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