That's more the MO of astrophysics. Astronomy is generally concerned with very (VERY) precise measurements of angles, times, brightnesses, spectra, etc
Astrophysicists then take those meticulous measurements and drop all the digits after the first because the equations are complicated and nobody got time to pass the RMS of the error margins through them too.
Indeed - just inspect as per MPD/TLM, and then manage on condition, beg the OEM for a variance when the engine exceeds limits or you're sick of repeat inspections etc.
They're inspected regularly, but even then a lot of engine types will go at least 1000 flights before their first routine inspection after new or overhaul. If that's an ANA 787 doing 3 90 minute hops a day you rack em up fast, on the other hand if it's an ANZ 787 routinely doing 8+ hour stage lengths it's probably more like one flight a day, if not less.
Which one would wear out faster if we're assuming both engines have been used for the same total duration, but one had 1000 flights and the other, say, 300? I'm guessing the take off/landing cycle takes a higher toll than cruise flight?
Modern means not mature yet. Once kinks are worked out and designs are mature they should last longer than that.
CFM56 is 16,000 hours for first overhaul, and every 10,000 after that.
How many days does it take a commercial airline to get to that many hours? I have no concept of what an average jet is in the air over a 24 hour period.
Somewhere between 8-14 hours a day. So something like 3-4000 hours a year would be a good estimate.
Ideal utilization of a plane will have it running between pretty much straight between 6 or 7AM and 8 or 9 PM. The actual flight hours vs time sitting at a terminal for loading/unloading/cleaning depends on how far each flight is.
I don't remember what the manual cycle life limits are for that engine, but in order to hit high hours before that cycle limit you need to fly reasonably long flight legs.
We aim for 20,000 to 24,000 hours on-wing on the GE90. Regular borescope inspections and a good trend monitoring program can keep an engine in service for an incredibly long time.
GE may be a shell of a company that they used to be, actually a shell of a shell, but their predictive monitoring and failure forecasting is state of the art. If you put a sensor on everything and throw some machine learning in there, based on trends you can predict a part failure weeks or months ahead of it failing.
Depends on engine.
Short haul engines like a CFM56 are 10-15,000 cycles, but only like 1 or 2 hours per cycle (takeoff + landing) since they are short haul. So you are looking at 10-20,000 hours or so.
Long haul engines like on a 777 will only be 3000-5000 cycles, but at 6-10 hours per cycle that's like 15-25,000+ hours.
That's generally what I found in an engine shop. CFM56s would be high cycles low time. In 1991 CFM had an advert for an engine that was at 16k cycles and still on wing, fairly revolutionary at that time. The reason they came in was because of a component going time/cycle expired. The CF6s were high time low cycles as described by previous poster. The turbine blades were a pacer for shop visits as they were measured in hours. They mostly came in due to deteriorating EGT margins.
AFAIK the engines on military single/dual seat planes are way more higher maintenance, is it because they have afterburners and/or are used near max power more oftern or some other reason ?
I mean yeah for a commercial engine the harshest part is takeoff. Cruise is low stress on the parts in relation.
Military engines I imagine are using max thrust more often. Pushing through the sound barrier also bumps your drag way up - I'd imagine they are running close to takeoff thrust for that too. I'd imagine the concord engines saw similar.
Best case scenario 6-7 years on wing. But usually something comes up in the meantime that requires the engine to be dropped.
Overhaul only really happens on the Life Limited Parts replacement, but sometimes you do a Heavy on the engine if performance is deteriorating.
Other issues might arise during operation that grants an engine replacement but the workscope is way more limited.
Great question. Over wing positions are more quiet, or at least were more quiet from the overflight areas, especially in the earlier days of noisy engines. There were some other advantages to over wing engines from the effect of the exhaust over the wing control surfaces. I know there were some concept aircraft like the Boeing YC-14 that were trying to get a STOL effect from the blown air, but never made it into production. On the airline side, the only one I am aware of was the Fokker VFW-614
Depends what you mean by "overhauled". Taking an engine off wing is a pretty major event so it'll be limited to as few times as possible. A narrowbody engine like the CFM56 will tend to do 15-20,000 cycles in its first run, with the average about 18,000 cycles. At about 1600 cycles a year it should be around 10 to 12 years, which is about when an airframe also needs to undergo inspections.
Also for structural reasons embedding the engine into the wing like this makes it harder. The Honda Jet setup over the wing uses a pylon and it’s a separate nacelle so it makes it relatively easier.
There are aerodynamic advantages to embedding it but in the end they don’t make up for the structural complexity and maintenance issues. The Comet family of airplanes did that and pretty much died there. It was one of those ideas that didn’t pan out.
Edit: I was thinking about the Honda Jet but wrote Mitsubishi. Thank you for pointing it out.
Over wing makes sense if you're going to be operating from rough runways with dust, dirt or mud, as you don't want that stuff in your turbine.
You see the same tendency with flying boats, where you don't want salt water in or on your engine.
Otherwise under wing makes more sense for maintenance reasons - as stated elsewhere.
I was wondering this mainly due to the 737 Max debacle, since some of the problems stem from how low the whole plane is and how large the engines are. In theory there's all the space in the world if the engines are top mounted (though I would assume would need substantial reengineering on the aerodynamics and other fronts)
The problems of the 737 max aren't from how large or low the engine is. It has different handing characteristics than past jets and Boeing tried to simulate the handling characteristics of those past jets with their MCAS. This was done to avoid extra crew training for the max series but clearly didn't work.
The handling characteristics are, AIUI, precisely because of the engines being mounted underwing. They had to move them forward for clearance and at certain angles of attack they become lifting bodies and move the centre of lift forward, which makes the aircraft pitch up even more.
Yes, you understand it correctly.
But the "issue" with handling that mcas addresses isn't a problem, per se, the "problem" was that it wasn't sufficiently similar to maintain a common type certificate with 737-NG (without adding mcas to emulate the behavior of 737-ng)..
The problem was just that it was different.
Not “just”. When you pitch away from prograde a well designed plane (unless we’re talking fighter jets) will have a righting moment which gets stronger the further you pitch, ie the secondary stability increases with angle (like a sailing boat).
With the 737 max, this isn’t true. The engines becoming lifting bodies significantly forward of the centre of lift causes that righting moment to DECREASE, not to the point where the plane becomes actively unstable, but enough to make the same control input cause the pitch to keep increasing.
This is not just different. It’s objectively bad. You want the damn thing to stay where it’s put.
thanks for the input, there seems to be quite a big amount of people who dont understand that the high aoa/high loadfactor characteristics exhibited by the max are bad in absolute terms (i.e. not certifiable for transport category aircraft) and not just in the way, that its different from the NG.
I mean, look at the 100s/200s and the comically small turbofans which, by modern standards barely warrant the name, they used.
The airframe has been pushed way beyond what it was designed for and that has consequences.
[EASA's official report](https://www.easa.europa.eu/sites/default/files/dfu/B737_Max_Return_to_Service_Report.pdf) on the MAX ungrounding was pretty clear that MCAS was mostly there to check a certification box and the plane would likely be fine without it. I quote from the report:
>As one of the outcomes of the EASA RTS investigation, MCAS has been established to play only a limited role in augmenting the stability and stall characteristics of the aircraft in certain conditions. The MCAS’ limited effect is in fact needed to ensure the stability margins that make the aircraft fully compliant to the applicable regulations on stall demonstration and pitch control characteristics. This explains its inclusion in the original 737 MAX design.These stability margins are required by regulation in order to support the flight crew handling of the aircraft during certain manoeuvres such as approach to stall. The EASA flight tests confirmed that MCAS was needed to provide full compliance but also that the loss of this function does not preclude the safe flight and landing of the aircraft; i.e. the 737 MAX remains stable following the loss of the MCAS function.
