Convetional rotorcrafts are hard to hover as when we increase collective, even tail rotor pitch needs to be compensated. But in coaxial design, will just increasing the collective do the job as torque doesn't change?


The difficulty in hovering a helicopter lies in the fact that hover is unstable. Any tiny divergence will increase over time, so you have to keep adjusting the controls all the time. And this does not differ between single rotor with anti-torque and dual-rotor.

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    $\begingroup$ I have no experience flying coaxial rotors, but I wonder if they are as difficult to hover as single rotor. The difficulty with single rotor is that a change of torque requires a change in anti-torque pedals, which will induce a side force that moves you off your position. This is then countered by opposite cyclic, which will stop the side-way motion, but now you have changed your power demand, so you will either slowly climb or descent. So then you change collective, but that changes the demand in torque and now you are full circle. I don't think co-axial hover has that much cross coupling $\endgroup$ – DeltaLima Feb 15 at 19:52
  • $\begingroup$ @DeltaLima, I don't have personal experience either. There shouldn't be basically any cross-coupling, but I wouldn't expect it to be that much of an issue—basically I expect that with a bit of experience, adding a bit of bank and yaw together with cyclic should become muscle memory, and the ratio is always the same. $\endgroup$ – Jan Hudec Feb 15 at 20:03
  • $\begingroup$ In the multi-rotor (be it coaxial or tandem), if the torques are not balanced the fuselage will spin round. This is basically the same problem as the single-main-plus-tail-rotor. $\endgroup$ – Guy Inchbald Feb 16 at 19:12
  • $\begingroup$ @GuyInchbald balancing of the torques is done with the torque pedals. But in multi-rotor the torques are always balanced with the pedals centered, because torque of both rotors changes the same with collective, while in single-rotor the torque of main rotor changes with collective and the torque of the anti-torque rotor does not, so the pilot has to add a bit. $\endgroup$ – Jan Hudec Feb 19 at 23:54

There is more than just compensating the tail rotor pitch (which I believe is somewhat compensated for anyways mechanically), which makes hovering a single-rotor helicopter challenging. You also need to tilt the main-rotor to compensate for the side thrust of the tail. If the main rotor is vertical you drift sideways.

However, keep in minds rotors are never perfectly identical. So you still need to compensate manually, if even a very small (possibly unconscious) amount.

I don't know if it's hard on a coaxial, but it should definitely be easier than on a single-rotor.

  • $\begingroup$ I don't think there should be much difference, really. Both are unstable in hover, and similarly sensitive. That's what matters. $\endgroup$ – Jan Hudec Feb 15 at 19:31
  • $\begingroup$ @JanHudec In the world of model helicopters at least, coaxial helicopters are known as much easier to hover and fly than single rotors because they are more stable. A larger significantly single rotor is still significantly more squirrely than a smaller coaxial. $\endgroup$ – DKNguyen Feb 15 at 19:59
  • $\begingroup$ The model controller is both less accurate and harder to manipulate in coordinated fashion, so I'd expect the cross-coupling to be much bigger issue there. $\endgroup$ – Jan Hudec Feb 15 at 20:05

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