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Without a tail rotor, how does a coaxial rotor helicopter achieve yaw control? Does a coax have to bank in order to turn?

The link goes to wikipedia's article, and down a ways it says:

Rotational maneuvering, yaw control, is accomplished by increasing the collective pitch of one rotor and decreasing the collective pitch on the other. This causes a controlled dissymmetry of torque.

However, I cannot understand what it's saying. AFAIK both rotors always spin at the same RPM because the mechanical engine links are built that way and it's set in stone (correct me if I'm wrong). So I can't see how one rotor makes more torque.

EDIT: related, is this kind of yaw, however it's achieved, slower or faster than yawing in a conventional tail helicopter?

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    $\begingroup$ Your quotation doesn't say the RPM differs; it says the blade angle differs, so 1 rotor works harder than the other, which causes the desired yaw. $\endgroup$
    – Ralph J ♦
    Sep 24, 2015 at 4:36
  • $\begingroup$ I would say it is unlikely that the two rotors are mechanically coupled, since any torque difference would by transmitted through the drive train. I think it is far more likely you simply have two engines. (take this with several grains of salt). $\endgroup$
    – Asad Saeeduddin
    Sep 25, 2015 at 14:58
  • $\begingroup$ @Asad They might not be mechanically coupled, I don't know. But I'm pretty sure both spin at approximately the same RPM because that's just how all single rotor heli engines work (as far as I know). If they're not coupled, a coax could just have 2 engines like you said. However, there's a thing called a free movement clutch or something like that, that allows a gear somewhere to decouple if one axis speeds over the other. So it may be possible that they are coupled after all. $\endgroup$
    – DrZ214
    Sep 26, 2015 at 20:48

4 Answers 4

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In a conventional helicopter the lift generated by the rotor is controlled by the collective pitch. Raising the collective pitch increases the Angle of Attack of the the rotor blades and consequently the lift, and inevitably increases drag. The drag is overcome by increasing the torque from the engine. Vertical movement is achieved by balancing the collective pitch against the torque while maintaining a relatively constant rotor speed.

The torque from the engine pushing the rotor one way results in a tendency of the fuselage to rotate in the opposite direction. This is conventionally countered by a tail rotor or other similar device, and varying the force generated by this rotor controls the yaw.

So far, so good.

In a helicopter with coaxial rotors there is no tail rotor, but the torque issue remains. Here, it's resolved by applying equal but opposite torque to each rotor.

Lift overall is controlled by changing the Angle of Attack of both rotors. If the same inputs are applied to both rotors then the torque remains balanced.

However, the Angle of Attack of one rotor can be increased while that Angle of Attack of the second is decreased, leaving lift overall unchanged. This will increase the drag on the first rotor, and hence the torque required to maintain rotor speed.

Simultaneously, the lift, drag and torque on the second rotor has been reduced, producing an imbalance in the direction of torque between the two rotors, and a consequent yaw moment around the axis of rotation.

All this can be achieved without a significant change in rotor speed. In fact, the exact rotor speed doesn't play a part in yaw control.

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    $\begingroup$ An accurate helicopter answer first time and not one use of the word "chopper"! Welcome to the site. Are you a helicopter driver? $\endgroup$
    – Simon
    Sep 24, 2015 at 6:01
  • $\begingroup$ This is starting to make sense. Ironic that counter-torque of a normal heli is a problem requiring a tail rotor, but in coax it's a benefit? Nevertheless I'm having trouble envisioning counter-torque from one axis that somehow does not impart the same torque to the other axis despite being coaxial. How does torque not transmit between the two coaxial shafts? $\endgroup$
    – DrZ214
    Sep 26, 2015 at 20:52
  • $\begingroup$ I understand that yaw is achieved by creating variable torque between the two rotors, which is accomplished by creating variable drag. But I still don't quite get how this is done. Aren't the two rotors powered by the same drive shaft? How is torque increased to only one rotor? Or is this increase in torque a natural side-effect of the increased drag? $\endgroup$
    – Rekov
    Aug 1, 2023 at 15:48
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In addition to @Airsick's excellent answer, some coaxial helicopters, like the Gyrodyne Rotorcycle, use tip brakes to achieve yaw authority.

Below are a couple examples of tip brakes, found at this website
http://www.gyrodynehelicopters.com/images/GCA_Rotor_Blade_PROBE_TYPE_Tip_Brake-ProgressivelyL5.jpg http://www.gyrodynehelicopters.com/images/QH-50_Rotor_Blade__Flag-Type_Tip_Brake_Open.jpg

A tip brake is a drag-inducing device on the end of the blade, that can be extended or retracted to increase or decrease drag. To yaw in one direction, the tip brakes would be extended on one set of blades, but not the other, generating a net torque due to increased drag in that rotor.

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    $\begingroup$ With a bit of delay: tip brakes are indeed used to achieve yaw authority but mainly, if not only, in autorotation. This is due to the fact that in autorotation differential torque is very limited or even reversed, as well explained for example here. $\endgroup$
    – sophit
    Dec 22, 2022 at 18:56
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When you increase the collective pitch of a rotor blade, it produces torque thus inducing a yaw in the opposite direction of the aircraft. By the same token, reducing the collective pitch reduces the amount of torque it had been inducing. This is evident in a single rotor helicopter with its need for an anti-torque rotor.

So in a coaxial rotor system, with the two main rotor blades counter rotating, increasing the collective pitch of the top rotors (thus increasing its torque effect on the main rotor shaft) AND reducing the collective pitch of the bottom rotors (thus decreasing its torque effect on the main rotor shaft) will make the aircraft yaw in the opposite direction that the top rotors are rotating.

So let's say that as viewed from the top of a coaxial helicopter, the top rotors rotate clockwise and the bottom rotors rotate counter-clockwise. When left pedal is applied (to point the helicopter's nose to the left or yaw counter-clockwise) the collective pitch for the top rotors is increased (increasing the torque counter-clockwise) while the opposite is done to the lower rotors (thus decreasing the clockwise torque). I hope this helps!

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  • $\begingroup$ Good first answer, welcome to SE Aviation. $\endgroup$
    – Koyovis
    Aug 17, 2017 at 5:04
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In simple terms, you increase the drag on the rotors going one way, and reduce it on the rotors going the other way.

Normally you increase the pitch of one rotor (meaning more lift and more drag), and decrease it on the other (meaning less lift and less drag). The lift balances out, but the difference in drag causes yaw.

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