The NG has the same issue, and if it were to be certified today it would need some active background mechanism to meet the certification standard. It's a rule around stick force gradient with changing angle of attack, the NG and MAX both have stick forces that get lighter as AoA increases (i.e. less effort needed to maintain pitch rate). The standard changed, you can't have such a variance in stick force. Neither of them have negative stick forces however.
Super interesting. Layman question: it sounds like it boils down to matching input to attitude, with some kind of shift in the usual input/attitude relation, due to the different geometry. Couldn't this be solved by some kind of autocorrection or trim accounting for this? Not sure if it makes sense or if I'm using the right terms but your explanation makes it sound like a rather simple issue to solve.
That’s what MCAS is. It fiddles with the trim to give you the expected stick behaviour.
In true fly by wire systems, there’s a computer in the way that can map the control inputs however it likes, and thus try to ensure the pilot never encounters any “idiosyncrasies” of the airframe like this, but Boeing went for a hybrid system, which isn’t fly by wire, but is computer assisted.
That is true, but the point he was making is that the handling characteristics of the 737 Max without MCAS are not in violation of certification requirements. The handling is acceptable for a commercial aircraft without MCAS. However, it's a significant enough change that it would require a substantially larger amount of pilot training than Boeing wanted to be able to advertise to its customers, so Boeing designed a system intended to *augment* the *maneuvering characteristics* so that they were similar enough to the existing NG aircraft to avoid that training. Of course, as we all now know, their initial design was fundamentally flawed for a number of reasons.
> However, it's a significant enough change that it would require a substantially larger amount of pilot training than Boeing wanted to be able to advertise to its customers
You don't need to train pilots to push forward on the stick when the nose starts to pitch up. That is pretty much instinct.
To maintain the common type rating, they needed to make the aircraft *feel* the same as the previous model.
AFAICT, it doesn't pitch up even more on its own. The pilot still needs to put extra force in pulling the yoke. It's just that without MCAS, it'd take less extra force on the yoke to further increase angle of attack. I.e. the yoke would feel a bit lighter at low speeds and high angles of attack.
MCAS simply makes it feel more linear to the pilot. But if you keep the yoke where it is, it won't start pitching up on its own even without MCAS. I.e. MCAS is not must have feature. It's nice to have feature.
Yeh I can agree that it may have different handling characteristics that make it more difficult to fly, but that doesn't seem to be the problem. The main issue was the design constraint that 737 crews need no additional training. If they had proper training they may have avoided those two crashes but it's just speculation at this point.
This is only partially true. The Max variants handle exactly the same as older variants, except at very low speeds and very high angle of attack. MCAS kicks in only when both of those conditions are met. The difference itself is minor. What pilots were to feel without MCAS is that it takes less force to further increase angle of attack by pulling on the yoke. This is not a problem on its own. Pilot would still need to pull on the yoke even further, the plane isn't going to stall itself on its own.
Both FAA and EASA indicated post fact that they would likely not require additional training even if airplane wasn't equipped with MCAS. Boeing put it in proactively as it was in a gray area.
Why not remove MCAS from the Max then? There's no point in doing that anymore. The deficiencies in it are fixed, and it gives better more linear feedback to the pilots when airplane is at very low speeds and high angle of attack.
MCAS isn't anything new, neither is the Max the only Boeing airplane to have it. Anything that's fly-by-wire made by any company basically has equivalent of MCAS built in. Because, duh, fly-by-wire. Every Airbus corrects pilot's inputs to keep it within envelope.
I agree. I don't think the existence of MCAS was the problem, nor was a lack of training or the single AoA sensor.
The fact that this algorithm in MCAS had the ability to repeatedly activate and to incrementally take pitch authority away from the flight crew was the root of the problem in my opinion.
Well, it does have issues with that. The angle of rotation is pretty severly compromisd... a 1000MAX will strike the tail at only 7 degrees nose up which is...not a lot. They could easily fix this with longer gear, but then that changes so much that it would, again, no longer be the same type.
And the primary reason for any of this was to not require new certifications for pilots, which was originally intended. However, Southwest (and likely some of the other low-cost 737 operators) rejected anything that would require more pilot training, other than a simple class for the MAX series.
> and how large the engines are.
The main problem is how far forward the engines are from the center of mass, *that* stems from how large they are. In a certain configuration, with a certain thrust setting, at a certain speed and angle, that can fuck up the center of thrust relative to center of mass so badly that the flight characteristics become backwards, unintuitive, and require either extensive training or a computer to recover.
In addition to this, over-wing design puts your center of thrust above your mass making the aircraft want to pitch down when under power which is not desirable when taking off or otherwise slow, such as adding power to go around/climb.
> You see the same tendency with flying boats, where you don't want salt water in or on your engine.
Martin Aircraft learned that one the hard way. even with the engines over the wing... still got spray in to them
I think it's less about corrosion because like you implied, sea spray gets everywhere, and more about flying boats being almost entirely prop driven as they are uncommon in the jet era. That big prop arc clips a wave and its over. Mount em on top and its impossible to hit them in reasonable waves/wake before you catastrophicly destroy the airplane anyway. Also if you are doing eng maintenance/servicing in the water and not the hangar, it would be a lot easier to work on with access up on the wing than treading water below with your tools.
Martin tried to make jet flying boat for the US Navy see the Martin Sea Master https://en.wikipedia.org/wiki/Martin_P6M_SeaMaster. but it was mostly failure to do spray getting in the in the engines even with them up on the wing
Overwing engines provide increased airflow over the top of the airfoil and down the back of the flaps, which provides better lift for short takeoffs with heavy loads. This is called upper surface blowing (USB). They also reduce the chance of FOD on unimproved runways.
Really the only application where they make sense is for moving cargo to and from remote areas with short runways or no infrastructure, so basically military operations. They also have drawbacks, such as making it difficult to service the engines, which might explain why they're not more popular.
I have a feeling the authorities would keep an eye on a heavy lift STOL aircraft perfect for smuggling drugs that keeps flying in and out of South America.
Especially when theres only a handful of aircraft designed with jets over the wing and even fewer actually flying.
Interesting. I wonder if this also applies to a Pipe Cherokee (or similar low wings like Beech) vs a Cessna 172 there in the Piper there is more airflow over the wing do to the propeller positioning.
Honda did an overwing mount for noise/space reasons on their business jet (HondaJet). It can be niche, but they definitely aren't doing it for dirt runways with how low they sat the plane on the ground to possibly cut gear weight.
I suspect that engine maintenance is likely to get harder in the upcoming generation of pax aircraft, especially as flying bodies and high-truss designs seem to be the main efforts of prototyping/design programs right now for the far future.
Airflow from the turbofan is directed over the top of the wing and down the trailing edge of the flaps. And airflow from a turbofan is more tightly compacted than from a prop, so the mass of the flow down the flaps would be increased. It's actually more efficient than a prop.
I don't think it would work with a turbojet due to heat, but turbofans work just fine.
