January 3, 2020, ©. Leeham News: We continue our series why e in ePlane shall stand for environment and not electric.
Our target is to lower air transport’s environmental footprint and we can achieve this more efficiently by using established technologies. As an example, I will describe a very promising concept that has fallen out of focus due to the hype around everything hybrid and electric.
Thrust for an aircraft is developed by accelerating air which passes the aircraft. All propulsion concepts are fundamentally some kind of air pump.
The pump’s efficiency is decided by how efficient we can generate the driving rotational force for the pump (defined as core or thermal efficiency) and how efficiently we accelerate the air with a fan or propeller (it defines our propulsive efficiency).
The propulsive efficiency is increased when we accelerate a lot of air to a low overspeed relative to the surrounding air. This is why turboprop airliners have the lowest fuel consumed per passenger mile (and hence low CO2 footprint). They accelerate a lot of air to a low overspeed.
The high bypass turbofans build on the same principle. A large bypass stream has a low overspeed out the back (the technical term is low specific thrust). As thrust is: air mass-flow * specific thrust we need to accelerate a lot of air to keep our thrust when we lower the specific thrust to gain efficiency.
The drawback of a low specific thrust engine is the thrust falls faster with increased speed (it’s called a higher thrust lapse with speed). This is why turboprops cruise at around 300kts whereas jet airliners cruise at 50% higher speeds, around 450kts.
The increase in turbofan ByPass Ratios (BPR) from around 5 to around 10 in the last decade has given us engine fuel consumption improvements of 15% between engine generations. But the next decade can’t double the BPR once again to 20. The engine gets too heavy when a nacelle shall encapsulate such large fans.
The CEO of SAFRAN, Philippe Petitcolin, said before Christmas we can expect a 5% efficiency improvement by 2025 for designs using the turbofan architecture like the CFM LEAP (SAFRAN is a 50% stakeholder in CFM, the other 50% partner is GE Aviation).
While there will be continued improvements in turbofan core and propulsive efficiency over the next years, it won’t give us our expected 15% TSFC gains for the next-generation engines. We need to lower the Specific Thrust further to get there, beyond what architectures like the CFM LEAP or Pratt & Whitney GTF can give us.
Both SAFRAN and GE Aviation have researched how we can keep a 15% improvement for the next generation engines for the large fleet of short- to medium-distance airliners we call single aisles. It’s based on the next steps in lowering Specific Thrust by going to “Open Rotor” engines.
Here, a bypass ratio of over 20 can be achieved (really a lower specific thrust accompanied with an increased air mass-flow, the increased BPR is just the method to get there) without the engine/nacelle getting too large and heavy. SAFRAN ran a dual contrarotation design in ground tests during 2019 as part of an EU Clean Sky research project, Figure 2.
It demonstrated we can now design such an engine with acceptable noise levels. It also demonstrated solutions to the safety problems of an open rotor design (for instance lost blade, where you design to the turboprop safety rules).
GE Aviation through its Avio Aero subsidiary ran a sister Clean Sky program called IRON together with Leonardo. It focused on a simplified open rotor design, called a Single Unducted Fan, USF. This has the simplicity of a turboprop (one rotating fan at the cold end of the engine paired with a fixed de-swirling fan stage, Figure 3) but keeps the speed and efficiency of an open rotor.
We recently visited Avio Aero in Turin to discuss the engine. The project leader for the IRON project, Jesus Lopez Ruiz, described GE’s route to the USF and its intriguing characteristics. In short, it combines the simplicity and efficiency of a turboprop with the speed of a turbofan.
GE and Pratt & Whitney built Open Rotor engines in the 1990s (Figure 4), but the aircraft projects using them didn’t happen.
The main problem was the engines were too noisy. But GE didn’t stop the open rotor work after these projects. It continued the research to understand the sources of the noise and gradually worked out how to build the engine to fix the noise problem.
This knowledge was then combined with the idea to simplify the engine by turning it around and making the rear rotor fixed, Figure 2. This keeps the efficiency of the open rotor but simplifies the design. The fixed second set of de-swirling blades are variable in pitch but the complex contra-rotating gearbox is replaced by the simpler gearbox we know from turboprop engines.
Lopez Ruiz said the IRON project had confirmed the efficiency and speed of the engine at the level of an open rotor. It also confirmed the complexity of the engine could be kept at a turboprop level.
