View Full Version : blade cross section width

06-14-2015, 02:00 AM
I see in propeller drawings that the length of the cross section, sometimes superimposed on the side view, sometimes exceeds the thickness of the blade at that station. I’m not referring to the chord of the cross section, but the width of the cross section relative to the thickness of the blade.

You can see this in the drawing of the Helice Normale on this site and I have attached a snippet from the drawing with sections 1 and 2 to illustrate.

Can someone explain this to me?

06-14-2015, 04:48 PM

IMHO, your snippet shows a projection of the blade thickness. So you have to divide the projected thickness by the cosine of the inclination of the blade vs the axle of the prop in each point. And then you have the blade width (length of the cross section).

If you have extracted the snippet from the Guéret book, all is explained inside. Btw, it is a correct practice to name the book from which you extracted something... Even it is in public domain ;)

Are you making props?


06-16-2015, 03:29 AM
I’m afraid I don’t understand your explanation. I believe blade thickness divided by cosine of inclination would give the chord of the airfoil section? I assume the plan view is taken perpendicular to the disc of the propeller and would not show any true chord distance.

It appears to me that (except station 1) each of the airfoil shapes, at their angle of attack, are shown with their trailing edge positioned on the trailing edge of the blade. This is exactly the view one would have if standing in the plane of the prop disc looking toward the prop axis.

My question is, why doesn’t the leading edge of the airfoils coincide with the leading edge of the prop?

Further, I cannot find any correlation between the width of the blade in the plan view on the right hand side, and the width suggested by the inclined airfoil sections, either the width drawn with solid lines or the width drawn with dashed lines labeled surface d’eveloppée plane.

If you can show me something I would appreciate it. In other propeller drawings I find the blade width correlates between the views. It is only this “Normale” drawing that I don’t understand.

Yes, I need to make a reasonable facsimile of a 1909 prop for a 1/4 scale Anzani 25 hp engine I am building. Not for flight but for test stand running. I believe this “Normale” propeller is correct for this engine but I would choose the “Rationale” as an alternate. It’s drawing makes more sense. Is there something about the Ratmanoff - Drzewiecki design that makes this difficult?


06-22-2015, 06:29 PM


On a propeller drawing, the cross sections are "true" cross sections, not viewed from a particular point, but drawn at their true size (prop drawings are 1/1 scaled).
Their trailing edges are not align with anything. They are drawn so that:
- their barycenter (center of mass) is on the neutral axis of the blade,
- their "inclination" is their true inclination ("H/2pi").

Anyway, you can't reduce all the dimensions to obtain a scale working prop, because the air resistance is in relation with the square of the speed (in first approximation).
You have also to consider that a prop is not made for an engine, but for an a/c fitted with an engine
Eventually, test prop are not designed like prop for flying, they have a specific design, and I don't know if any test prop for the Anzani 25HP has survived...


The added drawing is from Guéret book page 153 (B&W only original).