Why are turbine engines flat at the bottom?

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TJLanza@reddit

I hate to do this outside of a Star Trek group, but you are so confidently wrong.

Warp core is a misnomer, a colloquialism. The proper name is the matter-antimatter reactor assembly. It's called the warp core because the warp drive - which is very much the components in the the nacelles - is the single most power-hungry device aboard the ship. Also, because "matter-antimatter reactor assembly" is such a mouthful.

There are lower-performing ships that don't have a MARA but still have warp drive, and even some high-performing ones that do (some Romulan ships extract energy from a quantum singularity).

During “non-warp” time, it’s driven by the impulse engines, which are the little orange things on the back end of the saucer.

🤓🤓🤓

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IncredibleVelocity4@reddit

I hunt elk near a small Washington town called Naselle.

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Spaceinpigs@reddit

The starboard one is always causing problems

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Kidvette2004@reddit

Ones on the Discovery (in the future) are detached

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Hunter_Vertigo@reddit

Processing img faqlcl0gwdwg1...

"Sir there's an issue with the port nacelle. I'll need to bring it down for maintenance."

Once you change that, you’re pretty much changing what the entire plane is, and if you do that, you have to get all different kinds of recertification’s AND recertifying all your pilots, because at that point , you’re no longer flying a 737 max.

So in order to avoid all that, the new larger engines were moved slightly forward (and now higher ) on the wing of the 737 max. But as some of you reading might’ve already realized, now when you apply power to these engines, it makes the airplane want to naturally pitch UP even more.

Boeing knew this, so they installed a system in the airplane to counteract this. In the event that the airplane is uncontrollably pitching up, the computer will automatically counteract it and push the nose DOWN. You can already guess what happened.

Something went wrong and the airplane kept pushing the nose down when it shouldn’t have been. The pilots that were in this situation didn’t know what was happening and weren’t trained to disable the system in case of failure.

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747ER@reddit

Unfortunately most of this is not true. The 737 has had software that does the same thing as MCAS since October 1982, and most modern airliners have software that augments the flight controls for ease of handling like MCAS does. It’s not unique to the 737MAX, nor is it inherently dangerous if implemented correctly.

The problem was the way MCAS was designed, not the fact that it exists. It relied on only one sensor for disagreement logic, which meant that if you were a negligent airline that doesn’t bother fixing important broken sensors on your planes, then the software had the potential to do what you described. Keep in mind that similar software works just fine in the Airbus A320, Boeing 737 Classic/NG, and of course now the 737MAX after it has been fixed.

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JKT-PTG@reddit

Did the Ethiopian Air plane have a faulty sensor?

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747ER@reddit

Did the Ethiopian Air plane have a faulty sensor?

It’s not known, because Ethiopia produced such a biased and dodgy final report. They didn’t bother investigating any issues with Ethiopian Airlines because they didn’t want to find them responsible.

And there's still the issue of pilot training for that specific system being very lacking.

Well no, Ethiopian Airlines ignored Boeing’s urgent safety bulletin about this system. Those pilots should’ve had more than enough training to resolve that issue, and in fact, they correctly followed Boeing’s checklist when the failure happened. Then for some inexplicable reason, they undid everything they had just done, which made the nosedive worse and ended up being what killed them. Keep in mind that the NTSB found that the plane was still recoverable if the Ethiopian pilots had followed their training. As for LionAir, that same plane had nosedives multiple times that same week, each time the pilots followed the correct procedure and landed safely. If there was no procedure to resolve this issue, then how did all of these pilots follow the same procedure?

The “lack of information” thing was relevant, but vastly overplayed by the media during the early stages of the investigation. It only applied to the LionAir flight, and again, even if there was poor communication from Boeing, those LionAir pilots should’ve followed their generalised training and that would’ve saved them. But thanks for the comment, it’s important that people say these incorrect things so it gives others the opportunity to read what was wrong and what the correct answer is.

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alton_britches@reddit

"As for LionAir, that same plane had nosedives multiple times that same week, each time the pilots followed the correct procedure and landed safely."

747ER@reddit

This doesn’t conflict with what I said. The person I replied to said that it was compromised in both “design ethos, and application”. That is the statement I disagreed with.

