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counter rotating props

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I mean contra-rotating.

I'm looking for some education.

 

1. The S.American Cozy with the two engines, why didn't they tie both engines to a single prop.

 

2. Is contra rotating inline more efficient than say counter rotating, but one on each side of centerline? Yaw issues because one prop craps out not withstanding.

 

3. If you need to gear down because of an auto conversion anyway, would it be more efficient to build a contra box with two smaller contra props? This could give a fluff factor for high nose landing/takeoffs.

 

I've seen helicopter designed with contra rotors.

 

 

Except for the complexity of the system I can't find the effiency issues on the net.

 

Edit: Let me clarify that I can find lots of info on boat props, but air is few and far between. Tho from what I've found, could be a good way to go, for some.

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I mean contra-rotating. I'm looking for some education.

I'll attempt to answer some of the questions with some reasonably educated answers. Keep in mind that I might be wrong, but the answers is what I understand the issues are.

 

1. The S.American Cozy with the two engines, why didn't they tie both engines to a single prop.

 

My first guess would be the torsional vibration issues between the two engines. IIRC, they used drive belt and it's very difficult to get both engines to synchronize exactly. The builders also turned one of the engine around so it was much easier to do the contra rotation thing. Keep in mind that they had limited resources and made do with what they could get in Venezuela (where I think they build the plane.)

 

2. Is contra rotating inline more efficient than say counter rotating, but one on each side of centerline? Yaw issues because one prop craps out not withstanding.

 

Yes, because the rear propeller "straighten out" the airflow from the first propeller, thus improving the thrust.

 

3. If you need to gear down because of an auto conversion anyway, would it be more efficient to build a contra box with two smaller contra props? This could give a fluff factor for high nose landing/takeoffs.

 

No. The smaller propellers would actually be less effective. The main reason for the contra propellers in the first place has to do with engine power. Engines (both piston and turbine) reached a point where they delivered more power than the propeller could absorb and there's a point where you cannot add any more blades to a propeller, so some genus came up with the "contra propellers" as a way to make use of all that excess power.

 

The engines we use in the canards are not that powerful and all the available power is used by the single propeller. There's not much left to operate the "second propeller". There is also the issue of "blade interference", the pulses from the blades swinging by each others could set up some serious and destructive vibrations. That is why almost all (if not ALL) the contra-rotating propellers have at least five or more blades per propeller to spread out the pulses better. Contra-rotating propellers is also LOUD! Much louder than the "equivalent" single propeller.

 

I've seen helicopter designed with contra rotors.

 

Not my area of expertise, but If I had to guess, I'll say it's easier and probably cheaper to use a single rotor and a tail fan, which is why we don't see too many contra-rotating helicopters.

 

Except for the complexity of the system I can't find the efficiency issues on the net.

 

NASA is your friend here, they have reports and research papers on this issue.

 

Edit: Let me clarify that I can find lots of info on boat props, but air is few and far between. Tho from what I've found, could be a good way to go, for some.

 

Sure, if you have an humongous engine! :D Or, do like the Venezuela guys and use two engines.


WileEZ

"All of my ideas are suspect until proven otherwise!"

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