aoa of the propeller is the angle between the chord of propeller section and the relative airflow. but why would the forward speed become larger when the aoa is smaller?
A propeller is a moving wing section but unlike a wing it is moving in two planes, the rotational plane and the forward plane so your relative airflow to the propeller is made of two components, the rotation of the prop and the forward motion of the aircraft. When the aircraft moves forward at a faster rate the AoA reduces so it's really the AoA reducing as speed increases rather than the speed getting larger because the AoA has reduced.
To explain a little more. I shall attempt to draw a picture with words.
Draw an aerofoil section with an AoA with respect to its rotation and make the AoA very exagerated, i.e. more than the expected 4 or so degrees, probably about 20 degrees even more. Draw a line from the trailing edge of the aerofoil in line with the plane of rotation. Make the line at least twice as long as the aerofoil with the tip of the line extending out past the leading edge of the aerofoil. The length of the line represents the rotational speed of the prop.
Next draw a very short line out at right angles to the tip of the rotation line, towards the aerofoil, this is the forward speed of the prop. Now draw a line from the tip of the short line back to the base (trailing edge of the aerofoil). You should now have completed a right angle triangle. The last line represents the relative airflow to the aerofoil or prop.
If you know increase the length of the short line a slight amount to represent an increase in speed you should also see that the AoA has reduced.
also what's the use of constant-speed unit? maintain constant speed? in the book it says csu is a "constant rpm system that permits pilot to select propeller speed, and therefore engine speed, needed for any situation".
how could u have only one certain rpm but different airspeed and power?
arent the airspeed and power increase directly to the an increase in rpm?
A constant speed props is exactly that. Unlike a fixed prop that allows an increase RPM in a descent the constant speed prop increases the pitch to maintain the same RPM.
The constant speed prop stops or minimises the AoA decrease that would occur when speed increases.
RPM isn't the only factor regarding power. Power equals Torque X RPM. With a constant speed prop you can go faster by increasing manifold pressure (increasing torque) yet atill keep RPM constant.
Hope my explanations help.