Helicopters: Questions & Answers on Take-off

[fluidistic]

Hey,
I wonder how can these kind of helicopters can take off: http://www.google.com.ar/imgres?img...etgeGq5zoAw&page=1&ndsp=15&ved=1t:429,r:3,s:0.
The ones that have a double helices. I think the helices turns in opposite directions so that the net torque is zero and the helicopter doesn't turn on itself. If it is so, how can the helicopter take off? There should be no net air flow in a particular direction (up or down), unlike the helicopters with only 1 big helix.

Another question: where is the rotor, how big it is and in what direction does it turn in "common" helicopters?
I've seen a small helix in helicopters near the back end of it but it turned in the up/down direction unlike the big helix that makes the helicopter to take off. So I don't see how the torque of the big helix can be canceled by the small helix.
I'd like some clarifications, thanks a lot.

 

[boneh3ad]

First off, what you are calling a helix is what is typically called a rotor.

On a normal helicopter, the main rotor provides lift and forward thrust and the tail rotor provides sideways thrust to counteract the torque generated by the main rotor. By varying that thrust from the tail rotor, you can cause the helicopter to turn.

With those Soviet model double rotor helicopters, you just flip the direction of the blades on one rotor so they are both producing downward thrust.

 

[fluidistic]

Thank you very much. I understand everything now.
Last quick question: on common helicopters. About their small rotor that make the sideways thrust. They should create a torque (small I guess), how does the helicopter counters it? Since it is small I'm guessing inclining the big rotor would do the job but I want to be sure.

 

[JaredJames]

It's small, but you can incorporate it in the design of the helicopter - even using it to your advantage.

To counter it you simply create an equal torque in the opposite direction.

 

[Lsos]

The tail of a helicopter sticks out quite a bit, so it provides a long lever arm. The torque of the tail rotor is not what cancels the torque of the main rotor. The tail rotor creates a sideways force which, acting on the long tail of the helicopter creates a torque which cancels that of the main rotor.

[PLAIN]http://upload.wikimedia.org/wikipedia/commons/thumb/9/9f/Helicopter_tail_rotor.svg/250px-