eli5: Why do small jet aircraft, like regional jets and private jets, have engines on the rear, but larger jet planes have engines on the wings?
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It may have to do with the weight of the engine. A plane is an engineering marvel that is made to propel through the air and carry pilots, objects, and sometimes passengers, and itself. A small plane balances in the air differently than a large plane. You would have to look at the specific plane and think about balance to ascertain the position of the component parts. Obviously the way the plane is flown impacts the balance, and pilots must be aware of conditions of weather and weight of the souls on board.
Jet engines have to be placed a certain distance above the runway. This is both so it does not scrape the runway on touchdown when the body and wing flexes but also so that any debris and dust is not sucked into them. And these smaller aircraft do not have enough room under their wings for the engines and the clearance to the ground. If you put the engine on top of the wing you will be disrupting the airflow over the wings a lot more and you lose a lot of the wing surface. Putting the engines anywhere else on the body will put the exhaust right onto the rest of the airplane. So the only good place to put the jet engines is in the tail.
For larger airplanes the added complexity of having the engines in the tail is not worth the effort. Not only do you need to bring the fuel from the wing tanks all the way to the tail but the weight of the engines will also have to be carried by the wings so you need stronger beams between the wings and tail to carry the weight. So it is much better to have the engines mounted under the wings.
Typically, the wings on a smaller plane are closer to the ground. The plane isn’t as thick and long thin landing gear are too easy to break or too heavy to lift.
There isn’t enough space under the wing for the engine, and safe clearance against the runway. Mounting them to the body works as well, and the longer fuel pipes don’t add much weight.
It’s preferable to have the engines below the wings because it makes it easier to inspect and maintain the engines, and it keeps weight centered and reduces the amount of bending that the wings experience in flight.
The drawback is that in order to keep the engines from vacuuming up dirt and debris, you have to make the plane higher off the ground. This only really works for large planes intended for airports with ground equipment like jetways.
Smaller aircraft or aircraft intended to be self-sufficient in terms of getting people on and off can’t easily have the engines on the wings since they would be too close to the ground and so they go higher up on the tail.
Small jets are short, and that’s a design feature
Small jets like private jets aren’t always serviced by a jetway. Private jets in particular want to have a set of stairs they can put down for people to get on and off, this means that you can’t be more than about 3/4th of a fuselage height off the ground, for a Gulfstream G650 that’s 8′ in diameter that means its about 6 feet up.
Jet engines are large, this is another design feature as engines with larger fans at the front end up being more efficient. The engines on a Gulfstream G650 are about 6′ in diameter so to put them under the wing you need to ensure the wing is up quite a ways, it needs to be engine diameter + pylon length + safety margin off the ground andddd now you’re wayyyy above where the stairs can go
Bigger jets are pretty much always serviced by a jetway or a rollup staircase that is as tall as it needs to be to accommodate the plane. The concerns for these planes ends up being that working on an engine 40 feet in the air is kinda sketchy and the weight distribution of the plane ends up being better if they’re low and a hair forward of the wing. On small jets, the wings are set further back to adjust for this change in weight distribution. You always want your plane’s center of gravity to be in front of its center of lift, if it isn’t it will quickly try to correct that for you
Some jet planes used to have engines on the tail… MD-80, Ilyushin IL-62 (has 4 tail-mounted engines). The old Lockheed Tristar and MD11 had three engines, two on the wings, one in the tail. Maintenance and replacing an engine in the tail was much more difficult though
Lots of good reasons listed below. Also, having two jet engines in the tail that are very close to each other makes them very easy to fly if one engine goes out, because they are both close to the centerline.
On a large jet, swapping an engine out is much easier if they are hung under the wing.
Along with all the other reasons listed, if the engines are wing mounted that leaves less room for high lift devices (leading and trailing edge flaps) which results in longer runways needed for takeoff and landing. Short field performance is important for the smaller jets to enable them to have more options for airfields.
This is one reason the Boeing 727 used rear mounted engines albeit three rather than two.
well, another reason is flight. Smaller planes don’t need two or more engines, so it is impractical to have two. And you can’t go putting an engine on one wing and not on the other, so the only practical way to do this is to put the thrust (propellers in front, jet engines in the middle or built into the tailfin) along the centre of gravity (CoG) so that deviations in thrust least affect the movement of the plane.
Something else to look at is that if you were to look at the forces applied to larger planes’ engine thrust output, you can see that they will combine only above or below the CoG, never either side from it. It is also difficult to control the thrust output on the wings so that it doesn’t yaw uncontrollably to one side if the thrust is not exactly the same as the other side.
Also, bigger planes need increasing space, so putting the space-consuming engines on the outside saves a lot of space for cargo/passengers. It is also impractical to have 4 or 5 engines inside the fuselage as there would be little room (if the exhaust goes into another engine, the efficiency becomes so low, you may as well not have that engine.) So at best you can only really fit two engines in the fuselage, and the effort required to get the engines on the wings becomes worth it.
Also additionally to all the correct answers about engine clearance hight there were commercial airliners with rear mounted engines but due to the long momentum arm (both in the lateral and vertical plain) it was found to be undesirable in the case of an engine failure so that’s why the engines are mounted under the wings and forward.
Closer to the center of gravity so in the event of an engine failure the aircrafts have static stability and are controllable.
Smaller buissness jets have a smaller momentum arm and less powerful engines therfore are more staticly stable even in the event of a single engine failure.
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