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Sikorsky–Boeing SB-1 Defiant

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Consider this Sikorsky–Boeing SB-1 Defiant is being flying forward level and straight and being hover, which state is consumes more power? For both states, we assume that air is very still and calm for both states so we may neglect its effect, nor rain or anything else that makes one weather condition is different than the other. As we know, this helicopter is coaxial rotors and for moving forward it is powered by its tail rotor. Of course, that tail rotor consumes power so its power consumption we exclude from power calculation. What behind my question is for hover, the helicopter stay in one place and the rotors always in a disturbed air while when it is being moving straight forward and level (means no cyclic applied differently), some part of the rotors disk always gain new undisturbed air.

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    $\begingroup$ Does this answer your question? Do helicopters use more fuel when hovering? $\endgroup$
    – sophit
    Commented Jan 15, 2023 at 15:15
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    $\begingroup$ Not same case as it was not dealing coaxial rotors. For single rotor, there is cyclic application while with coaxial rotors there is not. $\endgroup$
    – AirCraft Lover
    Commented Jan 15, 2023 at 16:14
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    $\begingroup$ "Not same case" they both rely on exactly the same phisics and aerodynamis. "For single rotor, there is cyclic application while with coaxial rotors there is not" if there's no cyclic how is it controlled then? $\endgroup$
    – sophit
    Commented Jan 15, 2023 at 16:36
  • $\begingroup$ Where did you learn how to fly single rotor helicopter and that Sikorsky coaxial rotor? Single rotor's moving forward is based on cyclic application while this Sikorsky coaxial rotor its moving forward is pushed by its tail rotor. No cyclic applied when it is moving forward. $\endgroup$
    – AirCraft Lover
    Commented Jan 15, 2023 at 17:08
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    $\begingroup$ And to fly sidewards? Anyway forward thrust is given by a mix between forward cyclic and thrust from propeller. $\endgroup$
    – sophit
    Commented Jan 15, 2023 at 17:13

2 Answers 2

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This is actually very well researched and is discussed in a lot of papers on this topic, and can be nicely graphed for example in the following figure. Please note, I pulled all of the following graphs from this publication*.

Power consumed of two configurations. Taken from this publication

BL is a normal helicopter configuration, CCH stands for Coaxial Compound Helicopter. The paper exercises this on the example of a compounded Lynx helicopter as depcited in the following.

CCH configuration, taken from this publication

Look at the bottom right of the first graph, you will see the total power required over airspeed. You recognize from this graph, that the power for both configurations is medium high at hover, then drops as the airspeeds gets higher and reaches a minimum at around 75 knots, which is fairly typical. Then the increasing airspeed takes its toll, and you need more and more power to overcome the additional air drag. The only difference between a normal helicopter and a compound helicopter is that the top speed is higher for the compound helicopter as can be seen in that graph.

You specify in your question that you want to exclude the pusher propeller at the back, but the answer stays the same. Middle power levels at hover, low power levels at around 60-80 knots, and after that increasingly more power until you reach the power limit of your engine.

*I am in no form affiliated with the authors or this publication. Its simply the first one I found.

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If you exclude the power required for the tail propeller (per your thought experiment), the rotor disc will still require significantly more power to hover than to fly forward. A coaxial rotor does not reduce the aerodynamic benefit of translational lift, nor does it solve the ring vortex problem.

Even if the rotor disk is kept horizontal (no cyclic tilt) it will still benefit from translational lift. The upside to translational lift is not a function of cyclic rotor tilt. In fact my intuition is telling me that the flatter (more horizontal) the orientation of the rotor, the more it benefits from translational lift. A cyclic-tilted rotor disk is acting on air that already has downward movement through the disk by virtue of the forward motion of the helicopter in a nose-low attitude. Think “swimming upstream.” A horizontal disk is acting on still air that does not already have some downward momentum through the disk.

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