Thread: ID Please?
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Old 04-23-2019, 03:57 PM   #29
Forum Contributor
Join Date: Nov 2006
Location: France
Posts: 543

OK, a clinometer is the easier way!

You have to measure the angle between the flat side of the blade and the rotation plane of the prop, and this in different places.

First, have the prop on a table with the flat side of the blades upward.
Verify that the table is horizontal, placing the clinometer on the hub: it has to show "0" when the phone is paralelle to the blade width (= perpendicular to the length of the prop). If not, have something under the feet of the table until the clinometer show "0".

Now, measure the angle of the blade at three distances from the prop center: 93, 100 and 120 centimeters. For each, take note of the angle (A1, A2 and A3). For each measure, be sure the phone is exactly perpendicular to the prop length.

That's it!

In each place, the pitch (in centimeters) is:
- at 93cm : 186 x pi x tg(A1) : it is the "Régy pitch", measured at 2/3 of the blade,
- at 100cm : 200 x pi x tg(A2) : it is just for a control...,
- at 120cm : 240 x pi x tg(A3) : it is the "(French) military pitch" for props between 2.70 and 3.10 meters long.



PS: If the prop was exactly helicoidal, the three pitches would be the same. But the prop makers didn't make their props exactly helicoidal to compensate the distorsion of the wood when the prop was in use. Generally, the pitch was higher towards the tip, because the centrifugal forces "try" to flat the blades when the shape of the blades is "traditional".
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