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Flying a coaxial rotors helicopter


JonasMills
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From my (limited) understanding, pedal turns increase collective on one rotor and decrease collective on the other. Total lift remains the same, but the rotor with a higher collective obviously applies more torque.

 

Apparently the pedal effects reverse in autorotation, for reasons which I have not yet figured out.

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^ Negative.

 

A yaw turn with a tandem rotor helicopter is done simply by one rotor creating thrust in one direction (so the forward rotor creating thrust to the right) and the other rotor creating thrust in the opposite direction (so the aft rotor creating thrust to the left.)

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The major advantage of the coaxial helicopter is no tail rotor. All engine power can work toward the production of lift. The standard tail rotor configuration can rob as much as 10% of engine power for anti-torque.

 

Pitch and roll control is standard. Both rotors tilt their thrust vectors fore and aft for pitch, and right or left for roll. Directional control is commonly done by differential collective pitch, via rudder pedals. Producing a difference in torque between the two rotors by increasing collective pitch on one and reducing it on the other. The mechanics are such that total rotor thrust is constant and the unbalanced torques, between rotors, produces the desired yaw.

 

Another advantage is hover performance. The single rotor generates a vortex swirl off the tips. This produces drag and represents a power loss. The lower rotor of the coaxial brakes-up the swirl from the upper rotor, thus reducing drag and power required in the hover.

 

The distance between the upper and lower rotors was a design issue for safety and performance. They found as you increase the distance between the two rotors, the more the lower rotor was adversely affected by the downwash of the upper rotor.

 

While directional control was fine in powered flight, it was poor in autorotation. So, you have those oversize vertical fins along with minimum autorotation speeds for best control.

 

If you want to learn more detail, checkout the three links below. They are technical papers and general information on the subject. More than what can be covered in this forum. Also: Try your local R.C. hobby shops. Some of the high-end R.C. coaxial helicopters mirror the mechanics of the real thing.

 

http://www.humanpoweredhelicopters.org/articles/nasa-tp-3675.pdf

 

http://www.baldwintechnology.com/MTR_AHS06.pdf

 

http://www.helosim.com/coaxialarticle.htm

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Wait, there are RC coaxials that mimic the Kamovs?

 

Virtually all the ones I've seen have cyclic pitch on the bottom rotor, and altitude is controlled by changing rotor speed. The top rotor doesn't have cyclic pitch at all, because it's too hard to put a swashplate on there.

 

The only 'fully controlled' one I've ever seen is this, which is a beautiful piece of work but expensive ($6000 for the mechanics alone) and only available direct from the manufacturer.

 

 

 

C of G - that may well be correct. Apparently Kamov claim to have solved the directional issues in autorotation, and it'd be interesting to see exactly what they've done.

 

 

I've seen claims that coaxial rotors allow for a much smaller disc area. However, this would presumably be at the cost of some efficiency, as it's more efficient to generate thrust by accelerating a lot of air to a low speed than by accelerating a small amount of air to a high speed. It'd be interesting to see how much that loss in efficiency is offset by the lack of a tail rotor.

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Slayte,

 

If that's how it works, I could see transmission drag being the pedal reversal culprit.

 

 

That’s correct. In powered flight the method of obtaining directional control was to produce a difference in torque between the two rotors in addition to one or more large vertical surfaces with movable rudders connected to the pedals to obtain the desired turn.

 

However, in vertical or slow forward velocity autorotation (were vertical surfaces are ineffective), the alternative method was to produce a difference in drag between the two rotors. One innovation was to install movable drag vanes at each blade tip mechanically connected to the pedals. The drag vanes on one blade were deployed while on the other, the drag vanes remain retracted to obtain the desired turn.

 

Remember, in the case of our rotor, torque is a measure of how much force acting on an object causes that object to rotate. In this case friction or drag is in opposition to that rotation, so this drag vane scheme seemed to work in all flight conditions. However, there was a power penalty when using this type of drag vane.

 

Another scheme used brake shoes or the like (networked with the pedals) to grab either one or the other coaxial drive shafts in order to turn the fuselage.

 

Most all the mechanical innovations were a play on torque and drag.

Edited by iChris
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