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How would a airplane propeller without twist stall at the blade tip as airspeed increases?

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General Aviation

In the Pilot's Handbook of Aeronautical Knowledge, pages 6-4 and 6-5 state: "A propeller blade designed with the same angle of incidence throughout its entire length would be inefficient because as airspeed increases in flight, the portion near the hub would have a negative angle of attack while the blade tip would be stalled." As I've come to understand this somewhat recently, the relative airflow, with regards to the propeller, is a combination of the rotational speed of the propeller and the forward movement of the aircraft. So, as the airspeed (the forward movement of the aircraft) increases, the angle of attack of the propeller decreases. Conversely, as the RPM (the rotational speed of the propeller) increases, the angle of attack of the propeller increases. I understand that the portion of the propeller near the hub could eventually have a negative angle of attack as airspeed increases. What I don't understand is how the blade tip would be stalled if the AOA continues to decrease. Could anyone explain this to me? Thank you.

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1 Answers

  1. Russ Roslewski on Apr 16, 2015

    I noticed nobody has taken a stab at this one yet.

    I think you’re right. Aerodynamic stall occurs when the angle of attack increases past the critical AOA. Propeller AOA decreases with increasing airspeed regardless of where on the blade you’re measuring or what the pitch is, so I also can’t see how it would stall. In a constant-pitch propeller blade, the AOA at the tip would be less than near the hub. If the speed increases to where the AOA near the hub is negative, it would be “less” negative at the tip (so, possibly still positive).

    Admittedly, I’m having a hard time figuring out how an airplane could get up to this speed anyway. Maybe in a dive, since once the AOA goes negative on part of the blade, that part of the blade is no longer producing forward thrust. Could even be producing backward thrust. I imagine that would cause some interesting aerodynamics as well as interesting forces on the prop.

    It wouldn’t be the first time someone’s found an error in an FAA publication.

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