Yaw in Counter-rotating Helicopters

[DakarNick]

On a real, full-scale counter-rotating rotor blade system, how is yaw achieved?

 

With smaller R/C helicopters, each rotor is driven by it's own motor and one is sped up or slowed down to get the machine to rotate along it's rotor shaft axis.

 

On the one and only larger turbine-powered R/C counter-rotating heli I have seen, it is accomplished by a flying bridge that sits above the upper rotor head but still not 100% sure how it works. Since both rotors are coupled together by the transmission, they rotate at the same speed. The "tail (rotor) rudder" servo moves an internal shaft which moves the flying bridge up and down, which in turn adjusts the pitch. That's where I get lost. If the upper blades pitch one direction, the helicopter will bank that direction. So, the lower blades have to compensate. But how does that result in a yaw?

 

Here is the model page, pretty interesting: http://scalehelicopters.org/ka50.html

 

Do any of these mechanics of the larger R/C helicopters relate at all to full-scale helicopters?

 

Any help would be great, I am going to keep researching.

 

Thanks

Edited December 17, 2008 by DakarNick

[Jeff]

Yaw control is achieved just like you said. The collective pitch of one rotor is changed slightly (or perhaps both rotors, but in opposite directions), thus changing the torque. So, let's say the top rotor turns ccw... If you were to increase the collective pitch on that rotor, but not the other one, the torque on that rotor would increase, causing the helicopter to yaw to the right. On the other hand, if you were to decrease the collective pitch of the top rotor, then the torque of the lower rotor (cw) would be greater and the helicopter would yaw to the left.

 

I figured out how they get the collective control to work, changing the pitch of both rotors the same (two swashplates), but I could never get a good enough picture (or see one in real life) to figure out how to change the collective pitch independently to achieve yaw control. Hope I answered your question.

 

~Jeff

[copter doctor]

In a Helicopter like the Chinook, Yaw is accomplished by tilting the foward disk one direction and the aft the other to rotate the nose. For a turn to the left, the forward disc would tilt left and the aft to the right causing the aircraft to rotate.

[FauxZ]

http://www.pprune.org/rotorheads/222277-ho...ontrol-yaw.html

 

Discusses each.

[CharyouTree]

In a Helicopter like the Chinook, Yaw is accomplished by tilting the foward disk one direction and the aft the other to rotate the nose. For a turn to the left, the forward disc would tilt left and the aft to the right causing the aircraft to rotate.

 

I think what he means is a coaxial rotor system, not tandem.