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Rude_Internet1526@reddit

Why didn’t they just make the landing gear longer/taller?

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railker@reddit

They actually did for the 737-10, as that version's too long for the default 737 gear. Once that plane gets certified and sees service, you'll see 737s with a telescoping gear system.

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ramii99@reddit

Sure, it was a massive flaw introduced by how that system was designed (and by not mentioning it - cause otherwise the pilots could have recovered by using the "stab trim cut-off" switches). At least, it's been fixed, but at what cost... Just wanted to point out that the plane didn't need the system in order to be stable.

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Helpful_Equipment580@reddit

It's so annoying that the narrative of MCAS being required to stop 737 MAX stalls is so prevalent.

It was reported in almost every story about the MCAS crashes. One of Mentour Pilots videos about the MCAS crashes was the first time I heard an accurate description of the MAX and MCAS.

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Stoney3K@reddit

It's probably an important factor that Petter flew the MAX as a daily driver for Ryanair.

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joesnopes@reddit

So ... MCAS was another disaster that McDonnell Douglas brought to Boeing.

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jumpinjezz@reddit

I'm guessing the way C-17s are thrown around doing combat landings and take off mean their MCAS was probably tested a bit more. Reversing thrust at FL20 is a neat trick.

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fighterace00@reddit

That and the single point of failure for the notoriously quick to fail AOA probes which Boeing only provides one and the cockpit notification of failure is an extra upgrade.

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railker@reddit

Wasn't it only for just one specific scenario though? Super high AoA flaps up stall in manual flight (it doesn't activate with autopilot on or flaps down, we learned that from the crashes). Don't think it does anything to the airplane during any part of normal flight that weight and balance would've changed.

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the_Q_spice@reddit

Part of the issue was also that the LEAP has significantly more thrust than the 56. The CFM56-7B the 737NG uses produces 20,000-27,000 lbs of thrust whereas the LEAP-1B25 through -1B27 the MAX uses produces 25,000-29,300 lbs.

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autech91@reddit

The airplane equivalent of rolling the guards to fit some phat 20" rims

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mvpilot172@reddit

It was also designed to sit low enough to use stairs instead of a jet bridge. We are still dealing with 1960’s designs.

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VonThing@reddit

Too much design change = new aircraft type = new type approval required, pilots have to be re-certified etc…

Just to avoid different behavior on one specific maneuver, Boeing tried to solve it in software but didn’t inform the FAA, airlines or pilots which resulted in the earlier 737 MAX catastrophe.

The stupid part is, there’s a “runaway trim tab” procedure for the 737 but since pilots didn’t know that a system that needed to be triple redundant worked on a single input, the planes crashed until the crew could recognize the runaway electric trim tabs.

The stupider part is, captain’s and FO’s airspeed readings come from different pitot tubes so if one is faulty they will see different airspeeds. This is free.

But the “IAS DISAGREE” warning that comes on when the airspeed readings are different is a paid option on the 737 so most airlines don’t order it.

bp4850@reddit

Both statements are true. The JT8D is small in diameter, and the 737 was designed to be operated with little to no ground support equipment.

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Snobben90@reddit

Yeah they are true but the newest 737 aren't that low because of a design choice, but rather a certification choice.

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can-opener-in-a-can@reddit

So you’re saying, “Clearance, Clarence.”

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tolino978@reddit

Afaik they actually had to move the Auxilliary Gearbox to the side to make it work

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The original 737s were powered by low bypass turbofan engines. These engines were relatively round and narrow and looked like cigar shape. They fit nicely under the low slung 737-100/200; they were a capable engines and had good performance, but along came the era of more efficient and more powerful high bypass turbofan engines.

The high bypass engine has a much larger inlet nacelle to house the larger fan. To make the newer high bypass CFM-56 engine fit under the wings of the low rider 737, they had to squeeze out some ground clearance by flattening the the inlet nacelle of the two engines slung under the wings of these second generation/Next Gen 737s.

Has nothing to do with the engine. It’s because the aircraft is so low to the ground they do that to avoid ground strikes. Notice they don’t all look like that.

THIS is the aircraft I flew into retirement. No such design feature.

