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.
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.