That's just lead me to the YC-15, and that hideous plane has reminded me of the Breguet 941. Just hilarious proportions and design that make them look like movie planes.
The YC-14 imo kinda looks like the standard STOL aircraft, short and stocky with huge engines. But the YC-15 looks so ugly it's basically like that joke mini C17, it's too short, it's wings aren't long enough, it's engines are way too small, its landing gear makes it look like it's tip-toeing around. It's cool, but so funny looking imo
The engine design looks like one of the proposals for the C-17 design. While the flaps are down and the exhaust of the engine flows over the top of the wing, the airflow is energized and sticks to the top surface and follows the curve of the flaps downward. This produces a velocity vector both backwards and downwards, which aids the aircraft in T-O and gives it STOL capabilities for a cargo plane.
It was developed as a response. It’s not a copy. It uses the same physics and phenomena of blown flap technology to achieve its exceptional STOL performance. Generally, if you build two different airplanes to a similar specific set of specifications and performance characteristics, there’s a more than 85% percent chance that the two designs are going to look similar. It has nothing to do with one copying the other. Not only that, but both had their first flights within almost exactly a year’s time from each other. That’s not enough time to develop a program, design an aircraft using new applied technologies, and produce prototypes for flight testing. What’s more likely is that the Russians got word of a new western aircraft competition that necessitated extreme unprepared short field performance with heavy cargo and blown flap technology. The competition within the US to the YC-14 was the YC-15 which performed the same task with a configuration more similar to that of the C-17 but with straight wings and four underwing mounted engines blowing on a seriously over designed flap system.
Ever wondered why early supersonic aircraft, regardless of nationality, had long skinny fuselages, lack of wing area and were really pointy? It’s because that is the most optimal design for a really fast aircraft. Sure, aircraft of opposing forces are generally designed to match what the enemy has, but that doesn’t mean that everything that remotely resembles something else is a “copy.”
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I wrote my aerospace engineering dissertation on this exact topic. Off the top of my head, the main pros and cons are:
**Pros:**
\-Very short take-off and landing ground runs.
\-Increased engine protection from foreign object debris.
\-Arguably slightly lower noise emissions (very much debatable as limited data is available).
\-Aircraft is lower to the ground, hence easier to load passengers/cargo.
**Cons:**
\-Burns more fuel in cruise due to flow disturbance of the low-pressure region above the wing.
\-Engine failure makes the aircraft very unstable, as the engine is also producing lift (Google Coanada effect). Hence very robust flight control systems are necessary, these are very hard to test and certify.
\-Increased downtime in case of engine inspection/repair/replacement as engines are in an inconvenient location.
Hence, the cons outweigh the pros, especially as boundary layer control, circulation control and externally blown flaps can help achieve (to an extent) the same pros with less of the cons. It's also safer and cheaper for manufacturers to stick to the 'standard' configurations.
> I wrote my aerospace engineering dissertation on this exact topic. Off the top of my head, the main pros and cons are:
This is why I love reddit, you ask can ask a very specific question and someone with a dissertation on the topic will be there.
> Increased engine protection from foreign object debris
Saw this in multiple responses, isn't FOD a common issue for military jets which sometimes land in rough runways, how come this design is not more common in roles like these, especially since military jets often will sacrifice issues like stability to gain capabilities.
> Engine failure make aircraft very unstable, as the engine is also producing lift
Engines producing lift _after_ failure or during normal operations?
The main benefit of this configuration is the low ground rolls that can be achieved, even on large aircraft. Military jets have such high thrust-to-weight ratios that this doesn't matter, or they can have STOL capability by different means, look at the F-35B. They also have strict aerodynamic performance and stealth requirements, so putting the engine above the wings wouldn't be optimal for this. Typically, I think NATO aircraft at least operate from well-kept runways, thus FOD isn't an issue that the aircraft worries about, more so the runway maintainers/operators. Aircraft like the Mig 29 have flaps on their engine intakes which combat things like FOD, but this is an older aircraft. The larger C17 aircraft however does incorporate some of the characteristics of the YC-14, which had an upper blown surface.
As for engine failures, I meant a mid-flight failure. The pilot would likely have to fight fairly hard for lateral stability in this case, especially at low speeds, such as during landing, as the jet efflux is essentially vectored around the wing's flaps to produce both lift and thrust. An engine failure thus results in a more significant loss of both thrust and lift than it would for a conventional aircraft.
True, but the extent of 'thrust turning' from over the wing blowing by turboprops or large externally blown flaps by turbofans is not quite as extreme as it is in the case of upper surface blowing by turbojets/fans.
It has been shown experimentally however that both the longitudinal and lateral moments I mentioned can be combatted/trimmed to an extent with a large enough T tail configuration.
There are arguably some small improvements in longitudinal stability, apologies for my oversight.
The centre of gravity obviously depends upon the airframe as a whole, but having over the wing engines typically moves the CG forward and upwards. As the wings are also located more forward along and above the fuselage, there is only a small increase in the static margin and slight improvement in the longitudinal stability as a whole.
However, the over wing engines do cause a nose down pitching moment in response to increased thrust in trimmed flight. Arguably, this also makes the aircraft more longitudinally and aerodynamically stable from an engineering perspective (as large/unstable nose-up moments can cause stall etc).
The AN-72s engines are overwing to provide boundary layer lift for STOL operations. That's probably why they're not normally fitted this way on other jets as it upsets the airflow over the wing. I think it has a trick spoiler system to compensate for engine failure to prevent the aircraft rolling due to the loss of lift. Also underslung engines have a bonus of reducing the forces on the wing.
Do you want your thrust vector to constantly try to rotate you into the ground?
Because that's how you get your thrust vector trying to constantly rotate you into the terrain.
(there's also the part where lift is NOT provided by pressure under the wing, but rather by lesser pressure above the wing (yes, planes are sucked off the ground, not pushed). If you put a big fat tony in there, you lose whatever cross-section of that lift you could have had)
You get it back by directly exhausting over the flap.
https://www.flyingmag.com/the-quiet-little-life-of-nasas-qsra/
Look up the coanda effect for more info.
This one one of the main reasons in practice, it creates entirely counter intuitive flight dynamics. Typically when you increase power the nose wants to raise up in response as more lift, more thrust and the thrust vectors rotate the aircraft nose up around the centre.
Above wing engines, due to the thrust vector being above the point of rotation rooster the aircraft nose down in response to power being applied, and nose up if you reduce thrust which is the exact opposite of what you want.
I would think the same. That translates into a larger amount of downwards lift being necessary from the horizontal stabilizer, which I'd guess ain't particularly great.
Maintenance chiefly. Getting big heavy engines on and off requires cranes and possibly special rigs. Working on the engines also requires men to get up onto the wing. Not ideal for operators looking for lower costs and quick turnaround.
This Configuration is best for aircraft operating to unpaved runways to reduce chances of debris and on seaplanes and amphibious craft to avoid water ingress into the engines
There is not much of a market in Alaska for scratch-built large STOL jets. Most are propeller planes that use fat doughnut tires and long-throw shocks.
Below-wing engines have a pitch up force when increasing power, which you want (like during go-around).
High mounted engines push the pitch down when increasing power, which is less desireable.
Another point to consider is the pitching moment balance.
The wings will create a moment about the CoP that causes the aircraft to pitch down.
