Expert: Paul Soderman - 12/17/2010

Question
Please describe the relation between twist angle and the angle of attack.Say if the other parameters such as flow rate, velocity etc are known, and when the condition is given that the the particular section of the blade has a twist of a certain angle, how do you find the angle of attack for the same? I'm trying to get a relation between twist angle and the angle of attack for the entire blade.

Answer
The twist angle at any radial station plus the propeller pitch gives the total mechanical angle, beta, between the blade chord and the plane of rotation. We then need to compute the angle, theta, between the plane of rotation and the relative velocity vector.  The relative velocity vector is the vector sum of the velocity due to forward speed and the velocity due to rotation.  You could also try to correct for the effect of swirl, but that is small and can be neglected to first order.  The aerodynamic angle of attack, alpha, is then:

alpha = beta - theta

See Elements of Propeller and Helicopter Aerodynamics by D. Dommasch for diagrams and discussion if it is still in print.
Paul