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Far-Yellow9303@reddit

To add to the above comment:

The jet engines are the main source of power for almost everything on a plane and are absolutely festooned with equipment. Things like hydraulic pumps, fuel pumps, alternators... these would usually be wrapped around the engine in a neat packet. But on the 737 it's so close to the ground they don't put any equipment underneath, pushing it to the sides instead. This makes the engine appear flat on the bottom.

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BadahBingBadahBoom@reddit

To add to both these comments:

Whilst the 'why' these engines are flat-bottomed ('chipmunked'), it's specifically because these modern variants of engines are larger than the smaller 'cigar tube' shaped ones that were originally developed for the 737 back in the 60s.

That's why the landing gear being so short wasn't seen as a problem back then - actually this was a bonus as it meant passengers could disembark in smaller airports using the inbuilt stairs in the plane doors rather than relying on airport equipment.

Why are the modern ones bigger then? Efficiency. Engineers realised it was much more efficiency for most of the air to actually be pulled in and go around the combustion chamber of the engine. So to do this they made the intake much larger (called 'high-bypass turbofan' design). Unfortunately for planes with wings already so close to the ground this caused problems as there a limit to how tall the engine could be before it risked scrapping the ground.

Hence the readjustment of some of the components around the engine at the bottom to be moved to the sides giving the fattened 'chipmunk' design we see.

(This also played a part into need for MCAS in the even newer 737 version, the 737 MAX, as those engines were had even more bypass and were even larger and sohad to be moved higher and more forward on the wing causing inherent minor changes in plane handling.)

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Stoney3K@reddit

On the 737 the stubby landing gear wasn't even a "bonus", it was a design requirement as the aircraft was intended mostly for domestic flights in the US from regional airports, most of which didn't have jet bridges or sometimes even concrete runways.

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CaptainSnacks@reddit

Just to add a clarification that threw me for a loop - the original 737 had that requirement! By the time the Classic series was introduced in the mid-80s, most 737 airports had paved runways

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Neuvirths_Glove@reddit

Contrary to what you say, this configuration does not make the structure more efficient. On the other hand, this structure, called a wing box, takes quite a lot of space. On passenger aircraft, this space is mostly available below the floor, where the hold is.

Therefore, you can fit that structural part within the fuselage, making it stronger and more aerodynamic.

On the other hand, should you need high wings, there usually is a protrusion on top of the airplane and the space available is smaller.

😉

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Ataneruo@reddit

haha, well done 😅

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timelessblur@reddit

far from more words. It explain what they are doing to help get that space. I honestly really like the extra info as I always though they just made it flatter for ground clearance but it messed with the aero dynamics. I didn't know they moved a bunch of other crap around as well.

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seva2_0@reddit

True! I always wondered tho, why they decided to go with a fan case mounted gearbox instead of a core mounted one, regarding the 737's hight.

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arvidsem@reddit

Like the B-52, the 737 will never be retired. Unlike the B-52, fuel prices matter for the 737; so we keep getting newer bigger engines.

There are several advantages to having the engines below and forward of the wing. One of these is some bonus lift due to the high pressure area behind the engine.

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Jessie_C_2646@reddit

Short Brothers already figured out the aerodynamics.

Hmm….don’t recall saying communities…..but I agree. Wouldn’t be good.

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tylerscott5@reddit

737s are the only ones we see this on. They are low bois

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Lucy_4_8_15_16@reddit

Stupid question about the picture/caused by it does the 737 have her stabilizer higher relative to the main wing than the a320 cuz it looks almost higher of the ground that the airbus’s

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FlyJunior172@reddit

The A320 has a height of 38’7” from ground to the tip of the vertical stabilizer.

The same measurement on the 738 is 41’2”.

I can’t say for sure if the reasons I can think of are the reasons why Boeing actually decided to do that but the 737 was designed around small rural often dirt or gravel airfields. The ability to land on these fields requires certain flight characteristics that you don’t need on aircraft that are designed around what the vast majority of our modern airports are.

And those flight characteristics are based around, needing to fly slower in order to be able to takeoff and land on those shorter runways. And because control authority is proportional to area, a larger, vertical stabilizer with a larger rudder on it will give more control authority at those slower speeds than the smaller one on the A320 series

But again, while all of the above is true, I do not know if that is the specific reason why Boeing selected the proportions that they did for the vertical stabilizer. It is the reason why they selected the landing gear height with the original JT8D engines. That combination of the low landing gear height with the JT8D enables baggage handling for the 727 and 737 with no equipment on the field.

