Eli5: why are some airplane jet engines under the wings and some on the vertical stabilizer?


Eli5: why are some airplane jet engines under the wings and some on the vertical stabilizer?

In: 3

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.

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.

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.