Contrarotating propellors

[AAK]

How well do helicopters with two sets of blades rotating in opposite directions work? I know they are supposed to be ideal, but don't the mechanics reduce the effeciency?

[JesuBungle]

Idk if the effeciency would be any worse than a single rotor helicopter. The top blade does most of the work seeing as the bottom blade is getting only turbulant air. The lower blade is mostly just there to counteract the spinning of the top blade, so there is no need for a tail rotor. I personally don't like the design, not for effeciency reasons, but because there is extra engineering required in a counter rotating system. The mechanism is quite complex and more likely to break down. They are also a little on the sluggish side when it comes to yaw control.

[insane_alien]

it also eliminates the asymmetric lift problem and raises the max speed. you can get more payload for the same sized engine as well, since you don't have to "waste" energy on the tail rotor.

[gcol]

You also pack more power within a reduced effective blade operating diameter. Fixedwing conventional aircraft also used contra-rotating props for the same purpose. Not twice as efficient, but a useful compromise nonetheless.

 

 

Consider that the "Jolly green giant" also employs counter-rotating rotors, they just dont share the same shaft.

[bascule]

How well do helicopters with two sets of blades rotating in opposite directions work? I know they are supposed to be ideal, but don't the mechanics reduce the effeciency?

 

Are you talking about helicopters in general which have a main rotor and a tail rotor, or something like a Chinook with two rotors spinning opposite directions? There's also the Osprey which works on a similar principle, but is more half plane/half helicopter.

[AAK]

Sorry for not being specific. I mean like the following:

http://en.wikipedia.org/wiki/Image:Ka-226.jpg

http://www.barnstormers.com/EVENTS/020907/033.jpg

[Rebiu]

How well do helicopters with two sets of blades rotating in opposite directions work? I know they are supposed to be ideal, but don't the mechanics reduce the effeciency?

 

The most successful aircraft with this feature was the Tupolev tu-95

 

The US tried the concept

 

In the early eighties NASA,s The Advanced Turboprop Project: created this design that was more efficient yet competative with jet

Note the second set of blads are stationary.

 

Turbojets use a similar technique spinning rotors and stationary stators to provide air to the jet.

[Cap'n Refsmmat]

He's talking about helicopters.

[Rebiu]

Idk if the effeciency would be any worse than a single rotor helicopter. The top blade does most of the work seeing as the bottom blade is getting only turbulant air. The lower blade is mostly just there to counteract the spinning of the top blade, so there is no need for a tail rotor. I personally don't like the design, not for effeciency reasons, but because there is extra engineering required in a counter rotating system. The mechanism is quite complex and more likely to break down. They are also a little on the sluggish side when it comes to yaw control.

Does this look sluggish.

The Russian design helicopters fighters with this configuration. Their mission is to destroy attack helicopters. The design provides for a faster and more agile helicopter. The yaw issues occur only at slow speeds and as you can see this bird is meant to be flown fast. This configuration allows the helicopter to fly faster because as insane alien eluded it eliminate asymmetric lift problems created when they helicopter move forward at high speed. The problem with a single rotor is that the blade on one side of the aircraft advances into the airflow and therefore experiences greater lift that the retreating side.

[Rebiu]

How well do helicopters with two sets of blades rotating in opposite directions work? I know they are supposed to be ideal, but don't the mechanics reduce the effeciency?

Any spinning rotor creates a spiral in the airstream. This spiral represents an inefficiency. If one has a second rotor going the other way it takes advantage of the spiral by creating faster airflow over that rotor. In addition the sum of the spiral dynamics of both propellers creates less of a total spiral and results in an efficiency gain because spirals are inherently low pressure and pull the plane back.

[JesuBungle]

Does this look sluggish.

 

Sluggish speedwise? No. But YAW is sluggish on a counter-rotating design. A tail rotor will always win in this area.

[Rebiu]

Sluggish speedwise? No. But YAW is sluggish on a counter-rotating design. A tail rotor will always win in this area.

Your point is valid. My point is that at high speed yaw is less an aspect of maneuver as roll and climb are much more effective.

[Cap'n Refsmmat]

It is in a helicopter, when hovering to attack is quite common. Yaw control is necessary for quick maneuvering during hover.

[Rebiu]

It is in a helicopter, when hovering to attack is quite common. Yaw control is necessary for quick maneuvering during hover.

This is true of ground attack helicopters. This machine is meant to shoot down other helicopter and therefore hove maneuvering is not as important as high top speed.