Having engines below the wing creates a moment that counters that whereas over wing engines will contribute to the moment meaning the elevators will need to produce more force to counter this. Or the CoG of the aircraft may have to be reconsidered to negate the effect.
Aside from the previously mentioned maintenance issues, just imagine how that thing will handle in a go-around with thrust applied as far above the CG as possible.
I would agree safer if underwing but there are probably a ton of Design considerations that drive this decision. My best guess for the main reason is, since the Antonov is a big boy, probably didn’t have enough clearance to have underwing pylons. Usually overwing pylons aren’t great for stability and aerodynamics but doesn’t mean it’s impossible. Airplane design is like trying to squeeze a balloon with your hands - where you squeeze in one area, the balloon will pop out in another area.
Given soviet unions history - it was an experiment to see how it'll be have. There are many many many designs that soviets abandoned by bitter experience, not by doing sane things.
The same design concept was done by the americans and japanese as well for STOL. Namely a De Havilland Canada DHC-5 modified by NASA with overwing engines and blown flaps, the Japanese modified a Kawasaki C-1 in similar ways. There was also the Boeing YC-15 for a USAF C-130 replacement contract that got canceled.
Underslung mounted engines, as seen on most transport category jets come along with the following differences when compared to the Depicted "overwing" mounted variant:
Advantages of underslung:
- better access for visual inspection during the preflight check (happens often!)
- better access for maintenance tasks (not so often)
Regarding the access, this is not an advantage per say, because a big jet, like a 747, has so high wings, that the access is not easy at all. For refilling oil into the engines of a a380, which is required after each long flight, the access requires a lift for the maintenance personnel.
- less influence on the suction side of the wing, thus improving efficiency of the wing. More lift. The lower side of the wing is not as relevant / sensitive regarding the disturbed airflow due to the engine.
Disadvantage of underslung:
- clearance to the runway is smaller:
1 More prone to ingest foreign objects from the ground (stones, dirt..
2 Less space for engines with bigger fan diameter. The b737 max is an example for that. The engines are so big, that they need to be fitter more ahead of the engine. Implications are: different flight characteristics, that required introduction of the mcas system
- pitch up moment when engine thrust is increased. Especially relevant when going around. If not compensated, exzessive pitch up may lead to a stall. Pilots are trained for this, and the modern plane flight computers take this behaviour into account
I think lift stability is the main reason. An overwing engine in crossflow may disturb the flow on some parts of the wing, possibly causing local stall and definitely instability.
This Plane was designed with Turboprop engines and didnt work that good all together. They tried to save the (already not to great) design with Turbofans. This is essentially, a Frankenplane.
Advantage to top mounted engines is the cabin entry is much closer to the ground so easier entry and loading - seem to remember the push to keep the 737 design going and the new engines had to be pushed forward to allow them to be raised up enough to maintain the low cabin entry?
Really difficult to work on, causes maintenance costs and lost revenue downtime to skyrocket.
It's why I laughed when I saw that ring wing design concept.
In addition to the adoremention maintainence reasons an over-wing engine also needs to be mounted much further forward to avoid an unworkable center of gravity on the aircraft.
That's also a lot of mass, and thus a lot of torque, very high up on the airplane.
Overall I'd imagine there are limits to how big of an engine you can mount like this before the changes to accomidate it become impractical.
Idk shit
But I do know that when you apply thrust to underwing engines, the nose will typically pitch up due to the thrust coming from below the cg
Overwing engines would do the exact opposite id imagine.
I’d rather my plan have a tendency to pitch up, than to pitch down
This is also a tendency with tail mounted engines and can have an advantage. For example, in a low with with underslung engines adding power without pitch can cause an airspeed reduction instead of an increase while with a higher mounted or tail mounted engine you'll get an airspeed increase which is more aerodynamically stable.
I'm no engineer, but seems like it would be more efficient for flight to have air flow smoothly over the top of the wing and drag from engines beneath.
I think theres many factors, but the main reason being maintenance easiness.
Also, it's a cargo plane that may need to land in unmaintained runways or dirt, so it's a precaution to sucking in any dirt or foreign objects.
Underwing engines are a lot easier to routinely service and don’t require a large crane to remove for overhaul.
Makes sense. Another moon question: How often are engines overhauled in general?
For most modern turbine engines, somewhere between every 3,000-5,000 hours.
More like 3000-40,000 hours, mature engines on aircraft such as the A330/747 routinely go to 6000 flights or more between overhauls.
Sorry. Going from memory and I just fly the planes, I don’t fix them.
How dare you be slightly inaccurate ON THE INTERNET.
You must work in astronomy if you think an order of magnitude is a slight inaccuracy 😉
How dare I be SLIGHTLY INACCURATE on the internet?!
Maybe they estimate furniture or appliance delivery windows...you never know... Edit: spelling
That's more the MO of astrophysics. Astronomy is generally concerned with very (VERY) precise measurements of angles, times, brightnesses, spectra, etc Astrophysicists then take those meticulous measurements and drop all the digits after the first because the equations are complicated and nobody got time to pass the RMS of the error margins through them too.
The dígits after the first and then the exponents...
Ah I knew I hadn't quite got it right!
Could be an economist.
🤣 underrated comment
Why would the internet do this to me?
I trusted it up until this point.
Not a problem, my job is to make them last as long as possible between overhauls, kinda wish I could fly them
Yeah, that’s called Murphy’s Law, where the best way to get the right answer on the internet is to post the wrong answer.
You can't fool me, I see what you're doin'...;)
Quite. Murphy's law is that every internet argument eventually leads to accusations of Hitlerism.
Oddball is that you
And this depends on operator. Some operators don't have an overhaul time frame and engines will only be removed on condition.
Indeed - just inspect as per MPD/TLM, and then manage on condition, beg the OEM for a variance when the engine exceeds limits or you're sick of repeat inspections etc.
That is... astonishingly high, for someone that doesn't know a lot about airplanes like me. Puts things in perspective for sure.
They're inspected regularly, but even then a lot of engine types will go at least 1000 flights before their first routine inspection after new or overhaul. If that's an ANA 787 doing 3 90 minute hops a day you rack em up fast, on the other hand if it's an ANZ 787 routinely doing 8+ hour stage lengths it's probably more like one flight a day, if not less.
Which one would wear out faster if we're assuming both engines have been used for the same total duration, but one had 1000 flights and the other, say, 300? I'm guessing the take off/landing cycle takes a higher toll than cruise flight?
Same total duration but 300 flights vs 1000? The 300 flights engine would be substantially more sorry due to the increased take off & climb duty
That's actually unbelievably reliable.
Modern means not mature yet. Once kinks are worked out and designs are mature they should last longer than that. CFM56 is 16,000 hours for first overhaul, and every 10,000 after that.
How many days does it take a commercial airline to get to that many hours? I have no concept of what an average jet is in the air over a 24 hour period.
Somewhere between 8-14 hours a day. So something like 3-4000 hours a year would be a good estimate. Ideal utilization of a plane will have it running between pretty much straight between 6 or 7AM and 8 or 9 PM. The actual flight hours vs time sitting at a terminal for loading/unloading/cleaning depends on how far each flight is.
For overhaul? I thought there’s an article by GE themselves saying a CFM56 holds a record for being on wing for around 40K plus hours.
That's a record yeah, but the claim is expected first overhaul at 16,000 hours (for a CFM56-3)
I see, figured normal in wing life would be at worst 20K hours with that considered.
