Propfan | Hacker News Reader

Propfan

en.wikipedia.org

63 pointsstarpilot15 comments

So with renewed interest on aircraft emissions (due to Greta Thunberg et al.), I thought of drawing attention to this promising concept from the 80s. It's still being worked on by Safran: https://www.avweb.com/recent-updates/business-military/safra... with apparently the noise problems addressed. The basic idea is it generates similar amounts of thrust as a turbofan while consuming 35% less fuel. It was a great idea, but fuel prices were low and aeroacoustics wasn't at a state to handle geometry this complex. I wouldn't be surprised to see us flying on these in 50 years though.

Interesting! By from the 80ies you mean [1] https://en.wikipedia.org/wiki/Boeing_7J7 ?

Propfans are great in theory; extra efficiency! More power! Less weight! However in practice, things are not so rosy. The biggest downsides to propfans are Noise and Safety. Propfans are LOUD! Much louder than the current generation of turbofans. The lack of a fan-case means the tips of the blades are whirling right next to the passenger compartment. Which leads to safety; there's no fan-case to protect the passengers if one of the blades were to detach from the engine. Turbofans all have very stout fan-cases (they're the single heaviest part of all turbofan engines) and the certification processes include a blade-off test to make sure that the case can contain it. A blade puncturing the fuselage would be a big problem.

How does the blade detach scenarios differ from turboprops in this case?

Props are way slower moving and more resistant to breaking up than fans.

They also have far fewer blades.

Would it be impractical to engineer a limited portion of the fuselage to withstand this scenario?

Let's just make the props out of some material like safety glass. If it fan hits the s*&t, the blade shatters.

I suppose this might make things more catastrophic than they are already, but we're talking about these rare cases when a blade detaches. It's not a good moment.

Then duct them with a ring around them, and some sort of fins/chevrons like the [1] https://duckduckgo.com/?q=CFM+International+LEAP+chevrons&t=...

or something like [2] https://en.wikipedia.org/wiki/Ducted_fan

Furthermore i don't hear much difference between a Cessna 525A CitationJet CJ4 and a Piaggio P.180 Avanti at about a mile of distance, except the Piaggio has more of a buzzing/sawtoothy sound, and the CJ4 more howling.

Or take helicopters, an old Bell UH1(D) and a (meanwhile) not so new either Eurocopter 135 are worlds apart. The Bell UH1 a very loud 'carpet beater', the EC 135 almost unbelievably silent in comparison. I mention that because i have fresh memories from one gliding over my roof and balcony into a parking spacee in front of a supermarket in SAR/MEDEVAC role recently. And i'm not deaf either, because i can hear an A380 going from Paris to Shanghai at 11km/950kph trough closed windows with double paned glass in the silence of the night. Anyways, don't be so unimaginative, there are options!

The location where a fan could hit the fuselage at dangerous speeds is probably well concealed no? As in, once spinning it's probably feasible that the fan hit the fuselage on the back, in the area right next to the fans, but unlikely that it hits on the front, near the cockpit, far from the fan.

I wonder if it could be an effective strategy to re-enforce the parts of fuselage that could be hit instead of encasing the fans themselves.

All one has to do is look at the issues plaguing the current crop of geared turbo-fans to see where this really struggles in the real world. Attaching an ultra high RPM gas turbine to a relatively slow moving fan, whether a prop or ducted fan, is not an easy engineering exercise. The amount of heat generated in the gear box, the expensive materials that heat requires, and the operational needs of many start/stop cycles are making these engine styles difficult. That's not to say its not a worthy goal. It just seems that we're just on the cusp of making these things a reality and I think the geared turbofan will end up being the winner vs the propfan.

Is the idea with this design that it would allow for planes as fast as jet aircraft, but with turboprop efficiency? Is the implication that this might be a way to preserve air travel with a lower carbon footprint?

I am guessing the "Tic Tac" ufos are drones with prop fans.

No... these engines would still produce a very visible infrared signature from the exhaust gases. The Tic Tac ufo from what we understand had essentially zero IR signature.

Super interesting. Thanks for the info!

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