Coaxial Helicopters have two main rotor sets, one on top of the other, spinning in different directions to eliminate the need for a tail rotor. I've heard it said they're also more maneuverable, all else being equal.

So how is auto-rotation different (if anything) for these kinds of helicopters? Is it any easier? Do you get longer range and/or slower (safer) descent speed?

  • $\begingroup$ I really hope that you get an authoratative answer to this but I doubt that we have anyone who's flown a co-axial on the site. My guess, I can see no reason why it would be any different to any other helicopter. Do you have a specific reason to think that it might be? $\endgroup$ – Simon Sep 27 '15 at 19:44
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    $\begingroup$ @Simon Yes, sort of, because I've heard coaxes are more maneuverable (emphasis on 'heard'). I know coaxes are more efficient because all engine power goes into lift (no tail rotor, which wikipedia says can draw up to 30% of engine power), plus the contra-rotating blades can recover some of the sideways-swept air from each other and push it down instead of sideways. It's too bad coaxes are so rare, they seem fascinating and worthy of competing with conventional helis. $\endgroup$ – DrZ214 Sep 27 '15 at 20:01
  • $\begingroup$ Continuing with the guess work - more efficient means nothing in an auto-rotation, since there is no engine power to be lost, and sideways swpet air does not apply since the relative airflow is from under the disc and drives the inner region rather than the outer. $\endgroup$ – Simon Sep 27 '15 at 20:15
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    $\begingroup$ You might be able to answer your own question through a web search: kamov.net/kamov-net/coaxial-rotor-autorotation $\endgroup$ – Todd Wilcox Sep 28 '15 at 15:31
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    $\begingroup$ @Simon: I have flown a Kamov 26 briefly (from the right seat), but did never auto-rotate one. But I guess this is simple physics: If the inertia of the rotors is the same, autorotation should be very similar. $\endgroup$ – Peter Kämpf Mar 19 '16 at 7:37

There's one major difference between a coaxial helicopter and a conventional single main rotor -- single tail rotor helicopter during autorotation: yaw control.

A conventional helicopter has a main rotor which produces a torque. The tail rotor thrust, applied at the end of the tail boom, provides a counter-acting "anti-torque" to keep the helicopter pointing when the pilot wants it. As the pilot, when I change the collective pitch of the main rotor, to go into a climb or a descent, the torque required to drive the main rotor goes up or down, and the pilot counters by raising or lowering the tail rotor collective to change the tail rotor thrust to match. Additionally, if I want to yaw the nose to the left or right, I can use the pedals to raise or lower the tail rotor thrust in order to temporarily upset the balance between the main and tail rotor torques and steer the aircraft.

Now, consider how yaw control works for a coaxial helicopter. The torque of the upper rotor is now counter-balanced by the torque of the lower rotor. To yaw the helicopter, the pedal inputs are set up to upset the balance of torques by raising the torque on one rotor (by increasing the collective) and lowering it on the other (decreasing collective), while maintaining the same thrust.

Now, when I increase my rate of descent to autorotation, the engines are no longer providing torque to drive the main rotor--the rotor is being driven by the relative wind. In a conventional helicopter, the tail rotor is still being driven by torque transmitted from the main rotor through the drive shafts--so while there's no main rotor torque to balance, I can still use the pedals to change the tail rotor thrust and control my yaw. However, in a coaxial helicopter, when the torque on the rotors is low, differential collective does not have very much control authority: depending on the collective setting, pedal inputs can have a weak effect, no effect, or most surprisingly, a negative effect! To counter-act this, most coaxial helicopters have very large vertical fins to provide good low-speed directional stability and many have rudders to augment control.

For instance, here's a picture showing the empennage of the Kamov Ka-25 (from aviastar.org). Kamov Ka-25 via aviastar.org

More modern machines also have control systems that reverse the effect of pedal inputs on differential collective at the very low collective stick settings where control reversal can occur. Flight manuals for coaxial helicopters advise against reducing forward speed too quickly during autorotation, since the directional stability is provided primarily by the body and tail.

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    $\begingroup$ Practically speaking, I have to disagree with the disadvantage of yaw control (or lack thereof). One of the main reasons a traditional helicopter has to auto-rotate is a loss of tail rotor effectiveness. So all that yaw control that a tail rotor provides is gone. AFAIK, you're supposed to fly forward during auto-rotate no matter what kind of helicopter it is, and both types will benefit from some kind of tail extension or rudder or weathervane to help keep yaw stable. So it can't really be a disadvantage of one under the other when both types usually undergo the same loss of yaw. $\endgroup$ – DrZ214 Oct 1 '15 at 19:45
  • $\begingroup$ @DrZ214 So all that yaw control that a tail rotor provides is gone. Nope. Just entering autorotation will restore tail rotor authority. Once in auto, you have full yaw control. $\endgroup$ – Simon Mar 19 '16 at 8:01

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