Why is a propeller twisted, and why does blade pitch matter?
The engine turns a shaft; the shaft turns a pair of airfoil-shaped blades. Each section moves sideways through the air while air also approaches along the shaft. The blade meets that combined flow at an angle. Change the pitch or the incoming air speed to see the difference.
Orbit the paused model to see both the blade outline and its twist. The shaft and hub rotate together. Play to follow a full turn.
Increase blade angle
The blades rotate to a larger angle while keeping their manufactured twist. In Diagram, the chord line moves while the incoming-flow direction stays fixed.
Increase incoming air speed
At fixed RPM and pitch, faster axial inflow tilts the relative-flow direction. In this velocity triangle the angle of attack decreases.
Separate the two angles
Blade angle is measured from the plane of rotation. Angle of attack is measured from the relative-flow direction. They are equal only when axial inflow is zero in this simplified model.
The main parts
Horizontally opposed engine
An illustrative four-cylinder aircraft engine. Its job here is to supply shaft rotation; combustion and internal piston motion are outside this model.
Propeller shaft
Transfers rotation directly to the hub. This example has no reduction gearbox, so hub and shaft speeds match.
Hub and spinner
The hub joins the blades to the shaft. Changing blade angle rotates each blade about its lengthwise direction; the spinner streamlines the centre.
Twisted airfoil blades
Sections farther from the shaft move faster at the same RPM. The blade angle decreases toward the tip. Its manufactured twist stays fixed when you adjust pitch.
Section at 75% radius
The highlighted band is where the displayed blade angle, inflow angle and angle of attack are evaluated.
Incoming air
The arrows show the prescribed axial inflow before the propeller. They do not represent a computed wake, pressure field or thrust.