The blade is split into narrow radial strips ("blade elements"). Each strip at radius r sees a different resultant airflow because it moves faster than the strips closer to the hub, so a well-designed blade is twisted along its span. That built-in twist is exactly what "pitch" means for a variable-pitch propeller: the geometric pitch P is the distance the blade would advance in one revolution if it were a screw turning in a solid, so the local blade angle satisfies
tan(β(r)) = P / (2π r)
The air actually arrives at each strip along the helix it is really flying, at inflow angle
φ(r) = atan( V / (ω r) ), ω = RPM·2π/60
so the strip's local angle of attack is α(r) = β(r) − φ(r). Raising the collective pitch or slowing the RPM increases α on every strip at once; that is the whole point of a variable-pitch (constant-speed) propeller — it lets the engine hold its RPM while the pitch absorbs changes in airspeed and power.
Each strip then generates its own local lift and drag from thin-airfoil theory (CL ≈ 2π·α, clamped and rolled off past stall) acting on the resultant velocity W(r) = √((ωr)² + V²). Summing the axial and tangential components of every strip's force over the whole span and over all blades gives the propeller's total thrust and torque:
dT/dr = B·q(r)·c(r)·(C_L cosφ − C_D sinφ)
dQ/dr = B·q(r)·c(r)·r·(C_L sinφ + C_D cosφ), q(r) = ½ρW(r)²
Propulsive efficiency η = T·V ⁄ (Q·ω) — the fraction of shaft power actually converted to useful thrust power — peaks at a particular pitch for each airspeed, and collapses when strips near the root stall a coarse pitch at low airspeed, or when a fine pitch drives the tip to a negative angle of attack at high airspeed.