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To have fun with centre of mass, torque, and angular momentum.
The paper helicopter is very easy to make. Cut out a rectangular piece of paper approximately 4 cm by 15 cm. Fold the paper (along length) in equal halves and then unfold it. Cut along the fold crease upto half of the length. Now, cut about 1/3 through the paper laterally a little below first cut on both sides and fold these parts along length. Make folds as shown in the figure. Don't forget to add a paper clip at the bottom. The helicopter rotates when you drop it from a height of 2 meter or above. Why the blades rotates? If you fold the blades in opposite direction then direction of rotation changes! This simple toy can be used to explain concept of centre of pressure, torque, angular momentum etc.
The aerodynamics drag force acts on the blades. This force acts almost at the centre of each blade perpendicular to its surface. If the blades are perfectly horizontal then these forces cannot produce torque along vertical direction and helicopter will not rotate. Since blades are flexible, they tilt slightly upward. Since forces are perpendicular to the blade and blades are tilted, the forces will have both horizontal and vertical components. The horizontal components produce a torque about vertical and helicopter starts rotating. When the blades are folded in opposite direction, the direction of torque also gets reversed and helicopter starts rotating in opposite direction.
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