Question

What causes the rotor to spin, and what causes the balloon to rise?

Answer:

Why rotor spins?

Blade pushes air backward ⇔ air pushes blade forward

By pushing air backward, each blade ends up experiencing a forward thrust force (Newton’s 3^rd Law). In fact, the magnitude of the thrust force is given by the expression v dm/dt where v and dm/dt are the speed and rate at which air is being ejected from the balloon respectively.

Notice that (for the orientation depicted in the diagram above) all three thrust forces produce an anti-clockwise moment. The resulting anti-clockwise torque gets the rotor spinning.

Why helicopter rises?

For a real helicopter, the shape of the blades is crucial in generating an aerodynamic lift. For this toy helicopter, the upward lift is generated through a much simpler way. By slanting the blade slightly downward, air is ejected not completely horizontally, but slightly downward. As a result, a (rather weak) upward lift is generated.

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