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Double Fan Turbofan Engine

More fan, less ground clearance issues
  [vote for,

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.

bs0u0155, Feb 24 2020

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]


       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 ?
8th of 7, Feb 24 2020

       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.
bs0u0155, Feb 24 2020

       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.
neutrinos_shadow, Feb 24 2020

       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.
8th of 7, Feb 24 2020

       Consider a timing belt (instead of a gear) to reduce the wear increase and efficiency loss inherent in a gear.   

       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...   

       For maximum fan size, "simply" make the wings flap.
sninctown, Feb 25 2020

       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.   

       On the other hand you'd indeed get great fuel economy right up to the point of failure, crashing and dying.   

whatrock, Feb 25 2020

       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.
Voice, Feb 25 2020

       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,   

       Having the engines widely separated, out on the wings - as on a civil aircraft - is not an option for them.   

       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.   

       However, since the mission profiles are totally different, such comparisons are meaningless.
8th of 7, Feb 25 2020

       Make the turbine run a generator, and then elecrically power hundreds of ducted fans along the front edges of all of the wings.
pocmloc, Feb 25 2020

       The weight penalty would make that infeasable.   

       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.   

       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.   

       // great fuel economy right up to the point of failure, crashing and dying. //   

       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.   

       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.   

       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".   

       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 ...   

       What's wrong with this picture ?
8th of 7, Feb 25 2020

       Surely we're approaching this all wrong. We want as big a fan as possible, delivering bypass air as slowly (within reason) as possible.   

       We also have the fact that aircraft bodies are essentially very big tubes.   

       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.
MaxwellBuchanan, Feb 25 2020

       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.   

       The Heinkel "Lerche" <link> was a derivative; a VTOL tailsitter with a tubular wing.
8th of 7, Feb 25 2020

       Or have the passengers sitting on the outside
pocmloc, Feb 25 2020

       Baked. <link>   

       In a bizarre twist of fate, it was an original series 737, and landed safely.
8th of 7, Feb 25 2020

       //Consider a timing belt (instead of a gear) to reduce the wear increase and efficiency loss inherent in a gear.//   

       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.   

       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.   

       //At the moment, a single engine civil airliner wouldn't be certifiable for ETOPS//   

       Because it's Extended range TWIN engine Operation Performance standards.   

       //but that's just something humans have made up,//   

       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.   

       //systems that have a single point of failure with catastrophic consequences//   

       Single AOA sensor, single rudder, etc.
bs0u0155, Feb 25 2020

       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".   

       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.   

       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.
8th of 7, Feb 25 2020

       // So, stick the big fan at the front of the fuselage... //   

       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.
whatrock, Feb 26 2020

       Presumably the large fans are still ducted and the gear is hidden inside the duct?
sninctown, Feb 26 2020

       //the AOA sensor can be compensated by a technique that many pilots know called "looking out of the window".//   

       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.
Voice, Feb 26 2020

       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.
8th of 7, Feb 26 2020

       //the large fans are still ducted and the gear is hidden inside the duct?//   

       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.   

       You remove the ducting and you get a turboprop/propfan.
bs0u0155, Feb 27 2020


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