There is one drawback the GE USF shares with the SAFRAN Open Rotor design. It requires a clean sheet aircraft to be built around the engine concept.
The engines have a large diameter and the tips of the fans in an open rotor create vortices and these shall be kept away from the aircraft fuselage to avoid cabin noise and vibrations. Therefore, rear-mounted engines is the preferred configuration, like in the IRON project’s concept aircraft in Figure 1.
This, in turn, puts some challenges with the center of gravity and placement of the horizontal tail to control the aircraft in pitch. Leonardo found in the IRON project the three surface concepts known from Piaggio’s P180 Avanti provided an advantageous solution to the CG and pitch control problem of a rear-mounted USF (Figure 1).
The hype around electric and hybrid-electric aircraft has made all OEMs start ambitious research projects with demonstrator hybrid concepts flying over the next years.
At the same time, there is not a single project taking the USF off the drawing board to a flyable proof of concept. This is a shame, as I fail to see how the hybrids can deliver efficiency improvements (and, therefore, lower CO2) beyond what the USF can over the next decades.
It’s clear the hype has made us electric blind. Those who haven’t done the math think there will be an aeronautical Tesla popping up in the next years, and they don’t want to miss this train.
There is no train to miss. We lack stop-lights in the sky.
Thank you Bjorn. These articles are interesting and make a lot of sense.
Could a thin shroud be attached to the fan tips so it rotates with the fan to help with the noise?
Great article.I will need to read up further on this second OR engine design which I was not aware of.
Having said that of course A and B have just bought the 2 available regional aircraft builders -both with brand new models using the geared tan – so as you say ,they will not be in any hurry to upset the apple cart.
Neither will Russia as they too have new conventional aircraft.
Perhaps China with their ‘new’ MD 80 ‘knock off’.Not perfect of course but it is ‘modern’ and structured in the right way.
Thanks Bjorn, great to see an article on open rotors and unducted fans, and that development in that area is continuing. They are the limiting efficiency case for the recent trend in turbofan expansions.
Also the technology is applicable to electric propulsion, if that one day becomes feasible. So developing in this direction first with turbine engines, and then possibly later on with electric, seems like a good path forward.
It also opens up the market to clean-sheet aircraft designs that will push the technology forward more rapidly. The expense would be justified by both cost and carbon reduction.
Best wishes Bjorn (&Scott). Agree with your sentiments on spending so much energy on electric, flying cars.
It seems we have to invest in what the masses like to see, even if it doesnt makes sense environmentally. Throw in ‘earth’ and ‘grandchildren’ and physics/ logic is out the door.
Airlines play along, but have fleet departments people with excel dealing with reality.
Unducted CORC has been tested and a lot of research went into reducing the noise impact of engine pylon wake vortex hitting the fan. That makes the Piaggo’s so noisy compared to e.g. a B200.
The clean pull configuaration show by GE makes a lot more sense, but probably requires a T tail..
The location of the propeller disk in the Piaggo Avanti , well to the rear of the cabin makes its quieter inside that than the Beech 350 series and outside noise very similar, the distinctive note attracts attention. There is no pylon that you mention, as the engine nacelle is attached to the wing. Its the wing wake and the PT6 exhaust , which are ahead of the props, all combine to create a ‘square wave’ sound that people find very annoying.
GE might be on the right track to aviod an aft turbine counter rotating structure with pitch mechnism that sounds very expensive and problematic to get an expected 20 000 cycles on-wing Life.
Having 2-3 Engines tail mounted gives c.g. problems, I would not be surprised if a high winged airliner with 2ea big UDF’s + tail mounted small fuel cell driven combined APU/ UDF Engine for boundary layer acceleration making an additional fuel burn benefit.
One cash rich Aircraft builder has to be bold and go first paying both the Aircraft and Engine development, Mitusbishi+Honda or Embraer?
Turbo props , like piston propellor engines have had adjustable pitch blades for a very long time. Its a simple and very reliable method to do so, even simpler for non rotating blades.
The centre of gravity of a plane with rear engines is no more difficult than wing mounted engines, the length of fuselage forward of the wing is usually a much larger proportion than behind the wing.
It is more difficult to have variable pitch blades into the hot section than in the cold front especially if you have a reduction gear reducing blade speeds. You also gain alot by emininating the big nacelle and thrust reverser by having reverse pitch. Just look at the problems GE and Swiss had with the Coronado 990 CJ805-23 aft fan engine.
