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.
Sounds like a helicopter is not very efficient?
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:
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.
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.
> 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.
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.
Instead of reinforcing the fuselage, I wonder if just having a 1/4 nacelle that shields the passenger side would work.
>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.
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
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.
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.
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.
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.
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.
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.
Sounds like a helicopter is not very efficient?
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/
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.
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.
> 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.
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.
Instead of reinforcing the fuselage, I wonder if just having a 1/4 nacelle that shields the passenger side would work.
>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.
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
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.
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.
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.
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.