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I'm only a HS physics student, so please bear with my lack of jargon. My question goes like this:

In the scenario of two coaxial rotors spinning independently in two different directions, each with the same amount of torque, are the thrusts produced by each rotor the same as if the two rotors were side-by-side?

enter image description hereMy first guess was that they had to be different because of things like air displacement and pressure from the above rotor being applied to the below rotor, but it seems everything I find treats each rotor's thrust like it is completely unaffected by the one above or below it. Could someone shed some light on this? In what way is thrust from the above or below rotor affected by its companion? Thanks a lot!

  • $\begingroup$ It's a nontrivial problem (most things involving fluid flow are!), but basically you will lose some thrust because the lower rotor is seeing turbulent flow patterns rather than the "clean" air the upper rotor sees. How much effect depends on the vertical separation. You might be interested in reading about the long-abandoned "pogo" project (see wikipedia) $\endgroup$ Aug 28 '17 at 15:05
  • $\begingroup$ This configuration is commonly used on co-axial multicopters. From experience, the total thrust from a single axis (two motors, two props) is approximately 175% of the single propeller thrust. i.e. the second propeller experiences the turbulence and air velocity from the upper prop. Since the lower prop is not "biting" still air, it operates at a lower efficiency. This can be offset by adjusting the relative sizes of the rotors to some extent. $\endgroup$ Aug 28 '17 at 16:23

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