Question

Why there are two propellers in a helicopter???

Answer 1

what would happen if someone apply force on your leg only. you'll rotate and fall.
if there were only one propeller then helicopter will tilt and won't fly

Answer 2

Because with one hand you cannot clap, you have to take the help of your other hand same way, helicopter with one propeller with not able to fly it will just tilt and tilt and rotate on its own axis.
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