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I have a query on where propeller blade loading points. On this paper I found this picture:

enter image description here

In that paper, they say that, if we define a vector $\vec{R}$ from source (blade section) to observer, and a unit vector $\vec{n}$ aligned with the local force exerted by the blade on the fluid, the dot product $\vec{R} \cdot \vec{n}$ will be maximum when "the net loading exerted by the blade on the air points towards the observer". Specifically, they say that $\vec{R} \cdot \vec{n}$ has a "maximum magnitude when the blade is moving towards the observer". This occurs about at $n=10$ in the above picture.

What I'm wondering is: how is it possible that at $n=10$ (or a bit later than that) the loading is pointing in the direction of the observer? I mean, at $n=10$ the blade is showing its pressure surface to the observer and we know that the loading exerted by the blade on the fluid points away from the suction surface, instead, which is on the other side.

Thank you for your help.

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  • $\begingroup$ If it's a turboprop propeller it's a bit simpler. If it's a piston engine, you end up with a myriad of loadings beyond aerodynamic and centrifugal, the power pulse and inertial reaction every 180 deg on a 4cyl engine, made more complex by resonant flex of the blades in response to those pulses. A metal prop is a big tuning fork. It's a mess. $\endgroup$
    – John K
    Commented Dec 8 at 18:28
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    $\begingroup$ Why does the location of the observer matter? Is the observer in-plane with the disc, or at some distance in front of or behind the plane of the spinning blades? $\endgroup$
    – Ralph J ♦
    Commented Dec 8 at 18:29
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    $\begingroup$ The paper is behind a paywall so I cannot access it. But as the title of the paper clearly states, it addresses the noise of a contra rotating propeller so that the term "loading" might be related to noise loading rather than aerodynamic loading. $\endgroup$
    – sophit
    Commented Dec 8 at 18:54
  • $\begingroup$ Hi all, thanks for your replies. The "loading" the paper refers too is the loading exerted by a blade on the air. It is essentially the pressure force, which is normal to the blade surface. The observer is in the propeller plane to the right of the propeller. The observer location matters because the R vector is defined from a blade element to the observer. $\endgroup$
    – floyd123
    Commented Dec 9 at 13:41

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