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danit0ba94@reddit

All i see:

Exact_Package_7264@reddit

do ground strikes usually take engines out of commission or are they more of an annoyance? or both maybe? lol

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Jessie_C_2646@reddit

A larger problem is FOD. Engines which are too close to the ground suck up all sorts of things which aren't good for engines.

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saberplane@reddit

How do we know it's not good for them?

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danit0ba94@reddit

Processing img pa7ve7tivdwg1...

that jet was designed in the 60s

zephyrprime@reddit

it's too big*

It’s mainly a 737 thing rather than turbine engines in general. The aircraft sits quite low, so when they upgraded to larger diameter engines they had to reshape the nacelle to maintain ground clearance.

You see the same kind of compromise in other areas too intake shape, mounting position (slightly forward and higher on later variants), all to avoid redesigning the whole landing gear.

On aircraft with taller landing gear (like 787/A320 etc), the nacelles stay properly round because they’ve got the clearance to do it.

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Adventurous_Ad_4669@reddit

Ground clearance, 737 sit low to ground.

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texas1982@reddit

Because Boeing refuses to stop making the 737 bigger and bigger.

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shaggs31@reddit

Basically all 737s need to have the same basic dimensions. This is so that infrastructure doesn't need to be changed or updated to accommodate the new 737 variants. Since the original 737s where so low to the ground all newer, more powerful variants need to also be low to the ground. The engines of the newer 737 planes are bigger and the flatness on the underside is a way to allow this.

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Bever22@reddit

Because the earth is flat.

No, but really.

Page 20

2) Faulty Design and Performance Assumptions. Boeing made fundamentally faulty assumptions about critical technologies on the 737 MAX, most notably with MCAS. Based on these faulty assumptions, Boeing permitted MCAS—software designed to automatically push the airplane’s nose down in certain conditions—to activate on input from a single angle of attack (AOA) sensor. It also expected that pilots, who were largely unaware that the system existed, would be able to mitigate any potential malfunction. Boeing also failed to classify MCAS as a safety-critical system, which would have attracted greater FAA scrutiny during the certification process. The operation of MCAS also violated Boeing’s own internal design guidelines related to the 737 MAX’s development which stated that the system should “not have any objectionable interaction with the piloting of the airplane” and “not interfere with dive recovery.”62

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BoringBob84@reddit

Bobby

When you follow ridiculous hyperbole with juvenile taunts, it is difficult to take you seriously.

I'm quoting from this

It looks like you found a partisan source after the fact and quoted it out of context to confirm some other claim that is different from your original false claims. You alleged corruption in the FAA and delegation of all certification authority. Both are false.

The "faulty assumption" had been valid since the original 737 and was accepted by the FAA. Boeing verified it in simulations with pilots. It only became "faulty" when two crews didn't recognize the problem and do what they were trained to do (i.e., shut off a malfunctioning stab trim actuator).

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Professional_Act_820@reddit

Give up and stop moving the posts...lol

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Ok-Mushroom609@reddit

From going over speed bumps

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Nathan_Wildthorn@reddit

Ground clearance.

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Dolust@reddit

Being old has it's benefits. I do remember clearly this generation on tests having a lot of trouble with FOD.

The flat shape is too bend the airflow upwards so it does not picks every little piece of dirt from the ground. The original was literally the most powerful vacuum cleaner ever produced.

because short term profits win I guess.

Not every decision is a vast conspiracy of corporate greed. These decisions are driven by airline customers. They certainly don't want an all new airframe when a derivative aircraft can meet their requirements for far less money and lead time.

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buldozr@reddit

Yeah. Ask Norwegian who invested in the Max bigly, were they happy with it after all that happened?

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BoringBob84@reddit

We have discussed the MCAS crashes ad nauseum on this forum. There is no need to re-hash it for the millionth time. Of course, everyone who was affected was unhappy.

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buldozr@reddit

The crashes wouldn't have to happen if Boeing had a more suitable airframe to put the new engines on, instead of falling into the trap of satisfying their customers' incremental wishes for four decades and then cutting corners on safety because Airbus was eating their lunch.

