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Author Topic: How to measure and calculate pitch to diameter ratio  (Read 1255 times)
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David Lofthouse
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« on: April 08, 2015, 08:48:32 PM »

I have read a lot of debates about pitch to diameter ratios and I have seen some good posts about cutting a prop blank for a carved prop.....but I just want to measure the p/d on my plastic props.  I made up a block to hold and measure the prop, made some angle gauges to measure the pitch, but can somebody help me with the formula to get P/D.  My thanks.



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mkirda
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« Reply #1 on: April 08, 2015, 09:27:12 PM »

Take the pitch and divide by diameter.
That simple I think.

Regards.
Mike Kirda
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Warhawk
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« Reply #2 on: April 08, 2015, 09:28:58 PM »

David,

P/D = Pitch divided by the prop Diameter.  The Diameter should be pretty straightforward.  Just measure from one end to the other of the prop with a ruler.  The Pitch is the distance a propeller will move forward in one revolution, which is a bit more complicated to determine.  I usually make use a chart found in several texts.  In practice, you measure how far from the prop shaft hole (center of the prop) to the point where the blade makes a 45 deg. angle.  The chart will convert this into a pitch.

There is no magic P/D ratio that works best.  The P/D should be lower for low power motors, and may be larger for more powerful motors.  Peck-type props for rubber are supposed to be about a 1.25 P/D, however if the props are taken out of the mold too early, the pitch will be less.  Worse, the pitch may not be the same on each blade, which introduces vibration to the model, and robs performance.

There may be an article or two in the plans section on this site, so check that out.

Justin
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Warhawk
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« Reply #3 on: April 08, 2015, 09:32:45 PM »

David,

Just checked and there is a "Miscellaneous Article" on building a prop pitch gauge that is worth studying.

Justin
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Hepcat
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« Reply #4 on: April 09, 2015, 09:04:38 AM »

David,
The formula you are looking for is:
 
P/D = pi (r/R) tan(theta)  where: pi=3.142, r is the radius you are checking, R is the propeller radius, theta is the checked blade angle.

If you don't want to bother with the calculation I have given a table below. Look along the top row to the r/R value, run down to the measured angle and then go to the left hand column for the P/D ratio.

I should like to emphasize that the important part of a propeller is the part around the three quarter radius (r/R=0.75) which is where most of the thrust is generated.  The blade angle at that point and the corresponding P/D is regarded as the P/D of that propeller.  If blade angles at other parts of the propeller calculate to different P/D ratios these are just indications of errors from a true helical pitch propeller.  Such errors are common and do not usually cause much loss of performance as long as the P/D at 0.75R is correct.

John
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tom arnold
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« Reply #5 on: April 09, 2015, 09:59:34 AM »

Peck props (according to my measurements---which may not be exact) P/D are closer to .95, more or less depending on the cooling process as noted above. However, the Gizmo Geezer units have been repitched to a constant 1.25 on both blades. See the latest FAC newsletter for a method I stumbled on to repitch Peck props to any degree desired.
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Dave Andreski
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« Reply #6 on: April 11, 2015, 06:05:43 PM »

I always put my faith in the attached.
And there you have it.
All you need to know because of the carefully measured prop info.

Dave
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« Last Edit: April 11, 2015, 06:32:41 PM by Dave Andreski » Logged

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David Lofthouse
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« Reply #7 on: April 14, 2015, 02:52:54 AM »

Everyone. 
Thank you for all of this good information.  I now have what I need and I am measuring away....and learning a lot. 
Cheers
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