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Author Topic: Measuring prop pitch  (Read 1478 times)
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Pete Fardell
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« on: April 17, 2017, 02:19:01 PM »

If I want to get a quick, approximate figure for a readymade plastic prop's pitch, how far out from the centre should I be measuring at? Sorry if that's a daft question.
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« Reply #1 on: April 17, 2017, 02:57:16 PM »

If you mean P/D and close counts then you can use these.

@75%  23°=1

And so on.
Quick and approximate of course.


Instructions: Step One...Assemble the pile of sticks shown in pic "A" to look like the model airplane shown in pic "B"........
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« Reply #2 on: August 13, 2017, 10:26:36 PM »

IF ? this is important to you. Then Google and hunt down Plans for a DIY prop Pitch gauge.
 Invaluable imo. as Few! props are what they claim to be.
 Build one ...and be astounded at what you discover :-)
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« Reply #3 on: August 17, 2017, 02:56:16 PM »

And, what may be a minor thought, most pitch gauges I've seen measure the angle of the - usually flat - 'bottom' of the prop blade.

All propeller blades have some form of airfoil, even "flat plates" due to their angle to the airflow.

Airfoils may have their 'zero lift angle of attack' above the line of the 'bottom' surface, even if the blades are very thin but have a curvature. Thickness and possible leading edge radius may contribute  to the zero lift AoA deviation from the 'bottom' surface. A thick airfoiled blade with the same nominal pitch as a thinner blade airfoil may perform differently. The camber line is halfway between the upper and lower surfaces, thus usually curved, unless the airfoil is symmetrical. A thicker airfoil will have a greater camber line curvature compared to the straight line from furthest point on leading edge to the furthest aft point on the TE.

(In the 1940's Raul Hoffman released a book called Model Aircraft Aerodynamics Made Simple, or something very similar. He apparently passed on before he had a chance to organize the content into more usable, legible, sensible format. The book, as it is, ain't easy, but much can be mined out of it.)

Either Hoffman or Frank Zaic , in his Annual Yearbooks, suggested a way to SWAG a propeller's zero lift AoA: For a typical flat-bottom-type airfoil, split the angle between top and bottom surfaces, at the trailing edge. Most likely too crude, but it does convey the idea that the zero lift AoA occurs when the LE looks below the angle of the bottom surface.

Few of us need to consider the usually small differences this  presents, but much of what we do survives because of the tolerance of imprecisions. We can't, or very seldom can try to, approximate precision. Too many variables we can't control: Wind, height above Sea Level, temperature, even humidity, and variations in flight and power adjustments. Yet, we do fly - and usually pleasantly well!

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« Reply #4 on: August 17, 2017, 06:28:45 PM »

Thanks for reminding me about Hoffman's book.  I haven't looked at it for some years but I hope it is still on the shelf in the Garden room next to the Zaics.  Does anyone know anything of his background, in particular what he did for a living, because (although I don'r want to run down Frank in any way) I think Roul was more knowledgeable about the fundamentals of aircraft design.
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