This is going to be a quick note about propellers and why the angle of attack of the blades changes depending on aircraft speed and rotational speed of the propeller.

If the blades had the same geometric pitch throughout their length, portions near the hub could have negative AOAs while the propeller tips would be stalled at cruise speed.
- Pilot's Handbook of Aeronautical Knowledge, pg. 5-30


This is true, but why? Many folks will quote this to you but not be able to back it up. Once you draw out the picture, it's actually pretty simple. First, let's look at the propeller tips.

The tip of the propeller blade is moving at a faster speed than the hub. The propeller has a constant angular velocity (RPM) no matter where you look -- if the blade is set to turn at 2,300 RPM, then each portion of the blade will turn 2,300 times per minute. However, the outer portion of the blades have a farther total distance to travel for this constant rotational speed, so they end up with a faster linear speed.

Note: In all of the diagrams that follow, the propeller is rotating at a constant RPM. The difference in the component of the relative wind caused by the propeller's rotation is strictly due to where on the propeller blade we are looking (hub or tip).

Angle of attack is defined as the angle between the relative wind and the chord line of an airfoil. For a propeller, this relative wind has two components: one caused by the forward motion of the aircraft, and one caused by the rotation of the propeller. At the tips of the propeller, the component caused by the rotation of the propeller is exaggerated because the tip is moving at a faster speed than the hub. This pulls the relative wind away from the chord line and increases the angle of attack at the tip of the propeller. To make sure that the tip of the propeller doesn't stall, the geometric pitch of the propeller has to be reduced (smaller blade angle). In the diagram below, with the large geometric pitch, the blade would likely be stalled.

Propeller Angle of Attack at Cruise Speed (Tip of Propeller)
Angle of attack of a propeller blade at the tip during cruise flight


Now, what happens if we look at a part of blade at the hub? Let's assume that the aircraft's cruise speed is unchanged, so the only component of the relative wind that will change is the one caused by the rotation of the propeller. The component caused by the rotation of the propeller decreases because the hub is moving at a slower speed than the tip. In the diagram below, note that the angle of attack is reduced at the hub compared to the tip!

Propeller Angle of Attack at Cruise Speed (Hub of Propeller)
Angle of attack of a propeller blade at the hub during cruise flight


So, is it possible for the angle of attack at the hub of the propeller to be negative like the Pilot's Handbook of Aeronautical Knowledge suggests? Sure! If the component of relative wind caused by the rotation of the propeller becomes small enough, the angle of attack does in fact become negative. In the diagram below, the aircraft's cruise speed remains unchanged, but the speed of the propeller blades is reduced to show a negative angle of attack.

Negative Angle of Attack at Hub at Cruise Speed
Negative angle of attack of a propeller blade at the hub during cruise flight


In practice, propeller's have varying geometric pitches along their length to give constant thrust along the length of the propeller. At the hub, because of the reduced speed of the propeller, a bigger angle of attack is needed. At the tip, because of the increased speed, a smaller angle of attack is needed. Of course, for a fixed pitch propeller, this is most efficient at a specific RPM.

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