Coaxial rotors

July 19, 2013 by Tim McAdams

Traditional helicopter designs use a main rotor for lift and thrust, and a tail rotor to counter the torque applied to the fuselage. Another design, known as coaxial rotors, uses a pair of helicopter rotors mounted one above the other to produce both lift and thrust. Sikorsky’s high speed technology demonstrator the X2 uses this design as well as many Russian helicopters.

To neutralize the torque, the rotors spin in opposite directions creating equal and opposite torques that cancel each other and eliminate the need for a tail rotor. Yaw control is achieved by increasing the collective pitch of one rotor and decreasing the collective pitch on the other. Coaxial rotors also reduce the effects of dissymmetry of lift. Because they spin in opposite directions, both sides of the rotor disc have a retreating blade and an advancing blade.  

Another benefit of a coaxial design is a higher payload for the same engine power. A tail rotor consumes some of the available power produced by the engine. With a coaxial rotor design that extra power can be devoted to lift and thrust. Moreover, eliminating the tail rotor reduces noise, allows for a more compact design and increases safety on the ground.

The major disadvantage of the coaxial rotor design is the increased mechanical complexity of the rotor system. Two swash plates and their related linkages for both rotor systems need to be constructed on the same mast, which in itself is more complex because of the need to drive two rotors in opposite directions. This is offset somewhat by eliminating the intricacy of a tail rotor system. It would seem that the complexity of the rotor systems would increase the risk of a catastrophic failure. However, helicopters with this design have a good reliability record.

Sikorsky X2

Sikorsky X2

6 Responses to “Coaxial rotors”

  1. Ehud Gavron Says:

    A coaxial twin-rotor system does send more power to lift and thrust as it doesn’t require a tail rotor
    for anti-torque. However, both rotors are operating in less than ideal air as compared to one rotor
    above ETL.

    The twin-rotor (non-coaxial) Chinook and Sea-Knight helicopters spend the most of their engine power
    on thrust and lift and have both rotors operating in relatively clean air. Their disadvantage is that as
    both rotors rotate in the same direction the dissymtry of lift does limit them. EVEN SO the CH-47 is
    the fastest helicopter in the US arsenal.

    Ehud

  2. Alan Barnes Says:

    The rotors on the CH-47 and CH-46 do NOT rotate in the same direction, Ehud. If I remember right, the forward rotor is counter-clockwise and the aft is clockwise.

    As far as being the fastest, that depends on what you mean by fastest. Cruise speed? Max at level flight? VNE? Max speed at max weight? In most cases, the CH-47 is not the fastest.

  3. Dror Shannon Says:

    Actually, the fastest helicopter in the US arsenal is now the V22 Osprey, also not coaxial but also with counter-rotating rotors. Prior to that, it is my understanding that the CH-47 did in fact hold the helicopter speed records, largely because of the amount of power it produces.

  4. Bob Ness Says:

    I flew hooks in Nam. When we had Cobras as escorts we had to slow down so they could catch up. But, in a dive we couldn’t keep up with them. I absolutely enjoyed piloting that fine aircraft.

  5. Alan Barnes Says:

    Dror, while the V22 may operate similar to a helicopter at times, it is not a helicopter. They are in the powered-lift category. And that top-speed would be in airplane mode – hardly fair to compare that to helicopter speeds.

  6. Charlie Branch Says:

    Bob,
    Thank you for your service.

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