Because different airplanes are designed differently to account for amount and location of weight, payload, thrust, control, and other flight characteristics to achieve different goals or make different trade-offs, just like how everything is designed.

Why doesn't every car look the same, have the same engine, and use the same tire size?

They are easier accessible for maintenance under the wings. But the noise is less when they are mounted in the back of the plane.

There hasn't been a model designed with an engine on the vertical stabilizer since like the 70s/80s.

As for why, it has to do with reliability and the ability to make an emergency landing if 1 engine fails. Modern jet engines are reliable enough that only 2 of them are needed for the vast majority of airline flights. In the past, 3-engine or 4-engine designs would be used for airline routes that fly over oceans or over isolated areas where the nearest available airport in case of emergency is very far away. For further information about this, search for "ETOPS" (which is what the regulation is called). "4 engines 4 long haul" was also a phrase used in the past by certain airlines which preferred flying 4-engine planes, and by Airbus to market the A340 against competitors such as the Boeing 777.

And of course, with a 3-engine design, the only place to mount the center engine and still be balanced, is the vertical stabilizer.

What does ETOPS have to do with engine placement on the stab vs. under-wing? I don’t think you answered the question (which is about placement, not quantity).

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I answered how quantity is related to placement. You can't have 3 engines and have all of them under the wings, they wouldn't be balanced.

Pretty sure OP meant something like a business jet, with the two engines on the tail, compared to a big airliner with under wing engines. Not the old 3 engine liners.

Don't complain to me about assumptions you made about the OP

They very clearly said vertical stabilizer

>Why doesn't every car look the same, have the same engine, and use the same tire size?

Well... Why? What design requirements dictate the specific design choices?

This is a complete non-answer.

But there have been several planes with all the engines in back, like the Boeing 727 witt three engines, or the MD80 with two engines

That's outside the scope of the question asked by the OP. They asked about wing mounted and vertical stabilizer mounted engines.

> Well... Why? What design requirements dictate the specific design choices?

Similar ones. For a car, engine, and tire size: power, fuel consumption, grip, suspension, stability, cornering, ride comfort, cost.

I already listed some flight requirements that dictate design choices for a plane:

> amount and location of weight, payload, thrust, control, and other flight characteristics

Big planes have multiple big engines. It makes sense to mount big engines on the wings, because that's where you have lots of physical space for them, and that's where the lift is generated, so they're lifted by the wing directly. Trying to mount them to the fuselage would require heavy support structures to mount the heavy engines, increasing overall weight beyond the engines themselves, putting more stress on the connection between the wings and the fuselage. It also makes the maintenance much easier, because the engines are close to the ground, rather than high up. The downside is having engines close to the cabin. This is true both for jet and old piston engines.

A small plane has smaller engines, but also very little ground clearance. So it's pretty impractical to try to put them under the wing, and it'd complicate maintenance to put them inside the wing. At the same time, because of the size difference as compared to the airliner, mounting them on the body doesn't require as much reinforcing, so you don't suffer the same weight penalties. It has an added bonus of making the cabin quieter, which for a luxury jet is certainly a factor.

Yeah, exactly, you're just listing stuff, you're not even trying to actually say which ones matter for what.

Why do some cars have an engine in the front and others in the back? Why do some planes have straight wings and some swept? Why are missiles pointy but torpedoes round?

bEcAuSe ReAsOnS LOL

> Why do some cars have an engine in the front and others in the back?

Mass distribution and the torque that can be generated on a driveshaft depending on whether the drive wheels are at the same end as the engine or the opposite end. Mass distribution affects stability and control, just like it does with a plane, like I said.

Yes, all of airplane design can't be explained in a reddit comment. Center of gravity, lift, and the tail force to control the plane are the reasons for choosing the engine location on a plane. Otherwise an unbalanced plane is difficult and dangerous to fly, with unstable recovery modes. The center of gravity, center of lift, and the forces applied by the control surfaces have to be balanced to fly the plane with stability. Large, heavy engines are usually placed near the center of mass of the plane. Engines placed elsewhere have to be balanced in other ways to keep the whole plane level and well controlled.

This sounds like a reasonable ELI5 explanation, why not put that as the top level comment?

The biggest benefit that I remember from my design classes of having the engines on the tail is the reduction of adverse yaw. When you design planes, you have to design them in a way that if things break they don't crash. So planes need to be designed to be able to fly with only 1 engine. If the engines are on the wings, they are further from the center of gravity, and they'll make a bigger moment (or twisting force) on the plane. This makes it harder for the rudder to overcome, so you'll need a larger rudder and a more powerful actuator to control the rudder, which is more heavy. If the engines are on the tale this is less of an effect and you can design a smaller rudder and actuator (not exactly an apples to oranges comparison as you'll need to increase the structural parts of the tail to account for the engines). On top of this, it looks cool, which is a surprisingly big factor in aircraft design, especially for business jets. There are a few other benefits, for example less issues with tailwash from the engines wake and a few other things, but you get the gist.

