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I came across a video while learning about propellers where it showed the angle of attack increased from the root of the propeller to the tip of propeller whose pitch was fixed. I somehow cannot get my head around it. Is it because the tip moves faster than the root? If so, can anyone explain why moving faster changes the angle of attack at the tip?

I am pretty new to all this so sorry for the stupid question, & I tried searching on the site but could not find answers to this question explicitly.

Elroy D'souzA
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  • To be precise, the blade angle is twisted in order to (hopefully) provide a constant angle of attack along the blade. For a fixed pitch prop, this means a single value of forward speed/RPM. For variable pitch, you can control it better. – MikeY Mar 22 '19 at 16:34

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You are absolutely correct. The "relative wind" at any point on the propeller is a combination of the aircraft's forward motion plus the angular rotation speed of the prop, which is greater at the tips. This is why, to get the best Angle of Attack, the prop (airfoil) is designed with a twist or "washout" towards the tip.

Notice, with variable pitch props, as the planes forward speed increases, the entire prop AOA is turned forward to account for change in relative wind.

Fixed pitch props try to find a happy medium between low speed performance and cruise efficiency. For example, a 10 x 6 (finer pitch) gives better acceleration for take-off, but a 10 x 7 pitch uses less fuel at cruise.

Robert DiGiovanni
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  • You're thinking of washout,not downwash. Regardless, this refers to a different design consideration (tuning the angle of incidence for benign stall behaviour) rather than a propeller blade which has a twist even for constant angle of incidence. – Sanchises Mar 22 '19 at 16:35
  • Edited to washout. But it is the same thing for a different reason. A wing washout will change AOA because relative wind is relatively constant across the leading edge (though the other wing can be different in a slip). On a prop washout keeps AOA the same because the angular rotation creates different speeds along the leading edge. Good catch on the terminology, thanks! – Robert DiGiovanni Mar 22 '19 at 16:44
  • Thanks Robert. So just clarifying here ,that since the tip Has a higher angular velocity so It's angular rotational velocity vector will have a greater magnitude that at the root.So the resultant(which is the relative wind) of forward velocity vector(which is constant from the root to the tip of the prop) & the rotational velocity vector will Change at the tip & this changes the angle of attack at the tip , right ? – Elroy D'souzA Mar 22 '19 at 16:57
  • Yes, and I'll add in a consideration of stress load on the prop, which might make them "downwash" it even more, so "pull" or thrust is consistent across prop blade. Wind mill blades topping 60 meters making this an interesting study. – Robert DiGiovanni Mar 22 '19 at 18:13
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Just clarifying a bit further, the AOA change is actually opposite that stated in the original question about the video.

To quote the explanation on experimentalaircraft.info:

"The design is such that the blade is thick at the hub with a large blade angle and thin at the tip with a low blade angle." or 'washes out' from root to tip, as others have stated.

Thus a blade angle (and resulting AOA) decreasing from the root balances with higher angular rotation speed at the tip to provide a constant (or optimal) thrust along the span of the blade. Many blades also vary the chord and shape to optimize blade stress and efficiency under local airflow and to counter Mach effects.

blade angle change

user38251
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  • Thanks man .The the website also cleared my doubt about why RPM increases /Engine over revs when the aircraft enters into a dive. – Elroy D'souzA Mar 26 '19 at 12:34