Do coaxial rotors provide twice the amount of lift given same diameter and RPM?
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$\begingroup$ Related: Why does the Ka-50 (Hokum) have two main rotors, one on top of another? $\endgroup$– foootCommented May 18, 2017 at 18:25
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3$\begingroup$ Possible duplicate of What is the purpose of having contra-rotating props on an aircraft?. Although this is a question about rotorcraft, I'd say the question is basically the same: the propellers are just pointing in a different direction. But if there's a significant difference I'm happy to retract. $\endgroup$– PondlifeCommented May 18, 2017 at 18:45
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2 Answers
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What about the engine? Even if engine power was doubled, the coaxial rotor won't produce double the lift.
Airfoils don't like stacking neighbors.
(Source)
This double wing above (biplane) produced about 120% lift, not 200%.
Using the biplane above to illustrate a point, between the wings the pressure will neither be high enough for the upper wing, nor low enough for the lower wing.
In a biplane aircraft, two wings are placed one above the other. Each provides part of the lift, although they are not able to produce twice as much lift as a single wing of similar size and shape because the upper and the lower are working on nearly the same portion of the atmosphere and thus interfere with each other's behavior.
For the same engine output, we're looking at ~10% increase in lift.
Because the coaxial rotor does not require an anti-torque device such as a tail rotor, Fenestron, or Notar, a coaxial rotor helicopter requires about 10% less power than a single rotor/tail rotor helicopter.
For the same rotor diameter and power the coax will lift approximately 11% more weight. This equates to 30% greater useful load, which means more payload or fuel. The coax helicopter can hover at a 6000 ft higher altitude, or at a higher temperature (avxaircraft.com).
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$\begingroup$ Note however that the interference between blades is similar whether you double the number of blades in one rotor or mount two rotors above each other. And contra-rotation has its own advantages. $\endgroup$ Commented May 19, 2017 at 17:15
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$\begingroup$ Actually, it's more than 120%, but the power needed to produce that lift is at least the 200%. The power is the important factor here. $\endgroup$ Commented May 19, 2017 at 19:33
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Yes they can give twice the amount of lift if blade angle of attack is a variable, but require more power to do so. Momentum theory predicts:
a 28% power penalty on this configuration, when compared to running the two rotors in isolation, when the two rotors are operated at equal thrust.
a 22% power penalty when the two rotors are operated at equal rotor torque.
a 41% power penalty when the rotors have no vertical separation.
Experiments have shown a power penalty that is less than predicted by momentum theory: 16%.
Source: Principles of Helicopter Aerodynamics, J. Gordon Leishman, section 2.15.1
