h a l f b a k e r yYeah, I wish it made more sense too.
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The turbofan jet engine is a popular choice in the jet
engine buying community. It is fundamentally the same
as
a turbojet, but with a dramatically enlarged 1st stage
compressor, the fan, that moves a lot of air around
rather
than through the core. Moving lots of air slowly
compared
to
a little air very quickly turns out to be much more
efficient for most flying.
The downside of increasing the fan size is that the fan
size is increased. This is a major problem when the size
available isn't as large as the size needed. Such problems
occurred in the design of the 737 max, where the large-
fanned engines were placed high and forward,
compromising the aerodynamics. The size of fans is also
the most important reason behind why the B-52 was
never re-engined with modern turbofans. There would
not be the required clearance.
This issue clearly needs solving since it is inevitable that
airlines realize that there is money to be saved by
moving
from twin to single-engine operations. The fan on the
single engine hauling retrofitted 777s over the Pacific in
2028 will need to be extremely large.
My solution is to have two fans, side by side driven
through gears. Geared turbofans are already 1/2 baked.
and this
idea will add to that success by increasing the parts
count.
The two fans will, most importantly, look really cool.
Large
aircraft look cooler in proportion to how many engines
they
seem to have, see B-36 vs A330.
Huuuge Fans
https://en.wikipedi...oyce_Trent#UltraFan
[bs0u0155, Feb 24 2020]
Fractal Radial Engine
Fractal_20Radial_20Engine
Fishy [sninctown, Feb 25 2020]
Heinkel Lerche
https://en.wikipedi...wiki/Heinkel_Lerche
"The aerodynamic principles of an annular wing were basically sound ..." [8th of 7, Feb 25 2020]
Aloha Airlines Flight 243
https://en.wikipedi...Airlines_Flight_243
The infamous "verandah" service ... [8th of 7, Feb 25 2020]
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Will one of the fans be coaxial with the gas generator, and the other offset, or will the jet be positioned in between the fans, like the pinch of the hourglass ? |
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I thought of both, I like the symmetrical design the best.
Sort of like a single-engine dual intake fighter arrangement,
only with more biggerer fans. |
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Geared drives from turbines are completely baked; but, as
you say, not sure about turboFANs specifically. So there's
really no reason that this wouldn't work. |
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The advantage of an offset design with the driven fan at a 45 degree up angle is that it would produce direct airflow over the wing, like a turboprop, giving immediate lift rather than having to gain airspeed after throttle-up. |
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Consider a timing belt (instead of a gear) to reduce
the wear increase and efficiency loss inherent in a
gear. |
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Also consider a fan-based variant of my "Fractal
Radial Engine" concept. If two fans is good, it stands
to reason that 2^n fans might be better... |
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For maximum fan size, "simply" make the wings flap. |
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While the "twin intake single engine" approach would
indeed look more cooler I'm wondering if airlines would
dare design a single-engined large passenger jet like the
777. Currently, if one engine craps out you've got a
second to get you home, but if your one-and-only engine
craps out, you're fucked. |
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On the other hand you'd indeed get great fuel economy
right up to the point of failure, crashing and dying. |
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I read that it was determined an F-35 is more reliable with
one engine than two. This is because if an engine fails there's
a good chance it will take the other engine with it, and the
most common causes of failure are mechanics. So if you have
only one engine you have half the number of human errors
that can happen to it, and the extra engine doesn't make up
for that difference in risk. Or it could be a long-winded
excuse. Who knows. |
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In a fighter aircraft like the Eurofighter the engines are placed side by side for reasons of aerodynamics, protection, and to some extent stealth. Two engines do give a modicum of redundancy, |
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Having the engines widely separated, out on the wings - as on a civil aircraft - is not an option for them. |
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But transport aircraft - often based on civil airframes - do have outboard powerplants. It makes maintenance easier, redistributes the wing loading, and if a fan disc bursts then the chances of a cascade failure are minimized. |
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However, since the mission profiles are totally different, such comparisons are meaningless. |
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Make the turbine run a generator, and then elecrically power hundreds of ducted fans along the front edges of all of the wings. |
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The weight penalty would make that infeasable. |
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In land transportation, where all-up weight is not so critical, electric transmission is a viable alternative to a mechanical or hydraulic gearbox; but in an aircraft, it adds mass for no significant benefit. |
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For solar powered aircraft, where your energy is already electrical, then it's fine. But every transformation (mechanical to electrical and back to mechanical) incurs an efficiency penalty. If your energy source is mechanical (gas turbine), and the final use is mechanical (propulsion) then an intermediate system in such a weight-sensetive application is bad engineering. |
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// great fuel economy right up to the point of failure, crashing and dying. // |
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At the moment, a single engine civil airliner wouldn't be certifiable for ETOPS, which is where the money is; but that's just something humans have made up, nothing to do with physical law. There are single engine aircraft that are routinely used for commercial passenger transport. |
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Since in many fields humans are perfectly satisfied with systems that have a single point of failure with catastrophic consequences (cable cars, some bridges, etc.) the issue is merely one of user perception. |
