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?


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?

In: 312

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.

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.

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.

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