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A propeller produces thrust, the force of flight that pushes in the direction of the aircraft’s motion. It generates thrust by creating high air pressure acting on the back of the propeller, pushing in the direction of motion and low air pressure on the pront of the propeller, pushing in the opposite direction. Since propellers generate thrust through pressure differences, propellers are said to create “lift” in the direction of motion. An airfoil shape is typically defined as the cross-section of the wing, and the blades of a propeller are also airfoil shaped. If the propeller blades are flat plates rather than nicely contoured airfoil shapes, they will still produce thrust, but very inefficiently. Flat, paddle-like propeller blades were used by some would-be early flying machine inventors in the 19th century.