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As a wider issue, humans are pathetically bad - both personally and collectively - at understanding, assessing and comparing various forms of risk and threat, summaized as "Every day things fail to go wrong confirms our unjustified belief that everything is all right". |
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Humans build cities - major cities - right next to known active volcanoes, on top of known active tectonic faults, on shorelines vulnerable to tsunami and storm surge, and in tornado-prone areas. When those cities are damaged, they are rebuit on the same site ... |
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What's wrong with this picture ? |
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Surely we're approaching this all wrong. We want as big a
fan as possible, delivering bypass air as slowly (within
reason) as possible. |
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We also have the fact that aircraft bodies are essentially
very big tubes. |
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So, stick the big fan at the front of the fuselage, and make
all the passenger seat-backs taller and wider to serve as
windbreaks, and turn the rear end of the fuselage into the
outlet. |
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A tubular aircraft, in effect a tubular "flying wing", was proposed a century ago - by non other than Henri Coandă. There's nothing wrong with the concept, it was simply that his idea was way ahead of the engineering capabilities of industry in that era. |
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The Heinkel "Lerche" <link> was a derivative; a VTOL tailsitter with a tubular wing. |
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Or have the passengers sitting on the outside |
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In a bizarre twist of fate, it was an original series 737, and landed safely. |
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//Consider a timing belt (instead of a gear) to reduce the
wear increase and efficiency loss inherent in a gear.// |
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OK, I'll consider it. It does have a bit of Wright Brothers
charm. So, the belt drive on the motorcycle in the street
below me is about 30mm wide for a, say, 50kW engine. A
good-size fan will run at say 50MW so that's a simple 10^3
scale-up. Our belt drive will be 30 metres or so wide,
might ruin the view for the passengers in the forward
cabin. |
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Other issues include low service life, unsuitable
temperature range, susceptibility to water/oil/de-icing
fluid contamination and most seriously: speed. Reading
around, its recommended to keep belts below 50m/s. A
10cm pully on shaft running at full-chat take off power
would be moving at 23550 m/s, somewhat challenging.
No, a proper gearbox I think. Best practice would be to
speak to the chaps/chapesses down in the turboprop
division, they know their megawatt gearbox onions. |
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//At the moment, a single engine civil airliner wouldn't
be certifiable for ETOPS// |
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Because it's Extended range TWIN engine Operation
Performance standards. |
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//but that's just something humans have made up,// |
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aaaanddd.. I just made up ESOPS. That was easy. Maybe
we can keep ETOPS and just convince them that we have
TWIN fans, and they're doing most of the work anyhow. |
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//systems that have a single point of failure with
catastrophic consequences// |
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Single AOA sensor, single rudder, etc. |
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You can lose the rudder and still land safely, altho losing the whole fin is more problematic; and the AOA sensor can be compensated by a technique that many pilots know called "looking out of the window". |
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Yes, there are always SPOFs, but many of them aren't catastrophic. A failed nosegear can be unpleasantly loud and expensive, but is pretty surviveable. |
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Thankyou for the nicely crafted rant about drive belts, by the way. It's substantially shorter and less technical than the version we were preparing, but still manages to cover most of the salient issues (except tensioning, elongation, and asymmetric wear in side-loaded bearings). Well done. |
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// So, stick the big fan at the front of the fuselage... // |
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This dual purpose layout could probably be made to
work, with a few little mods, but getting your then-
necessary double Scotch delivered by the drink cart
would be problematical. |
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Presumably the large fans are still ducted and the
gear is hidden inside the duct? |
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//the AOA sensor can be compensated by a technique that
many pilots know called "looking out of the window".// |
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No! Bad! Very bad! Aviation history is littered with the bodies
of pilots who looked out the window and thought a mountain,
line on the ground, road, or cloud bank was level ground. |
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Hence the necessity for AOA sensors and the artificial horizon. But when those systems fail (and they do fail, all too often) then external visual cues are essential. |
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//the large fans are still ducted and the gear is hidden
inside the duct?// |
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Yep, geared turbofans are a thing. They are a thing
because even the slower spool on turbines wants to spin
at thousands of RPM. That quickly turns into a problem,
take the GE90 which has a fan radius in the 1.65m range,
at max thrust it likes to spin at >2000rpm which means
the blade tips are starting to break the speed of sound
(343ms or so). If you are gearing your fan you lose some
power in the gearbox, lets say 10%. But, you can gear
drive two fans with no additional losses. So now you can
get a bigger fan area (9.8m^2 vs GE90 8.5m^2) with tip
speeds well below the supersonic using 2x 1.25m fans.
There is an added advantage in that tested/certified fan
structures of that size range already exist.
Testing/certification is everything in aviation. |
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You remove the ducting and you get a turboprop/propfan. |
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