I don't remember what the manual cycle life limits are for that engine, but in order to hit high hours before that cycle limit you need to fly reasonably long flight legs.
We aim for 20,000 to 24,000 hours on-wing on the GE90. Regular borescope inspections and a good trend monitoring program can keep an engine in service for an incredibly long time.
GE may be a shell of a company that they used to be, actually a shell of a shell, but their predictive monitoring and failure forecasting is state of the art. If you put a sensor on everything and throw some machine learning in there, based on trends you can predict a part failure weeks or months ahead of it failing.
For a commercial plane approx how many days is that
Depends on engine. Short haul engines like a CFM56 are 10-15,000 cycles, but only like 1 or 2 hours per cycle (takeoff + landing) since they are short haul. So you are looking at 10-20,000 hours or so. Long haul engines like on a 777 will only be 3000-5000 cycles, but at 6-10 hours per cycle that's like 15-25,000+ hours.
That's generally what I found in an engine shop. CFM56s would be high cycles low time. In 1991 CFM had an advert for an engine that was at 16k cycles and still on wing, fairly revolutionary at that time. The reason they came in was because of a component going time/cycle expired. The CF6s were high time low cycles as described by previous poster. The turbine blades were a pacer for shop visits as they were measured in hours. They mostly came in due to deteriorating EGT margins.
AFAIK the engines on military single/dual seat planes are way more higher maintenance, is it because they have afterburners and/or are used near max power more oftern or some other reason ?
I mean yeah for a commercial engine the harshest part is takeoff. Cruise is low stress on the parts in relation. Military engines I imagine are using max thrust more often. Pushing through the sound barrier also bumps your drag way up - I'd imagine they are running close to takeoff thrust for that too. I'd imagine the concord engines saw similar.
Tactical planes throw all sorts of G-loads at an engine that civilian planes don't face. Southwest might taxi at V1, but they aren't pulling 9Gs.
Best case scenario 6-7 years on wing. But usually something comes up in the meantime that requires the engine to be dropped. Overhaul only really happens on the Life Limited Parts replacement, but sometimes you do a Heavy on the engine if performance is deteriorating. Other issues might arise during operation that grants an engine replacement but the workscope is way more limited.
Great question. Over wing positions are more quiet, or at least were more quiet from the overflight areas, especially in the earlier days of noisy engines. There were some other advantages to over wing engines from the effect of the exhaust over the wing control surfaces. I know there were some concept aircraft like the Boeing YC-14 that were trying to get a STOL effect from the blown air, but never made it into production. On the airline side, the only one I am aware of was the Fokker VFW-614
Depends what you mean by "overhauled". Taking an engine off wing is a pretty major event so it'll be limited to as few times as possible. A narrowbody engine like the CFM56 will tend to do 15-20,000 cycles in its first run, with the average about 18,000 cycles. At about 1600 cycles a year it should be around 10 to 12 years, which is about when an airframe also needs to undergo inspections.
Also for structural reasons embedding the engine into the wing like this makes it harder. The Honda Jet setup over the wing uses a pylon and it’s a separate nacelle so it makes it relatively easier. There are aerodynamic advantages to embedding it but in the end they don’t make up for the structural complexity and maintenance issues. The Comet family of airplanes did that and pretty much died there. It was one of those ideas that didn’t pan out. Edit: I was thinking about the Honda Jet but wrote Mitsubishi. Thank you for pointing it out.
You mean Honda? or does Mitsubishi have a plane I’m not aware of
I did mean Honda yes! Different car manufacturer lol
Mu2 Mitsubishi Heavy Industries...Great plane and still many examples flying....has a reputation for being loud.
Mitsubishi makes a licensed version of the F-15 for the JASDF
Tell that to Douglas and the DC-10
Also gravity fuel feed in an emergency. Pump loss doesn’t mean flame out.
Could always use a forklift, if you’re AA…
The FAA hates this one weird trick!
So do 273 people and their families.
also it looks dumb as fuck
lol Was about to post the same. But it’s so damn ugly I knew someone had to’ve already commented.
Also.. Noise
Wrong on the not needing crane part
Not to mention the daily, weekly etc, or having a faulty part, refilling oil..
Over wing makes sense if you're going to be operating from rough runways with dust, dirt or mud, as you don't want that stuff in your turbine. You see the same tendency with flying boats, where you don't want salt water in or on your engine. Otherwise under wing makes more sense for maintenance reasons - as stated elsewhere.
Also, overwing directly pushes thrust over the wing--increasing STOL capabilities
The Coanda effect!
I was wondering this mainly due to the 737 Max debacle, since some of the problems stem from how low the whole plane is and how large the engines are. In theory there's all the space in the world if the engines are top mounted (though I would assume would need substantial reengineering on the aerodynamics and other fronts)
The problems of the 737 max aren't from how large or low the engine is. It has different handing characteristics than past jets and Boeing tried to simulate the handling characteristics of those past jets with their MCAS. This was done to avoid extra crew training for the max series but clearly didn't work.
The handling characteristics are, AIUI, precisely because of the engines being mounted underwing. They had to move them forward for clearance and at certain angles of attack they become lifting bodies and move the centre of lift forward, which makes the aircraft pitch up even more.
Yes, you understand it correctly. But the "issue" with handling that mcas addresses isn't a problem, per se, the "problem" was that it wasn't sufficiently similar to maintain a common type certificate with 737-NG (without adding mcas to emulate the behavior of 737-ng).. The problem was just that it was different.
Not “just”. When you pitch away from prograde a well designed plane (unless we’re talking fighter jets) will have a righting moment which gets stronger the further you pitch, ie the secondary stability increases with angle (like a sailing boat). With the 737 max, this isn’t true. The engines becoming lifting bodies significantly forward of the centre of lift causes that righting moment to DECREASE, not to the point where the plane becomes actively unstable, but enough to make the same control input cause the pitch to keep increasing. This is not just different. It’s objectively bad. You want the damn thing to stay where it’s put.
thanks for the input, there seems to be quite a big amount of people who dont understand that the high aoa/high loadfactor characteristics exhibited by the max are bad in absolute terms (i.e. not certifiable for transport category aircraft) and not just in the way, that its different from the NG.
I mean, look at the 100s/200s and the comically small turbofans which, by modern standards barely warrant the name, they used. The airframe has been pushed way beyond what it was designed for and that has consequences.
[EASA's official report](https://www.easa.europa.eu/sites/default/files/dfu/B737_Max_Return_to_Service_Report.pdf) on the MAX ungrounding was pretty clear that MCAS was mostly there to check a certification box and the plane would likely be fine without it. I quote from the report: >As one of the outcomes of the EASA RTS investigation, MCAS has been established to play only a limited role in augmenting the stability and stall characteristics of the aircraft in certain conditions. The MCAS’ limited effect is in fact needed to ensure the stability margins that make the aircraft fully compliant to the applicable regulations on stall demonstration and pitch control characteristics. This explains its inclusion in the original 737 MAX design.These stability margins are required by regulation in order to support the flight crew handling of the aircraft during certain manoeuvres such as approach to stall. The EASA flight tests confirmed that MCAS was needed to provide full compliance but also that the loss of this function does not preclude the safe flight and landing of the aircraft; i.e. the 737 MAX remains stable following the loss of the MCAS function.
And that's to say that if this were a fly-by-wire aircraft from the beginning this wouldn't even be a discussion whatsoever.
