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I was watching this video by Veritasium on Youtube, where he talks with NASA engineers about the Mars helicopter. At the attached timestamp they talk about how a completely overlapping rotor configuration is actually more efficient than non-overlapping rotors.
This in direct contradiction with this post.
Which one of them is correct?
The thrust being the same, a coaxial design is going to need some 15% more power than a single-rotor helicopter. This increment in power is due to the aerodynamic interference among the upper and lower rotor and the fact that the lower rotor works in the slipstream of the upper one.
This decrease in efficiency in respect to a single-rotor design is partially offset by the fact that in a contra-rotating design the tail rotor is not needed anymore: a tail rotor normally uses some 10 to 15% of the total power. Plus a contra-rotating design is more compact since each rotor has to provide only half of the thrust and the radius can be reduced consequently.
So, both are right according to how one define efficiency: for Ingenuity compactness is for sure the way to define efficiency.