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747ER@reddit

The crashes wouldn't have to happen if Boeing had a more suitable airframe to put the new engines on

Of course, if Boeing had just built an untested, experimental aircraft with completely different systems and handling characteristics, then it would’ve entered service safely with no issues right? Why didn’t Boeing just build this imaginary “super safe” plane you’ve created?

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BoringBob84@reddit

cutting corners on safety

You are misinformed, but I am exhausted from trying to argue with people who believe what they read in sensational tabloids. Reputable sources (like crash investigation agencies) stick to the facts.

I will say this: those two accidents, like most modern aviation accidents, had many contributing factors, and not all of them were the fault of the manufacturer.

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samstown23@reddit

That decision sort of made sense at the time, development of the 737-300 began in 1979 after all. The oil crisis was the driving force behind the CFM56 and thus the second generation of 737s. They needed to replace the even older 727 and the MD-80 was the prime competitor in their eyes.

I believe you might be thinking about the 737NG (and yes, that would have been the ideal time to develop a new airframe, at least in hindsight). Problem was that Boeing, probably even correctly, realized their last major advantage was the type rating - and probably didn't have the money for a clean slate approach anyway.

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buldozr@reddit

Oh yeah, I actually forgot they are on the fourth generation now.

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MillwrightWF@reddit

I’ll never understand why they just didn’t engineer a landing gear that was just a bit taller. I get it needs to be certified, it’s structural, blah blah blah but with all the knowledge and smart people out there you can’t tell me that it could not of been done.

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Dependent-Fig-2517@reddit

it's just the fairing and it's to get a tad more ground clearance

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Longjumping_Ad_5407@reddit

Cause Boeing are tight arses and won’t develop a new modern narrow body jet.

The 737 was designed in the 1960s around a smaller but longer engine.

It was made to sit low to the ground so people could board via a stair car with a reasonable number of steps for most people to climb while carrying bags.

Modern engines are more fuel efficient but they're larger. So when those engines are fitted to a 737, the bottom of the engine cowl needs to be made flat at the bottom.

Raising the planes ground height would require a redesign of the landing gear and would require pilots to go through expensive training. It would be like a whole new aircraft.

So squashed looking engines are the compromise to all of those conditions.

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funnydud3@reddit

Layers and other paper pushers who ran the company for profits until it came crashing thought we should continue using a 50 yo design with no space for modern engines instead of designing something new. In the process they killed 2 planes full of people to maintain type certification and save a buck until it didnt.

Now we have this crush tin can looking engine and a bathroom so small inside the you can’t walk in without your man parts scraping the bottom of the sink, if we can call it that.

The real culprit is us, off course. We will buy a ticket from airline X if it’s $6 cheaper than airline Y.

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747ER@reddit

Second half of username checks out

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Also, due to the low ground clearance on these 737s there are no main landing gear doors since they would hit the ground when opened.

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747ER@reddit

This was done for different reasons. There are aircraft that are lower than 737s with MLG doors.

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iSingShoop@reddit

Thank you for the second photo

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Psorosis@reddit

years ago when I was a CAD/CAM user we used to make the layup ool to for the Naceles. I ws told the flat at the bottom was to increase drag of the plane landed on water causing the engines to sheer off rather than act as a pivot causing the plane to nose dive into the water.

Never could work out if I was having my leg pulled.

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Dad4Life0424@reddit

All great answers but IMO the answer can be traced back to Boeing becoming "switching to value creation for shareholder" model with caused them to kill the 757 and the trying to fill the vacuum with a stretched 737. https://m.youtube.com/watch?v=9QufEiFKxm0&pp=0gcJCdMKAYcqIYzv

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747ER@reddit

Boeing switching to a "value creation for shareholder" model which caused them to kill the 757

This opinion really needs to die out. Boeing is a business, not a charity. As much as you may love 757s, they are a far, far worse aircraft to operate than 737s for 99% of airlines. Boeing is not going to continue building a dead product for zero customers just because it looks cool or takes off in a short distance. Airlines are also businesses, and they do not purchase assets based on these same criteria.

If you wanted Boeing to survive the 2000s at all, then you’d be happy with them cutting the failing 757 loose and sticking with the reliable, popular 737 product.

