Counter-Rotating Props?

Question

Why?
I was watching TV and a show mentioned these. I can't see why they would be beneficial.
Propellers going the same direction seem to get the job done.
Ok, watching TV gets me into trouble, can anyone help me get out?
Just curious....

Answer 1

Counter rotating props are used on some aircraft for torque cancelling effect from what is called the "P" factor.
Normally an engine runs clockwise, a counter rotating prop engine would run counter clockwise.
For example...
In an emergency, a twin aircraft would operate on one engine
better with counter rotation..
Why?
If both engines rotated in the same direction this would make the Left Side engine the "critical engine". In other words, that engine would be the best single engine to run.
If the right side engine was only working the "P" factor would be farther away from the centre line of the aircraft, meaning more rudder input to cancell out the effect.
The "P" factor is caused by the down side movement of the prop.
This web page shows a normal engine set up turning in the same direction.
http://en.wikipedia.org/wiki/Critical_engine
You will notice the higher rate of yaw in the right picture.

The counter rotating prop would bring the "P" factor in closer like the Left Side engine making it easier to control the aircraft on one engine.

With helicopters counter rotating props cancel out the torque from the engine, thereby reducing the need for a tail rotor.

Yes propellers rotating in the same direction get it done, but aircraft safety is always first.
Counter rotating props make the workload for the pilot easier.

Answer 2

counter rotating props on a vessel and on an airplane counteract the effect of Torque: a single propellor turning in one direction tends to want to pull that way too...some sort of trim is necessary to counteract that. Two props turning opposite ways or towards each other eliminates that as a problem. Thety can actually also be used to help steer also, Ships with counterrotaing props can use that effect to help them rotate faster in a turn..

Answer 3

Ever wonder why a helicopter has a tail rotor? It's to keep it from spinning out of control due to the reaction force (torque) generated by the main rotor. "Every force has an equal and opposite force". The tail rotor can be eliminated by having two main rotors, counter rotating to cancel out each other's rotational forces. The blades have opposite pitch, however so that they work together to generate lift.

The tail rotor also serves to allow the craft to turn. In counter rotating twin rotor designs, the rotors have to be able to turn at different speeds from each other, to allow turning.

On airplanes, the counter rotating props on wing or tail mounted engines also serve the same function, torque cancellation. The airframe can be lighter as it doesn't have to be as rigid to resist the torque generated by the prop.