hckrnws

It's part of the tradeoff between momentum and energy that you should aim to move as high of a mass of air at as low of a speed as possible for efficiency.

When you put energy into a mass of air you impart energy of 1/2 MV^2, the kinetic energy equation, which you can think of as the energy you're leaving in the air as it's accelerated to a given velocity on exhaust from the engine. The V^2 part is a killer. This does not translate directly into momentum at all and the most energy efficient way to gain momentum is with a large mass that's accelerated to a low velocity. You can actually see this with the wings which keep the plane itself up. The wings impart enough momentum to hold the weight of the aircraft up by moving a lot of air at relatively low velocity which sacrifices very little energy for the upwards momentum gained.

So engines in aircraft have been getting bigger and bigger as well as slower and slower. It's basic physics, aiming to move as high of a mass at as low of a practical velocity as possible. The 737 max issues were an example of adding giant engines to an airframe not originally built for them due to the drive to move as much air at as low of a velocity as possible while still keeping the plane moving forwards. Passenger aircraft have been getting slower over the years, the 747 was faster than the newer 787's because we're looking for efficiency above all else these days. Going open bladed makes a lot of sense as we go further down this path.

This isn't true though, the 747's cruise speed is the same as the 787's at 0.85 mach. The 747 has a slightly faster max speed but that's not relevant for actual travel. The 777 has a slower max speed and cruise speed than the 787 despite being older. I don't think you can realistically draw a correlation between older/newer being faster or slower on wide body aircraft.

IDK if "bigger and slower aircraft" is what he meant, but rather "bigger and slower engines." Jets cruise @ mach .85 because that's the economic optimum set by wave drag, compressibility and passenger time costs. Hasn't changed in 50 years.

The relevant metrics are amount of air moved and speed the air is accelerated to, aka efficiency gains from propulsive efficiency- e.g. increasing bypass ratio, larger fan diameter, lower jet exhaust velocity

I agree with your points but they had literally stated that passenger aircraft were getting slower and provided the example of the 747 vs 787. So it was clear to me what he meant.

I remember first seeing one of those big ass fans in a planet fitness 15 years ago and being surprised how much air one big fans moves.

I'm curious about using a hybrid system where you have multiple electric fans. For instance 2 turbines and 4 fans. Advantage is smaller diameter for the same mass flow. And more redundancy. A negative is the weight of the electric motors and generators. If you added a battery you have some other advantages. Less pucker when you lose an engine. And better throttle response.

Another advantage is you can place the fans all along the wing getting you better stall resistance as the flow doesn't detach as easily. There's already a prototype of a hybrid plane that does this:

https://www.electra.aero/

You can go one further and just mount a squirrel cage fan in place of or on the front or top of the wing. https://en.wikipedia.org/wiki/FanWing

Or go further and use rotating drums: https://en.wikipedia.org/wiki/Flettner_airplane

Or you can use a horizontal-axis style helicopter rotor with variable pitch, and it gets you omnidirectional thrust (VTOL) https://en.wikipedia.org/wiki/Cyclogyro

There are a lot of interesting possible alternate histories (only requiring a few tweaks to physics) where fixed wings never really work and horizontal rotorcraft dominate, especially in a world where lighter-than-air craft are common - something like a hybrid between a zeppelin and a paddleboat.

There was never any possible alternate history where those alternative lift or propulsion approaches could dominate. The fundamental flaw is that in case of power loss they can't really glide or autorotate. Perhaps useful for some limited drone applications but not safe enough for humans.

Sounds like a helicopter is not very efficient?

Not for moving sideways at a constant altitude.

If you think about what a plane does to keep itself up, it sweeps through a curtain of air which ends up blowing downwards.

In a second it must blow down a large volume of air with enough speed to equal the impulse created by gravity in a second.

Basically m_air × v_down = m_plane × gravity × time

The energy you need to do this is the same quadratic, 1/2 m_air × v_down^2

A larger volume of air with a smaller v_down (a huge curtain of air of a fast plane with very wide wings) is more efficient then the smaller disk of air with high velocity of a helicopter.

But if the plane isn't moving forward the curtain has no volume and the plane stalls and falls. But helicopters have no trouble lifting off vertically.

Less efficient than an aircrafts wings over a long distance but very efficient for an aircraft with engines pointing straight down.

