Multi-Rotor question
 

An argument came up in the pub yesterday between me and a mate which raised a question concerning helicopters which hopefully someone here can answer.

The basis of the argument was as follows (and this is very basic stuff, so please forgive me). On a normal helicopter there is a single rotor. When this revolves to provide lift it generates torque which will cause the fuselage to revolve in the opposite direction to the rotor. To counteract this a tail rotor is fitted to provide a counter-thrust to the fuselage so it doesn’t spin. However if a helicopter has two contra-rotating rotors the forces between the two balance out so there is no need for a tail rotor to stop the fuselage spinning.

But (and here the argument started) there is still a requirement to turn the fuselage (to face the direction of travel, saves getting a crick in the neck when flying backward). In the standard single-rotor machine this is, I believe, done by varying the thrust from the tail rotor so it provides either too much or too little (which will turn the fuselage). However how is it done on a multi-rotor machine?

I claimed it could be done, at least theoretically, by varying the properties between the rotors (different pitch or, less likely, different speed for the two would produce a torque imbalance which would turn the machine). He insisted that they must have some external thrust, either from a tail rotor or variable ducting for the exhaust (for the turbine powered machines) to turn them (I cited the Chinook as not having a rotor but he insisted it has ducted thrust).

Does anyone know if this is possible (or even has been done) or am I talking through my posterial orifice again? (And there is a pint riding on this).

How to control a Chinook...
 

I believe I am correct when I say that there is definately no "notar" type system on a chinook or a tail rotor. Essentially when pedals are pressed in the cockpit both rotor systems are tilted in opposite directions.

I got this from another website:
"This is accomplished by pedal only inputs. By depressing one pedal over the other, cyclic inputs are put in both systems in opposite directions to pivot about the center of the aircraft. Both rotor systems receive equal cyclic inputs, and the helicopter just spins nicely at its center without the pilot having to move his cyclic control at all. A pivot around the tail is accomplished by heavy cyclic inputs by the pilot, and little or no pedal inputs. This will make the tail stay in one place, and the nose of the aircraft move laterally until it spins about the aft mast."

Needless to say it is a complicated system that requires a lot of linkages, a lot of control tubes, and a pilot who trusts his maintenance crew to make sure it all was put together properly!

Hope you won that pint....

MadsDad, as copterfamily states, there is no sideways thrust in the Chinook other than from the main rotors.

You may like to look at this thread for a more indepth explanation.

PS, the BV234 was/is the civil version of the Chinook.

Ah, but what about Kamovs?
 

When both rotors are on the same shaft, one above the other, applying different cyclic to both would serve only to bend the shaft (or if you're really out of luck, the tips would touch).

Being on the same shaft and generating equal amounts of lift and torque, they normally cancel each other out. When you want to turn on your axis, pressing, say, left pedal reduces the pitch on the anti-clockwise rotor and increases it on the clockwise rotor - result, fuselage turns anti-clockwise, or 'left'.

You win the beer.

Your only mistake was the statement "On a normal helicopter, there is a single rotor."

The single rotor is an abnormality. Future rotorcraft will have lateral symmetry, just like every other vehicle; and animal for that mater.
http://www.unicopter.com/No_Tail_Rotor.gif

With apologies to those who ride around in motorcycle sidecars.
:D

And now, the rest of the story
 

Discount just about everything stated by Fr O'Blivien in the above referenced post.

As previously stated if you push the "rudder" pedals the helicopter will spin about the center of the fuselage. This is differential pitch input in the fact that one rotor has a cyclic input to the left and the other rotor system has a cyclic input to the right. Pressing the opposite pedal will result in the helicopter spinning in the opposite direction. Cyclic input will not cause the blades to hit each other as they are locked in a fixed position relative to each other this is called phasing.

If the pilot moves the cyclic to the left or the right the helicopter will move in the direction of cyclic movement and once again cyclic input will not cause the blades to strike each other due to the fixed phasing. A coordinated pilot can input cyclic movement and at the same time press the pedals and the helicopter can be made to pivot either about the forward mast or the rear mast depending on the coordinated pitch input. A really coordinated pilot can perform a maneuver called walking the dog in which he alternates the pivoting from the front mast to the rear mast and back again.