Underwing versus on the back is not the same as why one on the vertical stabilizer or just the air intake there

If you have an odd number of engines you need to place one in the center line of the airplane and with a jet engine, you can put it in the front line with propellers.

You can but even the number of engines under the wing or on the body. 4 engines on the back have them placed side by side like a Vickers VC10

3 engines existed primarily because of ETOPS rating, you could fly longer over open water with 3 compared to 2 engines and the cost will be less than if you have 4 engines. A minor part is that it adds high-altitude takeoff performance in locations like Colorado which is at 1-mile altitude.

They have disappeared because engine performance and ETOPS regulations have changed so you are allowed to do the same flight with just 2 engines and it is a cheaper way to build and operate aircraft

Boeing 727 have all 3 engine in the rear, and McDonnell Douglas MD-11 had 2 under the wings and 1 in the back. So 2 engines under the wings or on the back are unrelated to the vertical stabilizer position of a third engine.

There have in fact been at least two planes designed in the last decade (or so) with an engine in the vertical stabilizer

The planes with single engines in the vertical stabilizer are due to a law that required twin-engine aircraft to always be within 60 minutes of an airport.This regulation came to be known as ETOPS: Extended range Twin engine Operation Performance Standards. (Or, "Engines Turn Or Passengers Swim)

This law did not apply to jets with 3 or more engines. If carriers wanted to fly longer, over-water routes, they had to follow coastlines, or fly planes with more engines. It's cheaper to operate 3-engined aircraft than 4-engined, so the market made 3-engined aircraft.

Planes with an odd number of engines can't divide them between the wings; they need the odd one in the fuselage. It has to point aft, so the tail is the natural choice.

With improved jet engine reliability, ETOPS standards have been extended from 60 minutes to 120 minutes in 1985, and 180 minutes in 1988. Some twin-engine aircraft are now certified to fly up to 370 minutes from a diversion airport. The era of trijets is rapidly coming to a close.

The reason depends on your "mission" . Underwing engines let you have bigger more efficient engines. This also make maintenance easier. The down side is you need long landing gear and they could suck stuff in when on the ground. This puts the aircraft body higher off the ground so you need extra stuff to get people and cargo in and out. So if you need your aircraft to be low to the ground or take off from dirt or grass you have to mount your engines up high. This why lot smaller jets have the engines mounted up on the tail. Down side is this also limits how big they can be, but now your aircraft can sit lower to the ground and not need things like a jet-bridge or extra equipment to load cargo.

Something to note is when you look at aircraft like the older 737 and A320. Boeing wanted to make the 737 as low to the ground as they could wile keeping under wing engines. This is why they have that odd pouch shape. Airbus just keep the landing gear longer. 737 was designed with the idea that could go in airports that might not have the gear to load cargo in to a taller aircraft. Also the newer regional jet tend to have engines back on the tail. same thing they need to be go in to airports with out jet-bridges.

E-jets and most business jets have tail mounted engines. the reason is mission. tail mounted engines let you have short landing gear and less chance of FOD damage on crappy airstrips. this lets you load pax and cargo with out needing extra equipment. even with under wing engines if you look at the 737 vs A320. the original 737 used turbo jets so they could keep the gear as short as possible when they change it to the CFM's they had relocate the some of the pumps and you ended up with the hamster pouch nacelles to keep the low gear. Airbus when they built the A320 years later airport infrastructure had improved a ton so they just keep the gear taller.

Main factors:

1 Engine noise: you want the engines to be away from the people. Having them in the tail is a good solution.

2 Structural weight: above a certain size, you gain a lot by optimizing the aircraft structural load. Engines and fuel are heavy. Having fuel and engines strapped to the wings means they are directly supported in flight. On the contrary, if you put the engine in the tail, the wing has to support the fuselage that then supports the engine, which asks for more structure, so more weight.

Conclusion: as long as the plane is small enough, tail engines are more silent and not so heavy. On bigger planes wing engines allow for a lighter structure, and due to dimensions, noise-wise they are still far enough from people.

Perks: wing engine are closer to ground and this saves a lot of maintenance time, and increases maintenance quality (the more comfortable is the technician, the better he can check things)

-source: I’m an aircraft mechanic (but not the designer, so take this with a bit of salt, design is not my job, shouldn’t be my job, damn I hate when I have to decide where to put the engines myself, that’s a red flag for the company)

OP wasn't asking about 727, DC-10, or L1011-style engines. They were asking about DC-9-style mountings, or the common tail mountings on business jets. Unfortunately wording on their part.

This is a great explanation. Thanks!

OP needs to clarify if they mean one engine in the middle of the vertical stabilizer (e.g. Lockheed L-1011), or two engines at the tail of the aircraft (e.g. MD-80). Though some have both (e.g. Boeing 727).

