this post was submitted on 28 Mar 2025
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How many blades do you have to add to a turboprop before it's promoted to an open turbofan and touted as a major new innovation?
Based on my image search engineering, the answer to your question is 2.
Based on my one semester of air breathing propulsion that I took 25 years ago, I'm guessing there is more going on inside the turbine part of the engine that both allows sustainable fuels that current turbofans can't and also allows compression ratios at lower fan speeds that allows an open fan with fewer blades. Again, I barely passed air breathing propulsion back then and haven't used ANY of that knowledge since, so I'm mostly talking out of my ass.
I've seen turboprops in museums and on the internet with around six or eight blades. When I looked on the Wikipedia page for propfan engines, which seems to be another name for an open turbofan, the distinction seemed to be mainly how the blades were shaped (like propellor blades or turbine blades) and how tightly-integrated everything is (you can swap the propeller out on a turboprop).
I don't think the number of blades is really important. After all if you just keep adding blades eventually you would get to a point of diminishing returns. That's around four blades which is why most only have four blades, unless they're made out of incredibly light material.
So if you have a lot of extra blades there probably is some additional engineering going on to make use of those extra blades in some way.
I was meaning that the blade count and detachability was the difference in definition between turboprop and propfan/open turbofan, not that it was necessarily the thing making the engine more efficient.
Mentour Pilot did a video about the CFM RISE open fan engines a few months ago, they’re somewhere between a turboprop and a geared turbofan. Able to cruise at turbofan speeds, but much higher bypass ratios like turboprops. They’re not technically new, but they’re possible now due to material advances. Pretty cool concept.