Propeller 2 blades with pitch close to the rotation's direction Propeller 6 blades with pitch close to the axis Propeller with normal pitch

Photo 1 and photo 2 from here, photo 3 is from here.

Here are three propellers. As in my understanding, the AoA or the pitch of a propeller should be close to its rotation's direction, around 15 degree on the root and less on the tip, more or less like what shown by picture 3. But this propeller with 6 blades (second picture) seems the AoA or pitch is not as that 'rule'. The AoA is close to the axis rather than to its rotation's direction.

Then my question is, how such propeller works?

  • $\begingroup$ I can see the tips. The way you are using the terminology is still incorrect. xxavier gave you the only possible correct answer. $\endgroup$ – Federico Sep 19 '19 at 11:46
  • $\begingroup$ So, what is the best terminology? $\endgroup$ – AirCraft Lover Sep 19 '19 at 11:48
  • $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – Federico Sep 19 '19 at 11:49

In the second photo is just a "feathered propeller" (flag position or rest position on ground to not be spinning around if the weather is windy). This pitch is not used in flight unless the engine is stopped (not working), in which case the feathered position is required to minimize drag on the aircraft from the propeller.

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    $\begingroup$ to be noted that pitch and AoA are related but not the same thing. $\endgroup$ – Federico Sep 19 '19 at 10:43
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    $\begingroup$ Very interesting. So, can I say that when that propeller is in use (working), the angle of attack or the pitch will be also like picture 3? I added another picture so it clearer. The third picture is fix pitch propeller. $\endgroup$ – AirCraft Lover Sep 19 '19 at 11:51
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    $\begingroup$ Yes, for the fixed pitch propeller the RPM's adjusting, will give the thrust you need for the take of /climb regime's. $\endgroup$ – George Geo Sep 19 '19 at 13:27

One can't speak about the 'angle of attack' of the blades of an stopped propeller in no-wind conditions, simply because the angle of attack is defined with reference to the relative wind direction, and when that reference doesn't exist, there is no angle of attack to speak of, either...

  • $\begingroup$ Just like my question to the above, so, can I say that when that propeller is in use (working), the angle of attack or the pitch will be also like picture 3? $\endgroup$ – AirCraft Lover Sep 19 '19 at 11:51
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    $\begingroup$ While absolutely, technically correct, for the average layman, that's a pedantic detail that doesn't answer the question. The fact that the prop in the 2nd picture is feathered as noted by GeorgeGeo is much more useful to the average layman in explaining why/how that particular prop "works". i.e., it isn't in a working position in that picture. $\endgroup$ – FreeMan Sep 19 '19 at 13:38
  • $\begingroup$ @AirCraftLover to an extent, yes. The AoA will vary depending on the power needs, the engine RPM and a variety of other factors. As I understand it, most variable pitch props will automatically adjust themselves to suit the conditions, but that's the subject for another question - one that I'm pretty sure has already been asked here $\endgroup$ – FreeMan Sep 19 '19 at 13:40
  • $\begingroup$ Thank you @FreeMan. I understand what is variable pitch propeller. I was just confused to see the pitch, was not thinking about the reason. $\endgroup$ – AirCraft Lover Sep 19 '19 at 19:14

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