Thanks everyone for the insight.
The propfan is a turboprop with an advanced propeller, with todays examples used on the A400M and AN-70.
So Peter, are you saying that there is basically no difference between a turboprop and a propfan then? That's what I suspected all along. Because the only difference I see between a modern turboprop and the GE36 is that GE decided to put the props at the back and have the high velocity exhaust drive them directly via a rear turbine, while modern turboprops use a reduction gearbox. The Progress D27 is basically a contra rotating turboprop?? But there has got to be a reason why they gave these things a different name
. I thought MAYBE the propfans/UDF are "fans" and the turboprops are "props"? (again, no satisfactory source on the internet that explains where you draw the line between a prop and a fan).
It appears we would agree that:
- Balancing a prop with many blades is not really an issue
- We have become so good at designing fancy swept prop blades that the cruise speeds they can achieve at a reasonable SFC are approaching those of subsonic turbofans, and the A400M is a living example.
- Ducting is not worth the weight and drag at high BPR because the fan will be really large.
I would say M0.7 is a very respectable speed for an airliner. Will we expect a gradual shift to props in the future then, even for transoceanic routes? Or is there some advantage to turbofans not related to speed and SFC that I don't know about? Perhaps noise?