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Ronaldis@reddit

This should be required viewing for all MBA curriculums.

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mastercontrolmodule@reddit

They used to be round but the concrete ate 'em away over time.

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SpiritedInflation835@reddit

The plane was designed by hamsters

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HeyIsntJustForHorses@reddit

It's not specifically the engines that require this but the overall design and generational updates of the 737.

The 737 was originally designed to be lower to the ground so when the aircraft was loaded and unloaded with airstairs, the airstairs wouldn't need to be too tall. Shorter landing gear means less material to build therefore saving cost. The lower/more compact design handles higher loads better. Out of a multitude of factors, the original engineers wanted to make the original design sit as low to the ground as possible.

The original models were fitted with low bypass engines that had a small diameter. The height of the aircraft was sufficient for the location of the small diameter. Over time, as new variants came out, they were typically fitted with higher and higher bypass engines. These newer high bypass engines had a larger diameter than the earlier engine models used. The larger diameter was too large for the original height of the gear and the location of the engines. The engineers needed to either make the gear taller to fit the newer, bigger engines or they could move the location of the engines. They determined that the cost of redesigning and recertification of the gear would be greater than just moving the engines. Over time, the engines were mounted higher and more forward on the wing.

This new position allowed for larger engines to be mounted. As they continued to update the aircraft, the engines continued to grow. Eventually they reached a point where they could no longer move the engines higher or more forward. They then decided to flatten the bottom of the nacelles to allow for adequate ground clearance as that would be the most cost effective method.

This is how we end up still flying around in a 1950's designed aircraft in 2030. When a crew parks the last 787 in the desert, a guppy crew will fly them out.

ohhhhhhitsbigbear@reddit

Laughs in KC-135

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clintkev251@reddit

This is most famously featured on the Boeing 737. Mostly because the basic design is very old, and at the time it first came out, engines had a much smaller diameter. Boeing has retained the same basic design as that original aircraft over the years for a variety of reasons, but now that engines are much larger in diameter, they needed to squish the bottom of the cowling a bit so that they still clear the ground

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BoringBob84@reddit

The narrow engines weren't the reason why the aircraft is low. The reasons are to be able to easily maintain the aircraft, and to load passengers and cargo in remote airports with little or no ground support equipment. The narrow engines made it easier to keep the aircraft low.

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clintkev251@reddit

They were the primary design constraint that engineers were able to base the height off of. Sure there are a lot of advantages to having the aircraft sit lower and the small diameter of the engines in the original version allowed engineers to maximize those advantages, but if it were designed from scratch today, the 737 would look a lot more like an A320 in all likelihood.

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BoringBob84@reddit

if it were designed from scratch today, the 737 would look a lot more like an A320 in all likelihood.

737 short landing gear

Aren't they doing some type of extendable landing gear on the Max10 ?

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Like being 5'3" with a foot long dong, hanging nuts sticking to your calf and coming unstuck like a raw slice of bacon off the side of a fridge

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aviation-ModTeam@reddit

This content has been removed for breaking one or more of the r/aviation rules.

Because the airframe should've been retired decades ago and that's the workaround for a larger, more efficient engine.

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BoringBob84@reddit

the airframe should've been retired decades ago

Sure, that is what the engineers want, but the airlines don't want to pay for it.

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username86992@reddit

Because they chose the wrong plane and killed the 757.

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sarahlizzy@reddit

Isn’t the 757 horrible for fuel efficiency?

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RealPutin@reddit

Yes

It was too much plane for 90% of ops

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sarahlizzy@reddit

A plane for a different era when jet fuel did not have uncertain availability or cost, I guess.

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BoringBob84@reddit

The 757 and the 767 were designed to not only have common parts and systems, but to be very fuel-efficient (by the standards in the 1970s). However, fuel did not get as expensive as predicted, and the 757 was expensive.

The 757 was intended to be more capable and more efficient than the preceding 727. The focus on fuel efficiency reflected airline concerns over operating costs, which had grown amid rising oil prices during the Yom Kippur War of 1973. Design targets included a 20 percent reduction in fuel consumption from new engines, plus 10 percent from aerodynamic improvements, versus preceding aircraft.

https://en.wikipedia.org/wiki/Boeing_757

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Apprehensive_Cost937@reddit

Boeing sold nearly twice as many 300/400/500 variants of the 737 as they did of the 757.

