Single Rotor Versus Coaxial Rotor Lift

[Kalagaraz]

I'm having a tiny debate with someone over whether a 2nd coaxial rotor generates additional lift for a helicopter by itself or the helicopter has additional lift only due to the elimination of the tail rotor so 100% power goes to generating lift?

My theory is that it does not generate additional lift in and by itself or if it does very minimal. I base this on the theory that the 1st rotor accelerates the air to high speed giving very little additional energy the 2nd rotor can add to the air to accelerate it to more to generate additional lift. So the 2nd rotor's only purpose is to eliminate the need for a tail rotor and generate additional lift by having more power from engine.

The counter argument is that the 2nd rotor does generate additional lift beyond that of simply the extra power from engine. He said something about friction I don't know wasn't really paying attention.

 

[cjl]

The second rotor does indeed generate lift. In order for the second rotor to counteract the torque of the first rotor, the same amount of torque must be applied to both rotors. This means that both rotors are doing work, and since both rotors have airfoils, they are both accelerating air.

That having been said, for a given engine power, I doubt the lifting power would be very different between a twin counterrotating rotor helicopter as compared to a standard configuration. In the twin rotor configuration, the power will be fairly evenly split between the rotors, while for the single rotor, the power will almost all go to the main rotor. As a result, I would think the lifting power would be about the same.

 

[rcgldr]

The second rotor would be similar to going from a 2 bladed propellor to a 4 bladed propeller. As far as lift force versus power goes, it's more efficient than spinning a single rotor of the same size at a much faster rpm and/or greater angle of attack to generate the same lift force, but it's less efficient than using a much larger single rotor, or a 4 bladed main rotor of the same size (no complicated coaxial setup). In the case of propellers, the main reason for going to more than 2 blades is ground clearance limitations during takeoffs and landings. I'm not sure why coaxail rotors are used for full scale helicopters. They are more stable than single rotor helicopters, which makes them popular as beginner radio control models.