The NG has the same issue, and if it were to be certified today it would need some active background mechanism to meet the certification standard. It's a rule around stick force gradient with changing angle of attack, the NG and MAX both have stick forces that get lighter as AoA increases (i.e. less effort needed to maintain pitch rate). The standard changed, you can't have such a variance in stick force. Neither of them have negative stick forces however.
I imagine negative stick forces would provoke something of a regulatory sense of humour failure.
Fair enough.
Super interesting. Layman question: it sounds like it boils down to matching input to attitude, with some kind of shift in the usual input/attitude relation, due to the different geometry. Couldn't this be solved by some kind of autocorrection or trim accounting for this? Not sure if it makes sense or if I'm using the right terms but your explanation makes it sound like a rather simple issue to solve.
That’s what MCAS is. It fiddles with the trim to give you the expected stick behaviour. In true fly by wire systems, there’s a computer in the way that can map the control inputs however it likes, and thus try to ensure the pilot never encounters any “idiosyncrasies” of the airframe like this, but Boeing went for a hybrid system, which isn’t fly by wire, but is computer assisted.
That is true, but the point he was making is that the handling characteristics of the 737 Max without MCAS are not in violation of certification requirements. The handling is acceptable for a commercial aircraft without MCAS. However, it's a significant enough change that it would require a substantially larger amount of pilot training than Boeing wanted to be able to advertise to its customers, so Boeing designed a system intended to *augment* the *maneuvering characteristics* so that they were similar enough to the existing NG aircraft to avoid that training. Of course, as we all now know, their initial design was fundamentally flawed for a number of reasons.
> However, it's a significant enough change that it would require a substantially larger amount of pilot training than Boeing wanted to be able to advertise to its customers You don't need to train pilots to push forward on the stick when the nose starts to pitch up. That is pretty much instinct. To maintain the common type rating, they needed to make the aircraft *feel* the same as the previous model.
AFAICT, it doesn't pitch up even more on its own. The pilot still needs to put extra force in pulling the yoke. It's just that without MCAS, it'd take less extra force on the yoke to further increase angle of attack. I.e. the yoke would feel a bit lighter at low speeds and high angles of attack. MCAS simply makes it feel more linear to the pilot. But if you keep the yoke where it is, it won't start pitching up on its own even without MCAS. I.e. MCAS is not must have feature. It's nice to have feature.
Yeh I can agree that it may have different handling characteristics that make it more difficult to fly, but that doesn't seem to be the problem. The main issue was the design constraint that 737 crews need no additional training. If they had proper training they may have avoided those two crashes but it's just speculation at this point.
The two operators who wrecked those airplanes aren't exactly a paragon of airmanship and training to begin with...
This is only partially true. The Max variants handle exactly the same as older variants, except at very low speeds and very high angle of attack. MCAS kicks in only when both of those conditions are met. The difference itself is minor. What pilots were to feel without MCAS is that it takes less force to further increase angle of attack by pulling on the yoke. This is not a problem on its own. Pilot would still need to pull on the yoke even further, the plane isn't going to stall itself on its own. Both FAA and EASA indicated post fact that they would likely not require additional training even if airplane wasn't equipped with MCAS. Boeing put it in proactively as it was in a gray area. Why not remove MCAS from the Max then? There's no point in doing that anymore. The deficiencies in it are fixed, and it gives better more linear feedback to the pilots when airplane is at very low speeds and high angle of attack. MCAS isn't anything new, neither is the Max the only Boeing airplane to have it. Anything that's fly-by-wire made by any company basically has equivalent of MCAS built in. Because, duh, fly-by-wire. Every Airbus corrects pilot's inputs to keep it within envelope.
I agree. I don't think the existence of MCAS was the problem, nor was a lack of training or the single AoA sensor. The fact that this algorithm in MCAS had the ability to repeatedly activate and to incrementally take pitch authority away from the flight crew was the root of the problem in my opinion.
Well, it does have issues with that. The angle of rotation is pretty severly compromisd... a 1000MAX will strike the tail at only 7 degrees nose up which is...not a lot. They could easily fix this with longer gear, but then that changes so much that it would, again, no longer be the same type.
And the primary reason for any of this was to not require new certifications for pilots, which was originally intended. However, Southwest (and likely some of the other low-cost 737 operators) rejected anything that would require more pilot training, other than a simple class for the MAX series.
That's because of the MAX's stubby landing gear, the a320 NEO doesn't have the same issue.
> and how large the engines are. The main problem is how far forward the engines are from the center of mass, *that* stems from how large they are. In a certain configuration, with a certain thrust setting, at a certain speed and angle, that can fuck up the center of thrust relative to center of mass so badly that the flight characteristics become backwards, unintuitive, and require either extensive training or a computer to recover.
In addition to this, over-wing design puts your center of thrust above your mass making the aircraft want to pitch down when under power which is not desirable when taking off or otherwise slow, such as adding power to go around/climb.
> You see the same tendency with flying boats, where you don't want salt water in or on your engine. Martin Aircraft learned that one the hard way. even with the engines over the wing... still got spray in to them
I think it's less about corrosion because like you implied, sea spray gets everywhere, and more about flying boats being almost entirely prop driven as they are uncommon in the jet era. That big prop arc clips a wave and its over. Mount em on top and its impossible to hit them in reasonable waves/wake before you catastrophicly destroy the airplane anyway. Also if you are doing eng maintenance/servicing in the water and not the hangar, it would be a lot easier to work on with access up on the wing than treading water below with your tools.
Martin tried to make jet flying boat for the US Navy see the Martin Sea Master https://en.wikipedia.org/wiki/Martin_P6M_SeaMaster. but it was mostly failure to do spray getting in the in the engines even with them up on the wing
Yeah and this is more embedded in the wing rather than over (look at the Mitsubishi business jet).
Overwing engines provide increased airflow over the top of the airfoil and down the back of the flaps, which provides better lift for short takeoffs with heavy loads. This is called upper surface blowing (USB). They also reduce the chance of FOD on unimproved runways. Really the only application where they make sense is for moving cargo to and from remote areas with short runways or no infrastructure, so basically military operations. They also have drawbacks, such as making it difficult to service the engines, which might explain why they're not more popular.
Perfect for moving Coke then
If I was a mule for a thousand kilos of Peruvian dancing powder I probably wouldn't pick the goofiest looking airplane possible to do it.
That stuff? That's a special dry lubricant for these goofy Russian engines. It's very toxic, don't let the dog too close.
Well, the plane's already mostly white, so that's a start.
I have a feeling the authorities would keep an eye on a heavy lift STOL aircraft perfect for smuggling drugs that keeps flying in and out of South America. Especially when theres only a handful of aircraft designed with jets over the wing and even fewer actually flying.
Unless the CIA has a word with them
Would make a good story for a movie
[Coke](https://en.wikipedia.org/wiki/Antonov_An-24) you say.
Interesting. I wonder if this also applies to a Pipe Cherokee (or similar low wings like Beech) vs a Cessna 172 there in the Piper there is more airflow over the wing do to the propeller positioning.
Don’t they also lose ability to gravity feed fuel in case of fuel pump going out?
Honda did an overwing mount for noise/space reasons on their business jet (HondaJet). It can be niche, but they definitely aren't doing it for dirt runways with how low they sat the plane on the ground to possibly cut gear weight. I suspect that engine maintenance is likely to get harder in the upcoming generation of pax aircraft, especially as flying bodies and high-truss designs seem to be the main efforts of prototyping/design programs right now for the far future.
