That one on the Ilyushin isn't a UDF, but the prototype for the An-70 engine (which is an improved NK-12). 13 fan blades in total on two shafts, 8 on the front prop and 5 on the back.
Here's a question: what became of this? Seemed like a good idea...
From what I picked up on was the safety concerns that whet with it. As it stands now, a duct engine seems safer because of its contained status. If a bird fly into a ductengine it simply get chopped up and spit out the back. Although there are accounts of birds hitting turboprops and becoming deadly projectiles.
Also, I am not sure, but I am pretty sure turbofans run at a must higher RPM than turboprops do. This engine is not really a turboprop, but a turbofan, not inside a duct.
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