Propeller Blade Angle

[Sidious]

Hey Everyone,

Looking at the PHAK (CH 5-3) it explains why the propeller itself is twisted.

It goes on to say how if the prop had the same angle of incidence throughout the length of the blade the portion near the hub would have a negative angle of attack while the blade tip would be stalled.

I can't picture how this happens. Does anyone have a good explanation or drawing that can explain this? Thanks-Matt

 

[tgrayson]

I can't picture how this happens. Does anyone have a good explanation or drawing that can explain this? Thanks-Matt

This could only be true if the blade angle started out with a stalled AOA. As the speed picked up, the root would become unstalled, and the unstall (new word) would move towards the tip.

 

[OldTownPilot]

the relative wind is the sum of the forward velocity of the airplane and the velocity of the propeller blade at that point. This relative wind angle is more vertical near the propeller tip as the velocity of the blade at that point is higher, and more horizontal at the hub. The local velocity of the blade is the local radius (in feet) times the rotational speed of the propeller (in radians per second). Picture a rectangle with a vertical blade speed and a horizontal forward motion. The diagonal from the propellor to the far corner is the relative wind. The horizontal forward motion is constant for all points and the vertical propellor blade speed is smaller near the hub, meaning a greater relative wind angle. The angle of incidence is selected to be slightly less than this relative wind for various RPM and airspeed combinations to provide a "unstalled angle of attack"


When the engine is at idle and the propeller is being driven by the airflow, most of the blade has a negative angle if attack.