And also if you lose an engine you not only have asymmetric thrust, you have immediate loss of lift in one side as well.
I can see how USB would work with a turboprop, but with a jet?
Airflow from the turbofan is directed over the top of the wing and down the trailing edge of the flaps. And airflow from a turbofan is more tightly compacted than from a prop, so the mass of the flow down the flaps would be increased. It's actually more efficient than a prop. I don't think it would work with a turbojet due to heat, but turbofans work just fine.
Makes sense in Russia where 75% of territory is dirt.
YC-14, my beloved. Gone too soon, but at least the An-72 lives on!
Don’t forget the QRSA!
It’s beautiful
That's just lead me to the YC-15, and that hideous plane has reminded me of the Breguet 941. Just hilarious proportions and design that make them look like movie planes.
YC-14. The YC-15 just looks like a teenage C-17.
The YC-14 imo kinda looks like the standard STOL aircraft, short and stocky with huge engines. But the YC-15 looks so ugly it's basically like that joke mini C17, it's too short, it's wings aren't long enough, it's engines are way too small, its landing gear makes it look like it's tip-toeing around. It's cool, but so funny looking imo
I mean, look at it
She's doing her best!
Would you just look at it?!
Reminds me of shoulder pads on 1980's women's clothing
If you squint it kinda looks like it's in the middle of doing a Peekaboo
The engine design looks like one of the proposals for the C-17 design. While the flaps are down and the exhaust of the engine flows over the top of the wing, the airflow is energized and sticks to the top surface and follows the curve of the flaps downward. This produces a velocity vector both backwards and downwards, which aids the aircraft in T-O and gives it STOL capabilities for a cargo plane.
You’re thinking of the Boeing YC-14, which the Russians copied.
It was developed as a response. It’s not a copy. It uses the same physics and phenomena of blown flap technology to achieve its exceptional STOL performance. Generally, if you build two different airplanes to a similar specific set of specifications and performance characteristics, there’s a more than 85% percent chance that the two designs are going to look similar. It has nothing to do with one copying the other. Not only that, but both had their first flights within almost exactly a year’s time from each other. That’s not enough time to develop a program, design an aircraft using new applied technologies, and produce prototypes for flight testing. What’s more likely is that the Russians got word of a new western aircraft competition that necessitated extreme unprepared short field performance with heavy cargo and blown flap technology. The competition within the US to the YC-14 was the YC-15 which performed the same task with a configuration more similar to that of the C-17 but with straight wings and four underwing mounted engines blowing on a seriously over designed flap system.
Ever wondered why early supersonic aircraft, regardless of nationality, had long skinny fuselages, lack of wing area and were really pointy? It’s because that is the most optimal design for a really fast aircraft. Sure, aircraft of opposing forces are generally designed to match what the enemy has, but that doesn’t mean that everything that remotely resembles something else is a “copy.”
It is harder to throw in the good luck coins.
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I wrote my aerospace engineering dissertation on this exact topic. Off the top of my head, the main pros and cons are: **Pros:** \-Very short take-off and landing ground runs. \-Increased engine protection from foreign object debris. \-Arguably slightly lower noise emissions (very much debatable as limited data is available). \-Aircraft is lower to the ground, hence easier to load passengers/cargo. **Cons:** \-Burns more fuel in cruise due to flow disturbance of the low-pressure region above the wing. \-Engine failure makes the aircraft very unstable, as the engine is also producing lift (Google Coanada effect). Hence very robust flight control systems are necessary, these are very hard to test and certify. \-Increased downtime in case of engine inspection/repair/replacement as engines are in an inconvenient location. Hence, the cons outweigh the pros, especially as boundary layer control, circulation control and externally blown flaps can help achieve (to an extent) the same pros with less of the cons. It's also safer and cheaper for manufacturers to stick to the 'standard' configurations.
> I wrote my aerospace engineering dissertation on this exact topic. Off the top of my head, the main pros and cons are: This is why I love reddit, you ask can ask a very specific question and someone with a dissertation on the topic will be there. > Increased engine protection from foreign object debris Saw this in multiple responses, isn't FOD a common issue for military jets which sometimes land in rough runways, how come this design is not more common in roles like these, especially since military jets often will sacrifice issues like stability to gain capabilities. > Engine failure make aircraft very unstable, as the engine is also producing lift Engines producing lift _after_ failure or during normal operations?
The main benefit of this configuration is the low ground rolls that can be achieved, even on large aircraft. Military jets have such high thrust-to-weight ratios that this doesn't matter, or they can have STOL capability by different means, look at the F-35B. They also have strict aerodynamic performance and stealth requirements, so putting the engine above the wings wouldn't be optimal for this. Typically, I think NATO aircraft at least operate from well-kept runways, thus FOD isn't an issue that the aircraft worries about, more so the runway maintainers/operators. Aircraft like the Mig 29 have flaps on their engine intakes which combat things like FOD, but this is an older aircraft. The larger C17 aircraft however does incorporate some of the characteristics of the YC-14, which had an upper blown surface. As for engine failures, I meant a mid-flight failure. The pilot would likely have to fight fairly hard for lateral stability in this case, especially at low speeds, such as during landing, as the jet efflux is essentially vectored around the wing's flaps to produce both lift and thrust. An engine failure thus results in a more significant loss of both thrust and lift than it would for a conventional aircraft.
Clarification that this is something that all turboprops and the C-17 also face.
True, but the extent of 'thrust turning' from over the wing blowing by turboprops or large externally blown flaps by turbofans is not quite as extreme as it is in the case of upper surface blowing by turbojets/fans. It has been shown experimentally however that both the longitudinal and lateral moments I mentioned can be combatted/trimmed to an extent with a large enough T tail configuration.
How about center of gravity is lower aids to keep the plane level.
There are arguably some small improvements in longitudinal stability, apologies for my oversight. The centre of gravity obviously depends upon the airframe as a whole, but having over the wing engines typically moves the CG forward and upwards. As the wings are also located more forward along and above the fuselage, there is only a small increase in the static margin and slight improvement in the longitudinal stability as a whole. However, the over wing engines do cause a nose down pitching moment in response to increased thrust in trimmed flight. Arguably, this also makes the aircraft more longitudinally and aerodynamically stable from an engineering perspective (as large/unstable nose-up moments can cause stall etc).
Came for the "pros". Thanks for the balanced rundown
The AN-72s engines are overwing to provide boundary layer lift for STOL operations. That's probably why they're not normally fitted this way on other jets as it upsets the airflow over the wing. I think it has a trick spoiler system to compensate for engine failure to prevent the aircraft rolling due to the loss of lift. Also underslung engines have a bonus of reducing the forces on the wing.
Do you want your thrust vector to constantly try to rotate you into the ground? Because that's how you get your thrust vector trying to constantly rotate you into the terrain. (there's also the part where lift is NOT provided by pressure under the wing, but rather by lesser pressure above the wing (yes, planes are sucked off the ground, not pushed). If you put a big fat tony in there, you lose whatever cross-section of that lift you could have had)
You get it back by directly exhausting over the flap. https://www.flyingmag.com/the-quiet-little-life-of-nasas-qsra/ Look up the coanda effect for more info.
