at that point for a certain P/D ratio and to make a triangle to check the blade angle at thatr point. To do this the formula can be simplified as follows:
blade angle theta = arctan( (P/D)/2.356). Just put your P/D into your calculator, divide by 2.356, press the arctan button and you have the angle to check your propeller blade at three quarter radius.
I am not much into No-Cals but I think they use P/D ratios of about 1.5 to 2. At P/D 1.5 you would need a blade angle of 32.5 degrees and for a P/D of 2 a blade angle of 40 degrees at the three quarter radius point.
John Barker - England
Received on Wed Dec 15 2010 - 14:31:12 CET
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