The blades are massive, push a lot of air relatively slowing compared to smaller engines. There's a reason most planes will stall when pointing straight up, despite in theory having more power to weight. Their prop efficiency is worse than a helicopters rotors.

> Airbus is also assessing shielding the area of the fuselage closest to the engines to minimize the risk of a blade off — one or more composite blades breaking, which could dent or puncture the fuselage and, in the worst-case scenario, strike a passenger.

sightly terrifying

The cowling of the current turbines serves the same purpose, but needs to cover 360 degrees of rotation, so it's heavier and draggier. The blades have a bit more angular momentum in the propfan than in a high bypass turbofan, but there's fewer of them.

Instead of reinforcing the fuselage, I wonder if just having a 1/4 nacelle that shields the passenger side would work.

The impact area of the fuselage looks much larger than an unrolled cowling, and thus significantly heavier to reinforce. The smaller cowling will save drag through.

It might hit the fuselage at a flatter angle than it would hit a nacelle, which would help.

>The cowling of the current turbines serves the same purpose, but needs to cover 360 degrees of rotation

this doesn't make sense. if you are not worried about fan blades flying off in directions other than the fuselage, why cover 360 degrees? (and if you are worried 360, then why open rotor?)

The cowling is its own structural support, so needs to be strong all around, otherwise it would fail on the other side and you'd get blade+cowling approaching the fuselage at high velocity.

not more terrifying than sitting in any turboprop airplane.

I had a sharp intake of breath after reading this and then clicking through to see the header image of the article.

Reminded me of this: https://youtu.be/j973645y5AA?si=QJrNJe0gT-zwpElD

Seems like quite an engineering challenge with this new design...

High bypass turbo fans do this as well, it's just in the fan/engine housing, not the fuselage.

Yeah I'd think you'd need some serious shielding to prevent a puncture

Isn't this like turboprops (already very efficient) with bigger propellors? I couldn't tell from the article, but quite possibly missed something.

This website has some nice explanations and GIFs: https://s2.smu.edu/propulsion/Pages/variations.htm

So the "open rotor" engine is actually a propfan engine?

Essentially yes, different engine companies have used different nomenclature over time. It seems that the "open rotor" terminology is being used to emphasize the improvements which have been made to blade design, noise, and general efficiency.

The inner nerd in me is so satisfied. Thanks for the link.

That website it gold!

I think it is the stators fixed to the engine nacelle, judging by the article image.

I cannot stop thinking about the fact that air travel wastes so much energy, just to float. Buoyancy is free.

It is insane that we are not doing materials research on how to capture vacuum in thin cavities.

Modern jetliners get a higher miles per gallon per seat than most US (ICE) cars. They easily beat any ICE car with a single occupant in miles per gallon equivalent. If you want to minimise fuel usage and can't find someone to share with, it's better to fly! The issue is almost entirely the distance travelled.

Cars also waste energy just to overcome friction while most of the times they do not change momentum (go straight at constant speed). But this is a discussion for another tire.

Aerodynamic drag accounts for more energy losses than tyre friction (and tyre friction can be relatively easily changed with tyre pressure or tyre type).

now do high speed rail..

It varies so much depending on passenger occupancy rates. Planes tend to run near 100%, and cars (at least in the US, although I'm sure other countries aren't much better) at near 20%.

Assuming 1mpg for the entire train, it needs at least 100 passengers to compare to a fully-occupied passenger plane.

Can you name a HSR route that exists between US cities that would challenge the near 100% occupancy rates? Seems relevant. We probably need to compare routes where there exists air travel and HSR to draw any occupancy rate choices, but this is clearly an aside.

The point is that it makes air travel ludicrous from an energy perspective where rail at high speeds (200mph) is possible

Edit: Now you've clarified your question.

There's nothing special about HSR when considering fuel (or energy) efficiency, except that it's probably less efficient over all due to increased air drag, and that it needs very particular infrastructure and passenger demand to make it work.

I'd hazard a guess that many (most?) flight routes are nowhere near popular enough to make them viable for train replacement.

Eg. The bullet train in Japan has a peak capacity of over 20,000 passengers per hour.

The most popular flight route in the US has around 3 million passengers per year, or ~340 an hour.

Air travel is SO inefficient that it ONLY makes sense to fly with a full plane.

Train travel is so efficient that running nearly empty trains is just accepted.

