Blade geometry, forces and construction
A propeller is a rotating aerodynamic system transferring shaft power to the surrounding air. Each blade section sees a combination of rotational and axial flow. Rotational speed increases with distance from the axis, so a useful blade has changing geometry along its span rather than one uniform section working under identical conditions everywhere. Treat the actual airfoil, velocity and reference station explicitly.
Angles and pitch
Blade angle is measured between the section chord and the plane of rotation. Angle of attack is measured between that chord and the relative airflow. Those are different references. A blade can retain the same geometric angle while its aerodynamic angle of attack changes with forward speed. For a fixed-pitch propeller at constant rotational speed, an increase in forward speed generally increases the helix angle and reduces the section's angle of attack under the simple model.
Geometric pitch describes theoretical advance per revolution in the screw analogy; effective pitch describes actual advance at the stated condition. Their difference is slip. If a question asks for percentage slip, identify the denominator rather than silently divide by effective pitch instead of geometric pitch. Propeller efficiency compares useful thrust power with shaft input power, retaining consistent units.
Loads and material behavior
Centrifugal force produces large outward loads on blades and their retention features. Aerodynamic loading also creates thrust and bending effects, and the pitch-axis geometry determines twisting moments. Do not infer the response of every propeller from one force acting alone. The hub, blade retention, bearings, stops and actual control arrangement work together.
Wood laminations, forged metal blades and composite constructions have different protection and inspection requirements. A composite blade may include a core, reinforcement, bonded erosion protection and a metallic retention interface, but the exact construction is model-specific. A surface that looks smooth does not independently establish the absence of hidden damage, a sound bond or acceptable remaining dimensions.
Limits of general theory
Blade stations provide repeatable locations for measurements; use the station and reference specified by the applicable data. High helical tip speed can introduce compressibility, noise and efficiency losses, but a generic rule cannot replace the approved diameter, speed and operating restrictions. This chapter teaches physical interpretation, not a universal blade-angle setting, repair dimension or permission to substitute another material.