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Prop pitch measurement, pitch=2 π r tan a

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dog

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Dec 29, 2019
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The prop that came with my project has the pitch
marked on it as 00.
Senenich M76AK cut to 68 and 9/16
measured the angle at a radius of 25.68"
and got 15 and a half degrees
pluged that into the formula found on the interweb,which is p=2πrtana and got 44.7
Does anyone recognise this formula or
have another one?
 

Dana

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Your math is correct. But, how accurately can you measure it? 1" of pitch is only 1/4 degree.
 

kent Ashton

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Concord, NC
Some prop-makers quote pitch from the flat side surface and some from the chord line. How does Sensenich quote it?
 

Pops

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I have a Senenich M 76 prop were the leading edge corner received a 3/8" chip when it hit the tow-bar when the engine was turned over one blade with the starter.
It's one that can't be shortened. So scrap aluminum. Hope your prop isn't the same model.
 

Dana

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I asked Sensenich recently and they said they measure pitch on the flat back face. But a metal prop is so thin, and the leading and trailing edges so sharp, that it's just about the same thing as the chord line.
 

dog

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Your math is correct. But, how accurately can you measure it? 1" of pitch is only 1/4 degree.
My first measurement of the angle was quick and dirty,though useing an old starett gauge.
In the past when I have needed accuracy that my
gear is not up to, I cheat, and do as good as I can
15 times or whatever and average the results,this has alowed me to set up things that would otherwise require ,sending stuff out or big buck tools.Long term durability has "proven" the work.
This will now be done.
Then test this prop's tracking.
The project came with a second A-65 case that is non serviceable and two cranks, plus I was gifted a box of mic heads,so I will use those parts to hold the prop on a bench and spin it and measure how it tracks.
The Sensenich site gave a slightly longer minimum
length than where this one is.
Prop weighs 27 1/4 pounds with a taper crank hub bolted to it, and has a tiny amount of pitting on the leading edge, as in I could count the pits on two hands.
Thanks for feedback.Will post results of refined pitch measurment and tracking(not worth another thread)here.
 

dog

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Dec 29, 2019
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301
Following through with the investigation of the propeller I have, I re-measured the pitch angle at 16.25 degrees which give a pitch of 47.02.
To do this I first mounted a crank in a A-65 case
and dialed it, and got less than 1 thou total run out.
Mounted the prop and checked for tracking and found something like 1/64 at the tip.
Length of each blade was the same.
Next I shimmed the case till it was level with my
bench at the 3/4 point of the propeller.
I used a square to do this.
Then useing a square and an angle gauge I measured the pitch angle of both blades.
I pencil marked the bench where I held the square
and pencil marked the prop for repeatability.
Both blades measured the same.
Attached a picture of basic set up and tools used.
Next will be figuring out a way to assemble and
dissasemble the fuselage,landing gear, and attach
the motor ,prop ,for a series of test runs(hopefully).
and from the prop measurements and RPM the motor will develope I think hp can be determined.
 

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Dana

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Note that you don't even need to mount the prop; you can measure the pitch with it laying on the bench (or on the plane with the crankshaft not level, as with a taildragger), all that matters is the difference between the blades. The average angle is the blade angle used in the pitch calculation.
 
Last edited:

dog

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Dec 29, 2019
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I cant for the life of me see what you are describing.
What I am seeing is the prop strait accross(level)
and then measuring the angle of the blade from a plumb line,which would be the same? for both blades, not seeing where there is a difference between the blades.
Litteraly scratching my head.
 

Dana

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If your prop was, for example, on a taildragger sitting at a 10° angle and you measure the blade angles on each side from the vertical, it would be 26.25° on one side and 6.25° on the other side. The average is 16.25, that's the blade angle you'd use in the pitch equation. (Sorry, I mistakenly said difference above instead of average, I corrected it). Or you can zero your measuring device on one side and read the total angle on the other, and use half of that. The point is you'll get the same angle regardless of the crankshaft angle, so setting it up with the crank dead horizontal isn't necessary.
 

dog

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Dec 29, 2019
Messages
301
Whew.
Totaly get that description now.Thanks.
And as there are some good (and free) phone apps
that give angle measurements,with a plane to hold
the prop ,the method you have given would be quick.
 

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