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How does an RC helicopter with only one pair of main rotors fly upside down?
RC helicopters have a really cool gadget underneat their main rotors: a swashplate. This can alter the pitch and roll of the helicopter by changing how the propeller blades are angled. Thanks to this cool contraption, when an RC helicopter turns over, the propeller blades change direction so that it's as if the helicopter is still the right way up.