Confusion about propeller angle of attack along length of blade
 

Hi everyone.

I'm new to the forum, and this is my first post. I thought I'd sign up to ask questions when I'm having trouble. I hope I've posted this in the right section.

I'm currently studying for my CPL exams using the Aviation Theory Centre textbooks. I'm currently reading through Aerodynamics and I have a little bit of confusion about a particular question in Chapter 2: Propellers.

The question is, 'Why is a propeller blade twisted?'

Now, I believe I understand why it's twisted. The the angle of attack is meant to be different along different lengths of the blade, to make it produce thrust more efficiently and evenly. The inside of the blade rotates slower than the outside, therefore the inside requires a higher angle of attack to produce the same amount of lift as the outside of the blade. The outside of the blade rotates faster and requires less angle of attack. The most efficient part of the blade is about 70% along its length towards the tip.

This was my thought process when answering the question. However, when referring to the answers in the back of the book, it says 'To achieve a constant angle of attack along the length of the blade'. I do not understand this. Why would the angle of attack be the same throughout the length of the blade? I thought that we had just established that the angle of attack along different lengths of the blade are different, not constant. What does it mean by 'achieving angle of attack'?

Thank you, and I appreciate anyone who can help me understand this.

Regards
Saki

The propeller blade has washout to even the distribution of lift, the same as helicopter blades do.

However, the angle of attack comes from the airflow around the blades as it is spiralling through the air, so there will be a high blade pitch angle for a relatively low one.

AoA is measured relative to the incoming airflow. In a straight wing in straight and level flight that is relatively easy as the entire wing is seeing the relatively (to the plane) stationary air coming at a single velocity and direction**.

In a propeller the airspeed changes along the length because the velocity of the propeller changes along the length.

To make the numbers simpler - suppose the tip of the propeller is going 100 mph due to rotation; the center of the propeller will be rotating at 0 mph.

Suppose further that the plane is flying at 100 mph.

Note - for most propellers these numbers don't work; however the method is the same for whatever airplane/propeller combination is being designed. Also I am ignoring that some propellers operate with tip speeds near the speed of sound where Mach/ compressability effects become significant and shock-wave formation alters the performance. Which is why it's better to buy a prop from a maker that has worked out all the particulars.

At the tip the propeller is going across wind at 100 mph and into the wind at 100 mph, so the relative wind is at 45º to the tip and going 140 mph. This is the 0º AoA basis for the tip.

At the center the propeller the prop is going 0 mph across the wind and into the wind at 100 mph, so the relative wind is at 90º to the tip and 100 mph. This is 0º AoA basis for the center (though few props get all the way to the center.)

The pitch of the prop will change/twist from 90º to 45º along the length of the blade from center to the tip just to maintain a 0º AoA at this condition. Change the airspeed or the tip speed and get a different angle at the tip. The center is always 90º.

**in low speed, <0.5M, in straight and level flight there is an upwash coming from ahead of the plane so the geometric AoA seen from the path of the plane is not the AoA that the wing encounters. It's not a great deal, but it is why ground-effect happens.

Looks like you initially confused Blade Angle and AoA. The blade has more blade angle at the root than the tip to keep the AoA constant due to different distances covered by the blade according to where on the blade we look. The tip travels much further rotationally than the root. This makes the RPM vector much longer and brings the relative airflow towards the plane of rotation of the propeller disc.

Thanks all!

I understand why I got confused. I didn't take into account of both the airspeed vector and rotational speed vector, which result in an approximate 45 degree angle of relative airflow.

Propeller Explained Video
 

This is a good video. It was used in the 90s to train engineers and pilots in propeller theory.