Aerodynamics ~ Yaw, Autorotation & the Coaxial configuration

[Dave_Jackson]

Help please.

Yaw is achieved in the coaxial helicopter by 'differential collective'. In addition, the two rotors are mechanically linked so that the blades always cross at the same azimuths.

I believe that during powered flight, when right pedal is applied; the collective pitch on the CCW rotating rotor increases, and the collective pitch on the CW rotating rotor decreases. The increase of the induced drag in the CCW rotor, combined with the decrease of the induced drag in the CW rotor causes the craft to yaw CW.

During autorotation, the pedal action is mechanically reversed so that the application of right pedal will still to cause the craft to yaw CW. By implication, this means that during autorotation the application of right pedal will cause the greater drag (induced &/or profile) to now be experienced by the other rotor.

I'll be damned if I can figure out the aerodynamic logic for this transposition of the drag.
Can anyone help?

[CJ Eliassen]

Dave,

In autorotation, the turning force is not the drag on the rotor, but the friction in the gear box. Therefore, the helicopter will turn with the rotor that has the greatest load. Does this help?

[Dave_Jackson]

CJ Eliassen thanks. The gearbox and tail rotor do consume a small amount of the energy, which would have otherwise gone toward a slower autorotation.


At very low differential collective settings the greater drag should be in the CW turning rotor (driven rotor) and the yaw will be CW, as desired. This very low differential collective probably has little yaw authority.

The problem appears to come about when a greater amount of differential collective is applied. In this situation, the stall region on the CCW turning rotor (driving rotor) will probable be so large that it will consume all the torque that its driving region is producing. The implication is that the yaw may now reverse and be CCW.
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The first quote below seems to support the above. The second quote give no reason as to how the problem was solved.

In an old thread, CRAN said ~ "Dr. Gareth Padfield picked the Russian engineers up on this point [weak yaw control during autorotation] at the conference and they sheepishly said that ...'yes, it is a problem, and we achieve direction control in autorotation with the moveable fins...' or words to that effect."

"The problem of coaxial-rotor helicopters' directional stability in autorotation has been solved in full." ~ Quote from Kamov web page. They do not say how it has been solved.
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Assuming that the above problem is correct, the following web page is submitted as a solution to this problem. It is submitted for;
1/ critique by the and
2/ to put this idea into the public domain and thereby deter its patent.

Rotor - Disk - Autorotation Yaw w/ Separate Root and Tip Control

[Specnut727]

Dave,

Here's a variation on the theme. I find this one easier to visualise by thinking about the torque transmitted to each of the rotors by the transmission. An equal and opposite torque is transferred from the transmission to the airframe. The airframe will yaw if there is an imbalance in these 2 torques, so increase collective on one rotor (torque increases) and decrease collective on the other (torque decreases) and balance, and yaw are controlled.

In autorotation, the rotors are driving the transmission, so it's all reversed.

I hope this helps, and look forward to other comments.

Spec.

[CJ Eliassen]

CJ Eliassen thanks. The gearbox and tail rotor do consume a small amount of the energy, which would have otherwise gone toward a slower autorotation.

Tail rotor?

[Dave_Jackson]

Spec,

Your description is a lot clearer.

As I understand it;

Raising the collective during autorotation causes; the Stall region to move out into the Driving region, and the Driving region to move out into the Driven region. On a helicopter with one rotor, this will reduce the rotor RPM but it does not effect the descent rate.

Raising the collective on one of the coaxial's rotors will also cause the three regions to move out, as mentioned above. It appears that the coaxial's problem is based upon the amount of the collective increase. Small collective increases will cause this rotor to become the Driving rotor, because it's Driven region has been reduced and its Stall region is insignificant. Larger collective increases will cause this rotor to become the Driven rotor, because its Stall region has now become the predominant region.

If this is true, then only small amounts of yaw control can be applied to the coaxial configuration during autorotation.

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CJ Eliassen,

Tail rotor?

You're correct. Mia culpa.

[vorticey]

well how does a chinook do it?

why dont the pedals just tip the discs for and aft alternativly. thatll work, but not on fully ridgid rotors i spose.
its gonna be real hard, in a big flair, to hold strait.

[CJ Eliassen]

vorticey,

A coaxial rotor is where two counter rotating rotors come from the same shaft. You are thinking about a tandem rotor system.

[Dave_Jackson]

vorticey,

The side-by-side and intermeshing configurations tilt the disks forward and back (opposed longitudinal cyclic) for yaw control. This method is excellent for the side-by-side. It is less effective for the intermeshing configuration, and is therefore usually supplemented with differential collective.

Opposed longitudinal cyclic should also work for absolutely rigid rotors on the intermeshing configuration. This is because the rotors have a lateral component to their thrust. The web page
Rotor - Disk - Yaw - Method Similar to Side by Side shows 147 ft-lb. of yaw torque for a 700 lb. GW craft. Also, the addition of a 3-degree pre-cone should even improve this yaw torque slightly.