If you want an odd number of engines on aircraft, one of the engines need to be in the middle of the aircraft. So the middle of the vertical stabilizer is a good place. This design made sense back when 2-engine airliners were not allowed to fly overseas routes, but 4 engines became expensive, so they opted for 3 engines. More recently, some small jets have a single engine, and at least one design (PiperJet) was developed with an engine in the vertical stabilizer.

https://old.reddit.com/r/explainlikeimfive/comments/11d3gw1/eli5_why_are_some_airplane_jet_engines_under_the/ja6wm82/

I'm aware of that but OP specifically used the term "vertical stabliizer", which excludes fuselage-mounted tail engines.

to layperson it looks the same

Do layperson say "vertical stabilizer"? Most of them just say tail

What about the HondaJet with its engines mounted on top of the wings?

Seems strange. If you have an engine failure in a 4 engine aircraft, why not just shutdown the opposing engine to match the thrust? Presumably if one of the wing engines failed in a trijet, it would be the same thing.

Exactly. The aircraft could continue with just the tail engine. In a 2 engine aircraft however you've run out of engines, unless you want to try to crab your way to landing (hence staying close to an airport)

You don't need to balance out the thrust in the event of an engine failure. You can fly with unequal configurations, it's just not efficient to do so by choice.

I always liked the MD-80..

Aircraft are designed with engine-out operation in mind. The vertical stabilizer provides sufficient yaw authority to operate with an engine out.

You don't shut down a good engine if you don't have to, because you might need it, and it might not start again. You might increase thrust on another engine on the same side, or reduce thrust in an opposing engine, but you're not going to shut a second engine down after one has failed.

You kind of missed the point and just compounded on what I was saying- surely if the other engines and the control surfaces can compensate for the failed engine the rule you laid out makes even less sense?

You misunderstand. Four engines is more expensive to operate than three which is more expensive to operate than two. Airlines what planes that are cheap to operate. Due to ETOPS regulations the cheapest two engine planes weren't the cheapest to operate on certain routes due to having to take longer routes to stay within x minutes of an airport. So they went one engine up. Three engines made the plane more expensive but this was countered by being able to fly more direct routes.

Four engines were used only on the largest aircraft due to needing all that thrust to fly. Modern engines are powerful enough to only need two and ETOPS regulations have greatly relaxed with increased engine reliability.

When a multi-engine plane loses and engine they never shut down an opposing one if they can avoid it. Planes can fly with asymmetric thrust, it's just not efficient. So while you would never design a plane with asymmetric thrust you don't crash when it happens.

Using fewer engine worse, more engine better.

ETOPS is (was) for planes with two engines. The thinking was that if one engine failed, you only have one engine in reserve. If it, too, fails, the passengers will be swimming for hours or days before help can arrive. Overland, the loss of both engines leaves the pilots with one final option before passengers are endangered: A dead-stick landing on the ground (See: "Gimli Glider"), or near enough to land for the passengers to be quickly rescued (See: "Miracle on the Hudson"). Flying out to sea, you can't expect a quick rescue after a forced landing. Regulatory authorities don't like it when you are operating with no redundancy left. They really hated the idea of twin engine aircraft flying over the ocean, just two failures away from catastrophe.

ETOPS initially said that If you wanted to fly a route that took you more than 60 minutes from land, you couldn't use a twin-engine plane. You had to find a plane with more than two engines.

You don't want to fly a three engines plane because it is less economical, but you are required by law to use at least three engines. You could use four, but that's even less economical. There was no practical need for three-engine aircraft (outside of extraordinarily rare double-emergency situations), but the law regulating twin-engine planes prohibited them from making these flights. Airlines needed three-engine planes to bypass this law.

ETOPS certification was incrementally expanded from 60 minutes to 370 minutes for some twin-engine aircraft and airlines. If you could prove your engines were reliable enough, and your maintenance program thorough enough, you could acquire additional certification to fly farther from land. With such certification available, more routes can be legally operated with twin-engine aircraft. Fewer routes legally require three engines. Presently, 99.7% of the Earth's surface is within 370 minutes single-engine flight time of a major airport. 99.7% of all possible flights can be undertaken with twin-engine aircraft. There was never much practical use for a three-engine aircraft in the first place; now there is only 0.3% of a legal requirement left. It's now cheaper to slightly alter one of the few remaining routes that twins can't legally perform than to maintain fleets of tri-jets to perform them. (Basically, they have to fly around the south pole, rather than directly over it, so they stay within 370 minutes of airports in South America, Africa, Australia, etc.)

Even 4-engine aircraft are being replaced with twins. Boeing ended production of the four-engined 747 in favor of the twin-engine 777 with nearly the same capabilities, because carriers prefer jets with two huge engines instead of jets with four smaller engines. Twins are simply more efficient to operate. The only thing that stopped them from using twins was the law, and the law has stepped out of the way.

You are correct that I don't understand your point, but I think the problem is that I failed to adequately explain the law. Your criticism doesn't seem to address a situation the law was trying to regulate.

What?

Which bit?

All of it. If you lost one engine, you would never unnecessarily shut down another. And if you lose one engine on a twin, it absolutely will continue on the remaining engine.