GEs UDF was a direct drive, no reduction gear.
The detail was given here
“What was unique in Adamson’s design,[GE36] which had been refined by engineer K.O. Johnson, was that in profile, the counter-rotating turbine stages were interlaced; the direction in which each row of blades spun was the opposite of the direction of the stages immediately upstream and downstream of it. The design had no stators, and the relative velocity between each stage was doubled. Counter-rotation effectively doubled the turbine’s rpm, so the turbine could be made smaller, simpler, and more efficient.
But think about the mechanics. The turbine blades that drove the aft propeller were attached to a solid shaft in conventional bearings. The turbine stages driving the front propeller were riding outside the aft set and could not reach a central shaft. The turbine blades were attached at the tip to an outer case, which was carried on inter-stage bearings and a ring bearing at the rear of the nacelle. This design had to allow for thermal expansion and the load imbalances that would occur if a propeller blade broke off.”
The project engineer for the Boeing 7J7 propfan project was Alan Mulally
Interesting that the Boeing fuselage concept was a twin aisle 2-2-2 layout.
“One week it was a single-aisle 90-passenger airplane, the next a 180-seat twin-aisle design. We saw things as being very fluid.”
Who said history repeats ?
Think the GE36 was a long way from being a commercial viable engine but a proof of concept engine they took to flight test. Still with higher Jet A prices and regulations new designs will come. In the old days the Air Force would lead the way and pay for super efficient UDF powered bombers. Now they buy last off the line PW4062 and CF6-80C2, GEnX2B engines (FedEx as well) and almost restarted JT8D production..
McDonnell Douglas offered GE36 or 578-DX powered MD-80 derivatives when the US Navy wanted to replace its 125 submarine detection aircraft in 1988, but Lockheed won the contract.
That’s why the European airlifter was another blown opportunity. I’m not familiar with all the details of that drawn-out saga, but I do know that the Germans and French were in favor of contra-rotating propfans at various times in the 1990s. After Rolls-Royce bought Allison in 1995, did RR even bother to compete the 578-DX against the TP400? Allison was the driving force behind the 578-DX (despite Pratt & Whitney’s top billing), and they designed its gearbox for a 30,000-hour service life. The 578-DX and TP400 both produced about 11,000 horsepower, but at 139″ the propfan diameter was much smaller than 208″ for the turboprop. PW-Allison had plans for a 16,000-hp version of the propfan, so there was more opportunity to adopt the technology for civilian application.
oops left off the reference
I always wondered what was meant by this statement. Is there something inherent about rear-engine planes that requires costlier maintenance? Or was the last part of that sentence not referring to the first part, and it just addresses the more expensive maintenance charges (at least initially) for the UDF?
Great option if noise levels meet the noise standards. Sounds like these are close. I wonder if they’ve done aircraft-level or only engine-level noise testing for these, though.
Most likely computer simulations verified in wind tunnels by pneumatic driven engines on scale models and on test stand. Hence a full scale UDF demonstrator 180 seat aircraft would be valuable (NASA found the money to order a SSBJ from Lookheed skunk works hence ordering a super efficinet demo aircraft from Boeing/Gulfstream)
While efforts to reduce fuel consumption, pollution and CO² emissions are laudable, a further 15% improvement will not be remotely enough to placate the climate alarmists.
Engine and plane makers, as well as air carriers, still think they can satisfy the Greens’ escalating demands, as they have not yet understood that they will never be able to do enough, even with offsetting all emissions.
The Flight Shaming movement and Greta Thurnberg’s histrionics are disturbing precursors of things to come.
Over the last 50 years, engines got 0.8-1% more efficient per year, 15% every 20 years. Air traffic grows around 4% per year. Do the math. You don’t have be a Greta, realism helps.
Even Greta will applaud a 15% reduction in Jet aircraft CO2 combined with automatic 4D traffic routes optimized for traffic and wind. So far the bigger bypass engines has also brought quieter and cleaner engines. One would think with the higher production numbers of jets that the engine makers would get a good return earlier and have money to design new engines meeting coming regulations.
How dae you.
My message is that we’ll be watching you. This is all wrong. I shouldn’t be up here. I should be back in school on the other side of the ocean. Yet you all come to us young people for hope. How dare you!
