Why do most modern jet airliners have engines under the wings?

Question

For example, the 737 has engine cowlings that are flattened on the bottom for more ground clearance, I presume. It seems much less common now to see planes with engines in the rear, too, like the MD-80.

It seems like it would be OK to have the engines above the wings. This would allow the landing gears to be shorter (maybe?) thus reducing the likelihood of landing gear shimmy. It would also reduce the likelihood of foreign object damage (FOD) in the engines.

Perhaps the engines are so often under the wings in order to simplify maintenance, but it seems like the landing gear shimmy and FOD problems are more important ones to solve.

Answer 1

The engines are easier to mount and support under the wing. They also operate in clean air no matter what angle of attack they are at. In side winds and side slip the engine intakes aren't masked by the body. They get better noise isolation from the cabin. They are less likely to ingest junk than tail mounted engines. They can fail without significant risk of serious airframe damage. They are easier to service. They are near and below CoG, so they don't have a negative effect on balance or stability.

On the down side they reduce ground clearance and reduce the amount of wing that can have trailing edge flaps.

I don't think that's a complete list, but the pros win.

Answer 2

Mounting the engines under the wings has many advantages, mostly that they are low to the ground and easier to maintain. Of course, it does mean that the landing gear must be taller to allow for proper ground clearance (at least in most cases), unless of course the wing is mounted on top of the fuselage (like on the BAe-146-/RJ/RJX family).

The advantages of rear-mounted engines are that the engines are mounted higher off the ground, which makes them harder to access for maintenance but also makes them less likely to ingest debris. Rear-mounted engines make sense on smaller airplanes, where mounting them under the wings would require the landing gear to be much taller. This is why most corporate jets and regional jets have rear-mounted engines. Most regional jets with underwing-mounted engines have high wings., like the BAe-146/RJ/RJX family, the An-148, and the 328JET. And yes, there are modern airliners with rear-mounted engines, like the CRJ and ERJ families, the 717, and the Tu-334. Rear-mounted engines make the cabin very quiet in the front, because they are far away from the engines, but unfortunately the passengers in the back get a loud ride. Ever sat in the last row of a DC-9 or MD-80? NOT a pleasant experience.

There are a few airplanes that have above-wing mounted jet engines, like the Be-200, the An-72, and the An-74. All of these aircraft have high wings. Placing the engines above the wings makes them less prone to FOD, but also harder to maintain. The Be-200 is amphibious and it needs to have the engines above the wings to keep them as far from the water as possible. The An-72 and similar An-74 have the engines release exhaust directly on to the upper surface of the wings. Because of the coanda effect, the curved surface of the wings bends the jet of exhaust downward to push the aircraft up. This makes the An-72/-74 capable of very short takeoffs and gives it a fast climb rate, but also makes it slow and inefficient.

Answer 3

Yep, modern jet airliners have engines under the wings mostly for maintenance concerns, but also for weight and balance issues, and besides... if the engine were on top of the wing, it would block the passenger's view!

Landing gear shimmy is primarilly from the nose gear, and placement of the engines wouldn't really make a difference.

Answer 4

in the early years of aviation, up through the mid forties, engines were placed both above and below the wing. they were also placed in the wing as well. wind tunnel testing showed that airflow over the wings needs to be smooth to reduce drag, and to prevent stalling. as aircraft grew in size, the underslung engine location also makes maintenance much easier. one more thing that pretty much ended engine being placed over the wings, it was found that on take off, aircraft with engines over the wing tended to nose down rather than up due to where the engines thrust was located. this was most prevalent on the dornier do29(?) seaplane. it had twelve engines all on top of the wing, six facing forward, and six facing rearward.

Answer 5

The engines are placed under the wing due to the speed of the air required to go over the wing.
The air going over the wing must be going faster than the air under the wing. If the air flow is slowed or disrupted the wing will stall and no lift would be created.
This is called the Bernoulli principle.
http://en.wikipedia.org/wiki/Lift_%28force%29
Very colorful answers here but they are under the wing for a reason and not just maintenance.

Answer 6

You need clean air around the wings to great lift. If the engine is mounted on the top of the wing, it will disturb the airflow causing turbulent flow over the wings' top surface. This will kill lift and cause more drag.
So simply, it is not efficient.
Moreover, putting it on top of the wing will make it closer to the cabin therefore more noise to the passengers.

Answer 7

It wouldn't be much of a jet if it didn't have engines. Jets require engines in order to fly. Without engines under the wings, a jet just sits on the ramp.

Answer 8

The new VLJ Honda Jet does have the engines on top of the wings (sort of) look at the picture http://www.vlj1.com/pages/HondaJet.html

Answer 9

I would think it would be WAY easier to service the engine, like remove it to overhaul it, the way they are now.

Answer 10

I have never seen a plane with engines mounted on the top of the wings before. I would think that since they are heavy it would be stronger to have them mounted hanging. Another thing to consider is the planes center of gravity, the fuel tanks are mounted low right? Wings would also not give as much lift with the engines on the top side