Contra Rotating Turbofan
 

Snecma had a proposal before of a turbofan with two contra rotating fans. It sounded good as you might have minimized drag because of the smaller wetted area of the engine, and increased bypass ratio as well.

https://www.flightglobal.com/assets/...x?ItemID=17425

How could it have fared against the PW GTF? UDF/UHB engines worked more similarly. What happened to it? :confused:

Claim was they would have been too noisy and blades off might create some havoc. If seen the An-70 fly once and it wasn't very noisy at all.

Would not the duct constrain the bypass ratio pretty severely?
Why would counter rotation help in this case? It adds considerable complication, so there needs to be a large benefit to justify it.

Not sure I buy the "increased" bypass ratio. LP drives the fans, and reduces the thrust available from the core, so one can SAY the ratio is increased, but to what advantage? Net thrust from both is what counts!

I think the fact that the design featured by the OP was 12 years ago and never saw the light of day speaks for itself.

Safran (formerly SNECMA) are reportedly now working on a more conventional unducted propfan design.

For the same reason that the spools in a modern multispool gas turbine engine rotate in opposite directions.

.....Which I presume is to reduce the gyroscopic effects?

I'm guessing all the same benefits of contra-rotating props but with the advantages of a duct. Reduced fan diameter, increased propulsive efficiency (need a caviat for that one). Noise reduction? The Tu-95 Bear is apparently, hellishly noisy!

With of course ALL of the disadvantages too. Complexity, weight, maintenance, development costs.

Yes. Its not as critical on a passenger aircraft as it is on a VTOL aircraft in the hover but I'd guess it gives your engine mounts and pylons an easier life.

I don't follow the premise of the OP?

Does the counter-rotation involve any compressors or just the fan/turbine?

What is the by-pass ratio for this?

If it's a multi-stage large by-pass fan then I might accept an influence on gyro-loads otherwise nil effect.

If talking gyro-loads, where do you expect the weak link to be, is it really the mounts which are easy to add strength vs weight? or is it the blade tip clearances in the engine blading?

The answer is known. Any advantage, regardless its origin or mechanicals, needs to be proven to a. Increase thrust without using additional fuel, b. Save weight to improve SFC, or c. Save money in manufacture.

Far greater minds than mine know the answer.

I don't see anything like this in development.

What am I missing?

I think they used to say the same about geared fans, winglets and other such fripary.

Time will tell. :ok:

"According to Snecma, such architecture enables the bypass ratio to be increased significantly, allowing the concept to meet, in combination with material and installation improvements, the goal set for the European Union’s VITAL project of an 18% reduction in fuel burn over current-generation engines."

IATA Technology Roadmap (P44)

There's also a link (dead) to a Flight article about it in the 24th April 2007 issue, if anyone has it to hand.

It's difficult to see much benefit to this. Counter-rotating props make sense because the second prop minimizes residual swirl (which is basically wasted energy) - at the price of massive noise from the second prop cutting through the vortices of the lead prop. With a ducted fan you remove the swirl with exit guide vanes so there is little wasted energy (at the price of increased weight and drag from the duct).
With a ducted counter-rotating fan you don't need the exit guide vanes, but you still need that big, heavy duct, and you're still making massive noise - yes the duct helps limit the noise, but it'll still be noisier than a simple fan. But the real down side is now you effectively have a two stage fan which means a much higher pressure ratio through the fan. A single stage fan (or compressor stage) is good for about a 1.5-1.6 pressure ratio. Double that and you're over 2, and anything over 1.9 means supersonic exhaust at takeoff. Very bad news for exhaust noise (we're basically talking a JT-8D fan pressure rise and resultant level of exhaust noise).
A two stage fan also contrary to maximizing efficiency - propulsion theory says max efficiency comes when you accelerate an infinite amount of air an infinitesimal amount - that's the fundamental reason why big turbofans are more efficient that pure jets and the reason why we keep moving to ever higher fan bypass ratios.
This looks like a whole lot of complexity with little or no benefit. The only way I can see this making any sense at all is for something designed to go supersonic (where you'd want a high fan pressure rise).

At the very superficial level at which I operate, it seems as if counter-rotating turbines, would have one efficiency advantage. Instead of wasting energy trying to rotate the stators, the swirl from one turbine would actually (help) turn the next one. I'm sure the energy lost to the stators is small (though it can't be zero), and that you'd lose much more in other ways if you tried to build counter-rotating turbines.

But am I right that this is a theoretical advantage of counter-rotating turbines? And if so, is there a corresponding advantage to counter-rotating fans (I can't quite seem to get my head around that).

You may find this GE36 design report a start, counterrotating without a duct.
ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900000732.pdf

See for example fig 5-9