You have stolen my dreams and my childhood with your empty words. And yet I’m one of the lucky ones. People are suffering. People are dying. Entire ecosystems are collapsing. We are in the beginning of a mass extinction, and all you can talk about is money and fairy tales of eternal economic growth. How dare you!
For more than 30 years, the science has been crystal clear. How dare you continue to look away and come here saying that you’re doing enough, when the politics and solutions needed are still nowhere in sight.
You say you hear us and that you understand the urgency. But no matter how sad and angry I am, I do not want to believe that. Because if you really understood the situation and still kept on failing to act, then you would be evil. And that I refuse to believe.
When you mention that GE…
“gradually worked out how to build the engine to fix the noise problem.”
…does that mean that the UDFs are in the same ballpark (noise) as current engines.. even the GTF?
Geared turbofans are quieter than direct-drive open-rotor engines, but O.R. engines can be as quiet or quieter than direct-drive turbofans.
The problem with IRON is that it applies to aircraft in the 90-to-130 seat capacity range. This is probably because the technology involves a single propeller that does most of the work, with a second propeller that’s there only straightens out the swirl. To handle A320/321 or B737-8/9/10 airliners, the propeller diameter would likely be considered too large for the airplane structure.
I noticed in Figures 1 and 3 that the engines are mounted on the horizontal stabilizers, instead of being attached directly onto the rear fuselage (like on the Boeing 727 and the MD-80).
BTW, the noise issue was solved in the late 1980s for the GE36 UDF and the Pratt & Whitney/Allison 578-DX. The reasons why open rotor aircraft didn’t sell were because of reliability concerns and the low/declining price of oil, so the fuel savings couldn’t offset the higher price of the new engines.
Doesn’t the risk of a blade failure on a UDF with two rear mounted engines mean….both engines are eliminated quickly?
How is that to be workable in any future regulatory certification environment post-MAX crisis? Most especially since, any company committing to a new 150-250 seat aircraft must sell to it’s board/decision makers a $5 billion investment. The public will be even more dubious now, than ever, of any new configuration. Even the public that believes in AGW.
As an engineer, one can look at a new structure or device and ask, “What would it take to make this thing fail?” It seems to me that the open rotor pusher configuration might some day fling some shrapnel out of the engine, or a fan blade out of the fan, and cause a lot of havoc in all the rest of the aft neighborhood.
Compared to a ducted turbofan, the RPM is much less and the blades are composite materials with less mass. So blade failures are not as energetic or catastrophic. As Bjorn said, the idea is to move a larger volume of air at lower velocity, to get the same thrust.
Still the tail control areas would need to be armored or protected in some way, but not significantly more than is required for fuselage protection of standard turboprops. With swept propellers, the debris cone can be 25 degrees, so a larger area than 5 degrees for straight propellers.
“Both SAFRAN and GE Aviation have researched how we can keep a 15% improvement for the next generation engines for the large fleet of short- to medium-distance airliners we call single aisles.”
With a 15% improvement from the engine and perhaps 5% from the airframe and wing those aircraft would almost be long haul aircraft.
You could improve industry wide efficency if all but the highest urgency air cargo went by sea or rail. Thats a looming problem as the dedicated air cargo fleet is ‘mostly old’ and the volume of low value goods is increasing
With Open Rotor I’m always thinking about shoulder wings with engines on it. Why wouldn’t I want the rotor air flow on the wings to create more lift?
I’m confused to make a plane 450kn fast. A400M has 422kn speed. Is the single rotor on the A400M not fuel efficient?
Airflow and noise wise undisturbed airflow ahead and behind the engine is a boon.
Would a VFW 614 / Honda Jet like arrangement work for a prop plane? ( i.e. engine on overwing pylons.)
another solution could be one front engine and one rear engine. ( see Do 335 ) blade out on either engine can’t hit the plane at all.
It seems to be a chellenge where to put the tail with overwing engines and more than 5m prop diameter.
The Do335 is a great solution when only 2 engines are needed, but might be noisy with engines inside the fuselage.
The IRON project looks great but the dimensions are wrong. The props need to be bigger than the fuselage.
The A400M with counter rotating fan blades would be even more efficinet than with its present single stage prop TP400’s and a pretty good test aircraft for an UDF replacing 2 engines on the A400M wing. The latest CFD optimized designs makes for Stage V noice complance and good efficienty at airline speeds. Lets hope for more EU money and flight tests.
