A 12 second loop showing a quadcopter from above with the front at the top. The propellers spin slowly so their direction can be seen. The front-left and rear-right propellers turn clockwise, and the front-right and rear-left propellers turn counter-clockwise, so each diagonal pair turns the same way. Curved arrows around the body show the opposite torque reaction from each pair, and the two reactions balance. A pre-flight check then ticks each propeller whose leading edge faces its direction of turn, and the aircraft lifts off level. With a wrong propeller fitted on the front-right arm, that propeller still spins counter-clockwise but its blade faces the wrong way. The check marks it with a red cross and no thrust, and the aircraft does not launch.
