OK, I am no aerodynamic engineer, but I will try to explain this as best as I can....

Why a 5x5 prop is stalled?? Simple, the pitch we see in these props is the theoretical forward advance the prop would make in one revolution, so in one revolultion, theoretically, this prop would advance 5 inches. Given it's small diameter, the angle needed to acheive this is higher, so the prop stalls. This pitch is measured at the 75% radius so this would give:

Radius at 75% -> (5/2) * .75 = 1.875 inches
Angle for 5in pitch = atan(pitch / (2 * pi * radius at 75%)) thus, with pi = 3.1416:

atan(5 / (2 * 3.1416 * 1.875)) = aprox. 23 degrees

Standard airfoils (without fancy mods to stall at higher AoAs) stall at 10 to 15 AoA....So at 23 degrees that prop is certainly stalled.

But as the model speed increases, the angle of attack of the prop decreaeses, due to simple vector physics. The AoA is:

Pitch angle - atan(RadialVelocity / ForwardSpeed)

Radial velocity (in m/s) is given by:

2 * pi * (RPM / 60) * Radius at 75% in meters.