It was a pretty simple commercial choice.

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username86992@reddit

Hindsight is 20/20 but I find it interesting that the new 757 variants and generations are getting more capacity with larger turbofans. Both would have been easier with 757 as the platform.

Not to mention, narrow body transatlantic capability is becoming increasingly important.

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RealPutin@reddit

Both would have been easier with 757 as the platform

The 757 wing is nearly 50% bigger than the MAX10's. The original 752s have 35% more thrust than the most powerful MAX engines. The 737 is creeping that way but it's absolutely not comparable.

narrow body transatlantic capability is becoming increasingly important.

It's becoming more important because we have these new narrow bodies capable of flying those routes at a viable price point. There was no version of the 757 that would ever have unlocked these routes from a profitability standpoint

There's absolutely a missing spot for a true clean-sheet MOM narrow body that covers the larger 737 to smaller 757 capacity range as our default narrowbody, but that never would've been a 757's job

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Apprehensive_Cost937@reddit

Much easier to lengthen a smaller, lighter airplane than to shorten a way too heavy jet with way too thirsty engines. A shorter 757 could never compete with A320ceo (let alone neo), and even A321neo is a lot more fuel efficient than a re-engined 757 could ever be. Even more important, at - checks notes - current, ehm, slightly pricey jet fuel.

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-burnr-@reddit

Beriev & Honda noises intensify

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mikkolukas@reddit

Full, comprehensive answer here, from Mentour Pilot:
Why are the Boeing 737NG engines FLAT?

tl;dw (from memory, last I saw the video)
Boeing have become a bit stuck on their 737s, which have their wings much closer to the ground than many other models. Newer, more fuel-efficient engines have a larger diameter, bringing the bottom of the engine very close to the ground - hence the special casing. The engine inside is standard.

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Realistic_Promise517@reddit

It’s for the ground clearance as the 737 sits very low.

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deleted_by_reddit@reddit

[removed]

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AutoModerator@reddit

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mrSProcras@reddit

to prevent this:

(dog scraping ass on the floor gif)

This appears on later B737 designs and it is largely because the B737 is being pushed beyond its useable lifecycle. The originals had smaller turbojet engines which weren’t as big as the currently used turbofans so they fit easily under the wing and were not close to the ground. As the airlines began asking that aircraft such as the A320 series and B737 carry more passengers and cargo the requirements for larger fuselages and engines climbed. As a result Boeing B737 designers had to move the engine forward on the wings and flatten the bottom of the larger cowling of the turbofan to avoid strikes on landing. This created the situation where when power was increased the 737 now has a nose up tendency because of the weight/thrust imbalance. Rather than create a new line of midsize jetliners Boeing has chosen to continue shoehorn “improvements” into an antiquated airframe. This was part of a series of design decisions and events that lead to Lion Air and Ethiopian air disasters.

The reality is that Boeing needs a complete rethink of their midsize airliner design if they are to compete with Airbus.

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Apprehensive_Cost937@reddit

This created the situation where when power was increased the 737 now has a nose up tendency because of the weight/thrust imbalance.

Every jet with engines below the wing has this tendency, unless they have some sort of flight control augmentation system.

The issue with the MAX has more to do with a bit unusual way the air flows around the engine nacelle, which slightly increases lift at high angle of attack. It does this regardless of the amount of thrust - idle or TO/GA.

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RunningWolf63@reddit

Every inch counts.

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lepape2@reddit

Be my guest if you want that nacelle to stand 1inch away from the ground on a swaying soft suspension platform that can move/roll at 130+knots while on the ground.

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froebull@reddit

Yet another example of Boeing adapting an older design to a more modern engine. In this case, they took the B737, which was originally designed for the much slimmer JT8D engine, and wanted to put a larger diameter turbofan on it, the CFM56. In order to do this, they had to maintain a certain minimum ground clearance, and that's how they did it.

The engine itself is round. It's the cowling and inlet that is oval shaped, mostly on the bottom.

In relation to other uses of the CFM56, the B737 application has the accessory gearbox located on the left side of the engine, rather than on the bottom, which is often the case.