TP400 is a 3 shaft design- 2 stage gas generator with a low pressure power turbine stage which connects to the prop gearbox.
I think one of the advantages of the prop-fan design is rear blades which avoid a drive shaft from the rear turbine back to the front, cant see it being used as a ‘pusher’ on an A400.
The little prop engines like PT6 have the engine facing to the rear to get around the same issue , but the mass flow of the prop fan is so much greater.
Personally I don’t see an issue with shoulder wings. But Antonov, which traditionally designed planes with a high-wing configuration, attempted to design a twin-propfan airliner in the late 80s/early 90s, and they decided to use low-mounted wings, a conventional tail, and engines mounted on the horizontal tailplane.
https://maksiemens.livejournal.com/3620.html (use in a translation website unless you can read Russian)
Single-rotating open rotor engines impart swirl, causing an energy loss of about 7%. The aft propeller of a contra-rotating open rotor engine recaptures the swirl energy created by the fore propeller, so CROR is a little more efficient than SROR. (But SROR is probably still more efficient than even the GTF.) Similarly, with contra-rotation the back prop more or less balances out the torque caused by the front prop, so vibration is mitigated compared with single-rotation. And as the Progress D-27 shows, a CROR setup can produce more power with a smaller propeller diameter than for SROR.
In business, fairly small differences in quality are enough to get a 2nd-place competitor slaughtered. I think Apple’s second CEO used to tell Steve Jobs about how Coke would outsell Pepsi by 10 to 1, and Coke definitely was not spending ten times as much per unit on their ingredients compared to Pepsi. So a 28-knot shortfall in speed could be a big problem, unless regulations are changed to favor slower aircraft. However, the A400M being limited to 422kn may not mean the TP400 has that limit. The TP400 could probably be modified to run efficiently at 450kn as well.
I don’t have a problem with shoulder wings too, but they might be heavier with an additionally higher tail.
Low wings with connected landing gears might have advantages too.
I guess the most common Western high-wing planes are the ATRs, Q400, and A400M, and they all have T-tails. Antonov built many high-wing craft with conventional tails, though, including the now-certified An-70 quad-propfan airlifter and the initial configuration of the proposed An-180 twin-propfan airliner discussed in my link. I wonder what design tricks the Western OEMs missed by having to resort to T-tails (or conversely, what risks [if any] the Ukrainians are ignoring by mating conventional tails with shoulder wings).
A more conventional configuration remains on option I guess.
Are the USF engine concept compatible with Truss Brace Wing configuration ( SUGAR project from NASA )?
Pro : ease cg , lowspecific thrust associated with High CL/CD ,folding tips for airport compatibility
Cons: flutter issues ,level of vibration on truss braced
NASA studies all pointed to that setup with a GTF on the wing.
There is also a double bubble center trussed cabin (Boeing bought out the mfg purportedly for the drones they did)
Good ideas out there, will the Airlines buy it? That is an impediment as well. Airlines have to go all in and ensure the public is good with it.
When we see Regional jets take main stage because props are considered totally inferior, then there is an issue.
More weight in the rear, add a bit of armor and your efficiency increase for the whole package goes away.
Trying to incorporate two different mfgs type open rotor engine in that location?
We have seen the rear mounted commercial engine setup go to zilch over time. Embraer gave it up, BBD gave it up, China gave it up.
Ducted rotor on the wing? Maybe has some merit.
We also need a leap in structure, the engine have been the vast majority of the improvements .
When a 60s era 737 form can compete with a newer A320, we clearly have gone no where in aerodynamics.
The form hasnt changed for the 737, but every thing else has changed. New engines – twice , new wing, new cockpits. The A320 series dates from the 1980s, so it too is getting on 40 years or so.
You may have noticed even a Tesla has the ‘form’ of a 1960s compact car like the Nash Rambler with a wheelbase 110 in or under. Check out ships ‘form’ or even locomotives from that era too.
Beech ‘King air’ derivatives keep on selling , but the light planes that went a different was like Starship , Learfan and now the Piaggo Avanti have failed or faltered. The Avanti has Bill Lears design genius fingerprints on it as well
The Avanti appears to be a beautiful design with numerous advantages, but also known for lack of support and risk of airport denial due to decreasing noise standards, especially on the West Coast.