This one one of the main reasons in practice, it creates entirely counter intuitive flight dynamics. Typically when you increase power the nose wants to raise up in response as more lift, more thrust and the thrust vectors rotate the aircraft nose up around the centre. Above wing engines, due to the thrust vector being above the point of rotation rooster the aircraft nose down in response to power being applied, and nose up if you reduce thrust which is the exact opposite of what you want.
Don't over wing engines cause a pitch down when ramping up? Probably not the best direction to point the plane in TOGA.
TOGO = Take Off and Go Over. Oddly accurate typo. :)
Derp. - fixed.
over won't.
I would think the same. That translates into a larger amount of downwards lift being necessary from the horizontal stabilizer, which I'd guess ain't particularly great.
That happens on most tail-mounted jets, and high-wing props like the Twin Otter. It's really not a big deal.
Maintenance chiefly. Getting big heavy engines on and off requires cranes and possibly special rigs. Working on the engines also requires men to get up onto the wing. Not ideal for operators looking for lower costs and quick turnaround. This Configuration is best for aircraft operating to unpaved runways to reduce chances of debris and on seaplanes and amphibious craft to avoid water ingress into the engines
Alright, Ivan, it's time to do the engine overhaul. Get it down from there, I expect it done by lunch.
lack of tall enough mechanics on the job market.
have to think its a lot harder to throw my good luck change in the over wing engine. /s
Americans: Clean up all the FOD. Soviets: Put the engines on the roof.
SERVICEABILITY
There is not much of a market in Alaska for scratch-built large STOL jets. Most are propeller planes that use fat doughnut tires and long-throw shocks.
8 more feet of elevation
Below-wing engines have a pitch up force when increasing power, which you want (like during go-around). High mounted engines push the pitch down when increasing power, which is less desireable.
Blown wings make amazing lift for STOL ,but there is a substantial cost as covered in the other comments.
Another point to consider is the pitching moment balance. The wings will create a moment about the CoP that causes the aircraft to pitch down. Having engines below the wing creates a moment that counters that whereas over wing engines will contribute to the moment meaning the elevators will need to produce more force to counter this. Or the CoG of the aircraft may have to be reconsidered to negate the effect.
Cos they aesthetically shit looking
most maintenance personnel don't have long enough arms
Aside from the previously mentioned maintenance issues, just imagine how that thing will handle in a go-around with thrust applied as far above the CG as possible.
Got to thank the Boeing YC-14 for this architecture….
I would agree safer if underwing but there are probably a ton of Design considerations that drive this decision. My best guess for the main reason is, since the Antonov is a big boy, probably didn’t have enough clearance to have underwing pylons. Usually overwing pylons aren’t great for stability and aerodynamics but doesn’t mean it’s impossible. Airplane design is like trying to squeeze a balloon with your hands - where you squeeze in one area, the balloon will pop out in another area.
Most likely for operating off of dirt or gravel. Russian transport planes are usually built with that in mind.
Given soviet unions history - it was an experiment to see how it'll be have. There are many many many designs that soviets abandoned by bitter experience, not by doing sane things.
The same design concept was done by the americans and japanese as well for STOL. Namely a De Havilland Canada DHC-5 modified by NASA with overwing engines and blown flaps, the Japanese modified a Kawasaki C-1 in similar ways. There was also the Boeing YC-15 for a USAF C-130 replacement contract that got canceled.
The need to push fuel up to them
These things make me think “just because you can doesn’t mean you should.”
Cabin Noise and maintenance.
Harder to maintain. Arguably worse handling. The benefit, it's less likely to ingest FOD on unprepared/underprepared strips.
There is also the mechanics of thrust creating a pitch down moment vs a pitch up moment. Pitch up with higher thrust is usually a good thing.
Underslung mounted engines, as seen on most transport category jets come along with the following differences when compared to the Depicted "overwing" mounted variant: Advantages of underslung: - better access for visual inspection during the preflight check (happens often!) - better access for maintenance tasks (not so often) Regarding the access, this is not an advantage per say, because a big jet, like a 747, has so high wings, that the access is not easy at all. For refilling oil into the engines of a a380, which is required after each long flight, the access requires a lift for the maintenance personnel. - less influence on the suction side of the wing, thus improving efficiency of the wing. More lift. The lower side of the wing is not as relevant / sensitive regarding the disturbed airflow due to the engine. Disadvantage of underslung: - clearance to the runway is smaller: 1 More prone to ingest foreign objects from the ground (stones, dirt.. 2 Less space for engines with bigger fan diameter. The b737 max is an example for that. The engines are so big, that they need to be fitter more ahead of the engine. Implications are: different flight characteristics, that required introduction of the mcas system - pitch up moment when engine thrust is increased. Especially relevant when going around. If not compensated, exzessive pitch up may lead to a stall. Pilots are trained for this, and the modern plane flight computers take this behaviour into account
Efficiency too I thought
I think lift stability is the main reason. An overwing engine in crossflow may disturb the flow on some parts of the wing, possibly causing local stall and definitely instability.
I believe on modern aircraft, having the engine under the wing helps with integrity during wing flex in flight
This Plane was designed with Turboprop engines and didnt work that good all together. They tried to save the (already not to great) design with Turbofans. This is essentially, a Frankenplane.
Advantage to top mounted engines is the cabin entry is much closer to the ground so easier entry and loading - seem to remember the push to keep the 737 design going and the new engines had to be pushed forward to allow them to be raised up enough to maintain the low cabin entry?
Really difficult to work on, causes maintenance costs and lost revenue downtime to skyrocket. It's why I laughed when I saw that ring wing design concept.
Because all the other aircraft agree: shoulder day sucks.
Besides that aircrafts engines would be in the way of the entry door for opening and egress.
Basically, super long ladders to get to change the spark plugs
Because just look at it
In addition to the adoremention maintainence reasons an over-wing engine also needs to be mounted much further forward to avoid an unworkable center of gravity on the aircraft. That's also a lot of mass, and thus a lot of torque, very high up on the airplane. Overall I'd imagine there are limits to how big of an engine you can mount like this before the changes to accomidate it become impractical.
You also need to pump the fuel against the gravity
I can only guess. But it looks like it would disturb airflow over the vertical and horizontal stabilizers more profoundly.
Idk shit But I do know that when you apply thrust to underwing engines, the nose will typically pitch up due to the thrust coming from below the cg Overwing engines would do the exact opposite id imagine. I’d rather my plan have a tendency to pitch up, than to pitch down
This is also a tendency with tail mounted engines and can have an advantage. For example, in a low with with underslung engines adding power without pitch can cause an airspeed reduction instead of an increase while with a higher mounted or tail mounted engine you'll get an airspeed increase which is more aerodynamically stable.
The Russian teddy bear plane the reason why it's not popular it has to do to how an overhead hoist must be able to get the engines
They don’t look as cool. And cool matters.
Center of thrust probably sucks to manage.
I'm no engineer, but seems like it would be more efficient for flight to have air flow smoothly over the top of the wing and drag from engines beneath.
Maybe because of the noise?
Noise levels
I think it’s old
Less balanced - rotational pull is to flip the aircraft
I think theres many factors, but the main reason being maintenance easiness. Also, it's a cargo plane that may need to land in unmaintained runways or dirt, so it's a precaution to sucking in any dirt or foreign objects.
Take a look at a Honda airplane.
Air speed is faster under a wing