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Free_Director2809@reddit

It's on 737s

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Enough-Astronomer-65@reddit

Thos is a feature on the 737 because of how low to the ground it is, as it came at a time when engines were low bypass, skinny and long. As engines evolved they got bigger, and raising the wings higher would have required a full redesign (obviously) so they just made the cooling flat on the bottom to clear the ground. Its one of the best ways to identify a 737 just by looking at it, look at the engines.

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GotAir@reddit

That’s why pilots give these planes white women names

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AverageAircraftFan@reddit

The 737 is OOOOOOLD. It was designed to fit a low bypass turbofan, the JT8D.

Due to not much air flowing around the combustion chambers, the engine was very slim—but long.

As technology got more modern, and Boeing saw no reason to replace the airframe or majorly redesign the landing gear system. As a result, new, higher bypass turbofans had to be flattened on the bottom due to ground clearance issues.

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Liko81@reddit

That is only for the 737-300 and later models. The original 737-100 had smaller engine nacelles. They were upgraded on the -300, and to make them fit without having to re-engineer more of the plane for longer gear, they thinned out the nacelle directly underneath the engine, moving some components to either side, creating the "hamster pouch" nacelles that have become iconic on 37s ever since.

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Patrizio_Argento@reddit

I can't believe I had to scroll so far to find someone mention nacelle. Love that word for some reason.

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AngelCatGamer@reddit

Would like to tack onto this comment that the hamster pouch is much more noticeable on the 737 NGs with the CFM56s. The new LEAPs just look square, NGs are giving blobfish mouth

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DawnSlovenport@reddit

The 737-300 launched 1979 in the also used and earlier version of the CFM56 and had the flattened nacelle as well. It didn't start with the NGs in the 90s.

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DutchBlob@reddit

737 Engine before his Boeing certification

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Healthy_Razzmatazz38@reddit

they start off round, but get worn down like breakpads when you tokyo drift

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KolibriRuf@reddit

they constructed the aircraft with other, older engines in mind. now they have access to more modern ones. more modern means that the bypass ratio is increased, so a small turbine powers a now much bigger fan, something that wasnt considered at the construction phase of the aircraft itself.

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euanmorse@reddit

Interesting question. Would you believe it is related to the Boeing 737 MAX crashes?

Boeing designed the original 737 to be low to the ground as this helped baggage handlers at smaller airports, making the plane more attractive to local air carriers and not just those that landed at large airports.

However, as engine deigns grew larger to increase bypass ratio - necessitating larger fans, this started to rub up against the design philosophy of the 737. Changing the height of the aircraft when it was on the ground would also impacts its flying and handling characteristics which would necessitate re-certifying the aircraft which would cost a lot in money and time not to mention pilots having to recertify as it would technically be a new ‘type’ of aircraft. Naturally, Boeing and the airlines wanted to avoid such an issue. Therefore, Boeing managed to convince the engine manufacturer to provide the engine with a flattened cowling for its New Generation 737s.

This worked until the latest generation of engines came along which would not fit regardless of a flattened bottom. Boeing’s solution was to mount them further forward. Now, this would surely change the centre of gravity and other flying characteristics you ask? Yes, it would. So Boeing added MCAS which meant the plane’s computer would accommodate for these changes, which avoided costly pilot retraining - a boon for the airlines. The best part? They didn’t really mention this to anyone or provide information during training about this system.

Eh, the MAX accidents were due to poorly designed software.

Only the nacelle is flat bottomed, the actual engine inside is still perfectly round.

On 737NG and MAX it’s done to increase nacelle ground clearance and it’s achieved by shifting the location of the accessory gearbox to the LH side of the fan case.

For comparison on the A320ceo with CFM56 the AGB is on the bottom of the fan case because there’s enough clearance.

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newtojersey32@reddit

Check out the size of the engine nacelles on the original 737

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m71nu@reddit

Actually, turbine engines are perfectly circular. You only see flat bottoms when a large engine is fitted to an aircraft with low ground clearance. In these cases, engineers move the auxiliary components (like the gearbox and pumps) from the bottom of the engine to the sides to 'squash' the outer casing. This is most notable on the Boeing 737; because it is a 1960s-era airframe, it was never originally intended to house modern, large-diameter high-bypass engines.

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gwdope@reddit

So they don’t hit the ground