How to calculate Blade angle?
In case you want to know at what angle does the blade 'see' the incoming wind, you first choose an station of the prop, say at 70% of the prop radius, measured from the axis. If the prop has a radius of 1,2 meters, that station is at a distance 1,2 x 0,7 = 0,84 meters from the axis. Let's imagine, to start with, that the plane is stationary, and the prop turns at 1500 rpm. That's 25 revs per second. In every revolution, our station moves a distance of 2 x π x 0,84 = 5,27 meters. Thus, the chosen station sees the wind coming parallel to the plane of rotation, at an speed of 5,27 x 25 = 131,75 m/s.
But if the plane is moving forward at 150 mph, that's a frontal airspeed of 67,04 m/s. That additional, frontal wind is 'seen' by the station as coming from 90º to the plane of rotation. If you now draw the triangle of velocities, you have one side measuring 131,75 and the other 67,04. The angle (with the plane of rotation) of the total relative wind obtained by (vectorially) adding the two winds is arc tan 67,04/131,75 = 27º
Hope that helps...