So more of a spirited but temperamental thoroughbred than a reliable and well-supported workhorse. I’m sure that has kept its production and sales low.
Very interesting design though, with distribution of lift across front, mid, and tail surfaces, and the large passenger cabin.
You’re right about markets converging on a basic design that works well, and then maintaining them with incremental innovative advances. Entirely new designs need really substantial advantages to make them competitive against an established market.
Federal Airport Noise and Capacity Act of 1990 no longer allows airports other than Santa Monica to ban aircraft like they did. Santa Monica has heavy restrictions on all types of planes, so its not really a surprise they are an anti noise hotspot ( for good reasons)
Places like Naples and Aspen might not like it but the locals cant stop them coming in.
The list of ‘banned’ aircraft at Santa Monica is long includes a lot of common types , including GIII models and other older jets
Thanks Duke, that is a long list! It may be that the distinctive square-wave note is just harsher on the ears, even if not measurably louder.
Trouble is also with the demand side.
Fuel savings from the MAX/NEO have made more routes viable and made it possible for more passengers to travel with more of their junk. Certain airlines like to boast about how new and efficient their airliners are, but unless they scrap the old ones instead of moving them on, it makes no difference to global co2 emissions. Only taxing fuel can make any real difference.
“… but unless they scrap the old ones instead of moving them on, it makes no difference to global co2 emissions. ”
At the end down the food chain they still replace older frames.
Same process that makes exporting less clean cars from Europe ( instead of scrapping them “polluter!” ) a good thing (TM) as at the end they replace even dirtier jalopies at their final destination.
( Counter: the electronics infested newer products are decidedly more costly or impossible to keep in running shape.)
I can’t understand your logic, the global fleet is still the global fleet. Airliners die mainly because maintenance or fuel becomes too expensive.
Current aircraft designs, with rear-mounted engines, and a T-tale, are very uncomfortable for passengers and crew, due to the high level of noise and vibrations generated in the rear section of the aircraft.
There is another downside to this type of design, any landing gear debris, can easily impact the engines.
As stated above,the optimal sector for the propfan is in the 100-150 seat regional marketplace.However that is also the sector under investigation for electric aircaft.
Just wondering,perhaps these technologies could be looked at in conjunction with some form of hybrid solution.
It is also looking increasingly likely that Boeing will drop the ‘MOM’ in favour of a NSA circa 200-250 seats.I think it’s very hard not to see this aircraft being powered by anything other than a second generation (x5 gearing?) geared fan, primarily from GE as they are Boeing’s prime engine supplier.
No doubt ( in terms of environment) further gains will be made up by the new airframe to reach the 15% improvement.
The trouble with that is GE has said no new tech engine from them for a NMA from Boeing. So it means its reused tech from the Leap, GEnx and GE9X, which still makes it still very interesting . No geared fan from GE, maybe from P&W though.
I am not sure they will have a choice!
If ( as is increasingly likely) Boeing do finally accept the need for a new 200-250 NB aircraft they will ‘have’ to up their game.Obviously the new aircraft will have a better range capability which is where a HBP geared fan scores heavily.
They have bought the required gearing technology’s and their existing core is world leading.They will not hand the market to P&W.
It’s not ( imho) going to happen overnight.2020 will be about getting the 737max back into service and restarting manufacturing.But mid 2021? Even 2022,they will probably announce a replacement for EIS 2028/9.
Public hates propellers, so they are not going to be less than thrilled with anything open rotor back
And so far while it looks good on the bench when it comes to a full install on the back , it wish more and thou still have that broken poop penetration issue (did not the shuttle think the light foam was no issue?)
This one has been the perpetual engine of the 80s on, still not viable. I suspect it will still be the next big think when I am pushing up dirt in 30 or 40 years (I hope)
you have a broken poop?
Nobody, really NOBODY wants to know that,
don’t post it here,
talk with your dog, he will understand you
The one thing we’ve learned about the traveling public over the past 20 years is that they will take the cheaper flight. Other thing being equal (route, flight time, schedule) a propfan flight costing $50 less than a jet flight will be a winner.
Bjorn -anybody,a question.
In the 60’s-70’s a huge range of rear engined aircaft were successfully launched by just about everybody both twins and ( a few) quads.Then it just died/went out of fashion-whatever –
I wonder why.