You're joking, right?

Passenger capacity is part of the design of air travel. Even so, a plane could be at 1/3rd capacity before it's less efficient than a singly-occupied car.

Trains are largely a relic of the Industrial Revolution - except for those places where population distribution has made it feasible to invest in specialised passenger rail, the degree of infrastructure investment required makes them economically infeasible given a blank slate today.

If we were really concerned about transport efficiency, long-distance bus routes are the answer. Per-seat energy usage is comparable to trains, but with a fraction of the infrastructure cost, and significantly more flexibility. Countries that have a blank slate and are only interested in maximum transport for minimum cost (ie, the developing world) have gone that way for a reason.

We accept nearly empty trains, despite them needing at least 30 passengers to be competitive from a fuel efficiency standpoint with a singly-occupied car, because trains are largely seen as a service. Very few passenger trains are economically viable without government support.

There’s an irony where tech folks use a forum to discuss ideas, amongst which involve Generative Language Models which aid and abet political astroturf campaigns that make ridiculous arguments to waste people’s energy when they use tech forums…

It’s like a drain and the usefulness of the forum is swirling around and largely depleted

The interesting problem for tomorrow’s internet is how to automatically root out this nonsense. That’s a browser addin / AI tool that would be useful. Take the comments and probabilistically score the nonsense factor. A new PageRank if you will.

The value is in the well trained bullshit detector. One that could have read the parent comment on everyone’s behalf and saved us all the bother.

Let the corporate/pr/oil industry shills exist in their own space, and enable legitimate discussion to continue

You need to state your premise to enable a reasonable discussion. I've been talking about energy or fuel efficency when it comes to public transport (I see you don't have any return for this, because the facts are indesputible).

However, your premise is that "people must use (I assume, electrified) HSR to stop burning oil". This is an entirely different discussion. In many ways, it is significantly less efficient when looking purely at an energy usage perspective, especially when considering new routes across sparsely-populated expanses with relatively low demand.

Not clear to me from the article - what's the different between an 'open rotor' engine and a turboprop (https://en.wikipedia.org/wiki/Turboprop)? At face value, both seem to be jet engines with propellers used on single-aisle planes?

There is still a jet stream coming out of the engine propelling the aircraft. Unlike a turboprop where only the propellor generates thrust.

> Unlike a turboprop where only the propellor generates thrust

Each of turbojets, turboprops and turbofans generate thrust with exhaust.

Turboprops have no meaningful thrust component from the exhaust.

Almost. Wikipedia says 10%, which is kinda meaningful but really not much. Even some piston engines (like in WW2 fighters) get 2% or so from exhaust.

Like a lot of people I think I hold a mental image of "jet" which is actually not helpful for a modern engine. All modern jets seem to have this massive rotational component, the turbine, and the fan outside the turbine chamber. so does a turboprop. And the basic propeller before that. Oh, the "fan" has more blades. Pshaw! a spitfire went from 3 blades to 5 across it's lifetime. post-spitfire engines had contra rotating props with many more blades. It can't just be about the NUMBER of blades can it?

So, there is the turbine. Is that directly coupled to the "fan" bit? If not, it's probably a turboprop, but even then I am unsure all visible fans on modern jets on the spool couple directly to the turbine.

The "jet" part is the combustion chamber. Everything else, you might as well consider turbines and propellers as "the same kind of thing" but then you're in a pub arguing which details make one a prop and the other a fan.

If you like Roger ramjet you're in the other kind of Jet: the one which is more like a rocket. Also, if you work in government service how are you passing the drug test with those proton energy pills?

Frank Whittle's biography is a great read. He had some hair raising moments. things OSHA would not be happy about.

> I am unsure all visible fans on modern jets on the spool couple directly to the turbine

They exist now [1][2]. The general term is geared turbofans.

If you want to mentally unfuck it a bit, the major variables are: combustion type (internal or turbine), gearing or not, ducting or not and bypass ratio. (Compression ratio and number of blades can come too.)

When one of these changes substantially, you get a change in engine type. When it changes a little bit, you get a blur.

[1] https://en.wikipedia.org/wiki/Pratt_%26_Whitney_PW1000G

[2] https://en.wikipedia.org/wiki/Rolls-Royce_Trent#UltraFan

Most modern engines have two turbines. A low pressure turbine and a high pressure turbine. The low pressure turbine is connected directly to the fan.

