The issues of propfans isn't just a comparison of fuel efficiency. There is also considerations of noise, efficiency at various altitudes, safety (turbofan cowlings are designed to contain fan breakage/other breakages), and size.
To elaborate, the big benefit is that they keep pieces of the fan blade and engine from cutting through the body of the plane at close to the speed of sound. It’s happened before and is not pretty.
Considering that attempts at propfan commercialization has more or less always ended up as a boondoggle for the past ~50 years[1], I'm not especially optimistic about seeing these engines make it beyond R&D.
It looks like several of those attempts failed for reasons that had nothing to do with the aircraft itself. A couple were canceled because they were being developed by the USSR, which broke up during the projects. A third was financed by Ukraine, and got cancelled due to budget shortages there. The Boeing and McDonnell Douglas ones were cancelled because oil became cheap enough that the fuel savings from the planes would not be enough to justify development costs.
https://en.wikipedia.org/wiki/General_Electric_GE36 - the Wikipeida article about GE's attempt to commericalize something similar in the late 80s/early 90s seems to infer that there is some options that meet noise requirements while still providing meaningful efficency gains.
From the article:
"Chapter 4 community noise standards of the International Civil Aviation Organization's (ICAO's) Committee on Aviation Environmental Protection (CAEP), which would go into effect in 2006 and be a reduction of ten effective perceived noise decibels (EPNdB) from the existing Chapter 3 standards that were established in 1977. The regulatory compliance, however, caused a five-percent reduction in fuel efficiency compared to the most efficient fan configuration."
"...has only a single fan blade and a variable geometry stator sitting behind it. This layout is expected to be more efficient and significantly quieter."
They actually have the potential to make less noise.
"Some think that propfan engines can potentially cause less of a community impact than turbofan engines, because the rotational speeds of a propfan are lower than that of a turbofan. Geared propfans should have an advantage over ungeared propfans for the same reason."
I'm not sure why these are being described as "wild looking" - they just look like a larger version of the engines you get on smaller turboprop planes - is it just me?
We're just not used to seeing them on large jets these days. We're so used to encased engines that hide away its mechanism.
To me, propellers feel like old, slow, noisy technology, even if they are efficient at keeping small planes up in the air. I'm sure they'd eventually look normal on jets, if this technology works out, but right now they look very unusual.
I fly (or flew, before the pandemic) on small turboprop planes fairly frequently, mostly between Scotland and Norway, and between different parts of Norway, from maybe 20 seats up to 40 - they are a common sight in small city airports.
They are indeed very, very noisy - inside the cabin, you almost have to shout to be heard. It's a strange kind of loud though, in that it always seems to make me sleepy!
It's hard to see from the front view, but in each engine, there are two rotors, one just behind the other. I can't recall seeing that on other engines. There's an image of it further down the page.
To the worryguts fearing damage to the airframe because of 'unplanned spontaneous disassembly', why would that be different from the countless turboprops in use already? Be it civilian or military.
edit: I expect this leading to something like 'slow steaming' in container shipping, topping out at somewhere between 750 to maybe 800 kph airspeed, instead of up to 950, as it is now.
Form-wise these just look wrong to me - there's something cool and high-tech about having turbofans as encased units. But function & efficiency should top aesthetics.
IIRC, ducted fans are almost always more efficient than open fan designs. It's curious the Celera 500L doesn't use a ducted fan along with a propeller specifically-optimized for such a configuration. (It's an ICE, not a turboprop.)
Unpopular opinion: We need to get away from FF motors, turbines, and airborne transportation unless they can be made GHG negative.
> We need to get away from FF motors, turbines, and airborne transportation unless they can be made GHG negative.
We need to get away from carbon emissions, not particular sources. We need to add a price to carbon and keep increasing it until e-fuels are price competitive.
No thanks. I've flown turboprops into tiny airports a lot and they are obnoxiously loud. Usually these are 45-60 min flights, more than that I think risks hearing damage.
