The next steps for Airbus' big bet on open rotor engines
aerospaceamerica.aiaa.org
aerospaceamerica.aiaa.org
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.
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 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
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.
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.
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...
It is insane that we are not doing materials research on how to capture vacuum in thin cavities.
Assuming 1mpg for the entire train, it needs at least 100 passengers to compare to a fully-occupied passenger plane.
The point is that it makes air travel ludicrous from an energy perspective where rail at high speeds (200mph) is possible
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.
Train travel is so efficient that running nearly empty trains is just accepted.
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.
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
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.
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...
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.
Big slow props, biplane wings, take twice as long, serve better coffee, give me somewhere to plug in my laptop charger.
I'll even wear a nice suit. Make flying An Occasion again.
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.
https://books.google.com/books?id=rgAAAAAAMBAJ&lpg=PA69&dq=t...