A Brain teaser on Prop pitch

[suckmywake]

I have a question on prop pitch. I understand the 4,5 and 6 series on octura props. But I was wondering if you reduce a prop diameter what is the pitch? I am thinking the pitch stays the same. I understand you loose some of the tip cupping of the prop as you remove the tip of the prop to make it a smaller diameter. Loosing cup will be like loosing pitch but as you cut down a prop the angle of the blades do not change so pitch does not change. It seems to me that most of the pitch of the prop is in the ramp of the prop. I also know pitch is not constant across the blade of the prop. I know it is not ideal to cut down a prop multiple sizes. I am asking in more of theoretical terms.

my example
Stock X447 = 47mm dia. with 66.8mm pitch
Stock X450 = 50mm dia. with 70mm pitch

So a X450 cut down to 47mm = 47mm dia. with (70mm pitch – (the cup you will have lost)).

Is this right in my thinking?

[Fluid]

The pitch remains the same regardless of prop diameter reduction (within reason). An x450 has a pitch of 70mm. Cut the prop to 47 mm and the pitch is still 70mm. The ratio is relative to the original prop diameter.



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[suckmywake]

Thanks Fluid.

[eric_laermans]

@Fluid

???

The pitch of a prop is the factor to multiply the diameter of the prop with. Also, an X450, where the '4' means multiply by 1.4, has an overall pitch of 50 x 1.4 = 70mm.

If you cut down this prop a little, the pitch stays the same, which means the factor to multiply the diameter of the prop with stays 1.4, also overall pitch will be 47mm x 1.4 = 65.8mm

[siberianhusky]

Good question, I can't even figure out where you are supposed to take a pitch reading from. I get hugely different numbers from a dial indicator depending on how far out from the hub I measure.
None of the prop people seem to want to share this simple piece of info, it's either really simple and they just want people to think they are smart or they don't have a clue and are just making crap up to baffle people.
Found if people don't want to share info it's usually one of the two.

[Fluid]

If you cut down this prop a little, the pitch stays the same, which means the factor to multiply the diameter of the with stays 1.4, also overall pitch will be 47mm x 1.4 = 65.8mm

No. Guys, the pitch is an actual value. An x450 prop has a diameter of 50mm and a pitch of 70mm. The 1.4 multiplier is only a way of describing the pitch value, it is NOT the value itself. If you cut an x450 down to 47, it becomes in effect an x547 with a pitch ratio of 1.49.

None of the prop people seem to want to share this simple piece of info, it's either really simple and they just want people to think they are smart or they don't have a clue and are just making crap up to baffle people. Found if people don't want to share info it's usually one of the two.

There is no secret. The measured pitch has to change from hub to tip - it's simple geometry. What matters is the calculated amount of movement per revolution. Most props are measured between 60% and 70% from the hub.

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[NativePaul]

The pitch will stay the same but the multiplication factor is only for the stock prop, if you cut it down enough you can turn a 1.4x pitch prop into a 1.6x, I have several of the same 40mm props that I cut by various amounts down to 33mm (just cut no bends or twists added) to try and get runtime for racing, I ran them back to back on the 110yds and there was less than 2mph between them with the 40mm being the fastest, the pitch had obviously remained the same but the more extreme cuts had lost a little efficiency compared to the Octura shape and needed the tips re-cupping.

[eric_laermans]

It seems against all laws of physics to cut down a prop a little and still get the same overall-pitch, measured in distance. I get the same feeling as during my first quantum-mechanics colleges. A strange feeling that it cannot be true, still hoping it to be true.

I apologize for the comment I posted above. I felt so sure about what I posted, never taking into account the strange laws that apply to props. :-)

[siberianhusky]

Ok that explains why I way getting numbers which didn't make any sense I think.
The prop is going to move forwards 70mm in an ideal viscous fluid with no slip.
So about 60-70% from the hub I should be able to measure about 35mm movement on the dial indicator through a 180 degree rotation?

[Fluid]

It seems against all laws of physics to cut down a prop a little and still get the same overall-pitch, measured in distance.

Feelings have little to do with physics...or geometry, which is what applies here.

What matters is how far a prop would progress through a solid medium for each revolution. If you take a wood screw and remove half the thread depth you have NOT changed the pitch of the screw's threads at all. Same with a boat or plane prop.

Take a look at an airplane prop. The pitch is the same for any spot on the blade, but the angle of the prop changes from hub to tip. This HAS to be for the blade to progress the same distance through a solid medium (which is what pitch measures) - it's geometry.

If it is still not clear I suggest you Google 'propellers' and read up on them. There is a lot more written on airplane props, and the design terms are the same.


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