Which bio?

This one is the one I have a hard copy of from 1954:

"jet" -the story of a pioneer by Sir Frank Whittle

https://www.amazon.com.au/Jet-Story-Pioneer-Pioneers-Aviatio...

Everything old is new again... McDonnell Douglas looked into the propfan thing. Boeing looked into the propfan thing. Now it's Airbus' turn. IIRC the technology has been ready for years but the passengers are freaked out by it.

Real problem was noise, not passengers. Immense advances in aeroacoustics over the past 40 years thanks to CFD is the main enabler here.

I think it’s a cool idea but I also know that the nacelles have a safety function of containing the rotor blades in the event of disintegration (e.g. from a bird strike).

If these fans have blades with anywhere near the same kinetic energy, I would be nervous.

Southwest 1380[0] is a case where the cowling didn't quite contain the thrown rotor blade.

They were very lucky that only one person died.

[0] https://en.wikipedia.org/wiki/Southwest_Airlines_Flight_1380

Turboprops have already existed for decades, so perhaps you should already be nervous.

I am assuming the target market for this is European short haul flights?

On something like a New York <-> Los Angeles flight I cannot imagine the turboprop beats a 737 in any performance or comfort category.

The article is not about turboprop but about open-rotor engines which are a modern variation of propfan engines.

Since you've undertaken the effort of distinguishing, please elucidate the difference in your own understanding of it, (and not whose brand or publication name is validating the terminology) in more detail.

Interesting - I'm curious to learn how ETOPS ratings apply to open rotor engines. Any experts can chime in?

Won’t this be absurdly loud?

I talked with one of the aeroacoustic engineers working on it, she says they expect to match noise levels of current engines.

This is discussed in the article, was there a specific part that was ambiguous?

TFW does say there is an opportunity for reduced noise. However, conventional turboprops are very loud compared to their jet counterparts.

Each revolution of a prop blade sends out a shockwave of air against the airframe. The strength of the shockwave is likely proportional to the instantaneous thrust of the engine, and more blades are likely to weaken or smooth it.

A turbofan has a nacelle to contain the shockwave, and avoid the whole noisy mess.

It's only discussed in a similarly ambiguous way - like that they know noise is a potential problem that they're working on. Though to be fair, the designers probably have no idea themselves, since apparently nobody has built a prototype engine that could be run at the rated thrust level in a way they could check the real-world noise and vibration on.

I would assume that these days you can simulate that increasingly accurately before you need a full-scale prototype.

They could also use active noise cancellation, which is already used in some turboprops like the Q400.

It was glossed over and buried.

Just an opportunity to sell premium quiet seats at the back, and pleb seats at the front.

With all seriousness, I am thinking whether there are parallels between this proposed plane and the Q400.

The Antonov An-70 has been in service with "open rotor" engines for 30+ years. It's superior to its western counterparts in every way. i.e. greater speed and payload with less fuel consumption than a C-130 or A400M.

Huh? Only two An-70 prototypes were ever built so it's not really "in service". The early propfan designs, while efficient, were too loud for widespread civil use. Newer open rotor designs are much quieter.

You know what makes the C-130 or the A400M superior? The fact there's more than one operational today.

Is this not... a propeller? A turboprop engine?

I’m not an expert but I think the distinction is that the blade tips in these reach supersonic speeds like in turbofans. That is a hard problem to fix because you don’t have the duct to contain the noise and catch the blades if one were to break.

> the distinction is that the blade tips in these reach supersonic speeds like in turbofans

Commercial engines are not designed to have anything to supersonic.

https://aviation.stackexchange.com/questions/22316/why-do-tu...

Sorry, you’re correct. You want to avoid supersonic tips as much as possible.

Modern turbofans permit supersonic tips during high-thrust regimes. (Part of the work in these new designs is releasing that constraint since those supersonic tips are a bastard.) It’s something sought to be avoided. But not at all costs at all times.

Then we can duct it.

I knew I had seen this before growing up as a child, Popular Science, 1985:

https://books.google.com/books?id=rgAAAAAAMBAJ&lpg=PA69&dq=t...

Russia has also just modernized their IL-114s and got an order from India.

Those are turboprop like the A400m or C-130. The article is about open-rotor engines which are a modern variation of propfan engines.

Thank you for the clarification!