Fr O’Blivien made a statement about the tilting of the transmissions when the pilot inputs forward cyclic. This is not the case. Movement of the cyclic either forward or rearward will cause one rotor system to increase collective pitch and the other to decrease collective pitch which causes the helicopter to move in the direction of the decreased collective pitch.

The pilot does not have any input relative to forward or aft cyclic stick movement other than to effect the collective pitch levels of the respective rotor system. When the helicopter reaches a forward speed of around sixty knots forward cyclic pitch is automatically applied to both rotor systems and at this time the pilot can lower his collective stick. When the speed drops below sixty knots the forward cyclic input is removed and most likely the pilot will have to increase collective pitch to a level it takes to fly with decreased weight.

:E :E

RE: Kamov
 

Good evening all!

Correct me if I'm wrong but I always thought that the big russian coaxial twin rotors use their downwash wich acts on the tailboom rudder to turn?

Lu,

Why is it the Chinook has only Pivoting and Swiveling Actuators on the rotor heads....One of each per head...driven by two independent hydraulic systems?

Kamovs
 

Woolf, you suddenly made me wonder if my information was correct. Then I thought about autorotation.

With the airflow up through the rotor (and of course the tail), what happens? If it's the sheets of metal on the tail that are used for yaw control, then the effect of the pedals will be reversed. If it's differential collective, they won't.

I'm imagining that yaw control by the tail fins would be achieved by pivoting them at the top and swinging them in the opposite direction to which you want to spot-turn.

CH-47 hydraulics
 

To: SASless

To the best of my knowledge the swash plates on the CH-47 have two each hydraulic actuators that impart both lateral (cyclic) input as well as collective input. Each swashplate has an extension arm that is about a foot or more in length. Attached to this extension arm is an electrical actuator that retracts to provide forward cyclic when the helicopter reaches approximately sixty knots. When dropping below sixty knots the actuator is commanded to extend returning the respective swashplates to the commanded collective setting.

The control geometry is such the when applying lateral cyclic the swashplate pivots on the extension attach bearing for the electrical actuator. When forward cyclic is imparted by the electrical actuator the swashplate pivots on the bearings on the hydraulic control actuators.

An added side note: The CH-47 is a cargo and troop carrier and at that time (below sixty knots) the rotor systems are operating at high collective settings and at high power settings. When the rotorheads are returned for overhaul many of the high value components are scrapped due to cracking from high stress levels.
This is especially true for the rear rotor.


:E :E

How the systems yaw
 

Oookay, well someone might beat me to this, but…

Tandem rotor system, like the Chinook:

To yaw, one rotor “banks” left while the other banks right, fuselage pivots in middle. I hear (but have no firsthand experience) that a good Chinook pilot can, through finesse with cyclic, collective and pedals induce the big tandem to pivot around the nose or tail.

Coaxial rotor system (contra-rotating rotors stacked on top of each other), like the Kamov:

To yaw, one rotor increases collective pitch while the other decreases collective pitch. The resulting difference in torque about the rotor driveshaft(s) causes the aircraft to pivot about the rotor mast. The vertical tail surface provides stability in forward flight

Synchromesh counter-rotating rotors (side-by side), like the Kaman:

One rotor pitches forward while the other pitches back, dragging the fuselage around a point between the rotor masts (a lateral version of the tandem, I suppose). Again, the vertical stab provides stability in forward flight.

That's as far as I know, anyway.

What Flingwing207 says is essentially correct, with one small exception. The "Synchromesh" description is totally correct for the Side-by-side configuration, and part of the answer for the Intermeshing configuration.

The Intermeshing configuration is sort of a hybrid between the Side-by-side and the Coaxial. Most Intermeshing helicopters use both opposed longitudinal cyclic and differential collective.

http://www.unicopter.com/B318.jpg