The reason I ask is that if Boeing have to replace the venerable 737 ( with its historic 1950’s 707 nose!) then they will need to futureproof it from future engine developments .Open Rotor is clearly one very possible development or indeed even higher bypass geared fans.
Either you build it with v long and therefore heavy MLG or you free them main wings and move the engines to the rear once more.
Would this be a bad thing?
Someone else can explain in more detail why rear engines faded away, but by the late 1960s Boeing had already come to the conclusion that wing-mounted engines were the most optimal. At the request of the US government, they shared this theory with Soviet engineers in exchange for info about how to work with titanium.
For some reason, this first-hand account from Boeing didn’t mention its triple-aft-engined Boeing 727, which held the all-time airliner sales record into the mid-1980s.
See my reply to Phil. It doesn’t mean the differences are so large it rules out rear-engined configs. So rear-engined airliners still have a play when other factors like the best place for an Open Rotor engine chips in.
It’s very much about the structural efficiency of the aircraft. It’s advantageous to have heavy weights attach where you have lift, hence fuel in the wings and engines there. Also with engines on the wings, you colocate in good proximity the forces (and hence structural reinforcements) of the engines and MLG. Engines on the wings degrade maximum lift somewhat compared to clean wings and engines at the back. But to have the engines sitting at the end of the tube and the MLG in the middle creates stresses at hard landings (stress is weight) and between wing and engines at gusts. So engines on wings is at the end of it all the most efficient config, all factors counted.
Phil, the rear mounted twinjets were viewed at the time as being safer for foreign object ingestion, and the clean wing allowed lower takeoff and landing speeds. The downside was excessive noise for rear passengers and the need for the heavier T-tail, and the stall prevention techniques that went with it.
Another advantage was the adoption of a third engine in the tail to make a trijet, which became allowed (by evolution of engine technology & ETOPS) for longer flights over water, with better fuel economy than the quadjets which had previously been required.
As engines & ETOPS evolved further to allow twinjets to fly the same longer routes, the trijet became the more expensive option, and mounting the much larger and heavier turbofans in the rear became problematic, and they went back under the wing as twins, which is the currently favored design.
So you are right that this issue will need to be reconsidered once more, in view of a possible resurgence of propfans. The engine nacelle itself can be much smaller, but clearance and fuselage/tail protection must be allowed for the larger fan.
The same tradeoffs may arise again, with some differences. The propfans would need to be lighter than the current turbofans, if they are mounted on the rear. If on the wings, that may favor a shoulder or mid-wing design. That changes many of the safety aspects of the design, if you don’t have a wing/wing box cushion beneath you in a crash, or you have fuel being spilled into the cabin from the higher wing.
All things that will have to be worked out in the ultimate design. One thought is a conventional wing & either single or twin tail with cruciform engine mountings. That solves some of the rear-mounting issues but creates a big one for CoG and tail structure. Somewhat lessened if either the cruciform mounts or the stabilizers became larger lifting surfaces. But still may be impractical.
Rob:Agree with your comments except the certification issue relative to the higher wing config for USF engine.
when you say “fuel being spilled into the cabin from the higher wing” :I think about the A400M config with a fuel central tank above the” cabin”. A400M has been certified in both civil and military category. There are likely many issues behind this civil certification point but a military aircraft should also usually stand for higher vertical acceleration ( hard landing , maneuver,..)
In figure 1 of this very interesting article I am skeptical about the weight penalty of the rear mounted engine pylon linked to the horizontal empennage and associated CoG issue. Vibration level on the highly critical horizontal empennage is also a concern.
Again USF engine installed on Truss Brace Wing (TBW SUGAR with high Cl/Cd ratio) config looks for me very promising if from a structural efficiency point of view flow interference between wing strut and propeller airflow does not lead to an important weight penalty . Are there some studies on this issue?
Thank you both for such good answers!Loved the linked article too.
Imho the MAX is not a ‘bad’ aircraft and will be made fully flight worthy.
However it does need to be replaced as it simply cannot meet the requirements ( range or capacity) for future narrowbody missions.
So we will see a new design from Boeing circa 2022.Just have to wait I guess.
Bjorn, you are going into fantasy land, even the Clean Sky IRON project aircraft model does not make sense in several ways.
It is PR, as typical of PC fad subjects. Rational voices will be ignored in the sky of blather.