Diagrams.ink / Everyday engineering

How a spinning propeller pushes air to make thrust

Each blade is a small wing moving in a circle. Tilt it more (pitch) or spin it faster (RPM) and it throws more air backward, so the air pushes the propeller forward. Drag the two sliders and watch the blade cross-section on the right.

Propeller on a test stand · three blades, bench test, no airplane movingAir flowing smoothly

Blade cross-section · slice at 75% of the blade length

1Blade a twisted wing2Hub & spinner3Motor4Air darker blue = faster5Thrust force on the stand
0 N

pushes forward

0°

blade pitch minus wind angle

0 m/s

air is faster behind it

0 kW

tip speed 0 m/s

slowerfaster
flattersteeper
Try this: slide the pitch up. More tilt grabs more air, until the blade stalls.Spin is shown slowed down · not to scale

Tilt, speed, thrust

The blade moves round and the air slips past it at a slant. The angle between the blade's chord line and that relative wind is the angle of attack α. At a small angle the airflow follows the blade and pressure is lower on the curved front face and higher on the flat back, giving a force (lift) at right angles to the wind. Part of that force points forward: thrust. The rest just resists the spin.

Too much angle and the air can no longer follow the blade. It separates and the force drops: a stall. Spinning faster feeds the blade more air per second, so the force and the power the motor needs both grow quickly.

A simplified model, not a design tool

Numbers come from a textbook blade-element + momentum estimate for a bench test (no forward speed): a 1.2 m, three-blade propeller, one air-speed through the whole disc, a simple airfoil lift curve with a stall, no tip losses or compressibility. Real propellers differ. Read the values as rough, relative guides.

α = pitch − inflow angle · thrust ≈ lift × cos(inflow angle) · thrust = 2·ρ·A·v²