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'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:
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.
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.
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.
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.
> 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.
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.
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.
>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 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?
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.
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.
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.
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.
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.
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.
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.
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.
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
https://www.electra.aero/
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.
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.
It is insane that we are not doing materials research on how to capture vacuum in thin cavities.
sightly terrifying
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?)
Seems like quite an engineering challenge with this new design...
Each of turbojets, turboprops and turbofans generate thrust with exhaust.
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.
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
"jet" -the story of a pioneer by Sir Frank Whittle
https://www.amazon.com.au/Jet-Story-Pioneer-Pioneers-Aviatio...
If these fans have blades with anywhere near the same kinetic energy, I would be nervous.
They were very lucky that only one person died.
[0] https://en.wikipedia.org/wiki/Southwest_Airlines_Flight_1380
On something like a New York <-> Los Angeles flight I cannot imagine the turboprop beats a 737 in any performance or comfort category.
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.
They could also use active noise cancellation, which is already used in some turboprops like the Q400.
With all seriousness, I am thinking whether there are parallels between this proposed plane and the Q400.
Commercial engines are not designed to have anything to supersonic.
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.
https://books.google.com/books?id=rgAAAAAAMBAJ&lpg=PA69&dq=t...